CHEMISTRY OF LIPIDS(Dr M PRIYANKA)

Chemistry of Lipids
MAHE INSTITUTE OF DENTAL SCIENCES & HOSPITAL
Chalakkara, P.O Palloor, Mahe – 673310
U.T. of Puducherry.ph:0490 2337765
Dr. M. Priyanka
Reader
Department of Biochemistry

1. Storage form of energy (triglycerides)
2. Structural component of cell membrane (phospholipids & cholesterol)
3. Precursor of many steroid hormones
4. Act as thermal insulator, electrical insulator
5. Activators of
FUNCTIONS OF LIPIDS

6. Helps in absorption of fat soluble vitamins
7. Lipoproteins – imp. cellular constituents
8. Fats serve as surfactants by  surface tension
9. Act as emulsifying agents (amphipathic lipids)
10. Improve taste & palatability of food
11. Components of
FUNCTIONS OF LIPIDS

Excessive fat deposits  Obesity (risk factor for
heart attack)
Abnormal cholesterol & lipoprotein metabolism
 Atherosclerosis & CVD
In DM, abnormal fatty acids & lipoprotein
metabolisms 
CLINICALAPPLICATIONS

&
Lipids complexed to other compounds
CLASSIFICATION OF LIPIDS
Based on chemical nature;

 Esters of FA with alcohol containing additional
(prosthetic) group.
 Sub classified depending on prosthetic group;
a. Phospholipids, containing phosphoric acid
b. Non-phosphorylated
II. COMPLEX or COMPOUND LIPIDS

1. Nitrogen containing glycerophosphatides
I. Lecithin (phosphatidyl choline)
II. Cephalin (phosphatidyl ethanolamine)
III. Phosphatidyl serine
A. PHOSPHOLIPIDS

2. Non-nitrogen glycerophosphatides
I. Phosphatidyl inositol
II. Phosphatidyl glycerol
III. Diphosphatidyl glycerol (cardiolipin)
A. PHOSPHOLIPIDS

3. Plasmalogens, having long chain alcohol
• Contain unsaturated fatty alcohol at C-1. Hence, ester
linkage is replaced by ether linkage.
• Usually nitrogen bases are choline, serine and
ethanolamine.
I. Choline plasmalogen
II. Ethanolamine plasmalogen
A. PHOSPHOLIPIDS

4. Phospho sphingosides
Contain fatty acid, long chain amino alcohol (sphingosine) and
bases or additional groups.
Eg: Sphingomyelin
A. PHOSPHOLIPIDS

F.A is linked to sphingosine by an amide bond & phosphoryl choline is esterified to C-1 hydroxyl of
sphingosine.

1. Glycosphingolipids (carbohydrate)
I. Cerebrosides (ceramide monohexosides)
II. Globosides (ceramide oligosaccharides)
III. Gangliosides (ceramide + oligosaccharides + N-acetyl neuraminic acid)
B. NON-PHOSPHORYLATED LIPIDS

2. Sulpholipids or sulfatides
I. Sulphated cerebrosides
II. Sulphated globosides
III. Sulphated gangliosides
B. NON-PHOSPHORYLATED LIPIDS

Derived from lipids (simple or compound) by hydrolysis or precursors of lipids.
III. DERIVED LIPIDS
Fatty acids
Steroids
Glycerol
Retinol
Prostaglandins
Leukotrienes
Terpenes
Dolichols etc.

 Proteolipids  Lipoproteins
IV. LIPIDS COMPLEXED TO OTHER COMPOUNDS

 Differ primarily in the length of the hydrocarbon chain & the position of the
unsaturated bonds.
 The most abundant have an even number of carbons with chains between 14 –
22 C’s.
 The most common fatty acids are 16 or 18 C’s.
FATTY ACIDS

Based on total no. of carbon atoms:
A. Even chain: 2,4,6 & similar series
Common
A. Odd chain: 3,5,7, etc.
CLASSIFICATION OF FATTY ACIDS

 Based on length of hydrocarbon chain:
a) Short chain fatty acids: 2 – 6 carbon atoms.
b) Medium chain fatty acids: 8 – 14 carbon atoms.
c) Long chain fatty acids: 16 – 22 carbon atoms.
d) Very long chain fatty acids: 

 Based on the nature of hydrocarbon side chain:
A. Saturated fatty acids: no double bonds
B. Unsaturated fatty acids: one (mono-enoic) or more (poly-enoic) double
bonds.
C. Branched chain fatty acids
D.

