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1. Lipids
By: Alemu Adela(MSc)
Department of Medical biochemistry
Collage of medicine, DBU

2. OUTLINE OF LECTURE
•Fatty acids
•Acylglycerols
•Phospho-glycerides
•Sphingolipids
•Sterols (steroids)

3. lipids
•Lipids are a group of structurally diverse compounds defined by
their hydrophobicity
•They are not very soluble in water but soluble in organic solvent.
• Widely distributed in nature both in plants and in animals.
•Differ from other organic compounds in their potential to be
utilized by the living organisms.
•Fats, Oils, Steroids, Waxes and the Fat-soluble Vitamins are lipids

4. BIOLOGICAL IMPORTANCE OF LIPIDS
•They are efficient energy sources.
•Serve as thermal insulators.
•They are structural components of the cell membrane.
•Serve as precursors for hormones (steroid hormones).
•Play crucial roles as enzyme cofactors, electron carriers, light-
absorbing pigments, emulsifying agents in the digestive tract,
hormones, and intracellular messengers.
•They also dissolve the vitamins, which are fat-soluble and assist
their digestion.

5. Fatty acids
•They are carboxylic acids with hydrocarbon chains ranging from 4
to 36 carbons long.
•Most common chains range from 12–20 carbons in length
•Usually, an even number of carbons in the chain, including the
carboxyl carbon
• pKa of -COOH is 4.5-5.0 --> ionized at physiological pH
•They differ from each other by the:
➢ Length of the tail
➢ Degree of unsaturation
➢Position of double bonds
•They are the fundamental building blocks of lipids.
•They are in a low oxidation state, and thus conversion to CO2 and
H2O is highly exergonic

6. Classification
1.Based on the presence/absence of double bonds
▪Saturated fatty acid
▪Monounsaturated fatty acid
▪Polyunsaturated fatty acid
2.Based on the number of carbon atoms in the chain
▪ Short-chain fatty acids (4-6)
▪Medium chain fatty acids (8-12)
▪ Long-chain fatty acids ( 14-22)
▪Very long chain fatty acid (>22)
3.Based on the biological importance
➢Essential
➢non-essential fatty acids

7. SATURATED AND UNSATURATED FAS
❑Saturated FAs have
no double bonds. (C-C)
Double bonds lower the
melting temperature
❑Unsaturated FAs
have at least one
double bond (C=C) in
one of the fatty acids
•.

8. SATURATED AND UNSATURATED FAS

9. Structural differences between saturated and
unsaturated fatty acids

10. Naming of fatty acids
•Fatty Acids Are Named After Corresponding Hydrocarbons with -oic
being substituted for the final –e
•Saturated acids end in -anoic, e.g. , octanoic acid
•Unsaturated acids with double bonds end in -enoic, e.g. , octadecenoic
acid
•Carbon atoms are numbered from the carboxyl carbon (carbon no. 1) or
as the α, β, and γ carbons.
•The terminal methyl carbon is known as the ω- or n-carbon.
•Δ is used to indicate the number and position of the double bonds
•ω9 indicates a double bond on the ninth carbon counting from the ω-
carbon

11. Naming of fatty acids
oleic acid

12. Saturated fatty acids
❖Contain only single C–C bonds.
❖Are closely packed.
❖Have strong attractions between chains.
❖Have high melting points.
❖Are solids at room temperature.
❖But short-chain saturated fatty acids are liquid
at room temperature

13. Saturated fatty acids

14. Unsaturated fatty acids…
•Cis-fatty acid: H‘s on same side of the double
bond; fold into U-like formation; naturally
occurring.
•Trans-fatty acid: H‘s on opposite side of double
bond;
–More linear; occur in partially hydrogenated foods

15. Unsaturated fatty acids
No. of
carbons
Name Structural
feature
Occurrence
16 Palmitoleic acidC16:1∆
9 Body fat
18 Oleic acid C18:1∆
9 Body fat
18 Linoleic acid C18:2∆
9,12
Vegetable oil
18 Linolenic acid
C18:3∆9,12,15Vegetable oil
20 Arachidonic acid
C20:4∆5,8,11,14 Vegetable oil
20 Timnodonic acid
C20:5∆5,8,11,14,
17
Fish oil, brain
22 Erucic acid C22:1∆
13
Mustard and
rape seed oil

16. TRANS FATTY ACIDS AND HYDROGENATION
Trans fatty acids
❑Are formed during hydrogenation when cis
double bonds are converted to trans double
bonds.
❑In the body behave like saturated fatty acids.
❑Are estimated to make up 2-4% of our total
Calories.
❑Are reported in several studies to raise LDL-
cholesterol and lower HDL-cholesterol.

