Lipids are a diverse group of organic molecules that are insoluble in water but soluble in nonpolar solvents such as chloroform or ether.pptx

Chemistry of Lipids
By: Amanuel Tobe (MSc in Clinical Biochemistry)
4/28/2024 By: Amanuel Tobe 1

Definition of Lipids
• Lipids are a heterogeneous group of compounds
of biochemical importance. Lipids may be defined as
compounds which are relatively insoluble in water, but
freely soluble in non-polar organic solvents, such as
benzene, chloroform, ether, etc.

Functions of Lipids

Classification of Lipids
• Based on the chemical nature, lipids are classified as:
1. Simple lipids: They are esters of fatty acids with glycerol or
other higher alcohols, e.g. TAGs (Triacylglycerol), waxes
2. Compound lipids: They are fatty acids esterified with
alcohol; but in addition they contain other groups.
• Depending on these extra groups, they are sub-classified into:
• a. Phospholipids: Contain a phosphoric acid residue. They
frequently have nitrogen-containing bases (eg, choline) and
other substituents.

Glycerol reacts with 3 fatty
acids (Esterification reaction) Fats/Oils ( Triacylglycerols)
5
Example of Simple Lipids: The triacylglycerols/TAG
4/28/2024 By: Amanuel Tobe

Example of Complex lipids: Phospholipids
Structure of Phospholipids
Phospholipid bilayer of
Cell membrane
6

Cont.
In many phospholipids the alcohol is glycerol
(glycerophospholipids), but in sphingophospholipids-it is
sphingosine, which contains an amino group.
b. Glycolipids (glycosphingolipids): Contain a fatty acid,
sphingosine, and carbohydrate.
c. Other complex lipids: These include lipids such as sulfolipids
and amino lipids. Lipoproteins may also be placed in this
category .
3. Derived lipids: They are compounds, which are derived from
lipids or precursors of lipids, e.g. fatty acids, steroids.

Triacylglycerols (triglycerides)=TAG
• Are the main storage forms of fatty acids
• The triacylglycerols , are esters of the trihydric alcohol glycerol
and 3 fatty acids.
• Mono- and diacylglycerols wherein, one or two fatty acids are
esterified with glycerol, respectively, are also found in the tissues.
• An acylglycerol comprises glycerol with one or more fatty acids
(the acyl group) attached through ester linkages.
8

Cont.
• Triacylglycerols rarely contain the same fatty acid at all three
positions and are therefore called simple triacylglycerols.
• Triacylglycerol containing two or three kinds of fatty acids are
called mixed triacylglycerols
• Unsaturated fatty acids, when present, are most often esterified to
carbon 2.
9

Cont.
• The fatty acid on carbon 1 is typically saturated, that on carbon 2
is typically unsaturated, and that on carbon 3 can be either.
• The presence of the unsaturated fatty acid(s) decrease(s) the
melting temperature (Tm) of the lipid.
11

Functions of Triacylglycerols
Energy source
• 9 kcal per gram
• Form of stored energy in adipose tissue
Insulation and protection
Carrier of fat-soluble vitamins
Sensory properties in food
12

Phospholipids
• There are two classes of phospholipids: those that have glycerol
(from glucose) as a backbone and those that have sphingosine
(from serine and palmitate).
• Phospholipids are amphipathic molecules
• They are of 2 types
A. Glycerophospholipids
B. Sphingophospholipids
13

A-Glycerophospholipids

1. Phosphatidic acid/phosphotidyl/Phosphatidate
• It is the simplest phosphoacylglycerol
(phosphoglyceride). Phosphatidic acid is
composed of diacylglycerol with a
phosphate group on the third carbon.
• It is precursor of the other
glycerophospholipids.
• Phosphatidic acid is important as an
intermediate in the synthesis of
triacylglycerols but is not found in any
great quantity in tissues.
15

General structure of glycerophospholipids (Phosphatidic acid & its
derivatives)
16

2. Cardiolipin/diphosphatidylglycerol
• It is formed by esterification of phosphatidic acid to
glycerol. When two molecules of phosphatidic acid
are linked with a molecule of glycerol, diphosphatidylglycerol
or cardiolipin is formed.
• It is the major lipid of mitochondrial membrane.
Commercially, it is extracted from myocardium. Decreased
cardiolipin level leads to mitochondrial dysfunction, and is
accounted for heart failure, hypothyroidism and some types of
myopathies.

Cont.
• Cardiolipin is also abundantly present in the cell membranes of
bacteria.
• Cardiolipin is antigenic, and is recognized by antibodies raised
against Treponema pallidum, the bacterium that causes syphilis.
• Used in serological diagnosis of autoimmunity diseases.
18
Structure of cardiolipin

3. Phosphatidylcholine or Lecithin
• This is a nitrogen containing phospholipid. The word
lecithin is derived from the Greek word, lekithos = egg yolk. It
contains glycerol.
• The alpha and beta positions are esterified with fatty acids.
Usually, the fatty acid attached to the betacarbon, is a PUFA
molecule .
• The phosphoric acid is added to the third position,
to form phosphatidic acid. The phosphate group is
esterified to the quaternary nitrogen base, Choline. The
molecules of lecithin exist as zwitterions.

