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Three-‐dimensional structure of the proteins
...
TOPIC 1
...
Levels of Protein Structure
...
Levels of Protein Structure
...
Over 300 different amino acids
exist in nature, but only 20 standard amino acids occur in proteins (also called standard aamino acids)
...
Amino acids consist of amino group (-NH2), which is basic and
carboxyl group (-COOH), which is acidic, therefore amino acids are amphoteric
...
Also attached to the carbon atom is a
hydrogen atom and distinctive side
chain (or R group)
...
Also, exist in D or L configuration, which are nonsuperimposable mirror images
...
Zwitterion: dominant
protonated
and
form of amino acids at neutral pH
...
Structure of the 20 standard amino acids: divided according to similarities in the
properties of their side chain (see handout for
structures)
...
2 configurations of peptide bonds: trans and cis
...
Trans: the 2 a-carbon atoms are on opposite sides of peptide bond
...
Also the carbonyl oxygen and amino hydrogen are trans
...
Proteins: molecular weight >5,000 Da e
...
insulin is around 5, 000 Da
Peptides: molecular weight <5,000 Da
Primary structure: linear sequence of amino acids in polypeptide chain, joined by peptide
bonds- determined by the sequence of nucleotide bases in gene encoding protein
...
Knowing primary sequence is
important for understanding the mechanism of action and determines the 3D structure of
proteins, protein function and evolutionary history
...
g
...
The a-helix is the
most abundant and stable configuration, with lowest energy
...
Also called 3
...
6
residues per turn and 13 atoms
of polypeptide backbone per
turn
...
The carboxyl
oxygen acts as a hydrogen
acceptor and the amino nitrogen
as a hydrogen donor
...
Van der
Waals
interactions
provide
additional stability
...
Stabilized by hydrogen bonds between CO group and NH group peptide
bond residues, far from each other, on same/different polypeptide
...
Β-turn (also called 310 helix): in order to fold into compact, globular structure, the
polypeptide chain often reverses direction, making a β-turn (or hairpin)
...
Tertiary structure: 3D arrangement of amino acids in polypeptide chain in space, as well as
side chain groups
...
Quaternary structure: spatial arrangement of subunits of a multimeric protein (protein
containing 2 or more polypeptide chains) and how they interact
...
g
...
Biologically active peptides: perform different functions in humans
...
• Tripeptide Glutathione (or γ-glutamyl-cysteinyl-glycine): important in redox systems,
particularly in RBCs
...
• Vasopressin: can increase blood pressure when administered in pharmacological
amounts
...
• Oxytocin: stimulates uterus contraction and induced labour in humans
...
• Enkephalines: are neuropeptides
...
All play role in control of pain
...
Endorphins bind to the same central nervous system
receptors as opiates
...
g
...
• β-lipotropin: hormone that stimulates release of fatty acids from adipose tissue
...
Include TRHC (Thyrotropin
Releasing Hormone) activates release of TSH (Thyroid Stimulating Hormone)
...
Protein folding
...
Protein folding:
Process by which the polypeptide chain folds into its stable, low energy, native conformation
...
According to Anfinsen’s law, the primary structure of a protein
determines its secondary, tertiary and quaternary structure
...
This occurs spontaneously, without input of
energy
...
Proteins do not fold via a ‘random search’, as this would require
millions of years
...
Short stretches of amino acid residues of the secondary
structure are formed, including regions of α-helices, β-sheets and
β-bends
...
Domains form
...
‘Molten globule’, with a compact, partially folded
conformation, substantial secondary structure and little tertiary
structure, forms
...
Conformation changes convert the ‘molten globule’ into the
native conformation, which is compact and folded, with tertiary
structure
...
Molecular chaperones:
Family of proteins, that ensure proteins are correctly folded, by binding to proteins and favoring
paths that inhibit inappropriate interactions and facilitate appropriate interactions
...
g
...
Hsp70: binds to a small stretch of
hydrophobic
amino
acids
on
the protein’s surface
...
Hsp70
dissociates by the re-binding of
ATP
...
Protein disulfide isomerase:
Catalyzes the formation of correct disulfide cross-links between cysteine residues in proteins, as
they fold
...
Prolyl-cis-trans isomerase:
Catalyze the interconversion of cis and trans peptide bonds
...
Includes:
•
•
•
•
•
•
Acetylation: introduction of acetyl group
...
g
...
Glycosylation: addition of sugars (mono or oligosaccharides) to proteins, to form
glycoproteins e
...
membrane proteins
...
Carboxylation: attachment of carboxyl group to protein, catalyzed by carboxylases
...
Protein function:
• Enzymes: catalyze chemical reactions, by converting substrates to products at the
enzyme active site
...
g
...
• Hormones:
protein
hormones
include
insulin,
glucagon,
ADH,
ACTH
(adrenocorticotropic hormone), GH (growth hormone), FSH, LH, parathyroid hormones
etc
...
• Regulation of gene transcription and translation: proteins associated with DNA;
repressors and enhancers
...
g
...
• Incorporated into cell or organelle membranes as receptors: bind effector molecular
and transduce signal from outside to inside
...
Enzymes: general properties, enzyme specificity
...
Isozymes
...
All enzymes are proteins, except ribozymes, which are RNA molecules that act as enzymes
...
Many enzymes require in addition to a substrate, a cofactor, for their activity
...
Metals e
...
Zn, Fe, Mn and Cu bind to the substrate or
stabilize intermediates in the reaction
...
g
...
Enzymes are specific, catalyzing only one reaction
...
In absolute, the
enzyme catalyzes a reaction with one substrate e
...
glucokinase only phosphorylates glucose,
not other hexoses
...
g
...
Reaction specificity is absolute
dehydrogenase
catalyzes
the
L-glutamate to a-ketoglutarate,
decarboxylation to GABA (gamma
acid) or its conversion to glutamine:
e
...
glutamate
oxidation
of
but not its
amino butyric
Enzyme Classification:
Enzymes are classified according to the
reaction they catalyze
...
Class 1: OXIDOREDUCTASES
...
g
...
Class 2: TRANSFERASES
...
g
...
Class 3: HYDROLASES
...
g
...
Class 4: LYASES
...
g
...
Class 5: ISOMERASES
...
g
...
Class 6: LIGASES (also called SYNTHETASES)
...
g
...
Isoenzymes (or Isozymes):
Isoenzymes are different forms of an enzyme, which catalyze the same reaction but differ in
their affinity to the substrate and regulatory factors, as well as stability
...
E
...
human sera contains isoenzymes of lactate dehydrogenase (LDH)
...
H and M can randomly combine with each other, forming 5 isoenzymes:
1
...
3
...
5
...
Clinical importance: under normal conditions, little LDH is found in blood, therefore the
appearance of some isozymes in blood is a sign of tissue damage e
...
LDH1 and 2 in serum is
found in patients with myocardial infraction and LDH5 in serum is found in patients with liver
disease
...
It consists of 3 isoenzymes:
1
...
MB CK2 in heart (increases in patients with myocardial infarction)
3
...
In contrast, alloenzymes are molecular variants of enzymes in individuals, families and ethnic
groups e
...
there are multiple variants of glucose 6 phosphate dehydrogenase
...
which has a higher free
is an intermediate state; it is neither S nor P
...
Rate of reaction is proportional to the number of molecules with free energy ≥
increase the rate of reaction by providing an alternative pathway in which
otherwise would be
...
Enzymes
is lower than
ΔG is the difference
in free energy
between S and P
...
e
...
e chemical affinity
Substrate induces conformational
change in active site-‐ this changes
the amino acids/other groups in
the enzyme
TOPIC 4 BIOCHEMISTRY: KINETICS OF ENZYME CATALYSIS, COMPETITIVE and NON-‐COMPETITIVE
INHIBITION, EXAMPLES OF INHIBITORS AS DRUGS
...
Rate of reaction: amount of product formed vs
time
...
ES complex can
dissociate to form E + S or proceed to form E and product (P)
...
At point A, the reaction is very rapid
...
Problem: how do you measure the rate of
reaction when it changes? In enzyme kinetics, we don’t measure all points,
just the initial rate of reaction
...
Therefore we can estimate Vmax by using
high [S]
...
Is a measure of the stability of ES
complex and affinity of E for S
...
A better estimate of Vmax and Kmax can be achieved by
plotting 1/V against 1/[S] in a Lineweaver-‐Burk plot:
Use reciprocal of the Michaelis-‐Menten equation, to draw
Lineweaver-‐Burk plot:
The rate of reaction of enzymes can be lowered by inhibitors
...
Reversible inhibition is subdivided into competitive and non-‐competitive
...
The competitive inhibitor binds
reversibly to the active site and rapidly dissociates from the enzyme-‐inhibitor (EI)
complex
...
Competitive inhibition can be overcome by increasing [S]
...
The competitive inhibitor increases the slope of the line
on the Lineweaver-‐Burk plot and alters the intercept on
the x-‐axis (since Km is increased) and leaves the intercept
on the y-‐axis unchanged (since Vmax remains the same)
...
This
changes the enzyme’s 3D shape, thus reduces its catalytic activity
...
The competitive inhibitor binds reversibly to the enzyme and rapidly
dissociates from the enzyme-‐substrate-‐inhibitor (ESI) complex
...
The affinity of the enzyme for the enzyme’s substrate is unchanged, so
Km remains the same
...
The non-‐competitive inhibitor increases the slope of
the line on the Lineweaver-‐Burk plot and alters the
intercept on the y-‐axis (since Vmax is decreased) but
leaves the intercept on the x-‐axis unchanged (since Km remains the same)
...
It is used in chemotherapy e
...
for leukemia
...
Thus,
and Sulfonamides
...
THF is a carrier of one-‐carbon units and is
important in metabolic pathways, such as purine and pyrimidine synthesis, thus affects DNA, RNA and
protein synthesis
...
They are used as antibacterial drugs
...
To do this, they compete with PABA
for the active sites of enzymes which synthesize folic acid
...
Thus,
sulfanilamides inhibit growth and cell division of bacteria
...
Ritonavir is an antiretroviral drug used to treat HIV
...
g
...
HIV-‐1 protease is important in the life cycle of HIV
...
Thus, ritonavir prevents HIV maturation
...
Control of Metabolic
Pathways, Examples: Glycogen Phosphorylase and Pyruvate Dehydrogenase
...
(1) Control of enzyme turnover (i
...
rate of enzyme synthesis and degradation)
(2) Control of enzyme activity (particularly of rate limiting enzymes in reaction pathway)
...
g
...
(2) Certain drugs e
...
phenobarbiturates induce Cytochrome P450
...
Enzyme synthesis is repressed by:
(1) Certain products of metabolic pathway e
...
the rate limiting enzyme in cholesterol synthesis,
HMG-CoA reductase, is repressed by cholesterol
...
e
...
g
...
Enzyme degradation is mediated by:
(1) Ubiquitin system (ATP dependent)
Ubiquitin is a small protein, present in all eukaryotic cells
...
Proteins destined for degradation are bound by several molecules of ubiquitin, with the carboxyl
(COOH) terminus of ubiquitin bound to lysine-NH2 groups of the protein
...
(2) Lysosomal-cathepsins system (ATP independent)
Control of enzyme activity:
Involves:
(1) Synthesis of proenzymes (inactive precursors) - also called zymogens
Proenzymes are inactive as their active sites are not completely formed or are inhibited
...
Proenzymes include digestive enzymes pepsinogen,
trypsinogen and procarboxypeptidase synthesized in the pancreas
...
(2) Covalent modification
Involves phosphorylation and dephosphorylation, catalyzed by protein kinase and protein
phosphatase, respectively e
...
glycogen phosphorylase is activated by phosphorylation, while
glycogen synthase, pyruvate dehydrogenase, acetyl CoA carboxylase, HMG-CoA reductase
etc
...
(3) Allosteric effects
Allosteric effectors bind to an allosteric site on the enzyme or a regulatory subunit
...
Allosteric effectors act as activators or
inhibitors
...
g
...
Control of metabolic pathways:
Involves:
(1) Feedback control, involving rate limiting enzymes
...
Feedback control occurs in
branching sites
...
TOPIC 6
...
Enzymes in disease diagnosis: Myocardial
infarction, Hepatitis, Acute pancreatitis, Parotitis
...
1 IU of enzyme activity catalyzes conversion of 1µmol
substrate to product per 1 minute
...
The reason
for testing enzyme activity is to show the changes that have occurred in a specific tissue or
organ due to damage or disease
...
Functional enzymes are present in the highest concentration, whereas non-functional
enzymes, at very low concentrations, but increase during disease
...
Therefore, measuring enzyme
activity is important in diagnosis as well as prognosis and treatment of disease
...
Also, few enzymes are tissue or organ
specific
...
Markers for Liver disease:
ü ASAT, ALAT and GGT
ASAT (aspartate amino transferase):
Enzyme that catalyzes the transfer of an amino group from glutamate to oxaloacetate, to form
α-ketoglutarate and aspartate
...
Exists in
cytoplasmic and mitochondrial forms
...
g
...
ASAT is non-specific, so ALAT must be tested
alongside
...
ALAT (alanine amino transferase):
Enzyme that catalyzes the transfer of an amino group from glutamate to pyruvate
...
ALAT increases with liver
damage, usually due to viral hepatitis
...
GGT is found in the liver, pancreas and kidney
...
However, it is not specific with the cause of liver disease; it
increases with many liver diseases
...
Higher levels of GGT can be found in chronic, heavy drinkers
...
ASAT and ALAT tests will indicate alcohol induced liver damage
...
Markers for Myocardial infarction:
ü CK and troponin, also LDH and myoglobin,
CK (Creatine Phosphokinase):
Enzyme that catalyzes creatine phosphate synthesis:
Creatine + ATP ↔ creatine phosphate + ADP
Found in the heart, brain, skeletal muscle
...
While total CK is non-specific,
CK-MB is specific for myocardial infarction
...
Troponin:
Protein that regulates muscle contraction
...
Exists in TnI,
TnC and TnT forms
...
After a myocardial infarction, TnI and
TnC increase within a few hours, but unlike CK-MB, remain elevate for 1-2 weeks
...
Myoglobin:
02-binding protein
...
Released more rapidly than CK and
troponin, but only remains elevated for ~24 hours
...
Found in almost all body cells, including the heart,
liver, skeletal muscle, kidneys etc
...
LD1 and LD2 are
found in the heart
...
3
...
Produced by the pancreas, as well as salivary glands
...
Lipase:
Enzyme for dietary fat digestion
...
Increased serum lipase may indicate pancreatitis
...
Nucleotide Structure and function
...
Energy-‐rich compounds
...
1
...
The
Nucleotides are the monomeric units of nucleic acids (DNA and RNA)
...
A nucleotide is a nucleoside, that forms a phosphate ester with the C5’OH group of
the sugar
...
(dCMP), deoxythymidine 5’-‐monophosphate (dTMP)
...
uridine
...
Nucleotides are not only the monomeric units of DNA and RNA
...
g
...
g
...
• AMP (Adenosine monophosphate): component of many coenzymes e
...
NAD+, NADP+, FAD,
coenzyme A etc, which are essential for metabolism of carbohydrates, proteins and lipids
...
• UTP (Uridine triphosphate): activates substrates e
...
UDP-‐glucose, for glycogen synthesis
(glycogenesis):
1
...
Glycogen primer (n) + UDP-‐glucose à glycogen (n+1) + UDP (catalyzed by
glycogen synthase)
• CTP (Cytidine triphosphate): synthesis of lipids
...
g
...
g
...
Also activation of cAMP-‐dependent protein kinases, which for
example regulate the effects of glucagon and adrenaline
...
2
...
The sugars are linked to one another by phosphodiester bridges
...
This is known as the sugar-‐phosphate backbone
...
The nitrogenous bases in DNA are: adenine and guanine (purines) and
cytosine and thymine (pyridmidines)
...