Have the general formula CH3-(CH2)n-COOH.
C16 (palmitic acid) & C18 (stearic acid) are most
abundant in body fat.
Human body fat contains 50 % oleic acid, 25 % palmitic
acid, 10 % linoleic & 5 % stearic acid.
SATURATED FATTY ACIDS

Common
name
No. of C
atoms
Chemical
nature
Occurrence
Acetic 2 Saturated;
small chain
Vinegar
Butyric 4 do Butter
Caproic 6 do Butter
Caprylic 8 do Coconut oil
Capric 10 do Coconut oil
Lauric 12 do Coconut oil

Common
name
No. of C
atoms
Chemical
nature
Occurrence
Palmitic 16 Saturated;
long chain
Body fat
Stearic 18 do Body fat
Arachidic 20 do Peanut oil
Lignoceric 24 do Peanuts
and brain
Propionic 3 Saturatued;
odd chain
Metabolism

UNSATURATED FATTY ACIDS
Common
name
No. of C
atoms
Chemical nature Occurrence
Palmitoleic 16 Monounsaturated
( 7)
Body fat
Oleic 18 do ( 9) do
Erucic 22 do ( 9) Mustard oil
Nervonic 24 do ( 9) Brain lipids

Common
name
No. of C
atoms
Linoleic 18 2 = bonds ( 6) Vegetable oil
Linolenic 18 3 = bonds ( 3) do
Arachidonic 20 4 = bonds ( 6) do

Common
name
No. of C
atoms
Timnodonic 20 Eicosa pentaenoic
( 3)
Fish oils,
brain
Clupanodonic 22 Docosa pentaenoic
( 3)
do
Cervonic 22 Docosa hexaenoic
( 3)
do

BRANCHED & HYDROXY FATTY ACIDS
Common
name
No. of C
atoms
Chemical
nature
Occurrence
Isovaleric 5 Branched Metabolic
intermediate
Cerebronic 24 Hydroxy Brain lipids

Similar to saturated fatty acids in the reaction of
COOH group but also show properties due to
presence of double bonds.
Exhibit geometrical isomerism at the double
bonds.

Because of the double bonds, unsaturated fatty acids
exhibit cis-trans isomerism.
In the cis-isomer, bulky groups are located on the
same side of double bond; where as in trans isomer,
they are on the opposite side of double bond.
CIS-TRANS ISOMERISM

 All the naturally occurring unsaturated fatty acids are cis-isomers.
CIS-TRANS ISOMERISM

They are not synthesized in the body & have to
be supplied in the diet.
They are also called as poly unsaturated fatty
acids (PUFA).
linoleic acid, linolenic acid and arachidonic acid
ESSENTIAL FATTY ACIDS

Arachidonic acid is the precursor of
prostaglandins & can be synthesized in body, if
the essential FAs are supplied in the diet.

Linoleic (C18) 9, 12 (6 family)
CH3 - (CH2)4 - CH = CH - CH2 – CH = CH - (CH2)7 - COOH
18 6 12 9 1
Linolenic (C18) 9, 12, 15 (3 family)
CH3 – CH2 - CH = CH - CH2 – CH = CH – CH2 – CH = CH – (CH2)7 - COOH
18 3 15 12 9 1
Arachidonic (C20) 5, 8, 11, 14 (6 family)
CH3 – (CH2)4 - CH = CH - CH2 – CH = CH – CH2 – CH = CH – CH2 – CH = CH - (CH2)3 – COOH
18 6 14 11 8 5 1

Functions:
Essential for the synthesis of eicosanoids (eicosa
= twenty; polyenoic FAs).
They are also required for membrane
PUFA

Medical Importance:
1.  Blood cholesterol.
2. Deficiency is rare with normal diet, but in rats causes poor growth, reproductive disorders and dermatitis.
3. Lipid transport may be impaired.
4. Infants consuming formula diets are susceptible to deficiency of essential fatty acids. They may develop skin
abnormalities
PUFA

• Complex molecules containing four fused rings.
• The four fused rings makeup ‘cyclo-pentano-
perhydro-phenanthrene’ or ‘sterane’ ring (steroid
nucleus).
• The most abundant steroids are sterols which are
steroid
STEROIDS

Structure: C27H460
 In animal tissues, cholesterol is the major sterol.
 Cholesterol is 3-hydroxy-5,6-cholestene.
 It is found in bile (chol-bile).

