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Mighty T
GLYCOLYSIS
(EMBDEN-MEYERHOF PATHWAY)
Glycolysis is the sequential degradation of a molecule of glucose(6-carbon compound) in a series of enzymecatalyzed reactions to yield two molecules of the three-carbon compound( pyruvate)
...

Site of reaction: All the reaction steps take place in the cytoplasm
...

Glycolysis was the first metabolic pathway to be elucidated and is probably the best understood
...

Glycolysis is an almost universal central pathway of glucose catabolism, the pathway with the largest flux of
carbon in most cells
...
It is the sole source of metabolic energy in some mammalian tissues and cell types (erythrocytes, renal
medulla, brain, and sperm)
...
It is the only pathway that is taking place in all the cells of the body
...
The glycolytic pathway provides carbon skeletons for the synthesis of non-essential amino acids as
well as glycerol part of fat
...
Most of the glycolysis reactions are reversible, which are also used for glucose synthesis
(gluconeogenesis)
...
Anaerobic glycolysis forms the major source of energy for muscles during strenuous exercise
...
It can be oxidized to yield ribose-5-phosphate for nucleic acid synthesis via the pentose phosphate
pathway
Phases of Glycolysis
Glycolysis has two phases, namely:
1
...
Payoff phase –Energy yield(which constitute the last five steps(step 6-10) in glycolytic pathway)
Chemical transformation in glycolysis
In the sequential reactions of glycolysis, three type of chemical transformation are particularly noteworthy:
(a) Degradation of carbon skeleton of glucose to yield pyruvate
(b) Phosphorylation of ADP to ATP by high energy phosphate compound form during glycolysis
...


Fates of Glucose
Glucose has three major fates;
1
...
Oxidized via the pentose phosphate (phosphogluconate) pathway to yield ribose-5-phosphate for
nucleic acid synthesis
...
It is stored as polysaccharides or sucrose
1

Mighty T
Steps in glycolysis
• Preparatory phase
1
...


This irreversible reaction is catalyzed by hexokinase
...
The type of hexokinase present in
hepatocytes(liver cells) is called hexokinase IV or glucokinase
...
Conversion of Glucose-6-phosphate to fructose-6-phosphate:Phosphohexose isomerase
(phosphoglucose isomerase) catalyzes the reversible isomerization of glucose-6-phosphate (aldose)
to fructose-6-phosphate (a ketose)

3
...
This enzyme catalyzes the transfer of a
phosphoryl group from ATP to fructose-6-phosphate to fructose 1,6- bisphosphate
...


4
...
Fructose 1,6- bisphosphate is cleaved
to yield two different triose phosphates, glyceraldehyde 3-phosphate, an aldose, and
dihydroxyacetone phosphate, a ketose
...
Interconversion of the Triose phosphates: only one of the two triose phosphates formed by aldolase,
glyceraldehyde 3-phosphate, can be directly degraded in the subsequent steps of glycolysis
...


•

The payoff phase

6
...
Phosphoryl Transfer from 1,3-Bisphosphoglycerate to ADP: This reversible reaction catalyzed by
phosphoglycerate kinase involves the transfer of high-energy phosphoryl from the carboxyl group of
1,3-bisphosphoglycerate to ADP, forming 3-phosphoglycerate and ATP
...


Mighty T
8
...
Dehydration of 2-phosphoglycerate to Phosphoenol pyruvate: The enzyme enolase promotes the
reversible removal of a molecule of water from 2-phosphoglycerate to yield Phosphoenol
pyruvate(PEP)
...
Transfer of the phosphoryl group from Phosphoenol pyruvate to ADP: The last step in glycolysis
is also a substrate level phosphorylation which involves the transfer of the phosphoryl group from
Phosphoenolpyruvate to ADP is catalyzed by pyruvate kinase forming Pyruvate and ATP

4

Energy yield in glycolysis

•

Energy yield(number of ATP generated) per molecule of glucose in the glycolytic pathway under
anaerobic conditions(oxygen deficiency)

Step

Enzyme

1

Hexokinase

——

–1

3

Phosphofructokinase

—––

–1

7

1,3-bisphosphoglycerate
kinase

ATP

1×2=2

10

Pyruvate kinase

ATP

1×2=2
Total=4-2=2

•

Source

No
...
of ATPs gained per
glucose molecule

1

Hexokinase

–––

–1

3

Phosphofructokinase

–––

–1

6

Glyceraldehyde-3phosphate dehydrogenase

NADH

3×2=6

7

1,3-bisphosphoglycerate
kinase

ATP

1×2=2

10

Pyruvate kinase

ATP

1×2=2
Total=10–2 =8

5

•

Energy yield per molecule of glucose when it completely oxidised through glycolysis plus Citric
acid cycle, under aerobic conditions
Pathway
Step
Enzyme
Source
No
...
5ATP and FADH2=2 or 1
...