Energy rich compounds (also in Topic 10):
Molecules containing high-‐energy bonds are themselves energy-‐rich compounds
...
As a result, energy-‐
rich compounds provide energy for biochemical processes that require energy
...
Class
Bond
Example
Phosphoanhydrides
– C – P – P
ATP
Acyl phosphates
O
1, 3-‐ Bisphosphoglycerate
║
– C – O ~ P
Enol phosphates
– CH
PEP (phosphoenol pyruvate)
The squiggly
║
symbol ~
– C – O ~ P
represents the
Thiol esters (thioesters)
C
Acetyl CoA
high energy
║
bond
– C – O ~ S –
Guanidine phosphate
|
Phosphocreatine
– N~ P
ATP is the most important energy-‐rich compound in living cells
...
It has two high-‐energy phosphanhydride
bonds
...
3kcal/mol energy
...
6 KJ/mol= -‐7
...
Redox systems and Coenzymes (also in Topic 3):
Many enzymes require small molecules known as cofactors, for their catalytic activity
...
g
...
OR (2) small, inorganic molecules called coenzymes
...
Tightly bound coenzymes are
with its cofactor i
...
called prosthetic groups
...
holoenzyme
...
The oxidation number of the
FAD, FMN
Riboflavin (Vitamin B2)
species decreases or increases, by gaining
NAD+, NADP+, NADH, NADPH
Niacin (Vitamin B3)
(reduction) or losing (oxidation) an e-‐,
+
+
Tetrahydrofolate
Folic acid
respectively
...
8
...
DNA
...
Adenine
2
...
Thymine
4
...
In 5’ -‐-‐> 3’ direction
2
...
DNA -‐-‐-‐-‐-‐-‐-‐> mRNA -‐-‐-‐-‐-‐-‐-‐> Protein
DNA replication – the process of self-‐reproduction on cell delivery
DNA transcription – the process of complementary RNA synthesis
DNA package
•
•
•
•
1 Nucleosome is made of 200 base pairs wrapped twice around a histone octamer
Histone Octamer is formed by 2 of each histones
§ H2A
H2A + H2B form dimer
...
§ H3
H3 + H4 form tetramer
...
Nucleic Acids: RNA
...
Adenine
2
...
Cytosine
4
...
Overview of Intermediary Metabolism
...
The
High Energy Bond
...
Degradation of macromolecules:
molecules stored in cells to be converted
Proteins -‐-‐-‐> 20 AA
into building blocks needed for synthesis of
Carbs -‐-‐-‐> Glucose
complex molecules
...
Degradation of main organic molecules to simpler molecules:
Pyruvate and Acetyl Co-‐A
3
...
Anabolism
•
•
Refers to the conversion of small molecules into large ones during biosynthesis
Anabolic reactions require energy (endergonic), derived from ATP à ADP + inorganic phosphate (Pi)
1
...
Synthesis of macromolecules and complex compounds from the main organic molecules
...
5
61
49
43
41
The causes for ATP being energy rich compound:
1
...
2
...
5 kJ/mol
3
...
The Role of ATP
•
•
•
May be used as a mediator between super energy rich compounds and energy poor compounds
Play a central role in energy transfer
ATP has 2 transfer levels:
ATP -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐> ADP + Pi
ATP -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐> AMP + 2Pi
Example: In protein synthesis GTP is hydrolysed into GDP + Pi – ATP is used as the phosphate donor to
regenerate GTP:
GDP + ATP -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐> GTP + ADP
11
...
Structure of Electron Transfer Chain
...
Regulation of ETC: Inhibitors, Uncoupling agents
...
Mitochondrial Oxidation
...
NADH Dehydrogenase
2
...
Cytochrome C Oxireductase
4
...
Processes that take place in the mitochondria:
-‐ Kreb’s Cycle
-‐ Oxidative Phosphorylation
-‐ B-‐Oxidation
The ETC collects the reducing equivalents from all processes and direct them to their final reaction with
Oxygen
...
1
...
3
...
5
...
7
...
9
...
11
...
Cytochrome B
ii
...
Cytochrome c1
Complex 3 transfers electrons to cytochrome C (mobile protein)
Cytochrome C will bind to Complex 4
Complex 4 will reduce O2 to H2O
The coupling of processes of [O] & [P]
OXIDATIVE PHOSPHORYLATION: AN ENZYMATIC PROCESS THAT SYNTHESISES ATP FROM ADP
•
•
•
Electron transport & Phosphorylation are said to be tightly coupled
Protons will go through ATP synthase to produce ATP
4 H+ + ADP à 1 ATP + Pi
Complex I: Takes H+ atoms from NADH, transports them to the outer membrane and splits them into
protons and electrons
...
Complex II: Translocated protons are transferred here
...
Complex
I
II
III
IV
Total:
No of p+ pumped
4
0
4
2
10
Regulation of ETC: Inhibitors, Uncoupling agents
...
Uncoupling proteins create a “p+ leak” allowing protons to re-‐enter the matrix w/o capturing any
energy as ATP
•
•
•
The energy is released as heat and the process is called NONSHIVERING THERMOGENESIS
Nonshivering Thermogenesis occurs in brown adipose tissue and is the ability to uncouple oxidative
phosphorylation from ATP synthesis to generate heat
UCPS occur in the inner mitochondrial membrane
12
...
Digestion and Absorption of Carbohydrates
...
•
•
•
•
Most abundant organic molecule in nature (60% poly, 35% di, 5% mono)
We need approx 200g of carbs per day
...
Digestion
1) Starts in the mouth
...
Enzyme = endo-‐glycosidase
...
2) Continues in small intestine
...
Cl-‐
activates both types of amylase
...
Enzymes
Maltase, Isomaltase, Sucrase, Trehalase and lactase are anchored to the plasma membrane of the brush border of
cells of the SI
...
Lactose intolerance:
1o – Defective/Lack-‐ Defective lactase enzyme at
birth (rare)
...
2 o – Acquired – Consequence of damage to the brush border: Tropical sprue/ celiac sprue
Glucose Transporters – Each of the transporters has different affinities for glucose and the other hexoses, which
largely dictates their function
...
GLUT-‐1
Erythrocytes, brain, fetal
tissues
...
NOT fructose
...
NOT fructose
...
Glucose sensor in pancreatic β
cells
...
NOT fructose
...
Insulin dependent
...
NOT
glucose/galactose
...
Indicates that they are functioning at maximal rate under physiological
concentrations of glucose
...
It allows it to change transport rate in proportion to the increasing glucose
concentration that occurs after ingestion of a carb-‐rich meal
...
Glucose metabolism
...
•
•
•
•
•
Glucose can be converted to glycogen for storage (particularly in muscle & liver)
Glucose can be metabolised in PPP
Glucose can be converted into other MS’s: Galactose, fructose, glucoronic acid, hexosamines
Glucose is synthesised in GNG
Glucose metabolites are used for fat and AA synthesis
GLYCOLYSIS
Main glucose degradation pathway, resulting in ATP production
...
Main strategy:-‐ 1
...
Chemical transformation into compounds possessing energy-‐rich bonds
3
...
Overall Process: Glucose + 2ADP + 2Pi à 2 Pyruvate + 2ATP + 2NADH2 + 2H2O
In Anaerobic conditions: Glucose + 2ADP + 2Pi à 2Lactate + 2ATP + 2H2O
•
•
•
•
•
•
•
•
•
Energy Gain = 2ATP (1) –ATP (3) –ATP (7) +2ATP (10) +2ATP === Total 2ATP
Anaerobic Glycolysis is used in muscles when energy requirement must be fast, e
...
in a sprint
...
Anaerobic Glycolysis also occurs in white muscle fibre, smooth muscle, intestine, renal medulla, skin
...
Lactate is used in liver for GNG and in the heart, by converting it to pyruvate à krebs
...
Oxidative Decarboxylation of Pyruvate
...
Regulation
...
Oxidative Decarboxylation of Pyruvate
•
•
•
•
•
Pyruvate must be transported into the mitochondrion before it can enter the TCA cycle
...
Pyruvate dehydrogenase complex is a major source of acetyl CoA
Oxidative decarboxylation in aerobic glycolysis requires Vitamin B1
Pyruvate Dehydrogenase Complex (PDC) – multi-‐enzyme complex, which catalyzes pyruvate oxidation and
decarboxylation
...
Functions – OXD decarboxylation of a-‐keto-‐
glutarate DH complexes
-‐ decarboxylation of a-‐keto derivatives of
branched a-‐amino acids; Leucine, valine and isoleucine
Regulation of the PDC
•
•
•
•
•
PDH Kinase [P] & inactivates Pyruvate DH
PDH Phosphatase dephosphorylates & [A] Pyruvate DH
Calcium [A] PDH phosphatase
...
Note: This is particularly important in skeletal muscle, where release of Ca2+during contraction stimulates the
PDH complex, and thereby energy production
...
Forms the coenzyme CoA
...
Regulation
...
Relationship with other metabolic pathways
...
The main function of this cycle is
oxidation of acetyl-coA and production of NADH and FADH for the respiratory chain
...
The oxaloacetate acts as a catalyst (Mediates
the process and is regenerated at the end of the cycle
...
(1 molecule of
Oxaloacetate mediates the oxidation of numerous acetyl-coA molecules
...
There are 8 reactions which begin and end with the same product
...
The net reaction of the Krebs cycle:
CH3CO-CoA + 3NAD + FAD + ADP + Pi + 2H2O → 2CO2 + 3NADH2 + FADH2 +
ATP + CoASH
Reactions
1
...
OA binds to the active site of Citrate synthase causing conformational changes which expose the
residues which then bind to the Acetyl-CoA to form Citryl-coA which is hydrolysed by Water to Citrate and CoA
...
2
...
The isomerisation is an endergonic and reversible
process
...
)
3
...
Oxidative decarboxylation coupled to the reduction of NAD to NADH2
...
This reaction is the main regulatory step in the cycle
...
)
4
...
There are also 5 coenzymes required
for the overall reaction; thiamine pyrophosphotase, lipoic acid, CoASH, FAD and NAD+
...
The lesser amount of energy in the thioester bond is
used for the generation of GTP in the next step of the cycle…
5
...
This reaction is an example of
substrate level phosphorylation
...
6
...
This reaction is bidirectional and begins with the single electrons from 2 adjacent methylene
groups (-CH2) of succinate to one FAD bound to the Succinate DH, leaving the Succinate to form a double bond,
converting it to Fumarate
...
7
...
8
...
One complete cycle of the Krebs cycle produces 3 NADH2, 1 FADH2 and 1 ATP
...
)
Regulation:
The main regulatory reaction is step 3 by Isocitrate DH
...
Lesser regulatory steps
...
~ Citrate synthase is inhibited by ATP, NADH and Citrate
...
Amphibolic (Both anabolic and catabolic) nature of the Krebs cycle means that in every tissue intermediates (citrate malate,
Oxaloacetate) are removed
...
α -Ketoglutarate and oxaloacetate can, for example,
serve as precursors of the amino acids aspartate and glutamate by simple transamination
...
§
α –Ketoglutarate is used in glutarate and γ-AMB synthesis, important in nervous tissue
...
§
Malate is used in Gluconeogenesis
...
The Krebs Cycle is the final common stage of oxidative degradation of lipids, carbohydrates, and amino acids
...
Topic 16|Aerobic glycolysis
...
Glycerophosphate shuttle and malate shuttle to transport reducing equivalents
to mitochondria
...
This occurs in the
cytosol of the cell and directly generates ATP by transfer of high-energy phosphates from intermediates to ADP
...
If the cell contains mitochondria then the reducing equivalents of NADH can be transferred to the
mitochondrial ETC and the pyruvate can be completely oxidised to CO2 in the Krebs Cycle
...
Most cells are dependent upon glucose as fuel
...
g
...
In other cells, glycolysis must be AEROBIC, e
...
Heart, Brain, Retina, Renal cortex, and red muscle fibers
...
Glycolysis
2
...
Degradation of acetyl-CoA in the Krebs cycle and production of reducing equivalents
4
...
Transport of pyruvate and NADH through the inner mitochondrial membrane
GLYCOLYSIS
C6H12O6 + 2 NAD+ + 2 ADP + 2 P
2 pyruvate(CH3(C=O)COOH + 2 ATP + 2 NADH + 2 H+
Transfer of REs (reducing equivalents (NADH)) across the mitochondrial membrane requires the use of a shuttle system because the
inner mitochondrial membrane is impermeable to NADH
...
Transport
of reducing equivalents across the inner mitochondrial membrane is mediated by glycerophosphate shuttle and Malate-aspartate
shuttles
...
GLYCEROPHOSPHATE SHUTTLE (Glycerol-3-phosphate shuttle)
This shuttle transfers REs from cytosol to mitochondria only
...
This shuttle is mainly located in the brain and in muscle cells
...
MALATE-ASPARTATE SHUTTLE
This shuttle system is located in liver and heart cells and transports cytosolic and mitochondrial equivalents
...
Malate for α –Ketoglutarate
2
...
These enzymes have cytosolic and
mitochondrial isoenzyme forms
...
It yields 3 moles of ATP per mole
of cytosolic NADH
...
-Oxaloacetate in the mitochondria is transaminated with α –Ketoglutarate to produce Oxaloacetate and glutamate
...
A total of 36 ATPs are gained if the G-3-p DH shuttle is used, where instead of 2 molecules of reduced NADH2 in the cytosol, 2
molecules of reduced FADH2 are produced in the mitochondria
...
The red muscle fibers contain many more mitochondria to carry out aerobic glycolysis
whereas the white muscle fibers perform anaerobic glycolysis preferentially
...
The lactic acid cycle
...
The lactic acid thus generated should not be allowed to accumulate in the muscle tissues
...
This lactate diffuses into the
blood
...
Lactate then reaches 49 liver where it is oxidized to pyruvate
...
This cycle is called cori's cycle, by which the lactate is efficiently reutilised by the body
...
The lactate in the liver can enter the GNG pathway and become
glucose which then enters the blood and then muscles
...
Unlike the Liver, muscle cannot supply Glucose to other
organs despite having Glycogen
...
GNG is an anabolic process and occurs in the
liver 80%, kidneys 15%, gut 5%
...
It is GNG which maintains the blood/glucose levels during periods of starvation/fasting and so it is of great physiologic importance
...
g Red blood cells, nervous tissue/brain, Kidney medulla and testes
...
GNG is technically a “reverse” of glycolysis where glucose is synthesised, not catabolised
...
The main substrates for the GNG pathway include lactate, pyruvate, glycerol (from lipid metabolism), intermediates from the Krebs
cycle, and amino acids
...
Reaction steps: Seven of the reactions of glycolysis are reversible and are used in gluconeogenesis
...
First step: ATP required, endergonic
...
Requires 2 mitochondrial enzymes, pyruvate
carboxylase (Pyruvate is carboxylated to form Oxaloacetate, this enzyme is only found in the mitochondria, so the pyruvate must
travel there) and PEP carboxykinase (Uses GTP to decarboxylase Oxaloacetate to PEP)
...
Carboxylation of Pyruvate
...
1
...
2
...
Elevated levels of acetyl CoA may signal one of several metabolic
states in which the increased synthesis of oxaloacetate is required
...
At low levels of acetyl COA, pyruvate carboxylase is largely inactive and
pyruvate is primarily oxidized in the TCA cycle
...
transport of Oxaloacetate to the Cytosol
Oxaloacetate, formed in mitochondria, must enter the cytosol where the other enzymes of gluconeogenesis are located
...
There are 3 pathways through which this may occur;
1
...
Oxaloacetate undergoes transamination to Aspartate
3
...
)
These products are transported to the cytosol
...
Decarboxylation of Cytosolic Oxaloacetate
Oxaloacetate is decarboxylated and phosphorylated in the cytosol by PEP-carboxykinase
...
PEP then enters the reversed reactions of glycolysis until it forms fructose 1, 6- bisphosphate
...
13)
D
...