20
The structure of
phosphatidylcholine
The Biosynthesis of
Phosphatidylcholine

B-Sphingolipids
• Are second largest membrane lipids, which contain two non-
polar and one polar head groups. Their alcohol is the amino
alcohol sphingosine, derived from serine and a specific fatty
acid, palmitate.
• Sphingolipids have two classes:
1. Sphingomyelins/Phosphosphingosides/
2. Glycolipids

General structure of sphingolipids
22
Sphingosine

1. Sphingomyelins
• Sphingomyelins are found in large amounts in brain and
nerves(hence their name, sphingomyelins) and apparently lack
in plants and the microorganisms.
• On hydrolysis, the sphingomyelins yield a fatty acid,
phosphoric acid, choline, and a complex amino alcohol
(sphingosine).
• They contain two nitrogenous bases: sphingosine itself and
choline.
• To the amino group of sphingosine and fatty acid is attached
by an amide linkage = forms ceramide
23

The structure of Sphingomyelin
24

Cont.
• Sphingomyelins are differ from lecithins in that
 they contain sphingosine as the alcohol instead of glycerol.
 they contain one fatty acid
• Sphingomyelin is an important constituent of the myelin of nerve
fibers , membranous structure that insulates and protects neuronal
fibers of the central nervous system.
25

Degradation of sphingomyelin
• Sphingomyelin is degraded by sphingomyelinase, a lysosomal
enzyme that hydrolytically removes phosphorylcholine, leaving a
ceramide.
• The ceramide is, in turn, cleaved by ceramidase into sphingosine
and a free fatty acid.
26

Niemann–Pick disease (=sphingomyelin lipidosis)
• Is a rare genetic disorder, inherited as an autosomal recessive
condition.
• The disease is caused by a deficiency of the enzyme
sphingomyelinase, which cleaves ceramide -phosphocholine
bond of sphingomyelin.
• As a result, sphingomyelin accumulates in large amounts in the
reticuloendothelial system since their synthesis is normal in rate
but their degradation is interrupted.
• Niemann–Pick cells (prototype of ‘foam cells’) are found in the
bone marrow, spleen, lymphoid tissues, liver, lung and tissues, of
virtually any organ.
27

Niemann Pick Disease…
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Derived Lipids
A. Fatty acids
• A fatty acid (FA) may be defined as an organic acid that
occurs in a natural triglycerides (fats and oils), but do occur
in the un -esterified form as free fatty acids, a transport form
found in the plasma carried by albumin.
• The chain may be saturated (containing no double bonds) or
unsaturated (containing one or more double bonds). Mostly
the double bond occurs at the 9th carbon counting from the
carboxyl group end. 29

Cont.
• Fatty acids that occur in natural fats are usually straight-chain
derivatives containing an even number of carbon atoms, (2,4, 6,
etc, carbon atoms).
• Odd chain fatty acids have 3, 5, 7, etc carbon atoms, are seen in
microbial cell walls and in milk.
• Depending on the length of the hydrocarbon chain, there are short
chain fatty acids with 2 to 6 carbon atoms, medium chain fatty
acids, with 8 to 14 carbon atoms and long chain fatty acids with 16
or more carbon atoms up to 24 carbon atoms.
30

Saturated and Unsaturated fatty acids
• Saturated fatty acids have single bonds between the carbons in the
chain, and unsaturated fatty acids contain one or more double
bonds.
• The most common saturated fatty acids present in the cell are
palmitic acid (C16) and stearic acid (C18).
• Monounsaturated fatty acids contain one double bond, and
polyunsaturated fatty acids contain two or more double bonds.
• The kinks made by double bonded C in unsaturated fatty acids
prevent the molecules from packing tightly together.
32

Saturated fatty acids Vs Unsaturated fatty acids
34
 Unsaturated fatty acids have kinks in their tails.
 The kinks in the unsaturated side chains prevent close
packing of the hydrocarbon portions of the phospholipids.