Lactate
II
...

• Under aerobic condition, it is oxidized to
III
...
Lactate Formation

6

When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of
pyruvate and NADH produced in glycolysis,NAD+ is regenerated from NADH by the reduction
of pyruvate to lactate
...
This reaction is catalyzed by lactate dehydrogenase
...
Ethanol formation: Yeast and other microorganisms ferment glucose to ethanol and CO2, rather than
to lactate
...

In the first step pyruvate is decarboxylated in a reaction catalyzed by pyruvate decarboxylase forming
acetaldehyde which is later reduced to ethanol by alcohol dehydrogenase

3
...


Relationship between Magnesium ion(Mg2+) and Adenosine triphosphate(ATP)
Magnesium is an essential element in biological systems, and it is present in every Organism
...
ATP is often called Mg-ATP(MgATP2-)
...
It
play an important role in the glycolysis
...
This association facilitates the
nucleophilic attack of the phosphate group, leading to the transfer on to the hexose sugar
...
Because of this low affinity, glucokinase can only
act when there is plenty supply of glucose
...


•

Phosphofructokinase(PFK-1) is the most important rate limiting enzyme for glycolysis
...
AMP acts as an allosteric activator
...
F-2,6-BP is formed from fructose-6-phosphate by the
action of PFK-2
...
When energy
is plenty in cell, glycolysis is inhibited
...

Pyruvate kinase is inactive in the phosphorylated state
...

Past questions on glycolysis

•
•
•
•
•
•
•
•

Discuss the fate of glucose in Embden-meyerhoff pathway
...

Name the inorganic substrate level phosphorylation in glycolysis
...

State the number of ATP generated in glycolysis
Discuss briefly the fates of pyruvate
...
The Brain alone requires about 120g of glucose each day, and the supply of
glucose from its stored form(glycogen) is not always sufficient; between meals and during longer fasts, or
after vigorous exercise
...
Glucose from the blood is the sole or
major fuel source in human Brain, nervous system, erythrocytes, testes, renal medulla, and embryonic
tissues
...
The
pathway is partly mitochondrial and partly cytoplasmic
...

Three reactions(1,3 and 10) of the glycolysis are essentially irreversible in vivo and cannot be used in
gluconeogenesis
...
Replenish blood sugar
Gluconeogenesis helps in the replenish of blood sugar; an action only performed by liver because glucose 6phosphatase is present mainly in liver
...


2
...
Gluconeogenesis is speeded up by substrate provided by protein
catabolism(glucogenic amino acids) and lactate
...
Generation of glucose
In mammals(human brain, nervous system, erythrocytes, testes, renal medulla, and embryonic tissues
depend majorly on the glucose(energy) synthesized by gluconeogenesis
...

ii
...

iv
...

Key glucogenic enzymes
•
•
•
•

Pyruvate carboxylase
Phosphoenol pyruvate carboxykinase
fructose 1,6-bisphosphatase
Glucose 6-phosphatase

Bypass reactions of gluconeogenesis
1
...
This reaction is irreversible and has a large ,
negative standard free-energy change and thus cannot be reversed by the action of pyruvate kinase of
glycolysis
...
Then pyruvate
carboxylase, a mitochondrial enzyme that requires coenzyme biotin as a carrier of activated HCO3-

,converts

the pyruvate to oxaloacetate:
Because the mitochondrial membrane has no transporter for oxaloacetate, and therefore must be
reduced to malate by mitochondrial malate dehydrogenase, at the expense of NADH:

Malate leaves the mitochondria through a specific transporter(malate aspartate shuttle) in the inner
mitochondria membrane, and in the cytosol it reoxidized to oxaloacetate with the production of
cytosolic NADH
...
Because this reaction is highly
exergonic
...


2
...
Reversal of this reaction is
energetically unfavorable
...


11

Muscle and brain lack this enzyme and so cannot carry out gluconeogenesis
...

bloodstream
...


2
...

4
...
Acetyl CoA is an activator of pyruvate carboxylase so that generation of oxaloacetate
is favoured when the acetyl CoA level is sufficiently high
...

Glucagon favours gluconeogenesis by the production of glucose from Its stored form(glycogen) which
prevents blood sugar from falling and thus favours gluconeogenesis and inhibit glycogen Synthesis
(glycogenesis) and glycolysis
Glucocorticoids induce the synthesis of hepatic amino transferases thereby providing substrate for
gluconeogenesis
...