This fructose 6-phosphate is isomerised to glucose 6phosphate in the reversible reaction number 2 of glycolysis
Regulation of Gluconeogenesis
The main regulatory step involved: pyruvate carboxylase reaction
...
Glucagon and cortisol are the hormones which stimulate GNG in order to return blood/glucose levels to normal
...
As you know there are 10 reactions in total and 3 of them are irreversible
...
1
...
Enzyme: Hexokinase
...
High amounts of glucose6-phosphate will inhibit hexokinase
...
2
...
Main regulatory step! Enzyme: PFK-1 (Phosphofructokinase) converts Fructose-6-phosphate
to Fructose 1,6 bisphosphate (glycolysis)
...
In Gluconeogenesis the opposite reaction occurs (Fructose 1,6 bisphosphate converts to Fructose-6-phosphate by enzyme
Fructose 1,6-bisphosphotase) This enzyme is inhibited by a product Fructose 2,6-bisphosphate and insulin
...
GLYCOLYSIS (Phosphofructokinase)
GLUCONEOGENESIS (Fructobisphosphatase)
ATP inhibits
F2,6-Bisphosphate inhibits
Citrate inhibits
Insulin inhibits
AMP stimulates
Glucagon stimulates
F2,6-Bisphosphate stimulates
Irreversible Kinase Reaction #10
...
(Glycolysis)
In Gluconeogenesis the reaction has 2 steps, 1- Pyruvate to Oxaloacetate and 2-Oxaloacetate to PEP
...
Hormonal control of this enzyme is what controls this
overall reaction step #10 in gluconeogenesis
...
High amounts of Fructose 1,6-bisphosphate stimulate PK
...
GLUCONEOGENESIS (PEP-CK)
Glucagon will stimulate PEP-CK
High amounts of Camp-dependent-proton-kinase will
stimulate PEP-CK
...
Topic 19 | The Pentose Phosphate pathway
...
There are 2 major pathways in the human body which catabolise glucose – Glycolysis and Pentose-phosphate pathways (PPP)
...
However, this pathway does oxidise glucose and under certain conditions can completely
oxidise glucose to CO2 and water
...
To generate reducing equivalents, in the form of NADPH, for reductive biosynthesis reactions within cells
...
To provide the cell with ribose phosphates (R5P) for the synthesis of the nucleotides, ATP, NAD, CoA, FAD and nucleic acids
...
Although not a significant function of the PPP, it can operate to rearrange the carbon skeletons of dietary carbohydrates,
monosaccharides- ribose and “non-hexoses” to glycolytic/gluconeogenic intermediates, e
...
The PPP has 2 stages: 1) Irreversible glucose oxidation to Pentose-5-phosphate AND production of NADPH2
...
The reactions of fatty acid biosynthesis and steroid biosynthesis utilise large amounts of NADPH
...
30% of the oxidation of glucose in the liver occurs via the PPP
...
The conversion of ribonucleotides to deoxyribonucleotides (through
the action of ribonucleotide reductase) requires NADPH as the electron source, therefore, any rapidly proliferating cell needs large
quantities of NADPH, and such is seen in cancerous cells
...
Reactions of the Pentose Phosphate Pathway: The first three reactions of the PPP are referred to as the oxidative portion and are the
reaction that yield NADPH
...
G6PDH = glucose-6-phosphate dehydrogenase
...
PGD = 6-phosphogluconate dehydrogenase
...
RPIA = ribose-5-phophate isomerase / phosphopentose isomerase
Transketolase and Transaldolase
...
The pentose phosphate pathway has both an oxidative and a nonoxidative arm
...
The reactions catalysed by glucose-6-phosphate dehydrogenase (G6PDH) and 6phosphogluconate dehydrogenase (PGD) both generate one mole of NADPH for every mole of glucose-6-phosphate that enters
the PPP
...
The non-oxidative reactions of the PPP are primarily designed to generate ribose-5-phosphate (R5P)
...
The primary enzymes involved in the non-oxidative steps of the PPP are transaldolase and transketolase
...
Like other enzymes that transfer two-carbon groups, transketolase requires thiamine pyrophosphate (TPP) as a co-factor
in the transfer reaction
...
The net result of the PPP, if not used solely for R5P production, is the oxidation of G6P, a six-carbon sugar, into a five-carbon sugar
...
The six-carbon sugars can be recycled into the pathway in the form of G6P, generating more
NADPH
...
Alternatively, it can be utilized by the gluconeogenic enzymes to generate more six-carbon sugars, fructose-6-phosphate
or glucose-6-phosphate
...
Blood glucose can be obtained from three primary sources: the diet, degradation of glycogen, and gluconeogenesis
...
Glycogen is considered the principal storage form of glucose and is found mainly in the liver and muscle
...
Glycogen is a polymer of glucose residues linked by
-(1,4)- and
-(1,6)-glycosidic bonds
...
It’s a good
source of fuel for anaerobic activity as it doesn’t require O2 for breakdown – These properties make it an efficient storage molecule
...
GLYCOGENESIS - activated when the body is in a state of rest or during high glucose level in the blood
...
Glucose is first converted into glucose-6-phosphate by the action of glucokinase
...
•
Glucose is converted into glucose-6-phosphate by glucokinase in liver or by hexokinase in muscles
...
(Passing through the obligatory
intermediate glucose-1,6-bisphosphate
...
Pyrophosphate is formed,
which is later hydrolysed by pyrophosphatase into two phosphate molecules
...
Initially, about eight UDP-glucose molecules are added to each tyrosine residue by glycogenin, forming
(1 4)
bonds
...
Branches are made by glycogen branching enzyme (also known as amylo- (1:4)
the end of the chain onto an earlier part via
more
(1:6)transglycosylase), which transfers
-1:6 glycosidic bond, forming branches, which further grow by addition of
-1:4 glycosidic units
...
Degradation of stored glycogen, occurs through the action of glycogen phosphorylase
...
The product of this reaction is glucose-1-phosphate
...
The glucose is removed from glycogen is an activated state, i
...
phosphorylated and this occurs without ATP hydrolysis
...
The concentration of Pi in the cell is high enough to drive the equilibrium of the reaction in the favourable direction
since the free energy change of the standard state reaction is positive
...
Therefore, any glucose released
from glycogen stores of muscle will be oxidized in the glycolytic pathway
...
Biochemistry 2015
21
...
Glycogenolysis
...
Major regulatory mechanisms: induction of
enzyme synthesis, changes in enzyme activity by phosphorylation and
dephosphorylation, allosteric control
...
Reciprocal regulation of glycogen synthase and glycogen phosphorylase
...
Regulation of Glycogen Metabolism
Stores of readily available glucose to supply the tissues with an oxidisable energy source are found
principally in the liver, as glycogen
...
A second major source of stored glucose is the glycogen of skeletal
muscle
...
Regulation is performed in a similar was as that of gluconeogenesis and glycolysis, on three different
levels:
1
...
Enzyme phosphorylation/ dephosphorylation
3
...
i
...
In diabetes, the quantity of
glycogen synthase decreases
...
•
Mechanism of covalent modification and allosteric control
Main regulatory enzyme of glycogen synthesis: glycogen synthase
•
Main regulatory enzyme of glycogen degradation: glycogen phosphorylase
•
Both enzymes regulated by phosphorylation/dephosphorylation mechanism, as well as
allosterically
•
Control of glycogen metabolism involves a cascade of events where an enzyme catalyses the
modification of a second enzyme, which
in turn catalyses the modification of a
third, and so on
•
Regulation of glycogen phosphorylase
and glycogen synthase under the control
1
Biochemistry 2015
of hormones: glucagon, epinephrine, cortisol and insulin
•
GLUCAGON AND EPINEPHRINE act as extracellular regulators: their mechanism of action
includes second messengers cAMP, IP3 and DAG
•
Cells possess different adrenergic receptors: α1, 2, β1, 2, 3
...
First messenger is the hormone molecule (does no
penetrate cell membrane)
...
cAMP may be degraded by phosphodiesterase, which is
activated by insulin
•
When epinephrine binds α2 adrenoceptors, adenylate cyclase is inhibited, which results in
the activation of glycogen synthase and inactivation of glycogen phosphorylase a
...
•
Epinephrine on α1: phospholipase C is activated, which catalyses the hydrolysis of membrane
phospholipid phosphatidylinositol-‐diphosphate into 2 products; inositol triphosphate IP3 and
diacylglycerol DAG
•
IP3 opens calcium ion channels in the ER, thus increasing its concentration in the cytoplasm,
where calcium binds to calmodulin
...
DAG activates PK-‐C
Pathology of Glycogen Metabolism
•
•
The deficiency impairs the ability of the liver to produce free glucose from glycogen and
from gluconeogenesis
...
This can lead
to enlargement of both
...
It is caused by an accumulation of glycogen in the lysosome
due to deficiency of the lysosomal acid alpha-‐glucosidase enzyme
...
C
...
•
The build-‐up of glycogen causes progressive muscle weakness (myopathy) throughout the
body and affects various body tissues, particularly in the heart, skeletal muscles, liver and
nervous system
•
Type V: Glycogen storage disease type V (GSD-‐V) is a metabolic disorder, more specifically a
glycogen storage disease, caused by a deficiency of myophosphorylase
...
•
Type I: Glycogen storage disease type I (GSD I) or von Gierke's disease, is the most common
of the glycogen storage diseases
...
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Biochemistry 2015
23
...
Control of Blood Glucose
Levels in Different Conditions: Well Fed, Fasting, Prolonged
Starvation
...
Regulation of Carbohydrate Metabolism
•
Glucoregulation is the maintenance of steady levels of glucose in the body; it is part of
homeostasis, and so keeps a constant internal environment around cells in the body
...
rise by 5mM
glucose conc
...
rise in fatty acid conc
...
o
GLUT-‐2: β cells,
liver,
kidney,
intestinal epithelium
o
GLUT-‐3: neurons
o
SGLT-‐1: intestinal
epithelium, kidney
o
SGLT-‐2: kidney
Processes Affected by Insulin and Glucagon
• Muscle: insulin stimulates glucose uptake and consumption
•
Liver: glucagon stimulates glucose synthesis and export
PROCESS
glucose uptake (muscle and adipose)
gluconeogenesis (liver)
glycogenesis (liver and muscle)
glycogenolysis (liver and muscle)
Insulin
+
-‐
+
-‐
Glucagon
0
+
-‐
+
Actions of Insulin and Glucagon
Insulin:
1
...
Activates glycogen synthase and inactivates glycogen phosphorylase (liver and muscle)
3
...
Insulin stimulates glucose-‐6-‐P to pyruvate via effect on phosphofructokinase 1
b
...
FAs made in liver are converted to TAGs and
transported in VLDLs to fat cells
c
...
Note-‐ stored fat
derived from glucose
Glucagon:
1
...
mM
2
...
Inhibits liver glycolysis and stimulates gluconeogenesis (by lowering levels of fructose-‐2,6-‐
bisphosphate and inhibiting pyruvate kinase
4
...
Adrenaline acts on muscle
(glycogenolysis), liver (glycogenolysis) and adipose tissue (lipolysis)
...
8 mmol/L (140 mg/dL) or slightly more
•
Fasting: normal blood glucose level (tested while fasting) for non-‐diabetics, should be
between 3
...
5 mmol/L (70 to 100 mg/dL)
...
5 mmol/L (100 mg/dL)
•
Prolonged starvation: blood glucose levels will remain relatively stable during prolonged
starvation
...
At first, the brain continues to use glucose, because, if
a non-‐brain tissue is using fatty acids as its metabolic fuel, the use of glucose in the same
tissue is switched off
...
This high blood sugar produces the classical
symptoms of polyuria (frequent urination), polydipsia (increased thirst) and polyphagia (increased
hunger)
...
(Also referred to as insulin-‐dependent diabetes mellitus, IDDM for short, and
juvenile diabetes
...
(Formerly referred to as non-‐insulin-‐
dependent diabetes mellitus, NIDDM for short, and adult-‐onset diabetes
...
It may precede development of type 2 DM
...
Classification of Lipids
...
Synthesis of Chylomicrons
...
Digestion of lipids, emulsifying (role of bile
salts), hydrolysis of various types of lipids
...
Absorption of lipids
...
Synthesis of
chylomicrons
...
Excretion of
chylomicrons from intestinal cells
...
Chylomicrons -‐ synthesis and importance
...
Structure
of apolipoprotein B48
...
Classification
•
Fatty acids and triacylglycerols: FAs stored as TAGs, serve as fuel
•
Glycerophospholipids and sphingolipids: in membranes and in blood lipoproteins
•
Eicosanoids: PUFAs containing 20 carbon atoms form the eicosanoids; regulate many cellular
processes
•
Cholesterol, bile salts, steroid hormones: cholesterol stabilises phospholipid bilayer; also
serves as precursor of bile salts and steroid hormones
•
Fat-‐soluble vitamins: lipids involved in various functions such as vision, growth,
differentiation, blood clotting, calcium metabolism, etc
...
Route, to a certain extent, depends on the chain length of the FAs
Lingual and Gastric Lipases
• Preferentially hydrolyse short and medium chain FAs (12C or less)
...
This means it is suspended
in small particles by bile salts
•
Bile salts are synthesised in the liver and secreted via the gall bladder into the intestinal
lumen
...
Emulsification is not digestion per se; it greatly increases the
surface area of the fat to be attacked by digestive enzymes from the pancreas
8
Biochemistry 2015
o
Solubilisation and transport of lipids in an aqueous environment: bile acids are lipid
carriers and are able to solubilise many lipids by forming micelles
...
Colipase
binds to dietary fat and t the lipase, causing it to be more active
...
30-‐40% of lipids are digested
completely, 40-‐50% digested into 2 MAG, 5-‐10%are non-‐digested TAGs
•
Pancreas also produces esterases and phospholipases
•
Pancreatic phospholipase produced in form of pro-‐phospholipase A2 (pro-‐ PLA2)
...
Active pro-‐ PLA2 hydrolyses FAs from position 2 of the glycerol
moiety, converting phospholipids into lyso-‐phospholipids, which are powerful detergents
•
Pancreatic esterase hydrolyses cholesterol esters (CEs) into cholesterol and FA
•
Deficiency of digestive enzymes leads to steathorea
Absorption
•
FAs, lyso-‐phospholipids and 2 MAGs are packed into micelles
...
Also packaged
into newly forming apoB48 lipoprotein
9
Biochemistry 2015
•
Continued lipidation in the ER inflates the particle, requiring stabilisation of its surface by
phospholipids
and apoA-‐IV
...
The apoA-‐1 originates
both from local intestinal synthesis and
from plasma
...
TAGs are transported by lipoprotein
particles because they are insoluble in water
•
General structure of lipoprotein:
10
Biochemistry 2015
o
o
•
Core consisting of a droplet of TAGs and/or cholesterol esters (CEs)
Surface monolayer of phospholipid, unesterified cholesterol and specific proteins
(apolipoporteins)
Lipoproteins differ in content of protein and lipids, and classified depending on their density:
o
CM: largest, low density
...
Enter blood via thoracic duct
•
Transport of dietary lipids in the blood performed by CMs
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Biochemistry 2015
25
...
TAG Transport by Lipoproteins
...
Importance of the liver in metabolism of chylomicrons
Lipid synthesis and VLDL in the liver
...
Transport of endogenous lipids to peripheral tissues
...
Abnormalities in the synthesis of VLDL -‐ fatty liver
...
LPL is produced by
adipose tissue, muscle cells, spleen, lungs, and neonatal liver
...
Reaction converts polar cholesterol into non-‐polar, and thus removes
cholesterol from membrane into interior
•
Abnormalities of lipoprotein metabolism cause various hypo-‐ or hyperlipoproteinemias
...
•
Chylomicrons and VLDL deliver TAG to cells in the body
...
Chylomicrons are synthesized by enterocytes
from lipids absorbed in
the small intestine
...
