Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

LIPIDS
Tariq Javed (PhD)

Learning Objectives
1
...
Or a compound
to be called as lipid?
2
...
Learn the three major groups
with examples of each group
...
Study the biomedical importance of lipids
...

• Chemically they are various types of esters of
different alcohols
...


• Bloor’s Criteria
• According to Bloor, lipids are compounds
having the following characteristics:
• They are insoluble in water
...

• Some relationship to the fatty acids as esters
either actual or potential
...


• Thus, lipids include fats, oils, waxes and
related compounds
...

• Distinction between fats and oils is a purely
physical one
...


They have the common
Property
• Relatively insoluble in water
• Soluble in nonpolar solvents such as ether
and chloroform
...
g, obesity, diabetes mellitus,
atherosclerosis, and the role of various
polyunsaturated fatty acids in nutrition and
health
...

• Fat is stored in adipose tissue, where it also
serves as a thermal insulator in the
subcutaneous tissues and around certain organs
...


• Combinations
of
lipid
and
protein
(lipoproteins)
are
important
cellular
constituents, occurring both in the cell
membrane and in the mitochondria, and
serving also as the means of transporting
lipids in the blood
...
Simple Lipids

II
...
Derived Lipids
IV
...
Simple Lipids
Esters of fatty acids with various alcohols:
(a) Neutral fats (Triacylglycerol, TG): These are
triesters of fatty acids with glycerol
...

• True waxes are esters of higher fatty acids
with cetyl alcohol (C16H33OH) or other higher
straight chain alcohols
...

• Vit A and Vit D esters are palmitic or stearic
acids esters of Vit A (Retinol) or Vit D respectively
...
Complex lipids:
Esters of fatty acids containing groups in
addition to an alcohol and a fatty acid
...
Phospholipids: Lipids containing, in addition
to fatty acids and an alcohol, a phosphoric
acid residue
...

• Examples: phosphatidyl choline (Lecithin),
phosphatidyl ethanolamine (Cephalin),
phosphatidyl inositols (Lipositols), phosphatidyl
serine, plasmalogens, sphingomyelins, etc
...

They contain a special alcohol called sphingosine or
sphingol and nitrogenous base in addition to fatty
acids but does not contain phosphoric acid or glycerol
...

(d) Aminolipids (Proteolipids)
(e) Lipoproteins: Lipids as prosthetic group to
proteins
...
Derived Lipids

Derivatives obtained by hydrolysis of those given in
group I and II, which still possess the general
characteristics of lipids
...

(b) Monoglycerides (Monoacylglycerol) and
Diglycerides (Diacylglycerol)
...

• Cholesterol and other steroids including Vit D
...


IV
...

• Carotenoids
• Squalene is a hydrocarbon found in shark
and mammalian liver and in human sebum
...


FATTY ACIDS
FATTY ACIDS ARE ALIPHATIC CARBOXYLIC ACIDS
Fatty acids occur mainly as esters in natural fats
and oils but occur in the unesterified form as free
fatty acids, a transport form found in the
plasma
...

• The chain may be saturated (containing no
double bonds) or unsaturated (containing one or
more double bonds)
...

• Depending on the R group (the hydrocarbon
chain), the physical properties of fatty acids may
vary
...
FA are
obtained from hydrolysis of fats
...

• Unsaturated FA: Those which contain one or more
double bonds
...


• Saturated fatty acids having 10 carbon or less
number of carbon atoms are called as lower
fatty acids, e
...
acetic acid, butyric acid, etc
...
g
...

• Milk contains significant amount of lower fatty
acids
...

(1) Mono unsaturated (Monoethenoid) fatty
acids: They contain one double bond
...

(2) Polyunsaturated (Polyethenoid) fatty acids:
There are three polyunsaturated fatty acids of
biological importance
...

• They have to be provided in the diet, as they
cannot be synthesised in the body
...
g
...
g
...


(d) Substituted fatty acids: In hydroxy fatty acid
and methyl fatty acid, one or more of the
hydrogen atoms have been replaced by – OH
group or – CH3 group respectively
...
g
...

(e) Cyclic fatty acids: Fatty acids bearing cyclic
groups are present in some seeds, e
...

