NPTEL – Biotechnology – Cell Biology
Module 10 Lipids
Lecture 27 Lipids I
10.1 Introduction
Lipids are organic compounds that are found in living organisms. They have variety of
structures and functions, and soluble in organic solvents due to their hydrocarbon
component. Scheme 1 illustrates some examples:
OH
O
Me
Me
O
OH
Me
H
Me
Me
Me
OH
H
H
Me
O
Cortisone
Limonene
in orange and lemon oils
Vitamin A
Hormone
O
O CH2
O
HC O
O CH2
O
Tristearin
Fat
Scheme 1
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10.2 Fatty Acids
Fatty acids are unbranched carboxylic acids having long hydrocarbon chains (Scheme 2).
Most of the naturally available fatty acids bears an even number of carbon atoms due to
their synthesis from acetate. Both saturated and unsaturated fatty acids are available. In
unsaturated fatty acids, the double bonds have cis-configuration and are separated by one
CH2 group. Thus, unsaturated fatty acids have less intermolecular interactions compared
to saturated fatty acids. As a result, the unsaturated fatty acids have lower melting points
in comparison to saturated fatty acids. The melting point decreases as the number of
double bonds increases.
Number of Carbons
Common Name
Melting Point
(oC)
Structure
Saturated
CO2H
12
Lauric acid
14
Lauric acid
CO2H
16
Palmitic acid
CO2H
18
Stearic acid
CO2H
20
Arachidic acid
CO2H
Unsaturated
16
44
58
63
69
77
CO2H
Palmitoleic acid
0
CO2H
18
Oleic acid
13
CO2H
18
Linoleic acid
-5
CO2H
18
Linolenic acid
-11
Scheme 2
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10.3 Waxes
Waxes are esters of long chain carboxylic acids with long chain alcohols. For example,
beeswax contains a 26 carbon carboxylic acid and a 30 carbon alcohol component, while
carnauba wax has a 32 carbon carboxylic acid component and a 34 carbon alcohol
component. The latter is an example for a hard wax, and is widely used as a car wax and
in floor polishes (Figure 1).
O
O
O
22
27
28
a major component of beeswax
O
31
a major component of carnauba wax
O
28
O
31
a major component of carnauba wax
O
22
O
27
a major component of beeswax
Figure 1
Waxes are also common in living organisms (Figure 2). For example, wax is found on the
surfaces of some leaves, where it serves as protectant against parasites and minimizes the
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evaporation of water. Similarly, the feathers of birds are coated with wax to make them
water repellent.
Wax Coated Leaves
Wax Coated Feathers
Figure 2
10.4 Fats and Oils
The solid or semisolid triglycerides at room temperature are called fats, while the liquid
is called oil (Figure 3). Fats are generally obtained from animals, while oils come from
plant products. In fats, saturated fatty acid is usually involved in the formation of ester
with glycerol, whereas in oils, unsaturated fatty acid is involved. In case of fats, the
saturated fatty acid chain can pack closely together causing them to be solids at room
temperature. In case of oil, the unsaturated fatty acid chain can not pack tightly together,
and therefore, that leads to usually have low melting points.
O
O
CH2OH
CHOH
HO
R
CH2O
O
HO
R'
CHO
O
CH2OH
Glycerol
HO
R''
Fatty Acids
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CH2O
O
R
R'
O
R''
Triglycerides
a fat or an oil depending on R, R' and R''
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Oil
Butter
Figure 3
Polyunsaturated fats and oils are prone to undergo oxidation by O2 via radical reaction.
The allylic H is the one that is easily removed because the resulting radical is resonance
 bonds. taste
npleasant
The oxidation reaction
stabilized by the
and smell associated with sour milk and rancid butter.
10.5 Soaps, Detergents and Micelles
Sodium or potassium salts of fatty acids are called soaps. They are obtained by basic
hydrolysis of triglycerides which is called saponification (Scheme 3). Scheme 4 shows
the three most common soaps used. In aqueous solution, the long chain carboxylic acids
arrange themselves in spherical clusters called micelles as shown in Figure 4. Each
micelle contains 50-100 long-chain carboxylates.