The function
of these lipoproteins is
to deliver energy-‐rich
triacylglycerol (TAG) to
cells in the body (pink
pathway)
...
This enzyme digests the TAG to fatty acids and
monoglycerides, which can then diffuse into the cell to be oxidized, or in the case of an
adipose cell, to be re-‐synthesized into TAG and stored in the cell
...
These then pass
from the intestinal lumen into the enterocyte, where they are re-‐esterified
to form triacylglycerol
...
These are then released by exocytosis from enterocytes into
lacteals, lymphatic vessels originating in the villi of the small intestine, and
are then secreted into the bloodstream at the thoracic duct's connection
with the left subclavian vein
...
The main apolipoprotein component is apolipoprotein B-‐48 (apo B-‐48)
...
The HDL donates apolipoprotein C-‐II (APOC2) and
apolipoprotein E (APOE) to the nascent chylomicron and, thus, converts it to
a mature chylomicron (often referred to simply as "chylomicron")
...
Chylomicron remnant:
§
Once triglyceride stores are distributed, the chylomicron returns APOC2 to
the HDL (but keeps APOE), and, thus, becomes a chylomicron remnant, now
only 30–50 nm
...
Metabolism of VLDL
•
VLDL synthesis occurs in the liver
...
Each VLDL particle
contains one apoB100 molecule
•
Main function: transport TAG form liver to
other tissues
•
2 sources of TAG in the liver:
o
FAs synthesised
carbohydrates
from
o
FAs taken up by the liver
o
dietary
14
Biochemistry 2015
•
VLDL interacts with HDL and receives additional apoC-‐II and apoE
...
VLDL converted to IDL
•
Some IDL taken up by liver, because cells need more TAG than cholesterol
...
The process involves the discharge of more TAG catalysed by LPL
•
Most of the apoC-‐II and apoE are lost during conversions of VLDL to IDL and IDL to LDL
•
The LDL receptor can still recognise and take up LDL containing only apoB100
•
Uptake of LDL depends on recognition by an apoB100/apoE receptor
...
•
By these processes, CE is delivered from LDL to the live and to extrahepatic tissues
15
Biochemistry 2015
26
...
Structure of apolipoprotein B100
...
Transport cholesterol to peripheral tissues
...
Exchange of HDL: synthesis; intravascular exchange of lipid
components of lipoproteins; esterification of cholesterol -‐ the role of LHAT,
participation in the transport of HDL cholesterol
...
Hypercholesterolemia
...
The cholesterol pool in liver cells also receives the
dietary cholesterol, which is contained in the chylomicron remnants that are formed through the
extraction of triacylglycerol from chylomicrons by lipoprotein lipase
...
Like
chylomicrons, VLDL interacts
with lipoprotein lipase and
thereby
turns
into
intermediate (IDL) and then
low density lipoprotein
(LDL)
...
Excess cholesterol is
exported from the cell by an active transporter (ABCA1) and delivered to high density lipoprotein
(HDL), which then carries it back to the liver
...
About 70% of plasma cholesterol occurs in this form
...
Uptake of cholesterol into cells occurs when lipoprotein binds to
LDL receptors on the cell surface
...
Disorders involving a defect in or lack of LDL receptors are
usually characterised by high plasma cholesterol levels
...
This is the case in the inherited
disorder familial hypercholesterolemia
...
They are composed of 50% protein, with
phospholipid and cholesterol as the remainder
...
The role of HDL is to transport excess cholesterol from the tissues (including the
arterial wall) to the liver for disposal
...
Seems to have 2 different tasks:
o
o
•
Collect cholesterol from plasma membrane and issue to the extracellular medium
Allow cholesterol esterification by LCAT (lecithin acyl transferase)
Cholesterol efflux from membranes depends on lipid content of apolipoprotein (HDL)
...
Conversion to HDL precursors
o
Cholesterol issued from the cells of peripheral tissues, either by passive diffusion or
through the action of recently identified ATP dependent transmembrane
transporter: ATP binding cassette (ABC1)
...
After the cholesterol becomes associated
with HDL, LCAT catalyses the conversion of cholesterol to CE
o
Through selective uptake of CE rich HDL, mediated by hepatic receptor SRB1
scavenger receptor
...
o
The enzyme is able to remodel HDL through the hydrolysis of surface phospholipids,
thus allowing CE to flow from the lipoprotein core to the plasma membrane
LCAT Reaction
• The HDL particle contains the enzyme lecithin cholesterol acyltransferase or LCAT for short,
which
converts
cholesterol
to
cholesterol esters
...
The
transfer of one acyl
chain from a lecithin
(phosphatidylcholine)
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Biochemistry 2015
molecule to cholesterol produces a cholesterol ester and lysolecithin
...
•
Cholesterol also undergoes esterification as it is packaged into chylomicrons and VLDL inside
intestinal and liver cells, respectively
...
Atherosclerosis can begin to develop in
adolescence and progress without any symptoms for many years
...
•
Hyperlipidaemia is a common and treatable cause of atherosclerosis
...
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Biochemistry 2015
27
...
Regulation of Lipolysis and Lipogenesis
...
Participation of adipolytic lipase,
hepatic lipase, lipiproteinlipase and digestive enzymes
...
General synthetic route, the synthesis in adipose
tissue and the intestinal cells
...
Rapid
hormonal control by phosphorylation / dephosphorylation of enzymes, slow
hormonal control
...
Metabolism of glycerol
...
Natural uncoupling agents -‐ fatty
acids, bilirubin, thyroid hormones, UCP
...
) enclosed by a monolayer
of phospholipids and hydrophobic proteins, such as the perilipins in adipose tissue or oleosins in
seeds
...
They are not unique to
animals and plants as Mycobacteria and yeasts have similar lipid inclusions
...
However, lipid droplets may also serve as a
protective agency to remove any excess of biologically active and potentially harmful lipids such as
free fatty acids, diacylglycerols, cholesterol (as cholesterol esters), retinol esters and coenzyme A
esters
...
Main lipases: LPL, pancreatic lipase,
adipolytic and liver lipase
...
Attached to the luminal surface of endothelial cells in capillaries
•
Pancreatic lipase is a TAG lipase
...
Converts IDL to LDL
•
Adipolytic lipase hydrolyses TAG to DAG and DAG to MAG
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Biochemistry 2015
Synthesis of TAGs
•
Conducted mainly in liver, adipose tissue and intestine
•
3 different pathways of synthesis: general, adipose and intestinal
General Pathway
• Occurs in liver, kidneys, brown adipose tissue, mammary glands and intestines
•
Pathway starts with glycerol 3 phosphate and
produces intermediate phosphatidic acid
•
Pathway starts with glycerol phosphorylation by
specific glycerol kinase
•
TAG, synthesised in the liver, is packaged with
cholesterol, phospholipids and proteins to form VLDL
Adipose Tissue Pathway
• Adipose tissue lacks glycerol kinase and can only
produce glycerol phosphate from glycerol via
dihydroxyacetone phosphate
•
DHAP is reduced by glycerol phosphate
dehydrogenase, producing glycerol 3 phosphate
...
Trigger GPCRs which activate adenylate cyclase
•
Insulin stimulates phosphoprotein phosphatase PRP!, which dephosphorylates lipase and
makes the enzyme inactive
•
Insulin also stimulates phosphodiesterase activity that lowers levels of cAMP
...
cAMP dependent PKA phosphorylates perilipin A, causing hormone sensitive lipase in the
cytosol to move to the lipid droplet surface
...
These 2 phosphorylations cause about a 50-‐fold increase in fat metabolism
Thermogenesis
•
Thermogenesis is the process of heat production in organisms
•
One method to raise temperature is through shivering
...
It is not 100% efficient, meaning while some of the energy becomes heat, a
portion is transferred to the kinetic energy that produces its characteristic muscular
twitches
...
Shivering is the process by which the body temperature
of hibernating mammals (such as some bats and ground squirrels) is raised as these animals
emerge from hibernation
•
Non-‐shivering thermogenesis occurs in brown adipose tissue (brown fat) that is present in all
eutherians (swine being the only exception currently known)
...
This uncouples oxidative phosphorylation, and the energy from the proton motive
force is dissipated as heat rather than producing ATP from ADP, which would store chemical
energy for the body's use
...
•
The low demands of thermogenesis mean that free fatty acids draw, for the most part, on
lipolysis as the method of energy production
...
Some hormones, such as norepinephrine and leptin, may stimulate
thermogenesis by activating the sympathetic nervous system
...
β-‐Oxidation of FAs with an Even Number of Carbon Atoms
...
Sources of fatty acids in the body
...
β-‐
oxidation
...
Transport of activated fatty acids across
the mitochondrial membrane -‐ carnitine shuttle
...
Energetic balance
...
Pathology -‐ deficiency of acyl-‐
CoA dehydrogenase
...
Most FAs present in human cells are long chain
...
The FAs form complexes with albumin in the blood, and are taken up by
muscle, kidney and other tissues; where ATP is generated by their oxidation to carbon dioxide and
water
...
On entering
the cytosol, free FAs are bound by a FABP
...
FAs are oxidised into activated 2C molecules;
acetyl CoA
Activation and Membrane Transport
•
Free fatty acids cannot penetrate any biological membrane due to their negative charge
...
Once in the cytosol, the following processes
bring fatty acids into the mitochondrial matrix so that beta-‐oxidation can take place
...
•
If the fatty acyl-‐CoA has a long chain, then the carnitine shuttle must be utilized:
o
Acyl-‐CoA is transferred to the hydroxyl group of carnitine by carnitine
palmitoyltransferase I, located on the cytosolic faces of the outer and inner
mitochondrial membranes
...
24
Biochemistry 2015
o
•
Acyl-‐carnitine is converted back to acyl-‐CoA by carnitine palmitoyltransferase II,
located on the interior face of the inner mitochondrial membrane
...
If the fatty acyl-‐CoA contains a short chain, these short-‐chain fatty acids can simply diffuse
through the inner mitochondrial membrane
...
The process
consists of 4 steps
...
A long-chain fatty acid
is dehydrogenated to create a
trans double bond between C2
and C3
...
It uses FAD as an electron acceptor and it is reduced to FADH2
...
Trans-delta2-enoyl CoA is hydrated at the double bond to produce L-B-hydroxyacyl CoA
by enoyl-CoA
hydratase
...
L-B-hydroxyacyl CoA
is dehydrogenated
again to create Bketoacyl CoA by Bhydroxyacyl CoA
dehydrogenase
...
4
...
Thiolase enzyme
catalyses the
reaction when a new
25
Biochemistry 2015
molecule of coenzyme A breaks the bond by nucleophilic attack on C3
...
The
process continues until all of the carbons in the fatty acid are turned into acetyl CoA
...
However, some tissues such as
the adrenal medulla do not use fatty acids for their energy requirements, but instead use
carbohydrates
...
Most severe: LCA type
which leads to the syndrome of sudden death in infants
...
Oxidation of Fatty Acids with an Odd Number of Carbon Atoms
...
Role of vitamin B12
...
Fatty acids that enter betaoxidation with an even number of carbons are converted entirely to acetyl-CoA, with the last round
producing two acetyl-CoA molecules from one four carbon fatty acid
...
Fatty acids that have an
odd number of carbons present a special challenge
...
Propionyl-CoA is not a
common metabolic intermediate in other pathways, so it must be converted into something else to
be effectively metabolised
...
PropionylCoA is converted to succinyl-CoA, an intermediate in the Kreb's cycle
...
Mutation of propionyl-CoA
carboxylase has been observed to cause a genetic condition called propionic acidemia
...
Lipolysis is carried out by lipases
...
Beta oxidation splits long carbon chains of the fatty acid into acetyl CoA, which can eventually
enter the TCA cycle
...
All B vitamins helps the body convert food (carbohydrates) into fuel (glucose), which is used to
produce energy
...
B complex vitamins are needed for healthy skin, hair, eyes, and liver
...
All B vitamins are water-soluble, meaning that the body does not store them
...
Vitamin B12 also works
closely with vitamin B9, also called folate or folic acid, to help make red blood cells and to help
iron work better in the body
...
Vitamins B12, B6, and B9 work together to control blood levels of the amino acid homocysteine
...
However, researchers aren't sure
whether homocysteine is a cause of heart disease or just a marker that indicates someone may have
heart disease
...
That may be because their diets are not as healthy or because they have less
stomach acid, which the body needs to absorb B12
...
Severe deficiency of B12 causes nerve damage
...
Ketogenesis and Ketolysis
...
Ketoacidosis
Ketone bodies are alternative energy sources for skeletal muscles, cardiac muscle, smooth muscles
of the digestive system and the kidney cortex
...
Ketone body is the nonsystematic name for acetoacetate, Beta-hydroxylbutyrate and acetone
...
Ketogenesis occurs exclusively in the liver’s mitochondria
...
Ketone bodies are synthesised from acetyl-CoA by condensation of two molecules through the
reversion of the last reaction of beta-oxidation
...
Acetoacetyl CoA reacts with acetyl CoA to form beta-hydroxyl-beta-methylglutaryl-CoA
...
This synthesis involves condensation and hydrolysis and this
reaction is exergonic and unidirectional
...
Acetoacetate can be reduced by NADH to form Beta-hydroxybutyrate
...
Acetoacetate and hydroxybutyrate are both ketone bodies formed during
the catabolism of a few amino acids
...
Ketolysis - After release from the liver into the circulation, ketone bodies are metabolised by
mitochondria of extra-hepatic cells
...
Acetoacetate reacts with succinyl-CoA, an intermediate of the Krebs
cycle, to form acetoacetyl-CoA and succinate - a reaction catalysed by thioforase
...
Ketone
bodies are able to change the regulation of carbohydrate metabolism, inhibiting PDH and pyruvate
kinase reactions, hence stimulating the conversion of pyruvate into glucose
...
In starvation and diabetes increased glucagon levels stimulates lipolysis leading to an elevation of
FFA concentration in serum and as a consequence to increased ketone body production by 12 mmol/
L
...
Fatty acids are liberated from adipose tissue and are
transported to the liver as a complex with serum albumin
...
Glucagon stimulates ketogenesis inhibiting the synthesis of fatty acids or triacylglycerol (TAG) thus
decreasing the consumption of fatty acids in the cytoplasm
...
Insulin effects are opposite to those of glucagon
...
Ketoacidosis occurs in adults if they consume less than 100g/day for a prolonged period
...
However, the liver can oxidise more fatty acids than it needs and thus metabolises it to acetyl-CoA
in excess of it’s needs and then synthesises acetoacetate and hydroxybutyrate (ketone bodies) for
export into other tissue as an additional metabolic fuel
...
When starving to slim, blood glucose drops as this source of energy is being used
...
Free fatty acids increases
from the fed to fasting state but then do not rise further
...
Diabetic ketoacidosis is the production of inordinate amounts of ketone bodies in diabetes resulting
in diabetic ketoacidosis
...
The excretion of ketones in the urine is called
ketouria
...
Fatty acid biosynthesis
...
Substrates: 1 Acetyl CoA which is derived from carbohydrate in PDH reaction, from BetaOxidation and amino acid degradation
...
Source of NADPH2 in
the liver is Pentoso-Phosphate-pathway and adipose tissue it is malic-reaction
...
Strategy: Biosynthesis of FA is a cyclic process, Malonyl-CoA is a 3 C precursor of the 2
C fragments that are incorporated stepwise during fatty acid biosynthesis
...
Transport of Acetyl-CoA - produced in the mitochondria - FA synthesis is a cytoplasmic
process - acetyl-CoA must be transported to the cytoplasm
...
This is exported from the mitochondria by means of antiport
system
...
The malate is transported into the mitochondrion
...
A second route for cytosolic malate is malic-reaction: Oxidative decarboxylation to
pyruvate, CO2, NADPH2
...
Formation of Malonyl-CoA - The acetyl-CoA carboxylase catalyses the synthesis of
malonyl-CoA
...