• Chaulmoogric acid obtained from chaulmoogra
seeds

(f) Eicosanoids: These are derived from
eicosapolyenoic FA
...

• Saturated acids end in “anoic” e
...
octanoic
acid and
• Unsaturated acids with double bonds end in
enoic, e
...
octadecenoic acid (oleic acid)
...
1)
...
i
...
carbon number 2 is
known as α-carbon, carbon atom 3 is β-carbon
and the end –CH3 carbon is known as the ωcarbon (‘Omega’ carbon)
...
g
...
g
...


(d) A widely used convention is to express the
fatty acids by formula to indicate:
• The number of carbon atoms
• The number of double bonds and
• The positions of the double bonds
...
formula C17H33
COOH) has one double bond between C9 and C10,
thus: 10 9 CH3 (CH2)7 –CH = CH (CH2)7 – COOH
• According to above criteria, it is expressed as 18 :
1; 9,
[18 indicates the number of carbon atoms, 1
indicates the number of double bond and 9
indicates the position of the double bond]
...

(a) Geometric Isomers: They depend on the
orientation of the radicals around the axis of the
double bonds
...

If the radicals are on the opposite side, a ‘trans’
form is produced
...

• They cannot be synthesised in the body and
must be provided in the diet
...


Polyunsaturated fatty acids (PUFA)

• The polyunsaturated fatty acids (PUFA) exist
in cis configuration in naturally occurring
lipids
...
They are generally considered to be
injurious to health
...

• Oils containing PUFA also have high content of TFA
...

• Trans fatty acids adversely affects multiple risk factors
for chronic diseases, including composition of blood
lipids and lipoproteins, systemic inflammation,
endothelial dysfunction, insulin resistance and
diabetes

Properties of Fatty Acids

• Triglycerides or triacylglycerols are also known as
neutral fat

• Oxidation of Fatty Acids
• All fatty acids undergo oxidation in the body
to give energy
...


NEUTRAL FATS

• Neutral fats are also called as triacylglycerols
(TAG) or triglycerides (TG)
...

• As per International Union of Biochemistry
(IUB) the correct designations are
monoacylglycerol, diacylglycerol and
triacylglycerol
...


• Naturally occurring fats and oils are mixtures
of triglycerides
...


• They are hydrophobic and insoluble in water
...
Oils are generally of plant
origin
...
Fats are mainly of animal origin
...
g
...


IDENTIFICATION OF FATS AND OILS
Sometimes it becomes necessary to
• Identify a pure fat
...

• Determine the proportions of different types
of fat in a mixture
...
The
products are glycerol and soaps
• Saponification number is defined as the
number of milligrams of potassium hydroxide
required to saponify one gram of fat
...

• Thus fats containing short chain fatty acids will
have more –COOH groups per gram than long
chain fatty acids and this will take up more
alkali and hence will have higher
saponification number
...

• It is an index of the degree of unsaturation
and is directly proportional to the content of
unsaturated fatty acids
...
g
...


Rancidity of Fat
• Fats and oils have a tendency to become
rancid
...

• Hydrolytic rancidity is due to partial hydrolysis
of the triacylglycerol molecules due to traces
of hydrolytic enzymes present in naturally
occurring fats and oils
...

• Many natural fats and oils may contain
antioxidants (e
...
vitamin E), which prevent the
occurrence of oxidative rancidity
...


• Repeated heating of oils would lead to the
formation and polymerization of cyclic
hydrocarbons
...

Acid Number
• Definition: Number of mg of KOH required to
neutralize the fatty acids in a gm of fat is
known as the acid number
...


Waxes

• They form the secretions of insects, leaves and
fruits of plants, e
...
Lanolin or wool fat,
beeswax, whalesperm oil, etc
...

• They are used as the base for the preparation
of cosmetics, ointments, polishes, lubricants
and candles
...
g
Phosphoglycerides

Functions of Phospholipids
• Structural: Phospholipids participate in the
lipoprotein complexes which are thought to
constitute the matrix of cell walls and
membranes, the myelin sheath, and of such
structures as mitochondria
• Role in enzyme action: Certain enzymes
require tightly bound phospholipids for their
actions, e
...
mitochondrial enzyme system
involved in oxidative phosphorylation
...