35
Saturated fatty acids Vs Unsaturated fatty acids…

Geometric Isomers in Unsaturated fatty acids
• Most naturally occurring unsaturated fatty acids have cis- double
bonds. A type of geometric isomerism occurs in unsaturated fatty
acids, depending on the orientation of atoms or groups around the
axes of double bonds, which do not allow rotation.
• If the acyl chains are on the same side of the double bond, it is cis-
, as in oleic acid; if on opposite sides, it is trans-, as in elaidic
acid, the trans isomer of oleic acid.
• Increase in the number of cis double bonds in a fatty acid leads to
a variety of possible spatial configurations of the molecule— e.g.,
arachidonic acid, with four cis-double bonds, has “kinks” or a U
shape.
37

Cont.
• This has profound significance on molecular packing in
membranes and on the positions occupied by fatty acids in
more complex molecules such as phospholipids.
• Trans-double bonds alter these spatial relationships. Trans
fatty acids are present in certain foods, arising as a by-product
of the saturation of fatty acids during hydrogenation, or
“hardening,” of natural oils in the manufacture of margarine,
and they seriously affect health.
38

Cont.
39
Trans fatty acids
 Naturally present in small
amounts (<1% of total calories) in
animal food sources, including
human milk
 Mainly introduced in the food
chain by the industrial process of
hydrogenation
 „
This process is used to make oils
solid at room temperature or
“partially hydrogenated oils."

Nutritionally essential fatty acids
• Essential fatty acids, or EFAs, are fatty acids that humans and
other animals must ingest because the body requires them for
good health but cannot synthesize them.
• The term "essential fatty acid" refers to fatty acids required for
biological processes but does not include the fats that only act as
fuel.
• Only two fatty acids are known to be essential for
humans: alpha-linolenic acid (an omega-3 fatty acid)
and Linoleic acid (an omega-6 fatty acid).
40

Why EFA cannot be Synthesised?
41

Cont.
42
Routes of synthesis of other
fatty acids (See De novo fatty
acids biosynthesis).
Palmitate is the precursor of
stearate and longer-chain
saturated fatty acids, as well as
the monounsaturated acids
palmitoleate and oleate.
Mammals cannot convert oleate
to linoleate or α-linolenate
(shaded pink), which
are therefore required in the diet
as essential fatty acids.

Functions of EFA(Biomedical Importance)
• Structural elements of tissues: Polyunsaturated fatty
acids occur in higher concentration in lipids associated with
structural elements of tissues.
• Proper development and functioning of the brain and nervous
system.
• Synthesis of prostaglandins and other compounds:
 Thromboxanes
 Leukotrienes
 Prostaglandins
• Structural element of gonads: Lipids of gonads also
contain a high concentration of polyunsaturated fatty acids,
which suggests importance of these compounds in
reproductive function.
43

Cont.
 Structural element of mitochondrial membrane: A deficiency
of EFA causes swelling of mitochondrial membrane and reduction
in efficiency of oxidative phosphorylation. This may explain for
increased heat production noted in EFA deficient animals.
 Serum level of cholesterol: Fats with high content of
polyunsaturated fatty acids tends to lower serum level of
cholesterol.
 Effect on clotting time: Prolongation of clotting time is noted in
ingestion of fats rich in EFA.
44

Cont.
 Effect on fibrinolytic activity: An increase in fibrinolytic activity
follows the ingestion of fats rich in EFA.
 Role of EFA in fatty liver: Deficiency of EFA produces fatty
liver.
 Role in vision: Docosahexaenoic acid (22:6n-3) is the most
abundant polyenoic fatty acids present in retinal photoreceptor
membranes. Docosahexaenoic acid is formed from dietary
linolenic acid. It enhances the electrical response of the
photoreceptors to illumination. Hence linolenic acid is necessary
in the diet for optimal vision.
45

B. Steroids
• Steroids contain a four-ring fused structure called the steroid
nucleus cyclopentanoperhydrophenanthrene .
• This system consists of 3 cyclohexane rings (A, B and C)
fused in nonlinear or phenanthrene manner and a terminal
cyclopentane ring (D).
• Steroids are complex fat-soluble molecules, which are present in
the plasma lipoproteins and outer cell membrane.
46

Cont.
• Cholesterol is probably the best known steroid because of its
association with atherosclerosis.
• Cholesterol is the steroid precursor in human cells from which all
of the steroid hormones are synthesized by modifications to the
ring or C20 side chain.
47

Cont.
• Although cholesterol is not very water soluble, it is converted to
amphipathic water-soluble bile salts such as cholic acid.
• Bile salts line the surfaces of lipid droplets called micelles in the
lumen of the intestine, where they keep the droplets emulsified in
the aqueous environment.
• Cholesterol is one of the compounds synthesized in the human
from branched 5- carbon units with one double bond called an
isoprenyl unit (2- methyl-1,3-butadiene), a methyl-branched
compound with five C atoms.
49

Functions of cholesterol
• For the synthesis of bile salts that are important in lipid
digestion and absorption.
• For the synthesis of steroid hormones that are biologically
important like the sex hormones estrogen and progesterone
& hormones of adrenal cortex.
• For the synthesis of vitamin D3.
• As a structural material in biological membranes.
• As a component of lipoproteins as transport forms of lipid
based energy.
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Lipids are a diverse group of organic molecules that are insoluble in water but soluble in nonpolar solvents such as chloroform or ether.pptx

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Lipids are a diverse group of organic molecules that are insoluble in water but soluble in nonpolar solvents such as chloroform or ether.pptx