ATP availability
Increased ATP(adenosine triphosphate) favours gluconeogenesis and vice-versa
...
Increased in the enzyme favours
gluconeogenesis and vice versa
...

Enumerate the regulation of gluconeogenesis
Briefly discuss the bypass reactions of gluconeogenesis
...
Kidney and muscle
D-fructose(ketohexose) present in free form in many fruits and formed by hydrolysis of sucrose in the
small intestine of vertebrates is phosphorylated by hexokinase
...

2
...


The fructose-1-phosphate is then cleaved to glyceraldehyde and dihydroxyacetone phosphate by
fructose-1-phosphate aldolase(aldolase B)
...

Dihydroxyacetone phosphate is converted to glyceraldehyde-3-phosphate by the glycolytic enzyme
triose phosphate isomerase
...

15

Fructose is not converted to fructose-1-phosphate, thereby leading to the accumulation of
fructose; the fructose is excreted through urine
...

Treatment: Treatment of dietary fructose
...
The defect is aldolase-B; hence
fructose-1-phosphate cannot be metabolised
...

Accumulation of Frutose-1-phosphate will inhibit glycogen phosphorylase
...

Fructose is also excreted in urine when urine gives positive Benedict’s test
...


•

The galactose-1-phosphate is then converted to its epimer at C-4, glucose-1-phosphate, by a
set of reactions in which uridine diphosphate(UDP) functions as a coenzyme-like Carrier of
hexose groups
...

In galactokinase deficiency galactosemia, there is:
High galactose concentrations are found in blood and urine
16

Infants develop cataracts, caused by deposition of the galactose metabolite(galactitol)in the
Lens
...

Epimerase-deficiency galactosemia
It has a similar symptoms with transferase-deficiency but it is less severe when dietary
galactose is carefully controlled
...


Mannose-6-phosphate is isomerized by phosphomannoseisomerase to yield fructose-6phosphate,an intermediate of glycolysis
...

Write comprehensively on the following terms: I
...
Fructosuria
Draw the metabolic pathway of fructose and related disease conditions
...
It takes place in all animal tissues but
chiefly occurs in the liver and skeletal muscle
...
The main function of glycogen is to provide glucose during
starvation, and it also serves in:
• Maintaining blood glucose level
...

• Storage form of carbohydrates in human body
...

Reactions of the synthetic pathway

1
...

2
...
The reaction involves the phosphoryl shift from C-6 to C-1 of glucose
...
Activation of glucose-1-phosphate
glucose-1-phosphate reacts with UTP(uridine triphosphate) to form active nucleotide UDPglucose(uridine diphosphate glucose) by the enzyme UDP-glucose phosphorylase
...
Action of glycogen synthase
The glucose moiety from UDP-glucose is transferred to a glycogen primer(glycogenin) molecule
...

5
...

• When the chains is lengthened to 11-12glucose residues, the glycogen branching
enzyme(glycosyl (4-6) transferase) catalyzes the transfer of a terminal fragment of 6 to 7
glucose residues from the non-reducing end of a glycogen branch having at least 11residues to
the C-6 hydroxyl group of a glucose residue at a more interior position of the same or another
glycogen chain, thus creating a new branch
...

18

•

•

Regulation of glycogenesis
Availability of substrate: In a well-fed state, when the blood glucose level is high, glucose6-phosphate, the substrate for UDP-glucose is high, which allosterically increases
glycogenesis
...

Hormone:
Glucagon and epinephrine are diabetogenic(increase blood glucose level) and thus antagonize
glycogen synthesis
Insulin which is an anti-diabetic hormone favours glycogenesis by lowering the blood
glucose level and storing it as glycogen
...
Glycogen phosphorylase b
...
Phosphoglucomutase
1
...
The alpha1,4linkages in the glycogen are cleaved
...
It cannot attack the 1,6linkage at branch point
...
Debranching by bifunctional(two) enzymes
This enzyme removes the glycogen branches thereby making glycogen available for accessibility
...
This enzyme
is alpha-1,4-alpha1,4 glucan transferase
...
Then alpha-1,6glucosidase(debranching enzyme) canhydrolyse the remaining glucosyl unit held in alpha-1,6
linkage at the branch point
...

3
...

The glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase
4
...
The free glucose is
released to the bloodstream
...
Type 1(glucose-6-phosphatase deficiency) Von Gierke’s disease: The deficiency of this enzyme
leads to high accumulation of glycogen in the kidney and liver, this causes hypoglycemia(low
glucose level), hepatomegaly(enlarged liver)
...
Type II(alpha-1,4 glucosidase deficiency) pompe’s disease:This is the most devastating of the
glycogen storage diseases
...
Alpha-1,4 glucosidase is not involved in
the main glycogen metabolism
...