O
O
CH2O
CHO
O
NaOH/H2O
R
O
R'
CHOH
O
R''
CH2OH
a fat or an oil
Glycerol
CH2O
NaO
CH2OH
R
+
NaO
R'
O
NaO
R''
Sodium Salts of Fatty Acids
Soap
Scheme 3
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CO2Na
CO2Na
Sodium oleate
Sodium stearate
CO2Na
Sodium linoleate
Scheme 4
polar head group
non polar tail
Diagram of soap micelle
Figure 4
Soap contains the cleaning property because nonpolar oil that carry dirt dissolve in the
nonpolar interior of the micelle and are carried out away with the soap during rinsing.
polar head group
oil
non polar tail
Diagram of soap micelle
Scheme 5
Since soap can undergo precipitation (soap scum) in hard water due to reaction with
calcium and magnesium ions, synthetic soaps (salt of benzenesulphonic acid) called
detergents have been developed that don’t form soap scum in hard water.
CO2Na
SO3Na
Detergent
soap
SO3Na
Detergent
Scheme 6
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10.6 Phospholipids
Lipids that contain a phosphate group are called phospholipids. There are two kinds of
phospholipids: phosphoglycerides and sphingolipids (Scheme7). These lipids form
membranes by arranging themseleves in a lipid bilayer. The polar hydrophilic groups of
the phospholipids are on the outside of the bilayar, while non-polar fatty acid tail form
interior of the bilayer.
Strucutre of Phospholipid
O
CH2O
Fatty Acid Tail
O
CHO
Hydrophobic
O
RO P O CH2
OPhosphate Group
Hydrophilic
Types of Phospholipids
H
HC C
O
CH2O
CHO
O
R
CHOH
O
C-NH C-R
O
CH-O P OCH2CH2N(CH3)3
OSphingomyelin
R'
O
CO2CH2O P OCH2CHNH2
OPhosphoglycerides
H2O
Hydrophilic Phosphate Head Group (Polar)
Hydrophobic Fatty Acid Tail (non polar)
Phospholipid Bilayer
H2O
Scheme 7
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Module 10 Lipids
Lecture 28 Lipids II
10.7 Terpenes
Terpenes are available in nature as a diverse class of lipids. Some of them are used as
spices, perfumes and medicines for thousands of years (Scheme 1). The terpenes having
functional groups such as OH and carbonyl group are called terpenoids.
OH
HO
Geraniol
Menthol
Peppermint oil
Geranium oil
β-Selinene
Celery oil
Zingiberene
Ginger oil
Scheme 1
The naturally available terpenes are usually made by joining together the isoprene units,
usually in head-to-tail fashion. For examples, Scheme 2 shows the head-to-tail joining of
the isoprene units.
Head
α-Farnesene (a sequiterpene)
Tail
Isoprene Units
Found in the wax coating on the apple skin
O
O
Head
Carvone (a monoterpene)
Spearmint oil
Tail
Scheme 2
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Scheme 3 shows examples for tetraterpenes having eight isoprene units which are
responsible for colours of tomatoes and carrots.
Lycopene (a tetraterpene)
Responsible for the red colour of tomotoes and watermelon
β-Carotene (a tetraterpene)
Responsible for the orange colour of carrots
Scheme 3
10.8 Vitamins
•
Vitamins A, D, E and F are lipids (Scheme 4). Cleaving of
-carotene generates

two molecules of vitamin A that plays important role in vision. Vitamin A is also
called as retinol.
CH3
CH2OH
HO
O
H3C
CH3
Vitamin A
Vitamin E
OH
H
O
O
HO
OH
Vitamin D
O
Vitamin K
Scheme 4
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Scheme 5 describes the chemistry of vision. In retinol, the cone cells are responsible for
vision in bright light, while the rod cells are responsible for the vision in the dim light. In
rod cells, the vitamin A is oxidized to aldehyde that undergoes isomerization of trans
double to cis double bond at C-11 carbon. The latter reacts with protein opsin to form an
imine with (C-11 Z)-retinal, giving rhodopsin. When rhodopin absorbs light, the cis
double bond isomerizes to a trans double bond. This geometry change causes an
electrical signal to be sent to the brain, where it is perceived as a visual image. The trans
isomer of rhodospin is not stable, thus, it undergoes hydrolysis to give (11E)-retinal and
opsin. (C-11 E)-Retinal is then transformed back to (C-11 Z)-retinal to complete the
vision cycle.