Acetyl-CoA carboxylase contains
covalently bound biotin, and the mechanism and bioenergetics of this reaction parallel
pyruvate carboxylase
...
Fatty Acid Synthase as a Multienzyme Complex
Biosynthesis of fatty acids is catalysed by a polyprotein called fatty acid synthase
...
Reactions:
To initiate biosynthesis, acetyl-CoA reacts with peripheral thiol, where it is bound as a
thirster
...
Condensation reaction between alpha-carbon of malonyl-ACP is driven by the concomitant
decarboxylation, yielding acetoacetyl-ACP synthase
...
The acetyl group from acetyl-CoA forms the
terminus of acetoacetyl-ACP, the last two carbon atoms at the omega-end of the fatty acid
...
The next step involves a dehydration to 2,3-ttrans-butenoyl-ACP (crotonyl-ACP) catalysed
by the beta-hydroxacyl-ACP dehydrates
...
Two NADPH2 molecules are required for each two carbon unit that is added
during biosynthesis
...
Malonyl-CoA next combines with ACP
...
Palmitate is released from the fatty acid synthase complex by an exergonic hydrolysis
reaction
...
Net reaction:
Acetyl-CoA + 7 malonyl-CoA + 14 NADPH2 ——> CH3(CH2)14COOH + 7 CO2 + 8CoA +
14 NADP + 6 H2O
Acetyl-CoA serves as the primer for biosynthesis; acetyl-CoA provides the two carbon
atoms farthest from the carbonyl group
...
All
carbon atoms of FA are derived from acetyl-CoA
...
1:1 ration - one mole of ATP per 1 mole of acetyl-CoA
from citrate
...
Total of 15 moles of ATP equivalents are needed for synthesis of palmitate
from citrate; 14 moles of NADPH2 would provide the energy for the synthesis of 35
additional moles of ATP
...
The ER
contains the enzyme activities that elongate palmityl-CoA - reduce, dehydrate, the
compound to produce fatty acyl-CoA containing 2 additional carbon atoms
...
The fatty acyl-CoA substrate for elongation can be either un/saturated
...
The elongation process is the
reversal of Beta-oxidation except that an NADPH2 reductase substitutes for the FADrequiring enzyme-catalysed reaction
...
A variety of fatty acyl-CoA
molecules can serve as substrate and the location of the double bond depends on the
desaturases
...
Regulation: Reaction catalysed by acetyl-CoA carboxylase is the main regulatory step in
fatty acid biosynthesis
...
abundance of cyclic citrate acts as a signal that fuel molecules are high
...
Hence, isocitrate dehydrogenase is a key regulatory
step in the Krebs cycle
...
This can occur because
isocitrate dehydrogenase is not maximally activated
...
Conditions that favour fatty acid synthesis produce an inhibitor of a step required before
fatty acid oxidation
...
Hormonal control: acetyl-CoA carboxylase activity is regulated by phosphorylation and
under control of glucagon (stimulating phosphorylation - inactivating the enzyme) and
insulin (activating phosphoprotein phosphates 1)
...
Liver: net result - production of ketone bodies (occurs under insufficient carb storage and
gluconeogenic precursors availability)
...
Insulin has opposite effect to glucagon and epinephrine leading to increased glucagon
triacylglycerides synthesis
...
32
...
Role of Lecithin and
Phosphatidylinositol
4,5-bisphosphate
...
General structure of Phosphoglycerides:
The two acyl groups appear in the orange area, linked to the glycerol (in the white area) by ester
(carboxylic ester) linkages
...
In the green area, appears the Phosphate linked through a
phosphoric ester linkage to the glycerol, but also to X, the hydroxylated compound whose OH
group has formed an ester linkage with the Phosphoric acid (Phosphate, at physiological pH)
...
Depending on the identity of X, the phosphoglyceride can be:
•
•
•
•
•
•
•
Phosphatidate (if X is an Hydrogen)
Phosphatidyl choline aka lecithin (if X is choline)
Phosphatidyl ethanolamine (if X is ethanolamine)
Phosphatidyl serine (if X is serine)
Phosphatidyl glycerol (if X is another glycerol)
Phosphatidyl inositol (if X is inositol)
Cardiolipin (if X is a glycerophosphatidate)
In the group of Phosphoglycerides are also included other compounds that do not have the
Phosphatidyl structure, since these compounds have linked to the C1 of glycerol an alkyl side
chain bound through an ether linkage, instead of the acyl side chain linked through an ester
linkage, characteristic of the phosphatidyl group
...
The most important representatives of this kind of phosphoglycerols are Plamalogens and Platelet
Activating Factor
...
Lecithin is a generic term to designate any group of yellow-brownish fatty substances occurring in
animal and plant tissues composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids,
triglycerides, and phospholipids (e
...
, phosphatidylcholine, phosphatidylethanolamine, and
phosphatidylinositol)
...
) or mechanically
...
It has low solubility in water, but is an
excellent emulsifier
...
This results in a
type of surfactant that usually is classified as amphipathic
...
In cooking, it is sometimes used as an emulsifier and to prevent sticking, for
example in nonstick cooking spray
...
It is suggested that LCAT plays a crucial role in reverse
cholesterol transport, a process depicting the removal of cellular cholesterol through efflux to high
density lipoproteins (HDL) and its delivery to the liver for eventual excretion from the body
...
Increasing number of large scale,
population-based studies failed to detect an elevated cardiac risk with reduced blood levels of
LCAT, suggesting that reduced LCAT activity may not be a risk factor nor a therapeutic target
...
Coordinated
modulation of a number of anti-obesity and insulin sensitising pathways has been implicated
...
PIP2 is enriched at the plasma membrane where it is a substrate
for a number of important signalling proteins
...
IP3/DAG pathway
IP3/DAG pathway
PtdIns(4,5)P2 functions as an intermediate in the IP3/DAG pathway, which is initiated by ligands
binding to G protein-coupled receptors activating the Gq alpha subunit
...
PIP2 regulates the function of many membrane
proteins and ion channels, such as the M-channel
...
In this cascade, DAG remains on the cell membrane and activates the signal
cascade by activating protein kinase C (PKC)
...
The effect of PKC could be reversed by phosphatases
...
Calcium participates in the cascade by activating other proteins
Regulation
PIP2 is regulated by many different components
...
Some of the factors involved in PIP2 regulation are Lipid kinases, Lipid Phosphatase
Lipid Transfer Proteins
Growth Factors, Small GTPases
Cell Attachment
Cell-Cell Interaction
Change in cell volume
Cell differentiation state
Cell stress
33
...
Sphingolipidoses
Sphingolipids, or glycosylceramides, are a class of lipids containing a backbone of sphingoid
bases, a set of aliphatic amino alcohols that includes sphingosine
...
Sphingolipidoses, or disorders of
sphingolipid metabolism, have particular impact on neural tissue
...
Other common R groups include
phosphocholine, yielding a sphingomyelin, and various sugar monomers or dimers, yielding
cerebrosides and globosides, respectively
...
The long-chain bases, sometimes simply known as sphingoid bases, are the first non-transient
products of de novo sphingolipid synthesis in both yeast and mammals
...
Ceramides and glycosphingolipids are N-acyl derivatives of
these compounds
...
The backbone is also amide-linked to an acyl group, such as a fatty acid
...
The preferred substrates for this reaction are palmitoyl-CoA and serine
...
Dihydrosphingosine is acylated
by a (dihydro)-ceramide synthase, such as Lass1p or Lass2p (also termed as CerS), to form
dihydroceramide
...
Metabolic pathways of various forms of sphingolipids:
Sphingolipidoses are labeled at corresponding stages that are deficient
...
It may be phosphorylated by ceramide kinase to
form ceramide-1-phosphate
...
Additionally, it can be converted to sphingomyelin by the addition of
a phosphorylcholine headgroup by sphingomyelin synthase
...
Finally, ceramide may be broken down by a ceramidase to form sphingosine
...
This may be dephosphorylated to reform
sphingosine
...
The complex
glycosphingolipids are hydrolyzed to glucosylceramide and galactosylceramide
...
Similarly,
sphingomyelin may be broken down by sphingomyelinase to form ceramide
...
This forms ethanolamine phosphate and hexadecenal
...
Cholesterol Synthesis
...
Regulation
...
Biosynthesis in the liver accounts for approximately 10%, and in the intestines approximately 15%,
of the amount produced each day
...
The acetyl-CoA utilized for cholesterol biosynthesis is derived from an oxidation reaction (e
...
, fatty
acids or pyruvate) in the mitochondria and is transported to the cytoplasm by the same process as
that described for fatty acid synthesis (see the Figure below)
...
All the reduction reactions of cholesterol biosynthesis
use NADPH as a cofactor
...
Additionally, these intermediates are used in the lipid modification of some proteins
...
Note that the cytoplasmic malic enzyme catalyzed
reaction generates NADPH which can be used for reductive biosynthetic reactions such as those
of fatty acid and cholesterol synthesis
...
SLC16A1 is the pyruvate transporter (also called the monocarboxylic
acid transporter)
...
Acetyl-CoAs are converted to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)
2
...
Mevalonate is converted to the isoprene based molecule, isopentenyl pyrophosphate (IPP), with
the concomitant loss of CO2
4
...
Squalene is converted to cholesterol
...
Synthesis begins with the transport of acetyl-CoA from the
mitochondrion to the cytosol
...
The phosphorylation reactions are required to
solubilize the isoprenoid intermediates in the pathway
...
The
abbreviation "PP" (e
...
isopentenyl-PP) stands for pyrophosphate
...
3g/day
...
The level of cholesterol synthesis
is regulated in part by the dietary intake of cholesterol
...
The greatest proportion of cholesterol is used in bile acid synthesis
...
Regulation of HMGR activity and levels
2
...
Regulation of plasma cholesterol levels via LDL receptor-mediated uptake and HDL-mediated
reverse transport
...
The enzyme is controlled by four distinct mechanisms: feed-back inhibition, control of
gene expression, rate of enzyme degradation and phosphorylation-dephosphorylation
...
Cholesterol acts as a feedback inhibitor of pre-existing HMGR as well as inducing rapid degradation of the enzyme
...
This ability of cholesterol is a consequence of the
sterol sensing domain, SSD of HMGR
...
The mechanism by
which cholesterol (and other sterols) affect the transcription of the HMGR gene is described below
under regulation of sterol content
...
The enzyme is most active in its unmodified form
...
HMGR is phosphorylated by AMP-activated protein kinase, AMPK
(this is not the same as cAMP-dependent protein kinase, PKA)
...
Phosphorylation of AMPK is catalyzed by at least 2 enzymes
...
LKB1 was first identified as a gene in humans carrying an
autosomal dominant mutation in Peutz-Jeghers syndrome, PJS
...
The second AMPK phosphorylating enzyme is calmodulin-dependent
protein kinase kinase-beta (CaMKKβ)
...
Visit AMPK: The Master Metabolic
Regulator for more detailed information on the role of AMPK in regulating metabolism
...
HMGR is most active in the dephosphorylated state
...
Phosphorylation of AMPK is catalyzed by at least 2 enzymes:
LKB1 and CaMKKβ
...
Conversely, insulin stimulates the removal of phosphates and, thereby, activates HMGR
activity
...
35
...
Cholesterol
Excretion
...
Bile salts - there are two species - primary and secondary
...
Secondary bile salts results from the action of intestinal bacteria on the primary bile salts
...
Hydroxyl group is added by 7 alphahydroylase
...
Remaining 5-carbon fragment contains a carboxyl group
...
The reaction
involves NADPH, molecular oxygen and the cytochrome P-450
...
pK of the bile acids is
about 6
...
Excretion of Cholesterol - cholesterol is susceptible to oxidation and easily forms oxygenated
derivatives known as oxysterols
...
Additional roles for oxysterols in human physiology include their: participation in bile acid
biosynthesis, function as transport forms of cholesterol, and regulation of gene transcription
...
These derivatives undergo degradation upon storage and it is essential to purify cholesterol
prior to use
...
Cholesterol is oxidized by the liver into a variety of bile acids
...
A mixture of conjugated and nonconjugated bile acids,
along with cholesterol itself, is excreted from the liver into the bile
...
The excretion
and reabsorption of bile acids forms the basis of the enterohepatic circulation, which is essential for
the digestion and absorption of dietary fats
...
Although, lecithin and bilirubin gallstones also occur, but less frequently
...
This cholesterol originates from the diet, bile, and
desquamated intestinal cells, and can be metabolised by the colonic bacteria
...
A
cholesterol-reducing bacterium origin has been isolated from human faeces
...
Many of these cholesterol-regulated genes are
homologues of fatty acid β-oxidation genes, but have evolved in such a way as to bind large
steroid substrates like cholesterol
...
This disease process leads to myocardial infarction
(heart attack), stroke, and peripheral vascular disease
...
On the other hand, high concentrations of functional
HDL, which can remove cholesterol from cells and atheroma, offer protection and are sometimes
referred to as "good cholesterol"
...
Conditions with elevated concentrations of oxidised LDL particles, especially "small dense
LDL" (sdLDL) particles, are associated with atheroma formation in the walls of arteries, a condition
known as atherosclerosis, which is the principal cause of coronary heart disease and other forms
of cardiovascular disease
...
Increased concentrations of HDL correlate with lower rates of atheroma progressions and even
regression
...
Still, because cardiovascular disease is
relatively rare in the younger population, the impact of high cholesterol on health is still larger in
older people
...
) Fate of Amino Acid Nitrogen
...
AMMONIA DERIVED FROM DEAMINATION:
Ammonia derived mainly from deamination of the α-‐amino nitrogen of amino acids is toxic
...
Deamination of the glutamine in the liver, releases ammonia which is efficiently converted to non-‐
toxic urea
...
Urea accounts for 80-‐90% of
excreted nitrogen in ureotelic organisms-‐terrestrial mammalians
...
Consequently aquatic organisms are ammoniotelic and birds are uricotelic
...
Creatinine is produced from creatine phosphate
...
These compounds are mainly excreted in the urine, lost in
faeces and through the skin
...
(e
...
bilirubin
excreted mainly in faeces)
...
5g of
nitrogen per day
...
Urea cycle was the first metabolic
cycle to be understood, it was described by Hanz Krebs and Kurt Henseleit in 1930-‐1932
...
The
synthesis is a cyclic process, which uses ornithine as the substrate and produces it as a product at
the end
...
SUBSTRATES:
The carbon and oxygen in urea are derived from carbon dioxide, which is combined with water and
this produces carbonic acid
...
The nitrogen atoms
are derived from ammonium ion and aspartate
...
• Ammonia is produced in peripheral organs and is transported to the hepar organ as
glutamate and alanine
...
Since reabsorbed
ammonia from intestine is moved to liver, normally this ammonia is converted in to urea
...
ENZYMES:
• Carbamoyl-‐phophate synthetase 1 (CPS-‐1) : mitochondrial liver enzyme
• Ornithine-‐transcarbamoylase (OTC)
• Citrulline synthase
• Arginine-‐succinate-‐synthase
•
•
Arginino-‐succinate-‐lyase
Arginase
REACTIONS:
NH4+ + HCO3-‐ + 2 ATP → NH2CO2PO3-‐2 + 2 ADP + Pi + 2H+
The urea cycle starts with carbamyl-‐phosphate synthetase reaction
...
Eukaryotes have 2 forms of
CPS : mitochondrial CPS-‐I (uses ammonia as nitrogen donor and participate in urea biosynthesis) and
cytoplasmic CPS-‐II (uses glutamate as nitrogen donor and participates in pyrimidine synthesis)
...
In the first step, ATP reacts with bicarbonate to for
energy rich carbonyl-‐phosphate intermediate and ADP is released
...
In the 3rd step,
carbamate accepts phosphoryl group from ATP to form energy rich carbamyl phosphate
...
Carbamyl phosphate reacts with ornithine-‐ amino acid to produce citrulline
...
The reaction is exergonic and
proceeds with a loss of the energy rich bond of the carbamoyl phosphate
...