3
...
Type 0(Liver glycogen synthase deficiency): This is the only type of glycogen metabolism
disease in which there is low glycogen present rather than overabundance of glycogen
...
It causes hyperglycemia
...
The cycle is therefore named
after him(Krebs cycle) afterward he proposed original name as TCA(tricarboxylic acid) cycle
...
During this process, energy is trapped
...
This is

the link between TCA cycle and glycolysis
...
Pyruvate with the help of a carrier can enter the mitochondria from the cytoplasm
...
All the enzymes of citric acid cycle are
located inside the mitochondria
...
Formation of Citrate: The first reaction of the cycle is the condensation of acetyl-coA with
Oxaloacetate to form Citrate catalyzed by Citrate synthase
...

2
...
Aconitase contains an iron-sulfur center which acts both in the catalytic addition or removal of
water leading to the formation of either citrate and isocitrate
...
Oxidation of isocitrate to a-ketoglutarate: In this reaction, the enzyme isocitrate dehydrogenase
catalyzes oxidative decarboxylation of isocitrate to form a-ketoglutarate
...

4
...

5
...

6
...

7
...

8
...
Citrate and Citrate synthase: The formation of citrate from oxaloacetate and acetyl coA is as
important part of control
...
Citrate
allosterically inhibits PFK, the key enzyme of glycolysis; stimulates fructose-1,6-bisphosphatase, a
key enzyme of gluconeogenesis and activates acetyl coA carboxylase, key enzyme of fatty acid
synthesis
21

2
...
The Krebs cycle is the
largest generator of ATP among metabolic pathways
...
Isocitrate Dehydrogenase : In the step 3 of TCA cycle, ADP acts as a positive modifier enhancing
the binding of substrate
...

4
...

Importance of the Citric acid cycle
1
...
It is the Source of reduced co-enzyme (NADH, FADH2) that provide the substrate for the respiratory
chain
3
...
It provides precursors for synthesis of amino acids and nucleotides
5
...
of ATPs generated

3

Isocitrate
dehydrogenase

NADH

3*1=3 (2
...
5)

4

a-Ketoglutarate
dehydrogenase

NADH

3*1=3(2
...
5)

5

Succinyl-coA
Synthetase/succinate
thiokinase

GTP

1*1=1

6

Succinate
dehydrogenase

FADH2

2*1=2 (1
...
5)

8

Malate dehydrogenase

NADH

3*1=3 (2
...
5)

Total ATP=12or 10
Note:
•
•
•
•
•
•
•
•

Krebs cycle is also called Tricarboxylic cycle(TCA cycle)due to the presence of three carboxylic
group in the structure of tricarboxylic acid(citric acid) which is a member of the cycle
...

All reactions in this cycle are reversible with the exception of reaction 1,3 and 4
...

The members of the cycle can be formulated as (At comedy international, Alfa Sule Search for
more offer)
NADH =3 ATP / 2
...
5 ATP
The citric acid cycle has eight steps
22

•
•

The main function of this cycle is Energy production(ATP)
A total of 12ATP is generated per cycle

Past questions on Krebs cycle
•
•
•

Enumerate the energy produced by the TCA(Krebs) cycle
Write a comprehensive note on tricarboxylic acid cycle
Discuss the importance and regulation of TCA cycle

MightyT
Pentose phosphate/ Phosphogluconic pathway of Glucose oxidation
This is another fundamental important pathway of Glucose 6-phosphate Catabolism(the major catabolic fate
is Glycolysis) that take place in rapidly dividing cells such as those of bone marrow, skin and intestinal
mucosa
...
The cell uses pentoses to make RNA, DNA and
coenzymes such as ATP, NADH, FADH2 and coenzyme A
...

Pentose phosphate pathway(PPP) has two phases:
•
•

Oxidative phase (Glucose 6-phosphate is degraded to yield pentose phosphate and NADPH)
Non-oxidative phase (Pentose phosphate is recycled to Glucose 6-phosphate

The oxidative phase reactions( production of Pentose Phosphates and NADH)

1
...

2
...

3
...
This reaction generates a second molecule of NAD
23

4
...

isomerase converts ribulose5-phosphate to its aldose isomer, ribose 5-phosphate
...

Note: In some tissues, the pentose phosphate pathway ends at this point

Mighty T

The Nonoxidative phase reactions(recycles pentose phosphates to Glucose 6-phosphate)
5
...

6
...