H2N-opsin
Oxidatation
CH2OH
Isomerization
CHO
Vitamin A
N opsin
visible light
Activated rhodospin
Rhodospin
N-opsin
O
+
H2N-opsin
Scheme 5. The Chemistry of Vision
OH
H
OH
O
light
H
H
HO
HO
OH
Vitamin D
7-Hydrocholesterol
Scheme 5
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H+/H2O
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•
Vitamin K is required for proper clotting of blood. The deficiency of vitamin K is
rare due its presence in the leaves of green plants as well as its synthesis by
intestinal bacteria.
•
Vitamin E is a radical inhibitor (anti-oxidant) found in biological system. It is
water insoluble and traps radicals formed in non-polar membranes.
•
The body synthesizes vitamin D from 7-hydrocholesterol, found in skin, in the
presence of ultraviolet light.
10.9 Steroids
Steroids are lipids having a tetracyclic system composed of three six membered rings and
one five membered ring. Their non-polar character allows them to cross the cell
membranes, so they can leave the cells in that they are produced and enter their target
cells.
12
11
9C
1 10
2
A
3
4
D
16
8 14 15
B
5
13 17
7
6
The Steroid Ring System
In animals, the most abundant member of steroid is cholesterol, which is the precursor of
all other steroids. Cholesterol has 8 asymmetric centers, thus 256 stereoisomers are
possible. However, in nature only one exists. Cholesterol is an important component of
cell membranes and related to heart disease.
H
H
H
HO
Cholesterol
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Steroids can be broadly classified into five groups: glucocorticoids, mineralocorticoids,
androgens, estrogents, and progestins.
•
Glucocorticoids are involved in metabolism of glucose, proteins and fatty acids.
Cortisone is an example for this type of steroid. Cortisone is used as antiinflammatory agent to treat arthritis.
•
Mineralocorticoids are responsible for the increased reabsorption of Na+, Cl- and
HCO3- ions by the kidneys that can lead to an increase in the blood pressure.
Aldosteron is an example of a mineralocorticoid.
•
Antrogens are male sex hormones. They are responsible for the development of
male secondary sex characteristics during puberty. Testosterone is an example of
androgens.
•
Estrone and estradiol are female sex hormones known as estrogens. They are
responsible for the development of female secondary characteristics and secreted
by the ovaries. Estrogens also regulate the menstrual cycle.
•
Progesterone is the hormone that is essential for the maintenance of preganancy.
It also prevents ovulation during pregnancy.
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HOH2C
O
O
OH
H
H
H
H
H
H
H
Cortisone
Aldosterone
OH
O
H
OH
H
H
H
H
H
H
HO
H
HO
Testosterone
Estrone
OH
O
Estradiol
COOH
H
H
H
H
O
Cholesterol
O
O
H
O
HO
H
HO
HOH2C
OHC
H
H
HO
O
Progesterone
H
OH
Cholic Acid
Scheme 6
In addition to the precursor of all the steroid hormones in animals, cholesterol is also
precursor for the synthesis of the bile acids. For example, the bile acid, cholic acid
synthesized in liver, stored in gallbladder and secreted into the small intestine, where it
acts as an emulsifying agent so that fats and oils can be digested by water-soluble
enzymes.
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10.9 Synthetic Steroids
The potent physiological properties of naturally occurring steroids have led to the
development of synthetic steroids.
For examples, stanozolol and dianbol are drugs
developed in this way exhibit the same muscle-building effect as testosterone.
MeOH
Me
Me
N
MeOH
Me
H
H
Me
H
N
H
Stanozolol
H
H
H
O
Dianabol
Some of the synthetic steroids have been found to be more potent than natural steroids.
For examples, norethindrone is better than progesterone in arresting ovulation, while RU
486 terminates preganancy within the first nine weeks of gestation. These oral
contraceptives have structures similar to that of progesterone.
Me
Me
N
MeOH
MeOH
Me
H
H
H
H
H
H
O
O
RU 486
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Northindrone
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