17. Cont’d
•Hydrogenation –Hydrogen atoms are added to
monounsaturated or polyunsaturated fats
•Act more like saturated fats
•Resistant to oxidation
•Prolong shelf life
•Becomes solid at room temperature
•May cause CVD, DM and other problems

18. ESSENTIAL FATTY ACIDS
A.Linoleic acid: ω-6, C18:2
9, 12
.
CH
3-(CH
2)
4-CH=CH-CH
2-CH=CH-(CH
2)
7-COOH
B.Linolenic acid: ω-3, C18:3
9, 12, 15
.
CH
3-CH
2-CH=CH-CH
2-CH=CH-CH
2-CH=CH-(CH
2)
7-COOH

19. Importance of essential fatty acids
•Components of all cellular and sub-cellular membranes
•Useful in the prevention and treatment of atherosclerosis as they help
in the transport of blood cholesterol and triglycerides.
•Important in preventing and treating fatty liver as they have a
Lipotropic effect on the liver, i.e., they mobilize the fats from the liver to
other tissues.
•Essential for skin integrity and normal growth.
•Important structural elements of gonads and, hence, are essential for
normal reproduction.
•Important for the health of the retina and vision.

21. Simple lipids
❖Esters of fatty acids with various alcohols.
1.Fats (Triacyglycerol): Esters of fatty acids with
glycerol. Oils are fats in a liquid state.
2.Waxes: Esters of fatty acids with higher molecular
weight monohydric alcohols

22. TRIACYLGLYCEROLS (TAG)
•In a triacylglycerol,
Glycerol forms ester bonds with three fatty acids.

23. TRIACYLGLYCEROLS (TAG)
❖They are naturally occurring esters of glycerol with various fatty
acids
❖All the fats and oils from vegetables or animals are triglycerides
❖Simple triacylglycerols contain the same fatty acids, e.g.,
tripalmitin.
❖Triacylglycerols with mixed types of fatty acids are called mixed
triacylglycerols
❖Naturally occurring fats are mostly of the mixed type
OLIVE OIL: Contains a high percentage of oleic acid, which is
a monounsaturated fatty acid with one cis double bond.

24. Physical Properties of Triacylglycerols
•They are hydrophobic and insoluble in water.
•Oils are liquids at 20oC; they are triacylglycerols, which contain a higher
proportion of unsaturated fatty acids or short chain triglycerides. Oils
are generally of plant origin.
•Fats are solids at room temperature and contain mainly saturated long
chain fatty acids. Fats are mainly of animal origin.
•When the constituent fatty acids have a higher chain length and are
predominantly saturated, ‘hard fat' is formed, e.g. pig fat.
•Fats containing medium chain triacylglycerols or unsaturated fatty acids
are soft fats, e.g. butter, coconut oil. Coconut oil contains mainly
medium chain TAG, e.g. Lauric and Myristic acids.

25. Storage of Energy as Fat
•The triacylglycerols are the storage form of lipids in the adipose
tissue.
•In a 70 kg normal person, body stores contain about 11 kg of
triacylglycerol, which is roughly equivalent to 100,000 kcal.
•If the same calories were stored as hydrated glycogen, the total
weight of this alone would have been 65 kg!
•When stored as TAG, water molecules are repelled and space
requirement is minimal.
•Excess fat in the body leads to obesity.

26. Hydrolysis of Triacylglycerols
•This occurs in the body during digestion of dietary fat and
mobilization of TAG from adipose tissue.
•Triacylglycerols in the body are hydrolyzed by enzymes, lipases
which are hydrolases
•Triacylglycerol is sequentially hydrolyzed to diacylglycerol and
monoacylglycerol and finally glycerol plus 3 fatty acids

27. Saponification
•Saponification: hydrolysis of triacylglycerols by alkali,
–the process is 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.
•It is an indication of the molecular weight of the fat, and is
inversely proportional to it.
•Human fat has a saponification number of 194–198, butter has
210– 230 and coconut oil has

28. Iodine Number
•Iodine number: The number of grams of iodine taken up by
100 grams of fat.
•It is an index of the degree of unsaturation and is directly
proportional to the content of unsaturated fatty acids.
•Higher the iodine number, higher is the degree of
unsaturation,
–e.g. iodine number of butter is 28, and that of sunflower oil is 130

29. Rancidity of Fat
•Rancidity: the appearance of an unpleasant smell and taste for fats and
oils.
•Hydrolytic rancidity: partial hydrolysis of the triacylglycerol molecules due
to traces of hydrolytic enzymes present in naturally occurring fats and oils.
•Oxidative rancidity: partial oxidation of
•Unsaturated fatty acids to epoxides and peroxides by peroxides and free
radicals.
•If it occurs in vivo will affect the integrity of biomembranes, leading to cell
death.
–Many natural fats and oils may contain antioxidants (e.g. vitamin E)

30. Compound lipids
.
When a lipid contains an element or moiety in addition to
fatty acids and alcohol present in simple fats, it is known
as a compound or conjugate lipid