Required at the stage:
• Conversion of prothrombin to thrombin by active
factor X, and Possibly also in the activation of
factor VIII by activated factor IX
...

• Cofactor: Phospholipids are required as a
cofactor for the activity of the enzyme lipoprotein
lipase and triacylglycerol lipase
...


Glycerophospholipids
Glycerophospholipids
(phosphoglycerides), are common
constituents of cellular membranes
...

Hydroxyls at C1 & C2 are esterified
to fatty acids
...


CH2OH
H

C

OH

CH2OH

glycerol

Formation of an ester:
O
R'OH + HO-C-R"

O
R'-O-C-R'' + H2O

Phosphatidate
O
O
R1

C

H2C
O

O

CH
H2C

C

R2

O
O

phosphatidate

P

O

O

In phosphatidate:
 fatty acids are esterified to hydroxyls on C1 & C2
 the C3 hydroxyl is esterified to P
...
g
...

The 2 fatty acids tend to be non-identical
...


O
O
R1

C

H2C
O

O

CH
H2C

C

R2

O
O

P

O

O

H
OH

OH
H
OH

phosphatidylinositol

OH
H
H

H
H

OH

Phosphatidylinositol, with inositol as polar head group,
is one glycerophospholipid
...


O
O
R1

C

H2C
O

O

CH
H2C

C

R2

O
O

P

CH3
O

CH2

O

CH2

+
N CH3
CH3

phosphatidylcholine

Phosphatidylcholine, with choline as polar head
group, is another glycerophospholipid
...


• Cephaline

• Phosphatedylserine

Sphingolipids are derivatives of the
lipid sphingosine, which has a long
hydrocarbon tail, and a polar domain
that includes an amino group
...

Other derivatives of sphingosine are
commonly found as constituents of
biological membranes
...


OH
H
C

CH

H3N+

CH
HC
(CH2 )12

OH
H2 C

O

OH
H
C

CH

NH

CH

C
R

ceramide

HC
(CH2 )12
CH3

sphingosine

CH3

In the more complex sphingolipids, a
polar “head group" is esterified to the
terminal hydroxyl of the sphingosine
moiety of the ceramide
...

Sphingomyelins are
common constituent of
plasma membranes

O


O

H2C

sphingosine
O

fatty acid

Sphingomyelin

OH
H
C

CH

NH

CH

C
R

HC
(CH2 )12
CH3

Sphingomyelin, with a phosphocholine head group, is similar in size
and shape to the glycerophospholipid, phosphatidyl choline
...

A ganglioside is a
ceramide with a polar

CH2OH
O

OH
H
OH

H

H
H

OH

O

H

H2C

OH
O

cerebroside with
-galactose head group

H
C

CH

NH

CH

C
R

HC
(CH2 )12
CH3

head group that is a complex oligosaccharide, including the acidic
sugar derivative sialic acid
...


Steroids and Sterols

• The steroids are often found in association
with fat
...

• All of the steroids have a similar cyclic nucleus
resembling phenanthrene (ring A, B and C) to
which a cyclopentane ring (ring D) is
attached
...

• A side chain at position 17 is usual
...

• Most important sterol in human body is
cholesterol
...
Its molecular formula is C27H45OH
...

It has an –OH group at C3
...

It has two –CH3 groups at C10 and C13
...


Properties
• It occurs as a white or faintly yellow, almost
odourless, pearly leaflets or granules
...

• Its melting point is 147 to 150 C
...

Endogenous: Synthesised in the body from acetyl
CoA, approximately 1
...


• Forms of cholesterol: Cholesterol occurs both in free
form and in ester form, in which it is esterified with
fatty acids at –OH group at C3 position
...

• Esterification of cholesterol:
• Some cholesterol esters are formed in tissues by the
transfer of acyl groups from acyl-CoA to cholesterol by
acyl transferases
...


OTHER STEROLS OF
BIOLOGICAL IMPORTANCE
• 1 7-Dehydrocholesterol
It is an important sterol present in the skin
...
Ergosterol
It is a plant sterol, first isolated from ergot

Dehydrocholestrol

Ergosterol

Books
• Murray R, Rodwell V, Bender D, Kathleen M,
Botham P, Weil A et al
...
28th Ed

---