The remaining three-carbon fragment from xylulose is glyceraldehyde 3-phosphate
...
Action of Transaldolase: The other unique enzyme in phosphogluconic pathway is Transaldolase
which catalyzes the transfer of a three carbon fragment from sedoheptulose 7-phosphate and
condenses with glyceraldehyde 3-phosphate, forming fructose 6-phosphate and the tetrose
erythrose 4-phosphate
...
Action of Transketolase catalyzes the transfer of two carbon unit(C-1 and C-2) from a ketose donor
xylulose 5-phosphate to an aldose acceptor erythrose 4-phosphate, forming fructose 6-phosphate
and glyceraldehyde 3-phosphate
...

Transaldolase : catalyzes the transfer of three carbon unit
...
Regeneration of glucose 6-phosphate: Two molecules of glyceraldehyde 3-phosphate formed from
these reaction can converted to glucose 6-phosphate(a precursorof oxidative phase of pentose
phosphate pathway)through the process of gluconeogenesis
...


Regulation of HMP shunt pathway
•

Availability of coenzyme

The pathway is mainly regulated by the level of NADP+
...
The oxidative phase is therefore
24

controlled by the level of NADP+ and non-oxidative phase by the requirement of pentoses
...

•

Hormonal action

The action of insulin induce glucose 6-phosphate dehydrogenase and therefore will increase the overall
pathway
...
It serves as another catabolic fate of glucose 6-phosphate which lead production of specialized products
such as Ribose 5-Phosphate, NADPH needed by the cell
2
...

3
...

4
...

5
...
The generated NADPH is used in preventing oxidative damage caused by the deficiency of glucose 6phosphate dehydrogenase(the first enzyme of the pathway)
7
...
In human, NADPH is needed for the normal maintenance of Red blood cell where it serve to generate
reduced Glutathione(GSH), a deficiency of which result in haemolysis of the cell
...
NADPH is required for preserving the transparency of lens
...
Most of the drugs and other foreign substances are detoxified by the liver microsomal P450 enzymes,
with the help of NADPH
...

•
•
•
•
•

State the importance of hexose monophosphate shunt in man
State the importance of the NADPH and ribose-5-phosphate in pentose phosphate pathway
...

Write an essay on the metabolism and importance of pentose phosphate shunt
...


25

Mighty T
Biosynthesis of Triglycerides and Phospholipids

Biosynthesis of Triglycerides (Triacylglycerol)
Humans can store only a few hundred grams of glycogen in liver and muscle, barely enough to supply the
body for 12hours
...

In a man weighing 70kg, the total TAG found is 15kg which can support basal energy for 12weeks
...

When the diet contains more fatty acids than are needed immediately as fuel, they are converted to TAG in
the liver and packaged with specific apolipoprotein into very low density lipoprotein(VLDL)
...

The starting material in TAG biosynthesis is Sn glycerol-3-Phosphate
...
Phosphorylation of glycerol: The enzyme glycerol kinase catalyzed the transfer of a phosphoryl
group from a nucleoside triphosphate (ATP) to the C-3 of glycerol yielding glycerol-3-Phosphate
...
It occurs
in the liver
...
Reduction of Dihydroxyacetone phosphate(DHAP): The glycolytic intermediate
Dihydroxyacetone phosphate which is one of the triose phosphate produced from the cleavage
of fructose 1,6 bisphosphate is reduced by the action of the Cytosolic NAD-linked glycerol-3phosphate dehydrogenase to yield glycerol-3-phosphate
...
Denovo production by a process known as glyceroneogenesis via pyruvate: Glyceroneogenesis
is the shortened version of gluconeogenesis, from pyruvate to Dihydroxyacetone
phosphate(DHAP) followed by the conversion of DHAP to glycerol-3-phosphate by the action of
the Cytosolic NAD-linked glycerol-3-phosphate dehydrogenase
...


Mighty T
Process of TAG biosynthesis

27

The first stage of biosynthesis of TAG is the acylation of the two free OH group of Sn glycerol-3-phosphate
by two molecules of fatty acyl-CoA to yield diacylglycerol-3-phosphate(Phosphatidic acid) which can be
converted to TAG or glycerolphospholipid
...


28

Reactions in the biosynthesis of glycerolphospholipid

29

The first stage in the biosynthesis of glycerolphospholipid are shared with the pathway of Triacylglycerol,
which involves the acylation of the two free OH group of Sn glycerol-3-phosphate with two molecules of fatty
acyl CoA, a reaction catalyzed by acyl transferase to yield Diacylglycerol-3-Phosphate(Phosphatidic acid)
...
Displacement of Cytidine Monophosphate(CMP)
through the nucleophilic attack by the hydroxyl group of serine yields Phosphatidylserine(PS)
...
PE may also be converted by the addition of 3-methyl groups to its amino
group
...