Both citrulline and ornithine are amino acids, but
they are not found in proteins
...
It involves the condensation of aspartate with ureido
group or citrulline
...
Pyrophosphate is hydrolysed by the enzyme pyrophosphatase
...
Arginino-‐succinate is split using arginine-‐succinase (Arginino -‐succinate-‐lyase) to yield arginine and
fumarate
...
The fumarate produced can be
converted to aspartate via the fumerase and malate dehydrogenase reactions to form oxaloacetate,
followed by transamination
...
The final reaction of the urea cycle is that of the enzyme arginase catalysing the hydrolysis of or
arginine to form the product of the pathway (urea) and the regeneration substrate (ornithine)
...
Ornithine then enters mitochondrion and the process is repeated
...
To
form the non-‐toxic excretion product urea, the urea cycle thereby coverts 2 amino groups:
• One from ammonia
• One from aspartate
• (Also a carbon atom from bicarbonate)
SUMMARY:
NH3 + HCO3 + Aspartate + 3 ATP + H2O → UREA + Fumarate + 2ADP + AMP + 4Pi
The energy cost is recovered by the energy released upon the formation of the urea cycle substrates:
• Ammonia is produced mainly in the glutamate dehydrogenase reaction, the process is
accompanied by formation of NADH
...
These two molecules of NADH insures 6 ATP molecules
...
REGULATION:
The CPS-‐1 is the main regulatory enzyme
...
The remaining enzymes of urea cycle are controlled by the concentrations and availability
of their substates
...
Contrary diet rich in carbohydrates reduces the level of urea cycle enzymes
...
Renal disease is often associated with the elevation of blood urea nitrogen (BUN), which is not toxic
at such low concentrations
...
One postulated mechanism of ammonia toxicity is depletion of α-‐ketoglutarate metabolite of Krebs
cycle
...
In general,
they are severe defects, associated with mental retardation, seizures, coma and early death
...
When liver function is
seriously compromised, for example in individuals with massive cirrhosis of severe hepatitis,
ammonia circulates in the blood
...
TREATMENT:
• Diet with low protein content
• Frequent meals with low food quantity
• Neomicyn-‐supresses bacterial flora thus lowering the intestinal production of ammonia
Guidelines to the differential diagnosis (DDX) of neonatal Urea Cycle Disorder (UCDs)
...
This diagnosis can be confirmed by the abscence of acidosis or
ketosis
...
The baby maybe irritable at first, followed by vomiting and increasing lethargy
...
If untreated, the
child will die
...
Moderately high levels are indicative
of arginino-‐succinate lyase deficiency (ALD) and extremely high levels are indicative of arginino-‐
succinate synthetase deficiency (ASD)
...
Clinical symptoms are most severe when the UCD is at a level of carbamoyl phosphate synthetase I
(CPSI)
...
Poor
feeding and eventually coma and death if not recognised and treated properly
...
Several UCDs manifest with late onset such as in
adulthood
...
In general the treatment of UCDs has, as common elements:
• the reduction of protein in diet
• removal of excess ammonia and replacement intermediates missing from urea cycle
Administration of laevulose reduces ammonia through its action of acidifying the colon
...
Antibiotics can be administered to kill intestinal ammonia producing bacteria
...
) Other Ways of Detoxification of Ammonia: Glutamate
Dehydrogenase Reaction ; Oxidative Deamination of Amino Acids ; The
Role of Glutamate ; Biosynthesis of Glutamine
...
Normal diet contains about 100g protein/day
...
Most of the amino acids
are used for the synthesis of proteins in the liver and in other tissues, and other nitrogen containing
compounds
...
During fasting, muscle protein may be degraded into amino acids, some of which are partially to
produce energy
...
These enzymes include:
• Transaminases
• Glutamate dehydrogenase
• Glutaminase
• Dehydratases
• Deaminases
Nitrogen must be removed before degradation of carbon skeleton of amino acids
...
5g of nitrogen is released
...
In the liver , this ammonia
is converted to urea (which is non-‐toxic and water soluble) and it is readily excreted in urine
...
Other nitrogen excretion compounds are:
• Creatinine is produced from creatine phosphate
• Uric Acid is degradation product of purine
• Ammonia is released from glutamine (acts as urine buffer within kidneys by reacting with
protons to form ammonia ions(NH4+) )
...
Small amounts of nitrogen containing compounds are formed from the degradation of
neurotransmitters, hormones (also excreted with urine) and others like bilirubin (excreted mainly
with faeces)
...
It is involved in both synthesis
and degradation
...
This enzyme can either accept NAD or NADP as its redox coenzyme
...
Thus it
forms α-‐imino glutarate
...
It can incorporate
ammonia to produce glutamate or release ammonia from glutamate
...
This process provides one source of ammonia that
enters the urea cycle
...
e
...
After ingestion of a meal containing protein, glutamate levels in liver are elevated
...
Because
high conc
...
Hence the ammonia produced is converted to urea
...
These properties suggest a
physiological rartionale, when AA are needed for energy production, the activity of the
dehydrogenase is greatest
...
Dehydrogenase is found to be inhibited by several steroid hormones
and thyroxin
...
The chemical properties of the
enzymes purified from liver and kidney have been intensively studied, but the biological functions
are still not understood
...
The amount of enzyme is sufficient to metabolise large quantities of substrate
...
Also D-‐amino acid oxidases, oxidise, N-‐methyl-‐
glycine (sarcosine) and glycine
...
It catalyses the
reaction :
2 H2O2 → O2 + H2O
D-‐amino acid oxidase occurs mainly in the kidney
...
Superficially the over a reaction is catalysed by the amino acid oxidases, appears to resemble the
complex of reactions initiated by GDH and followed by the mitochondrial oxidation of NADH
...
E and is catalysed by glutamate
...
An additional quantity of ammonia is produced by enteric
bacteria, which is then absorbed into the portal venous blood, which thus contains higher levels of
ammonia than the systemic blood
...
Ammonia concentration in blood is low at about 10-‐20µg/dL
=100ml
...
D-‐Amino
Acid
Oxidase
(IMAGE: Action of D-‐amino acid oxidase
...
)
THE MAIN PROCESS INVOLVED IN AMMONIA DETOXIFICATION:
These are:
• Reductive amination
• Biosynthesis of glutamine
• Ammoniogenesis in kidney
REDUCTIVE AMMINATION:
The reaction is one of two mechanisms for incorporation ammonia ions into amino acids by fixation
of ammonia-‐by reverse glutamate dehydrogenation (GDH) reaction
...
The reaction is
reverse of the oxidation of glutamate and is catalysed by the enzyme glutamate dehydrogenase
(GDH)
...
Regulation:
GDH is inhibited by ATP and GTP and is activated by ADP
...
At high
concentrations of ammonia, the reaction is used to include ammonia
into the glutamate and vice versa
...
BIOSYNTHEIS OF GLUTAMINE:
This is another reaction involving the detoxification of ammonia
...
It is catalysed by the enzyme glutamine synthetase
...
This reaction
consumes energy in the form of ATP, the reaction mechanism involves formation of a ɣ-‐glutamyl-‐
phosphate intermediate
...
ATP + Glutamate + NH3 → [ ɣ-‐Glutamyl-‐phosphate Intermediate] → Glutamine + ADP + Pi
The kidney, the gut and the cells with rapid turnover rates such as those of the immune system are
the major sites of glutamine uptake
...
The excess of nitrogen not used for glutamine synthesis is transferred
to pyruvate to form alanine -‐ this carries this nitrogen back to the liver
...
The main metabolism of
glutamine carbon atoms in the gut, is oxidation to CO2 or it conversion to the carbon skeletons of
lactate, citrulline or ornithine
...
AMMONIOGENESIS
Glutamine is reconverted to glutamate by the enzyme glutaminase
...
Glutamine + H2O → Glutamate + NH3
Glutaminase is important in the kidneys as it is where ammonia is generated and then excreted in
urine as ammonium salts
...
Ammonia is produced by renal tubular cells and its production is increased in metabolic acidosis
...
In intestinal enterocytes, α-‐
ketoglutarate is used in anaplerosis (chemical reactions that form intermediates of a metabolic
pathway) of TCA (Citric acid cycle), since activities of pyruvate dehydrogenase (DH) and pyruvate
carboxylases are low
...
NH4+ is released from glutamine by glutaminase and by glutamate
dehydrogenase, resulting in the formation of α-‐ketoglutarate
...
The α-‐ketoglutarate is oxidised to CO2, converted to glucose (used in renal medulla), or converted to
alanine to return ammonia to the liver for urea synthesis
...
Renal
glutamine utilization for proton excretion takes place over the requirements of other tissues for
glutamine
...
Specific transporters in the renal tubular cells transport protons from
these cells in to the tubular lumen in exchange for Na+
...
Ammonia (NH3), which is uncharged, enters the
urine by free diffusion through the cell membrane
...
This ammonium ion cannot be transported back in to the cells and is
excreted in urine
...
However this is increased to a greater
extend during fasting and during metabolic acidosis
...
Asparagine is produces in a
transferase reaction with glutamine
...
Aspartate + ATP + Glutamine → Asparagine + Glutamate + AMP + 2Pi
Asparaginase catalyses the hydrolysis of asparagine to yield aspartate and ammonia:
Asparagine + H2O → Aspartate + Ammonia
For both reactions, asparaginase and glutaminase are exergonic
...
Asparaginase decreases the level of asparagine-‐essential for tissue growth
...
Glutamine nitrogen is derived mainly from the BCAA
...
) Decarboxylation of Amino Acids
...
Their Importance and Degradation
...
These reactions produce amines that often serve as neurotransmitters
and hormones
...
1) Glutamate:
(GABA)
(Image: glutamate is dearboxylated at carbon 1 to form an amine : ɣ-‐aminobutyric acid (GABA)
...
It binds several types of receptors:
• Metabotropic
• Ionotropic
• Those acting on adenylyl cyclase (affects Ca and K channels)
• Cl channels
Some steroid hormones (e
...
progesterone) binds GABA receptors, which leads to a sleepy state and
an anaesthetic effect
...
This would lead to membrane hyperpolarization and neuron
inhibition
...
Pyridoxine-‐dependant Epilepsy
It is a rare disease characterised by recurring seizures that can begin in utero, at birth or during
infancy
...
This is because, as mentioned before pyridoxal
phosphate plays important role in synthesis of GABA
...
2) Histidine:
(Image: Histidine is decarboxylated by histidine decarboxylase to form histamine
...
)
Histamine:
• mediates inflammatory and allergic reactions
• causes dilation of blood vessels
• increases vessel permeability
• causes redness
• causes edema
• mediates shock
Within the lungs it causes bronchio-‐constriction
...
Antihistamine drugs compete with histamine for receptors binding histamines-‐used as anti-‐shock
medicine
...
3) Aromatic Amino Acids:
The aromatic amino acids undergo decarboxylation during their metabolism to produce compounds
that serve as neurotransmitters and/or as hormones
...
Tyrosine is decarboxylated to produce Catecholamines: dopamine, norepinephrine
and epinephrine
...
)
Formation of serotonin (5-‐hydroxytryptamine [or 5’HT] ): tryptophan is first hydroxylated in a
tetrahydrobiopterin requiring reaction by the enzyme L-‐tryptophanhydroxylase (TPH)
...
This enzyme provides the rate-‐limiting step
for serotonin synthesis, (similar to L-‐tyrosine hydroxylase)
...
Serotonin is principally found stored in 3 cell types:
• The serotonergic neurons in the CNS and in the intestinal myenteric plexus
...
Simlairly, serotonergic neurons also
have the capcity for amine uptake via serotonin transporters
...
In the central nervous system(CNS), serotonin is believed to play an important role in regulation of:
• Anger
• Aggression
• Body temp
...
Serotonin
stimulates nervous system
...
Low levels of serotonin may be associated with several disorders, namely:
• increase in aggressive and angry behaviours
• clinical depression
• obsessive-‐compulsive disorder (OCD)
• migraine
• irritable bowel syndrome
• tinnitus
• fibromyalgia
• bipolar disorder
• intense religious experiences
If the neurons of the brainstem that make serotonin (serotonergic neurons) are abnormal, there is
risk of sudden infant death syndrome (SIDS)
...
Selective serotonin re-‐uptake inhibitors (SSRIs) increase the extracellular level of the
neurotransmitter serotonin by inhibiting its re-‐uptake into the presynaptic cell
...
(Low levels of serotonin in these areas
are what can cause mood disorders such as depression
...
Psychotropic drugs
(chemical substances that change brain function and results alterations in perception, mood and
consciousness
...
Serotonin is a potent vasoconstrictor and a stimulator of smooth muscle contraction as well as
acting as a neurotransmitter
...
Serotonin affects libido-‐high serotonin concentration decreases sexual desire
...
Seratonin undergoes acetylation by acetyl CoA, followed by methylation by
S-‐adenosylmethionine to for melatonin
...
Melatonin may also be involved in regulating reproductive functions
...
4
...
It is a vasoconstrictor and is
metabolized when it is degraded by the enzyme Monoamine Oxidase (MAO)
...
) to occur
...
In
foods, it is often produced by the decarboxylation of tyrosine during fermentation or decay
...
Tyramine is also found in chocolate, alcoholic beverages,
fermented foods like cheeses (except ricotta, cottage cheese, etc
...
5
...
They are used in phospholipid
synthesis
...
6
...
e
...
Serotonin is oxidised in 5-‐hydroxy-‐indol-‐acetic acid by MAO-‐A
...
of
substrate which permits the enzyme to achieve half the maximum rate of reaction) value than MAO-‐
B
...
g
...
g
...
Interestingly however, immunohistochemical studies have suggested that serotonin –containing
neurons may themselves contain only MAO-‐B
...
This is principally done by an
isoform of aldehyde dehydrogenase (ALDH2) located in the mitochondria, to produce 5-‐
hydroxyindole acetic acid as the major excreted metabolite of serotonin
...
However this pathway is considered to be insignificant
...
Catabolism of carbon skeletons of amino acids
...
Surplus amino acids are used as metabolic fuel
...
This is then
oxidatively deaminated to yield ammonium ions (NH4+)
...
Examples :
1
...
2
...
The nitrogen atom
in glutamate is converted into free ammonium ion by oxidative deamination
...
This reaction is close to equilibrium in the liver, and the direction is determined by the conc
...
Its usually driven forward by the rapid removal of NH4+ produced from the
hydrolysis of the Schiff base intermediate
...
PLP includes a pyridine ring that’s slightly basic, attached to an OH group which is slightly acidic
...
The most important functional group on PLP is the aldehyde, which
forms covalent Schiff-‐base intermediates with amino acid substrates
...
The α-‐amino group of the amino acid substrate displaces the ε-‐
amino group
...
The AA-‐PLP Schiff
base formed remains tightly bound to the enzyme by multiple noncovalent interactions
...
The Schiff base between the AA
substrate and PLP, the external aldimine, loses a H+ from the α
-‐carbon atom of the AA to form a quinonoid intermediate
...
The ketamine is then hydrolysed to an α
-‐ketoacid and pyridoxine phosphate (PMP)
...
The 2nd half takes place by the reverse of the preceding pathway
...
Overall reaction:
AA1 + Ketoacid2 -‐-‐-‐à AA2 + ketoacid1
...
The nitrogen
removed is transferred though glutamate, to alanine, which is released into the bloodstream
...
This
transport is called the glucose-‐alanine Cycle
...
The carbon skeleton of the
diverse set of 20 fuctional AA are funnelled into only 7 molecules: pyruvate, acetyl CoA, α
-‐ketoglutarate, succinyl-‐ CoA, fumarate and oxaloacetate
...
AA that are
degraded to pyruvate, alpha-‐ketoglutarate, succinyl CoA, fumarate or oxaloacetate are called
glucogenic AA
...
Only Leu and Lys are solely ketogenic
...