CHOLESTEROL

Sources:
Brain, spinal cord & neurons.
Egg yolk is also rich in cholesterol.
Steroids are non-saponifiable lipids, as they
contain no fatty acids & can not form soaps.
CHOLESTEROL

Functions:
1. Cholesterol and its esters are important
components of cell membrane & lipoproteins.
2. Steroids with diverse physiological functions are
derived from cholesterol.
CHOLESTEROL

 Vitamin D: 7-dehydrocholesterol derived from
cholesterol is provitamin of vitamin D.
 Bile acids: required for the formation of bile salts.
 Hormones of adrenal cortex: cortisol, corticosterone &
aldosterone.
 Female sex hormones: progesterone & estrogen.
 Male sex hormones: testosterone & androsterone
Derivatives of Cholesterol

Ergosterol: Provitamin of vitamin D found in yeast &
plants.
Sitosterol: Present in plants.
Cardiac glycosides like ouabain & streptomycin, an
antibiotic.
Coprostanol found in feces is derived from cholesterol.
Wool fat sterols like agnosterol & lanosterol.
Other steroids

• Derived from arachidonic acid.
• They are prostanoids, leukotriens (LTA) & lipoxins (LX).
– Prostanoids include prostaglandins (PG), prostacyclins (PGI) &
thromboxanes (TXA).
• Often word prostaglandins is used to indicate all
prostanoids
EICOSANOIDS

• Since they are initially found in prostate gland,
they are named as prostaglandins.
• But later they are identified in all cells & tissues
except erythrocytes.
PROSTAGLANDINS

Structure:
Chemically prostaglandins are derivatives of a 20 ‘C’
prostanoic acid.
Prostanoic acid is a cyclic compound (cyclopentane
ring) with two side chains.
PROSTAGLANDINS

There are some six or more types of prostaglandins.
They differ in the substituents on the cyclopentane ring.
prostaglandin A (PGA), PGB, PGC, PGD, PGE, PGF, PGG & PGH.
Most widely distributed prostaglandins are PGA1, PGA2,
PGE1, PGE2, PGE3, PGF1, PGF2, PGF3.
PROSTAGLANDINS

Structure:
Contain a second five-numbered ring in addition
to the one common to all prostaglandins.
PROSTACYCLINS

Structure:
So named because they are identified first in
thrombocytes.
Contain a six numbered heterocyclic oxane ring.
THROMBOXANE

Structure:
Found in leukocytes.
Contain no cyclic ring.
HPETE derived from arachidonic acid serves as precursor
for leukotriens and lipoxins.
LEUKOTRIENS & LIPOXINES

Functions:
Function as local hormones & act on several organs &
produce physiological as well as pharmacological effects.
1. Heart: PGE ↑ cardiac output & myocardial contraction.
2. Blood vessels:
EICOSANOIDS

3) Blood pressure: PGA & PGE ↓ B.P, useful as anti
hypertensive agents.
4) Brain: PGE produce sedation & tranquilizing effect in
cerebral cortex.
5) Kidney: PGA & PGE ↑ excretion of Na+, K+ & CI-. They
may ↑ urine volume by increasing plasma flow.
EICOSANOIDS: Functions

6. Lungs: Prostaglandins dilate bronchi, useful in the
treatment of asthma.
7. Nose: Prostaglandins relieve nasal congestion.
8. Stomach: Prostaglandins ↓ acid secretion in stomach,
useful in the treatment of peptic ulcers.

9. Uterus: Prostaglandins ↑ contraction of uterine muscle.
So they are used in the termination of pregnancy.
Prostaglandins also has role in fertility.
10. Metabolism: Prostaglandins influences several
metabolisms by altering cAMP level. For example, they
inhibit lipolysis in adipocyte by ↑ cAMP level.
11. PGE: involved in inflammation.

12. Prostacylins: inhibit platelet aggregation.
13. Thromboxanes: causes platelet aggregation & clot
formation.
14. Thromboxane A2: regulates acquired immunity. It
causes construction of smooth muscle cells.

15. Leukotreins: involved in the regulation of neutrophil &
eosinophil function.
– Act as mediators of immediate hypersensitivity reaction.
– The slow reacting substance of anaphylaxis (SRS-A) is a
leukotriene, has a major bronchoconstrictor role in asthma.
– Some leukotriens act as chemotactic agents.
16. Lipoxins: vasoactive & immuno regulatory substances.