Discuss the biosynthesis of phosphatidylcholine
...


30

Mighty T
Cholesterol biosynthesis

Cholesterol is a precursor of steroid hormones; it is an essential molecule in many animals(humans),it is not
required in mammalian diet
...
In a man weighing 70kg, a total of about 140g of cholesterol is found
...
Cholesterol 27C compound suggests a complex pathway
...

Site of cholesterol biosynthesis: livre, adrenal cortex, testis, Ovaries and intestine
...

The enzyme involved in the synthesis of cholesterol are partly in the endoplasmic reticulum and partly in the
cytoplasm
...

2
...

4
...

Conversion of Mevalonate to Two activated isoprene
...

Conversion of squalene to Cholesterol
...
Synthesis of Mevalonate from acetate :Condensation of acetyl CoA leads to the formation of
acetoacetyl CoA, which later condenses with the third acetyl CoA to form B-hydroxyl-B-methyl
glutaryl CoA(HMG CoA) , these reactions is acted upon by Thiolase and HMG-CoA synthase

respectively
...
This reaction acted upon by HMG CoA reductase is the committed and rate limiting step
...
Conversion of Mevalonate to two activated Isoprenes: Mevalonate is converted to two activated
isoprene by the addition of three phosphate groups from ATP molecules leading to the formation of
an intermediate; 3-phospho-5-pyrophosphate Mevalonate
...


The phosphate group and the carboxyl group leave, producing a double bond in the five carbon product;
Isopentenyl pyrophosphate, which is one of the two isoprene units of cholesterol biosynthesis
...


32

3
...
Geranyl pyrophosphate also undergo
head-tail condensation with Isopentenyl pyrophosphate by the action of the enzyme prenyl transferase
leadingto the formation of Farnesyl pyrophosphate
...

33

4
...
NADPH reduces the oxygen atom
of O2 to H2O
...
In animal
cells, this cyclization results in the formation of lanosterol, which contain the four rings characteristics
of the steroid nucleus
...

34

Fate of Cholesterol
• Degradation to CO2 : In human tissues, this conversion does not occur)
• Conversion to Bile acids: The major pathway, more than 50% is converted to bile acids and
excreted in faeces
...

• Formation of cholesteryl ester through acyl CoA transferase(ACAT)
• Formation of steroid hormones (testosterone, progesterone)
Regulation of cholesterol biosynthesis
The regulatory enzyme is HMG CoA reductase
...
Regulation at transcription: Long-term regulation involves the regulation at transcription of the gene
for HMG CoA reductase
...
When
cholesterol in diet is low, synthesis is increased
...
Insulin and thyroxine increase the activity of HMG CoA reductase
3
...
Drugs: Lovastatin and other statin group of drugs are competitive inhibitors of HMG CoA reductase
...


Bile acid formation
Bile acids are synthesized in the liver from cholesterol
...
All of them have an alpha-oriented hydroxyl group at position 7
...
Primary bile acid: They are synthesized in the liver from cholesterol
...
Secondary bile acid: They are produced in the intestine from primary bile acid by the action of
intestinal bacteria, they are produced by deconjugation
...
Cholic acid is
thus converted to deoxycholic acid and chenodeoxycholic acid to lithocholic acid
...

Role of bile acid
• Elimination of cholesterol from the body
• It increases the absorption of fats
• Bile acid-containing micelles aid lipases to digest lipids
...

•
•
•
•
•

Past questions on cholesterol biosynthesis
State the unique steps in cholesterol biosynthesis
Briefly describe the rate limiting step in cholesterol biosynthesis
Discuss the regulatory mechanisms in cholesterol biosynthesis
Mention one bile acid and discuss its biosynthesis from a two-carbon compound
Write the role of bile acid in the catabolism of fatty foods
...

Mighty T
De novo synthesis of fatty acids
The process of fatty acid synthesis was studied by Feodor Lyen(Nobel prize, 1964)
...
The process occurs in liver, adipose tissue, kidney, brain and mammary
glands
...

The starting material for denovo synthesis of fatty acid is acetyl-CoA
...

Transport of Acetyl CoA from mitochondria to cytoplasm
Pyruvate is converted to acetyl CoA in the mitochondria
...
Hence, the acetyl CoA units is first converted to citrate and transported from the
mitochondria into the cytoplasm through a tricarboxylic transporter
...
The oxaloacetate then
return to the mitochondria as malate or pyruvate
...
The enzymes are grouped into three units;
• Condensing unit : It is the initial substrate binding site
...