Some of their carbon atoms emerge in acetyl CoA or acetoacetyl CoA, whereas others in potential
precursors of glucose
...
Degradation of AA classified into groups with similar c-‐atoms
...
For
example, the transamination of alanine yields pyruvate directly
...
The α-‐amino groups of Ser and Thr can be directly deaminated
to pyruvate and α-‐amino-‐β-‐ketobutyrate respectively using serine and threonine dehydratase, in this
reaction PLP is the prosthetic group
...
Cofactor for enzyme:
N^5-‐N^10-‐methylene-‐THF that provides C1 unit necessary for reaction
...
2nd group-‐ C4 -‐oxaloacetate
Oxaloacetate is an entry point into metabolism for Aspartate and asparagine
...
Asparagine is hydrolysed by asparaginase to NH4+ and aspartate, which is
then transaminated
...
These AA are 1st converted to glutamate,
which is then oxidatively deaminated by glutamate dehydrogenase to yield alpha-‐ketoglutarate
...
The amide bond in the ring of this
intermediate is hydrolysed to the N-‐formimino derivative of glutamate, which is the converted into
glutamate by the transfer of its formimino group to tetrahydrofolate (THF), a carrier of activated
one-‐ carbon-‐ units
...
Proline and Arginine are both converted to glutamate gamma-‐semialdehyde, which
is then oxidized into glutamate
...
For Arg: ornithine
...
Propionyl
CoA and methylmalonyl CoA are intermediates in the breakdown of these AA
...
For Ile – α-‐keto-‐β-‐methylvalerate)
2) Oxidative decarboxylation to corresponding acyl-‐CoA
...
Thiolytic cleavage yielding acetyl-‐CoA and propionyl CoA, which is
converted to succinyl-‐CoA
...
Leu degradation includes carboxylation and hydrolysis to yield Acetyl-‐CoA and acetoacetate
...
The α-‐keto acid
dehydrogenase which catalyses oxidative decarboxylation of the transamination metabolites of
branched chain AA is a multienzyme complex
...
Lys degradation in liver by formation of α-‐keto glutamate-‐lysine intermediate scharopine the
pathway includes reactions from other pathways : transamination, decarboxylation of alpha-‐
ketoacids, similar reaction to beta-‐oxidation and alpha-‐ketoglutarate DH
...
A genetic defect in 1st
enzyme involved leads to hyperlysinemia, hyperlysinuria along with mental and physical retardation
...
Methionine degraded to succinyl CoA
...
44
...
Biosynthesis of non-‐essential AA
...
Asp, Ala and Glu are formed by the addition of an amino group to an α-‐ketoacid (α-‐ketoglutarate,
oxaloacetate and pyruvate) by a transamination reaction
...
Within a transaminase, the internal akdimine is
converted into PMP by reaction with glutamate in a multi-‐step process
...
The aldimine is cleaved to release the newly forme AA to complete the
cycle
...
Glu is the precursor of Gln, Pro and Arg
...
3-‐phosphoglycerate is the precursor of Ser, Cys and Gly
...
formed
transfer of side chain methylene group of srine to tetrahydrofolate
...
This
interconveersion is catalysed by serine hydroxymethylaseferase
...
The sulphur atom
of Cys is derived from homocysteine, whereas the carbon skeleton comes from serine
...
Its conversion to Cys and
ketobutyrate is catalysed by cystathioninase
...
Shikimate and Chorismate are intermediates in the biosynthesis of aromatic AA (Trp, Phe, Tyr)
...
Oxidative decarboxylation
yields p-‐ hydroxyphenypyruvate
...
45
...
Inhibitors of folate reductase as drugs
...
This cofactor consists of 3 groups: a substituted pteridine
ring (tetrahydropterin), p-‐aminobenzoate and a chain of one or more glutamate residues
...
They obtain THF from
their diets or from microbes in their intestinal tracts
...
This unit exists in
3 oxidation states and the most reduced form carries a methyl group, whereas the intermediate
carries a methylene group
...
The
fully oxidized C1 unit, CO2 I carried by biotin rather than THF
...
The 2nd important source of C1 groups is the cleavage of Gly
...
Trp degradation gives N^10-‐Formyl-‐THF
...
Used in Ser and Gly synthesis
Clinical Importance:
Folate deficiency causes megaloblastic anemia
...
Drugs analogous to folic acid or its components:
Methotrexate-‐ used as cystostatic in chemotherapy
Sulfonamides-‐ frug that is an analog of p-‐aminobenzoic acid
...
SAM (S-‐adenosylmethionine)
It participates in the synthesis of many compounds that contain methyl groups
...
Synthesized by transfer of adenosyl group from ATP to sulfur atom of methionine
...
SAH is then hydrolysed to homocysteine and
adenosine
...
Vitb12 is the
coenzyme that mediates this transfer
...
Supplementation with pyridoxine, folic acid, B12 or betaine reduces
homocysteine concentration in the blood
...
of endothelial
asymmetric dimethylarginine
...
can be raised by intense long duration exercise as it could increase the load on
methionine metabolism
...
Minimal levels of homoc = about 6
µmol/L
...
In patients with vitb12 deficiency N^5-‐methylTHF cannot be converted to free THF
...
The exhaustion of THF leads to deficiencies in THF
derivatives needed for dTMP and purine synthesis
...
Deficiency of b12 causes methyyylmaonyl CoA to accumulate in the brain, which could act as a
competitive inhibitor of malonyl CoA in FA synthesis OR it may act as a substitute for malonyl CoA in
the formation of FA unclear…research
...
46
...
Inherited Enzyme Defects-‐Phenylketonuria and Alkaptonuria
The degradation of Phe begins with its oxidation to tyrosine, a reaction catalysed by phenylalanine
hydroxylase, located in the liver
...
Phe is an
essential AA, but Tyr is not
...
The reductant here is tetrahydrobiopterin,
an electron carrier derived from the cofactor biopterin
...
The active form of biopterin is the fully reduced form, 5,6,7,8 Tetrahydrobiopterin is
initially formed from 7,-‐dihydrobiopterin by NADPH in a reaction catalysed by dihydrofolate
reductase
...
(3) This α-‐keto acid then reacts with O2 to form homogentisate, using the enzyme p-‐
hydroxyphenylpyruvate hydroxylase (a dioxygenase bacuse both atoms of O2 become incorporated
into the product, one on the ring and one u the carboxyl group )
...
(5) 4-‐maleylacetoacetate s then isomerized to 4-‐fumarylacetoacetate by an enzyme that uses
Glutathione as a cofactor
...
Phenylketonuria-‐ An autosomal recessive disease caused by an absence or deficiency of
Phenylalnine hydroxylase
...
So there are elevated levels of the AA in blood
...
If undiagnosed and untreated, PKU can result in impaired postnatal cognitive
development
...
Conc
...
2mmol/L-‐
7mg%
...
The aim is to provide just enough Phe to meet the
needs for growth and development, which is to maintain levels in blood between 3 and 12 mg/dL
...
In both cases neurologic symptoms
and mental retardation are seen in a few months after birth
...
So the homogentisate produced cannot be further metabolized, it accumulates and is
excreted in urine, which turns dark on standing as homogentisate is oxidized and polymerized to a
melanin-‐like substance
...
Commonly recommended
treatment: dietary restriction of Phe and Tyr and large dose of ascorbic acid (vitC)
...
47
...
Epinephrine
It is widely known as Adrenaline and is a hormone and neurotransmitter
...
Adrenaline is a natural stimulant made in the adrenal gland
of the kidney from Tyr
...
Norepinephrine
Also underlies the “flight or fight” response, directly increasing heart rate, triggering the release of
glucose from energy stores, and increasing blood flow to skeletal muscles
...
It is released from adrenal medulla
into the blood as a hormone and as a neurotransmitter in the CNS and SNS where its released from
noradrenergic neurons
...
It is produced in several areas of the brain, including the substantia nigra
and the ventral tegmental area
...
L-‐DOPA = 3,4-‐Dihydroxyphenylalanine…the reaction forming this also requires 5,6,7,8-‐
tetrahydrobiopterin as a cofactor
...
Adenosylmethionine provides the methyl group needed for conversion of noradrenaline to
adrenaline
...
The pacemaker for the
synthesis of these catecholamines is Tyrosine Hydroxylase
...
They are stored in vesicles until released by transient
change in electrical potential along the axon
...
Receptors-‐ adrenergic : α1-‐activates PhospholipaseC
Α2-‐ inhibits adenylate cyclase (AC), reducing cAMP levels
Β1, β2, β3-‐ activates AC rises cAMP levels
Dopamine receptors : D1A activates AC and D1B inhibits
Serum conc
...
8 nmol/L, Adrenaline = 0
...
Catecholamines are NT which are metabolized to urinary excretion products
...
Oxidative deamination of neurotransmitters catalysed by Monoamine oxidase (MAO) present in the
cytosol of presyp terminal, adjacent glial cells, ery, producing ALDEHYDES
...
ALSO, the hydroxyl group can be methylated by aldehyde
reductase to produce 3-‐methoxy-‐4hydroxy-‐phenyl-‐etilenglycol
...
MAO also inactivates SEROTONIN
...
Two types of MAO : MAO A – specific to NA and serotonin; inhibited by clorgyline
MAO B – specific to dopamine and histamine ; inhibited by deprenyle
Stress
Epinephrine, cortisol and corticosterone are major stress hormones
...
The level of epinephrine is controlled in part through its biosynthesis by the final enzyme in the
catecholamine pathway, phenylethanolamine N-‐methyltransferase (PNMT)
...
Adrenocorticotropic hormone (ACTH) and the SNS enhance activity of
tyrosine hydroxylase and dopamine β-‐hydroxylase, thereby stimulating release of epinephrine
precursors
...
Diseases
Parkinson’s disease : decrease in dopamine levels in CNS because of deficiency of a few cells that
produce dopamine
...
Characterised by tremor, bradykinesia (slow movement and unstable posture)
...
This is because it can
cross the blood-‐brain barrier
...
the dopamine and L-‐DOPA released by this tissue is presumed to
replace that lost due to degeneration of substantia nigra
...
Excess of dopamine in the limbic cortex is observed in Schizophrenia
...
Melanins and Thyroid hormones-‐ Synthesis and degradation
...
MIT = monoiodotyrosine, DIT = diiodotyrsine The thyroid
gland concentrates iodine against its cocentr
...
the ratio of I-‐ in thyroid to it in serum is 25:1
...
Iodination reaction catalysed by iodoperoxidases
...
Mature thyoglobin is inactive but its active forms T3 and T4 are
produced by lysosomal proteolysis of thyr
...
As T3 and
T4 are nonpolar, they are transported in the bloodstream bound t plasma carrier proteins, mainly
thyroxine binding globulin but also prealbumin and albumin
...
Abnormal levels of thyroid hormones
Hypothyroidism is characterized by lethargy, obesity and cold dry skin, whereas Hyperthyroidism has
the opposite effects
...
Second is thyroid gland failure called autoimmune thyroiditis or Hashimoto’s thyroiditis, a form of
thyroid inflammation caused by patient’s own immune system
...
Melanins
L-‐DOPA is the precursor for the black skin
pigment melanin
...
Albinism-‐ could result from defectiveness of enzymes that are involved in melanin production
...
Viteligo-‐ local deficiency of melanin is treated with psoralines-‐pland compounds
...
The
acute form is associated with liver failure, a cabbage like odour, death within 1st year of life
...
Treatment: low tyrosine, low Phe diet
...
Metabolism of Tryptophan
...
The main products of degradation : Alanine (from 3-‐Hydroxykynurenine), acetaoacetate
...
1st step = oxidation by tryptophan pyrolase-‐dioxygenase
...
2nd step = Hydrolysis of formyl group by kynurenine formylase
...
4th step = kynureninase is PLP requiring enzyme that catalyzes conversion of hydroxykynurenine to
alanine and hydroxyantranilate
...
Picolinic acid – primary chelator in the body for minerals like Cr, Zn, Fe, Mo, Cu, Mn
...
It is a ferro-‐porphyrin metalloprotein, induced in liver by
corticosteroids and Trp
...
Abnormal metabolism : Occurs in VitB deficiency
...
Pellagra results from dietary deficiency of nicotinamide, resutin in reduced levels of NAD+ and
reduced synthesis of Trp
...
Metabolism in red blood cells
...
So some equations
might not make sense as I’m clueless as to how to type them out
...
Sorry in advance
...
Red Underlining= Metabolite
...
v RBC functions= Oxygen transport (lungs to cell)
Carbon transport (cells to lungs)
Buffer Role (H+ + HCO3~ à H20+CO2)
v Biconcave shape
...
v Number of RBC in circulation = 2
...
v Haemoglobin levels = 14-‐18 g/dL
v Haematocrit level (the volume of packed red blood cells)= men-‐42-‐52% women-‐ 37-‐47%
...
1% of RBC population is replaced daily
...
RBC have high affinity glucose transporters – work at 75% of their
Tmax (transport maximum)
...
Glucose enters cell
...
2,3-‐BPG= reduces the affinity of Hb for 02 – decreases 02 binding
...
Nucleotide: (number 6 in pic) RBC contains adenosine deaminase in
order to breakdown adenine
...
So metabolism of glucose produces lactate* and ATP*
...
When G6P is reduced to F6P, NADPH2* is produced
...
GSSG is the oxidized form of GSH, it is oxidized during the conversion
of H2O2 àH2O
...
AUTO-‐OXIDATION of Haemoglobin to methmeglobin* produces
superoxide*
...
H2O2 is converted to H2O + O2
by catalase
...
H2O2 can also be converted to OH’ + OH-‐ by iron
...
If the iron is oxidized to 3+ then the methmeoglobin
formed cannot transport oxygen
...
For this reason RBC has a cytochrome b5 reductase enzyme,
which reduces cytochrome b5 along with NADPH2 (produced in PPP), this is the NADPH-‐ dependant methemoglobin
reductase system
...
Enzyme used in PPP to convert G6P
to F6P
...
Cant convert GSSG àGSH, thus it cannot dispose of H2O2/02’
...
The presence of Heinz bodies
indicate oxidative stress
65
...
Neutrophil functions:
1
...
3
...
5
...
Pentose Phosphate Pathway
...
Phagocytic cells à(metabolism of pathogens) initiates acute inflammatory response by the following
actions:
• Increase of vascular permeability (increases blood flow)
• Increase in neutrophils (migration to infected sites)
• Activation of platelets
• Decrease in swelling (if microorganism is dealt with)
STEP ONE: MIGRATION
...
They pass
through the capillaries and adhere to the endothelial cells – through use of INTEGRINS (adhesive surface proteins)
...
STEP TWO: ACTIVATION
...
(Activation of neutrophils is
similar to activation of platelets which is explained further on) The Ag-‐Ab complexes interact with the Neutrophil
receptors
...
IP3= increases intracellular Ca2+ levelsàessential for induction of respiratory burst
...
Neutrophil is activated
...
Activated neutrophil engulfs the bacteria and digests it, the leukocytes now as an increase in its oxygen
consumption, this is called a respiratory burst
...
It reacts with electrons releases from the ETC
...
Superoxide’s outside the
phagosome in the cytosol is converted to H2O2, via superoxide dismutase
...
STEP FOUR:
After activation the neutrophils release serine proteases into the extracellular environment
...
Can activate receptors and cleave the adhesive molecules
...
In order for them to be regulated, the
body produces anti proteinases, which are present in extracellular fluid and plasma
...
Responsible for the green colour of pus
...
(in the place of Cl,Br or SCN could also
be used) Also produces a tyrosyl radical both are used to kill bacteria
...
Platelets undergo three steps of activation so haemostasis can
occur
...
Adhesion to exposed collagen in blood vessels à binds via specific receptors (integrin), also binds to Van
Willebrand Factor (a glycoprotein secreted by endothelial cells)
2
...
3
...
Phospholipase C hydrolyses phosphatidylinositol bisphosphate into DAG and IP3
...