• Lipids like triglycerides are insoluble in water because
they contain non-polar hydrophobic hydrocarbon chain.
• Similarly, cholesterol ester is also insoluble in water
because of hydrophobic steroid
LIPID LAYERS, MICELLES AND LIPOSOMES

• Lipids like cholesterol, phospholipids & bile salts contain
both water soluble polar head group & water insoluble
non-polar tail.
• Since they have two very different kinds of groups, these
molecules are called as ‘amphipathic molecules
Amphipathic Molecules

• When amphipathic molecules like phospholipids are
present in water, their polar head groups orient towards
water phase & hydrophobic tails towards air.
• As a result, a unimolecular lipid layer is formed at water
air interphase.
LIPID MONOLAYER

• When amphipathic lipids are present beyond a
critical concentration in aqueous medium, they
aggregate into spheres.
•
MICELLES

• In the sphere shaped micelles, polar head groups
of amphipathic lipids are on the exterior whereas
non-polar tails are in the interior.
• Bile salts can form micelles.
MICELLES

Structure:
• When phospholipids are present in water oil
mixture, their polar head groups orient towards
water & non-polar tails towards oil.
• As result, a lipid bilayer
LIPID BILAYER

• Lipid bilayer is formed even in the absence of oil phase
because of hydrophobic attraction.
LIPID BILAYER

Function:
• Lipid bilayer is the basic structure of cell membrane.
LIPID BILAYER

Structure:
• They are also micelles but they may be composed
of various types of amphipathic lipids.
• They are formed when micelles of a particular
lipid combines with other lipids.
MIXED MICELLES

• During the digestion & absorption of lipids, micelles of
bile salts combines with products of lipid digestion &
forms mixed micelles.
Functions:
• Formation of mixed micelles is very important for
digestion & absorption of lipids.
• Mixed micelles are also formed during cleansing action
of soaps & detergents.
MIXED MICELLES

Structure:
• When a lipid bilayer closes on itself a spherical vesicle
called as ‘liposome’ is formed.
Functions:
• Used as a carrier of certain drugs to specific site of body
where they act. They can deliver drugs directly into cell
because they easily fuses with cell membranes
LIPOSOMES

Functions:
• They are used in cancer therapy to deliver drugs only to
cancer cells.
• In gene therapy also they are used as vehicles for genes.
LIPOSOMES

Hydrogenation:
• Conversion of unsaturated FAs into corresponding
saturated fatty acids by hydrogenation of the double bond.
• Since hydrogenation converts liquid fat to solid fat, it is also
called as hardening.
e.g: Vanaspathi (dalda
PROPERTIES OF FATTY ACIDS

(+)2H (+)2H (+)2H
Linolenic  Linoleic  Oleic 

Halogenation:
• When treated with halogens under mild conditions,
unsaturated FAs can take up 2 halogen atoms, at each
double bond to form the halogenated derivative of the FA.
Oleic acid + I2  Di-iodo oleic acid
• It is an index of the degree of unsaturation

Salt formation:
• Saturated & unsaturated FAs form salts with alkali.
CH3-COOH + NaOH  CH3COONa + H20
• Na & K salts of long chain FAs are called as soaps.
• Ca & Mg soaps are insoluble. Ca soaps are used in grease

Ester formation:
• Both saturated & unsaturated FAs form esters with alcohol,
glycerol.
Glycerol + FA  Mono acyl glycerol
Monoglyceride + FA  Di acyl glycerol
Diglyceride + FA  Tri acyl

Saponification:
• When fats are boiled with bases like KOH or NaOH; glycerol
& soaps (salts of fatty acids) are formed. This process is
called as saponification

Saponification number:
• No. of mg of KOH required to saponify 1 gm of fat.
• It is an indication of the mol. wt. of the fat, & is inversely
proportional to it.
Human fat: 194 – 198
Butter: 210 – 230

Rancidity of fat:
• When natural fats are exposed to atmospheric O2, they
develop bad smell & taste. It is called as rancidity.
• Rancidity of fat develops even on prolonged storing. It is
due to formation of lipid peroxides.
• Atmospheric O2

Hydrolytic Rancidity:
• Partial hydrolysis of the TGL due to traces of hydrolytic
enzymes present in naturally occurring fats & oils.
Oxidative Rancidity:
• Result of partial oxidation of unsaturated FAs with
resultant formation of epoxides & peroxides of small mol.
wt. FAs by peroxides & free radicals.

Antioxidants:
• As vitamins E and C prevent peroxide formation, they
are added to food fats to improve storage quality.
• Diseases like cancer, diabetes, atherosclerosis are
due to the formation of lipid peroxides in the body.

Antioxidants:
• Many neutral fats & oils may contain antioxidants,
which prevent the occurrence of oxidative rancidity.
• PUFA are more easily oxidized; so vegetable oils
with a high content of PUFA are usually preserved
with addition of antioxidants.

References
• Text of biochemistry- DM.Vasudhevan
• Text of biochemistry for under graduates-RAFI
MD
• Text of Medical biochemistry-Dr.S.K.Gupta

CHEMISTRY OF LIPIDS(Dr M PRIYANKA)

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