• Reduction unit: it contains keto acyl reductase, dehydratase, enoyl reductase and acyl carrier
protein(ACP) catalysing step 4,5,6 respectively
• Releasing unit: it is involved in the release of the palmitate synthesised
...

Step 1: Carboxylation of acetyl CoA
The first step in the fatty acid synthesis is the carboxylation of acetyl CoA to form malonyl CoA catalysed
by acetyl CoA carboxylase which requires a coenzyme biotin(carrier of activated carboxyl group)
...
But it is the rate-limiting enzyme
...
The acetyl transacylase(AT) catalyses the transfer of the acetyl group to the cysteinyl SH group of
the condensing enzyme(CE)
b
...

Step 3: Condensation
The acetyl(2c) and malonyl(3c) units are condensed to form beta-keto acyl ACP(acetoacetyl ACP)
...
The enzyme is called condensing enzyme(keto acyl synthase)
Step 4: Reduction
The acetoacetyl ACP is reduced by NADPH dependent beta-keto acyl reductase(KR) to form betahydroxyl fatty acyl ACP
...

Cycling of Reactions
The butyryl group(4c) is now transferred to the SH group of the condensing enzyme and another malonyl
CoA to the SH group of Acyl carrier protein
...
The cycles are repeated a total of seven times, till the 16Carbon palmitic acid is formed
...

The end point is palmitic acid(16c) in the Liver and adipose tissue
...
Cow milks contain odd numbered fatty acids
...

Give the total and net ATP generated from the oxidation of the following fatty acids:
16C,15C, 17C, and 24C fatty acids
...
Discuss
Give an account of the total ATP generated from the total catabolism of a named saturated
fatty acid
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Fatty acid Beta-oxidation

This is the process by which fatty acids are broken down by various tissues to produce energy
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Site of reaction: It occurs in the Mitochondrial matrix
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Beta-oxidation involves the splitting of fatty acyl molecules to a two carbon units(acetyl CoA)
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Note: The oxidation of fatty acid in the mitochondria(site of beta-oxidation), occur only after its activation in the
cytoplasm(site of FA synthesis) by the action of co-enzymeA to form acyl CoA
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Preparative steps for beta-oxidation
1
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This activation takes place in the
cytoplasm
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This reaction is catalyzed by the enzyme thiokinase or fatty
acyl CoA
synthetase
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Transport of activated fatty acid
Fatty acids are activated in the cytoplasm; but the beta-oxidation is in the mitochondria
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The long chain fatty acyl CoA cannot pass through the inner
mitochondrial membrane; therefore it binds to the hydroxyl group of carnitine
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Enzymes of Carnitine and their functions
• Carnitine Acyl transferase-I(CAT-I): It transfers the fatty acyl group which cannot cross the mitochondrial
membrane to the hydroxyl group of carnitine to form acyl carnitine
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Carnitine acyl translocase: A protein translocase will carry the acyl carnitine across the membrane to the
matrix of mitochondria
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Mighty T
Reactions of Beta-oxidation
Beta oxidation involves four major reactions: a
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Hydration c
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Cleavage

1
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FADH2when oxidised in electron transport chain will produce
2ATP molecules
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Hydration: This reaction is catalysed by enoyl CoA hydratase, which involves the addition of H20 to enoyl CoA
to form beta-keto fatty acyl CoA
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Oxidation: The beta-hydroxyl acyl CoA is oxidised again by the action beta-hydroxyl acyl CoA dehydrogenase
to form beta-keto acyl CoA, with the production of NADH
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Cleavage: The beta-keto acyl CoA now undergoes thiolytic cleavage, splitting off a molecule of acetyl CoA and
leaving behind a fatty acid with a 2 carbon atoms less
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Note: The newly formed fatty acyl CoA will sequentially undergo further cycles of steps 1,2,3 and 4 of Boxidation until Fatty acid is completely converted to acetyl CoA
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Therefore in beta-oxidation, the acetyl CoA produced is one greater than the cycle produced
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No of cycle =No of acetyl CoA—1
2
Examples: Calculate the ATP generated from the complete oxidation of (a) Palmitic acid (b) stearic acid
Solution
Palmitic acid=16carbon(it undergoes 7cycles of B-oxidation and produces 8acetyl CoA)
• 8acetyl CoA × 12ATP(Krebs cycle) =96ATP
• 7FADH2 × 2ATP(step 1)
=14ATP(Beta-oxidation)
• 7NADH ×3ATP(step 3)
=21ATP(Beta-oxidation)
Gross total
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=131-2ATP for activation =129ATP
Stearic acid=18carbon unit(it undergoes 8cycles of B-oxidation and produces 9acetyl CoA)
• 9acetyl CoA × 12ATP
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=16ATP
• 8NADH × 3ATP
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=148ATP
Net yield
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The Propionyl CoA is then converted to Succinyl-CoA, which later get
converted to acetyl CoA(2C) and finally enters the TCA cycle
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=12ATP
Energy yield from Propionyl conversion=18ATP-1ATP=17ATP
Note: Odd chain fA oxidation yields equal Number of cycle and acetyl CoA with one Propionyl CoA
Example 2: Calculate the number of ATP generated in 25c fatty acids
Number of Cycle =Number of acetyl CoA = N-3
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= 22
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=22ATP
• 11NADH × 3ATP
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=17ATP
Total energy yield= 204ATP-2ATP for activation = 202ATP