One of the chemicals released is ADP, can activate additional platelets
...
The alteration of shape results in a collagen induced activation of platelet phospholipase A2 = release of arachnoidic
acid and this is converted to thromboxane A2, which causes further platelet aggregation
...
Regulation of clotting and thrombosis:
Endothelial cells produce Prostacyclin
...
It also stimulates the activity of adenyl cyclase, thus increasing cAMP activity, resulting in a
decrease in intracellular Ca2+ levels
...
Biochemistry of the liver
...
Role of
cytochrome P450
...
IT’S A BIT LENGHTY
...
Excretory: Bile Acids: formed in the liver from the cholesterol
...
Synthetic: proteins: albumin, via globulin, b globulin, deamination of glutamate(released nitrogen group=urea)
3
...
Via oxi,red,conjugation
...
This causes a series of changes in the liver:
Stimulates: Glycolysis Inhibits: Gluconeogenesis
Glycogenesis (formation of glycogen from sugar) (synthesis of glucose from non carbs)
Increase in fatty acid synthesis
...
Processes occur in order to limit blood glucose levels
EARLY FASTING STATE: Blood glucose levels drop
...
Increase in glucagon
...
The liver and muscles use fatty acid as a fuel source
in order to compensate for the lack of glucose
...
EXTENDED FASTING STATE: When glucose levels are extremely low, adipose tissues release fatty acids, which are
converted by the liver to ketone bodies, which are used by the brain
...
(Gluconeogenesis)
The nitrogen is excreted as urea
...
Xenobiotic transformation takes place in the liver
...
Occurs in smooth endoplasmic reticulum, mitochondria, cytosol and lysosomes
...
BIOTRANSFROMATION:
In the first phase new functional groups are inserted to the molecule of xenobiotic
...
These changes take
place predominantly in smooth endoplasmic reticulum
...
Examples are as
follows: hydrolytic cleavage (e
...
esterases, peptidases), oxidation, reduction, alkylation, dealkylation (O-‐ or N-‐),
desulfurization of thiol compounds (change of S for O) or methylation (O-‐ or N-‐)
...
Oxidation (or rather oxygenation)
reactions are catalysed by enzymes oxygenases – the most important are so called mixed-‐function oxygenases
...
MFO have both oxidase and oxygenase ability, i
...
in reaction the substrate is oxygenated and NADPH is oxidised
...
NADPH + H+ + O2 + RH → NADP+ + H2O + R-‐OH
Fe3+-‐P-‐450-‐RH + e-‐ (NADPH) → Fe2+-‐P-‐450-‐RH
O2-‐Fe2+-‐P-‐450-‐RH + e-‐ (NADPH) → Fe3+-‐P-‐450 + R-‐OH + H2O
RH – substrate (xenobiotic)
Enzyme system of cytochrome P450 is huge heme protein family (heme binds O2)
...
Many types of cytochrome P450 are found in the liver, in the adrenal cortex (steroid hormone
synthesis) and in other organs
...
In long-‐term load by particular compound there is in a period of few days induction of synthesis of enzymes of
endoplasmic reticulum
...
This leads tofaster biotransformation of all substances metabolised by this
particular enzyme (i
...
faster biotransformation of even different compounds than that one that caused induction)
...
Described effect dissipates when inducing substances is removed
...
There are persons with high
activity of particular enzyme, therefore they are able to metabolise particular substances quickly (quick
metabolisers) and at the same time there are persons with low activity of the same enzyme – slow metabolizers
...
2) Dehydrogenases:
a) alcohol dehydrogenase (ADH) is localised in hepatocyte cytoplasm
...
ADH is inducible enzyme – it is induced by ethanol
...
Result of the first phase
First phase may have these results:
1) increased polarity of xenobiotic
2) inactivation of xenobiotic (detoxification)
3) bioactivation of xenobiotic (pharmaceuticals X carcinogens) – there is potential danger of damage
Second (conjugation, synthetic) phase – conjugation
Conjugation is process when xenobiotic is bound to high polar endogenous compounds(this is however not
absolutely true – e
...
binding of methyl)
...
e
...
Both
enzymes (transferases), and conjugation agents (endogenous substance that must be in active form– e
...
UDP-‐
glucuronate) are required for conjugation processes
...
Organism utilizes these conjugation agents:
1) Glucuronic acid (UDP-‐glucuronate, glucuronidation, UDP-‐glucuronyltransferase)
2) Sulphate (PAPS = phosphoadenosine phosphosulfate – active sulphate, esterification)
3) Acetate (acetyl-‐CoA, acetyltransferases)
4) Cysteine (glutathione, glutathione-‐S-‐transferases)
5) -‐CH3 (SAM = S-‐adenosyl methionine, methyltransferase)
6) Glycine, glutamine (amidation)
1) Glucuronic acid
Glucuronic acid is the most common conjugation agent
...
Only this form is able to react with xenobiotic molecule – conjugation is catalysed by the enzyme UDP-‐
glucuronyltransferase
...
In addition bilirubin or steroids (i
...
endogenous substances) are also excreted as
glucuronic acid conjugates
...
Metabolism in brain and nervous tissue
...
Brain and nervous tissues are composed of: high cholesterol levels, cerebroblasts, gangliosides and phospholipids
...
However
when starvation occurs ketone bodies (topic 30) are used as an emergency fuel source
...
CARBOHYDRATE METABOLISM:
GLUCOSE: in the CNS (Brain&Spinal Cord) glucose undergoes aerobic glycolysis
...
Due to its large
consumption of oxygen, when the body is deprived of oxygen the CNS goes into a coma and is damaged irreversibly
...
As the nerve cells use lactate as a fuel source (no B-‐
Oxidation in nerve cells) ATP gained from glycolysis is used in the Na+-‐K+ATPase pump to maintain the
transmembrane potential
...
GALACTOSE: converted to glucose and galactocerebrosides
...
g
...
There are 40 different types of neurotransmitters
...
After use neurotransmitters are
catabolized to urea and excreted
...
In the
astroglial cells (of the brain and spinal cord) glutamate and aspartate are synthesised using amino acids from
branched chain amino acids (BCAA)
...
Glutamine travels to the cells of the body and acts as a glutamate pre-‐
cursor
...
•
•
•
The rate of neurotransmitter synthesis, within the neuron, is regulated by a feedback mechanism
...
The rate of synthesis of neurotransmitter is usually independent of dietary supply of the pre-‐cursor AA or of
the concentration of pre-‐cursor AA
...
•
HYPERAMMONIA: ammonia diffuses into the brain and inhibits glutaminase
...
This occurs in astroglial cells
...
Glutamate is then converted to GABA
...
The facilitator neuron releases modulator which binds to the pre-‐synaptic neuron
...
Generates an action potential which is conducted
along the axon
...
2
...
4
...
6
...
Metabolism specificity in renal cortex:
Glycolysis, Gluconeogenesis, Renal glutaminase,
Buffers, Amino acid metabolism in the kidneys and
guts
...
Medulla: Anaerobic Glycolysis: poor blood supply
...
In fasting conditions the blood glucose levels drop, this initiate’s renal gluconeogenesis
...
Glutamine is produced in the skeletal muscle
...
The a-‐ketoglutarate produced enters the
citric acid cycle/Krebs cycle
...
AEROBIC ONLY
...
The NH4+ created
can not diffuse out of nephron
...
H+ is formed from the breakdown of H2CO3
...
H+ also binds to phosphate ions
...
AMINO ACID METABOLISM IN THE KIDNEYS AND
GUT:
•
•
The kidney metabolises proteins of low molecular weight (9
...
Albumin 3g a day
...
Through synthesis, degradation, filtration, reabsorption and urinary excretion of AA
...
SYNTHESIS: Serine (3-‐4g)
Cysteine-‐ from methionine
...
(phenylalanine hydroxylase)
Arginine (2g)
2
...
Citrulline – used in urea cycle
...
Produces 10-‐12g of body total
...
1
...
NO is a hormone mediator(insulin, IGF-‐1) and a vasodilator
...
3
...
4
...
69
...
Energetics of muscle contraction (depending on
load and type of muscle)
...
Creatine metabolism
...
Protein degradationà occurs during fasting to produce a supply of AA’s for the body stimulated by
cortisol
...
Main product of degradation = ALANINE
...
released in to blood taken up by the liver or oxidized to glutamine
in the muscles and other tissue
...
The alanine is released into the blood and
taken up by the liver
...
This mechanism aids in the removal of
ammonia from muscles
...
BCAA’s metabolism: first step is always transaminated
...
It can also be converted to
glutamate or alanine
...
Rate-‐limiting enzyme in
oxidative pathway is a-‐keto acid dehydrogenase
...
Used in the same proportions initially but when
carbon stores are depleted FA’s are used as primary fuel
...
Adrenaline increases glycogenolysis, to supply emergency energy to the muscles in flight or fight
response
...
Adipose tissue: high energy reserve
...
Degradation of TAG àGLYCEROL + FA’s [by hormone sensitive lipase]
FA: bind to albumin in blood and used as a fuel source
...
Converted to glycerol-‐3-‐phosphate
...
Glycerol in the blood is an indicator of lipolysis
...
Degradation of ketone bodies (produced in fasting state)
Produces and consumes 3
...
Generation of this energy
depends on the cardiac environment: blood flow, hormones, nutritional intake
...
This cycle can also be found
in adipose tissue
...
LCFA breakdown produces Acetyl-‐CoA&NADH-‐ their
presence inhibits pyruvate dehydrogenase
...
So glucose is
no longer transported into the cell
...
Glucose uptake=60-‐70%
LCFA uptake= 20%
ENERGETICS OF MUSCLE CONTRACTION:
Energy supply of the muscle
...
Each fibre type differs in its use of energy
...
Compare fiber type using table
...
Its phosphorylated to phosphocreatine
...
Another immediate energy
Source:
ADP+ADPàATP+AMP
...
Alanine + Glycine àGlycoamino
...
Where it is phosphorylated to
phosphocreatine
...
•
Increase in blood creatine levels indicates renal failure
...
Biochemistry of Connective Tissue
...
Structure of
glycosaminoglycans
...
Collagen and elastin are the main proteins of skin, connective tissue, sclera and cornea of the eye, and
blood vessel walls
...
They exhibit special mechanical properties
resulting from unique structure obtained by combining
specific
amino acids into regular elements
...
There are nineteen types of collagen
...
Collagen of bone occurs as fibers arranged at an angle to each other to resist mechanical
shear from
any direction
...
In the cornea of the eye, collagen is stacked so as to transmit light with a minimum of scattering
...
Collagen molecules possess a special triple-helical structure
...
The three polypeptide chains are stabilized by hydrogen bonds
...
This allows interaction
between triple-helical molecules that leads to aggregation of collagen monomers into long fibers
The cc-chains of different collagen types vary in the amino acid content but are almost the same size,
approximately 1000 amino acids in length
...
The most common collagen
...
Collagen contains hydroxyproline and hydroxylysine, which are
...
These
residues result from the hydroxylation of certain proline and lysine residues after their incorporation into
polypeptide chains (posttranslational modification)
...
The hydroxyl group of the hydroxylysine residues of collagen may be glycosylated
...
Biosynthesis of collagen takes place in fibroblasts (or in the related osteoblasts of bone and chondroblasts
of cartilage)
...
Like proteins
synthesized for export, the nascent collagen chains contain a special amino acid sequence at their Nterminal ends
...
The signal sequence
facilitates the passage of the polypeptide chain into the cisternae of the REP
...
The pro-a-chains are processed by proline and lysine residues hydroxylation to form hydroxyproline and
hydroxylysine residues
...
In the case of ascorbic acid
deficiency, collagen fibers cannot be cross-linked, that leads to decreasing the tensile strength
...
Some of the hydroxylysine residues are modified by glycosylation with glucose or glucosylgalactose
...
The procollagen molecules are translocated to the Golgi apparatus, where they are packaged in
secretory vesicles, that are secreted into the extracellular space
...
Collagen molecules spontaneously associate to form ordered, overlapping staggered-pattern fibrils
...
The resulting aldehydes condense with lysyl or hydroxylysyl residues in
neighboring collagen molecules to form covalent crosslinks
...
Degradation of collagen
...
Collagen degradation in the extracellular matrix is
accomplished by collagenases that cleave icollagen fibers into smaller fragments that can be phagocytosed
and degraded by lysosomal enzymes
...
Ehlers-Danlos syndrome; This disorder is a heterogeneous group of generalized connective tissue
disorders that is characterized by
stretchy skin and loose joints, which may be due to any
one of several enzyme deficiencies
...
Retarded wound healing
and a rotated and twisted spine leading to a hump-back appearance are common features of the disease
...
Elastin
In contrast to collagen, which forms fibers that are tough and have high tensile strength, elastin is a
connective tissue protein with rubber-like properties
...
Elastin fibers are
found in the lungs, the walls of large blood vessels, and elastic ligaments
...
Elastin is also
rich in proline and lysine, but as compared with collagen contains little hydroxyproline and no
hydroxylysine
...
The cross-links that are formed by four lysine side chains from four separate elastin chains
are named•a desmosine cross-link
...
Elastin degradation
...
The physiologic role of (IPAT is inhibiting of neutrophil elastase, a protease that is released into
the extracellular space and degrades elastin of alveolar walls as well as other structural proteins in a
variety of tissues
...
Monocytes and alveolar macrophages
synthesized it as well, that is important in the prevention of local tissue injury by elastase
...
This proteolytic activity is inhibited normally by Œ1-AT
...
Smokers with cq-AT deficiency have a considerably elevated rate of lung destruction and poorer survival
rate than do nonsmokers
...
3
...
These compounds have
the special ability to bind large amounts of water, thereby producing the gellike matrix that forms the basis
of the body's ground substance
...
Glycosaminogiycans stabilize and support cellular and fibrous components of tissue and maintain the
water and salt balance of the body
...
Cartilage is rich in ground substance, whereas tendon is
composed primarily of fibers
...
An example of specialized ground substance is
the synovial fluid which serves as a lubricant in joints, tendon sheaths, and bursae
...
The amino sugar is either
D•giucosamine or D-galactosamine in which the amino group is usually acetylated
...
The acidic sugar is either D-glucuronic acid or L-iduronic acid
...
Because of their large number of negative charges, these heteropolysaccharide chains tend to be extended
in solution and are surrounded bv a shell of water molecules
...
When a solution of glycosamifloglycans is compressed, the water is
"squeezed out" and the glycosaminoglycans are forced to occupy a smaller volume
...
Structure of proteoglycans
...
A proteoglycan monomer found in cartilage consists
of a core protein to which the linear carbohydrate chains are covalently attached
...
The species of glycosaminoglycans include
chondroitin sulfate 4 and keratan sulfate
...
The association is not covalent, but occurs primarily through ionic interactions between the
core protein and the hyaluronic acid
...
SYNTHESIS OF GLYCOSAMINOGLYCANS
The polysaccharide chains are elongated by the sequential addition of alternating acidic and
amino sugars, donated by their UDP-derivatives
...
The synthesis of the glycosaminogiycans is analogous to that of glycogen except that the
glycosaminoglycans 14re produced for export from the cell
...
Sulfation of the carbohydrate chain occurs after the
monosaccharide to be sulfated has been incorporated into the growing carbohydrate chain
...
Sulfotransferases cause the sulfation of the
carbohydrate chain at specific sites
...
The protein is then glycosylated by
membrane-bound transferases as it moves through the ER
...
Glycosaminoglycans are degraded intracelularly after phagocytosis and fusion with lysosomes
...
MUCOPOLYSACCHARIDOSES
The mucopolysaccharidoses are hereditary disorders that are clinically progressive and characterized by
accumulation of glycosaminoglycans in various tissues, causing skeletal and extracellular matrix
deformities
...
Hunter's syndrome and Hurler's syndrome are among the
mucopolysaccharidoses resulting from the deficiencies of enzymes required for the degradation of
heparan sulfate and dermatan sulfate