31. Phospholipids
•.
•The most abundant lipids in cell membranes
•Composed of glycerol, two fatty acids, phosphate, and an amino
alcohol
Phosphatidic acid is the simplest Phosphoglyceride in which "X" =
-H

32. Phosphoglycerides and their corresponding polar
alcohol groups
Phosphoglycerides
Phosphatidic acid
Phosphatidylethanolamine (Cephaline)
Phosphatidylcholine (Lecithin)
Phosphatidylserine
Phosphatidylinositol
Phosphatidylglycerol
Alcohol groups
-H
Ethanolamine
Choline
Serine
Myo-inositol
Glycero

34. CARDIOLIPIN
➢Cardiolipin is an important component
of the inner mitochondrial membrane
➢This is the only human
glycerophospholipid that is antigenic
➢For example, cardiolipin is recognized
by antibodies raised against
Treponema pallidum, the bacterium
that causes syphilis.
➢Cardiolipins are used in serological
diagnosis of syphilis and autoimmune
diseases.
.

35. Phosphatidylcholines (Lecithins)
•The most abundant phospholipids of the cell membrane
•The body’s store of choline: important in nervous transmission
•Dipalmitoyl lecithin: a very effective surface-active agent and a
major constituent of the surfactant preventing adherence
–Its absence from the lungs of premature infants causes respiratory
distress syndrome

36. Phosphatidylinositol
•Minor components of cell membranes, but play an important
part in cell signaling and membrane trafficking.
•Phosphoinositides may have 1, 2, or 3 phosphate groups
attached to the inositol ring.
•Phosphatidylinositol 4,5-bisphosphate (PiP2) is cleaved into
diacylglycerol and inositol trisphosphate upon stimulation by a
suitable hormone agonist
•Diacylglycerol and inositol trisphosphate are second messengers

37. SPHINGOLIPIDS
•Composed of a backbone of sphingosine
(18 - C alcohol) combined with fatty acid
with amid bond
•Predominate in the myelin sheath of
nerve fibers
•Sphingomyline prominent in
myelin, a membranous sheath that
surrounds and insulates the axons
of some neurons
.

38. SPHINGOLIPIDS
•They are believed to play a role in cell signaling and in apoptosis
•Sphingomyelins contain no glycerol, and on hydrolysis, they yield a
fatty acid, phosphoric acid, choline, and sphingosine
•The combination of sphingosine plus fatty acid is known as
ceramide

39. SPHINGOLIPIDS
•There are three subclasses of sphingolipids
1.Sphingomyelins
2.Neutral glycolipids
3.Gangliosides

40. Sphingomyelins
•Contain phosphocholine or phosphoethanolamine as their polar head
group sphingomyelins resemble phosphatidylcholines
SPHINGOLIPIDS

41. SPHINGOLIPIDS
Glycosphingolipids
•Have head groups with one or more sugars connected directly to
the —OH at C-1 of the ceramide moiety
1.Cerebrosides: have a single sugar linked to ceramide
–Those with galactose are found in the plasma membranes of neural cells
–Those with glucose are found in nonneural cells
2.Globosides: are glycosphingolipids with two or more sugars,
usually D-glucose, D-galactose, or N-acetyl-D- galactosamine

42. SPHINGOLIPIDS
Gangliosides:
•Have oligosaccharides as their polar head groups and one or
more residues of N-acetylneuraminic acid

43. SPHINGOLIPIDS

44. STEROIDS
❖Based on a core structure consisting four fused
rings. of which three are 6- membered rings
and one is 5-membered ring,
❖Cholesterol is the most common steroid in
animals and precursor for all other steroids in
animals

45. CHOLESTEROL
•Up to 40% of mammalian cell membranes
•Small polar head makes it weakly amphipathic
Function
❑It is important component of cell
membrane
❑It is precursor for bile acid
synthesis
❑It is precursor of vitamin D
synthesis
❑It is precursor for steroid hormones
synthesis

46. Summary
•Lipids are water-insoluble cellular components of diverse structure
•Almost all fatty acids have an even number of carbon atoms
(usually 12 to 24); they are either saturated or unsaturated, with
double bonds almost always in the cis configuration
•Trans fatty acids increase risk for coronary heart disease
•Triacylglycerols contain three fatty acid molecules esterified to the
three hydroxyl groups of glycerol
•Triacylglycerols are present in many foods and are primarily used
for storing energy and providing insulation in animals

47. Summary
•Glycerophospholipids contain fatty acids esterified to two of the
hydroxyl groups of glycerol
•phosphatidylethanolamine and phosphatidylcholine are
Common glycerophospholipids
•The sphingolipids contain sphingosine instead of glycerol
•Sphingolipids are abundant in the plasma membranes of
neurons,
•Sterols have four fused rings and a hydroxyl group.
•Cholesterol is the major sterol in animals

48. The end