2

Plasma protein
Total plasma is 400-600mg/dl containing one-third of cholesterol, triglyceride, phospholipids
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They are complexed with proteins to form lipoproteins
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Classification of Lipoproteins
Lipoproteins are classified into five(5) major types depending on the density by :
• Ultrafiltration
• Electrophoretic mobility
1
...

3
...

5
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Apoprotein increase the solubility of lipoprotein
Separation by centrifugation: Lipoprotein with High lipid will have a low density and so Will float
on centrifugation while the low lipid content lipoprotein will sediment and have high density
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Apolipoprotein(protein portion)
The protein part of lipoprotein is called apolipoprotein(apo-Lp) or apoprotein
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Intestinal cells produce
small quantities of apo
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Apo-A-I: it activates lecithin-cholesterol acyl transferase(LCAT)
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It
is anti-atherogenic
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Apo-B-100: it is the only apoprotein in LDL; it binds to LDL receptor on tissues
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Apo-B-48: it is the apoprotein of chylomicrons
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B-48 is so named because it is only 48% of the size of B-100
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Apo-C-ii: It activates lipoprotein lipase
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Apo-E: it is a universal lipoprotein, an arginine rich protein
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It is involved in the cellular transport of lipid
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Chylomicrons
They are the transport form of dietary triglycerides from intestines to the adipose tissues for
storage; and to muscle or heart for their energy needs
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VLDL
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VLDL carries triglycerides(endogenous triglycerides) from the liver to peripheral tissues for
energy needs
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LDL
It transports cholesterol from liver to the peripheral tissues
...
It is also referred to as bad
cholesterol because about 75% of the plasma cholesterol is incorporated into the LDL
particles
...

4
...
This is called reverse cholesterol
transport by HDL
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HDL is anti-atherogenic
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The triglycerides stored in the adipose tissue
are not inert
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• Adipose tissue in a well-fed condition
There is active lipogenesis(synthesis of lipid) in the adipose tissue
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In a well fed condition, glucose and insulin levels are increased
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Increased activity of glycolytic enzymes
2
...
Increase in activity of acetyl CoA carboxylase
4
...
It inhibit hormone sensitive lipase
• Adipose tissue in a fasting condition
The metabolic pattern totally changes under conditions of fasting
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Under starvation, a high glucagon, ACTH, glucocorticoids and thyroxine have lipolytic effect
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• Adipose tissue and Diabetes mellitus
Lipolysis isenhanced and high FFA level in plasma is noticed in diabetes mellitus
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These receptors are decreased, leading to insulin insensitivity in
diabetes, when there is no insulin, lipolysis will be enhanced
...
This leads to Obesity
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But the insulin
receptors are decreased; and there is peripheral resistance against insulin action
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Such TAG cannot be metabolically reutilized and forms the dead bulk in obese individuals
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There is always a balance between
factors causing fat deposition versus factors causing fat removal from the liver
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Factors causing fatty liver
Mobilisation of non-esterified fatty acid(NEFA) from adipose tissue
Reduced removal of fat from the liver
Excess calorie intake
Toxic injury to the liver
Alcoholism

Why alcoholism causes fatty liver ?
In the metabolism of alcohol, it is oxidized to acetaldehyde
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As the availability of oxalo-acetate
is reduced, the oxidation Of acetyl CoA through citric acid cycle is reduced
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Past questions
✓
✓
✓
✓
✓
✓
✓
✓

What are the five(5) main types of lipoproteins
List the factors that tend to 1
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Decrease liver fat
Discuss the role lipoproteins in the flow of cholesterol to and fro the liver
Differentiatebetween bad cholesterol and good cholesterol
Write a concise note on lipid metabolism in adipose tissue
Discuss cholesterol homeostasis in the cells
Write a short note on reverse flow of cholesterol and its significance
Write a brief note on the classification of lipoproteins

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