biochemistry lecture 3

8/3/2019 Biochemistry Lecture 3

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Chemistry of Lipids

Lecture No. 3


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What are Lipids?


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Lipids or Fats

Lipids are heterogenous group ofcompounds related either actually orpotentially to the fatty acids.

They are water-insoluble organicsubstances found in cells, which aresoluble in organic, non-polar solvents such

as chloroform, ether, acetone, benzene, n-hexane, TCA, CCl, etc.

They are widely distributed in nature.


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What are the functions of lipids? The importance of lipids can be seen from their varied

functions. They are: 1. As membrane structural components

2. As intracellular storage depots of metabolic fuel

As transport form of metabolic fuel

As regulatory substances (some hormones)

As protective form of the cell walls of many bacteria, ofthe leaves of higher plants, of the exoskeleton of insectsand the skin of vertebrates

As enzymes cofactors (some vitamins)

As transport form of some neurotransmitters(acetylcholine)

As receptors in nerve ending membranes

As determinants of immunological specificity

As part of the outer coating between the body of theorganism and the environment to prevent infection andexcessive loss or gain of water


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Classification of LipidsA. Classification by White based on the presence or

absence of glycerol

1. Fatty acids2. Lipids containing glycerol

a) Neutral fats (triacylglycerols)

b) Phosphoglycerides or phospholipids

c) Cardiolipin

d) Plasmalogens

3. Lipids not containing glycerol

a) Sphingolipids

b) Alipathic alcohol and waxes

c) Terpenesd) Steroids

e) Prostaglandins

4. Lipids combined with other classes of compounds

a) lipoproteins and proteolipids

b) Glycolipids


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Classification by Bloors

1. Simple Lipids

a) Fats and Oils

b) Waxes

2. Compound Lipidsa) Phospholipids

b) Glycolipids

3. Derived Lipids

a) Fatty acidsb) Alcohol

c0 Sterols


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Fatty Acids

Fatty acids are the most abundant occurringlipids. They are the building blocks of theseveral classes of lipids and neutral fats,phosphoglycerides, glycolipids, cholesterol

esters and some waxes. They are theproducts of fat hydrolysis

They are mainly straight chain, alipathic,monocarboxylic acids, ranging in chain

lengths from 4 -24 C atoms Type formula: R-COOH

Fatty acids may be saturated or unsaturated


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Fatty Acids


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Saturated fatty acids

Saturated fatty acids belong to acetic acidseries with general formula CnH2nO2 -. Theycontain up to 8 C and are liquid at roomtemperature. Longer chains of saturated FA

are solids. The systematic name ends inanoic

Examples: Lauric or duodecanoic

Myristic or tetradecanoic

Palmitic or hexadecanoicStearic or octadecanoic

Arachidic or didecanoic


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Triacylglycerols (TAG)

Storage of fatty acids in organisms islargely in the form of TAG or fats.

These substances are triesters of fattyacids and glycerol.

The long chains of fatty acids areextraordinarily for energy storage,

because they contain C in a fully reducedform and will therefore yield a maximumamount of energy on oxidation.


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Unsaturated fatty acids

Unsaturated fatty acids contain double bondand their systematic names end in enoic.They are unstable and reactive due to the

presence of the double bond therefore theyare capable of taking up molecules of H2O, H,I2, O2, B2 at each double bond

Double bonds in naturally occurring fattyacids are always in a cis- form as opposed totrans- configuration.

The presence of the double bond gives riseto the possibility of geometric isomerism


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Oleic acid

Oleic acid is

CH3(CH2)7CH=CH(CH2)7COOH

Oleic acid is the most abundant fatty acid in

nature forming about 50% of the total fatty

acids in many fats and is found in allnaturally existing fats


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Indispensable fatty acids

The body has the ability to desaturatefatty acids but more unsaturated fattyacids which are necessary for nutrition

and growth can be formed in the body. They have to be supplied from the diet.

They become indispensable items in

the diet. These are linoleic acid, linolenic acid

and arachidonic acid.


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Indispensable fatty acids

Linoleic acid cis-9 octadecadienoic acid

18:2,9,12

Linolenic acid 1,5 octadecatrienoic acid18:3,9,12,15

Arachidonic acid cis-5,8,11,14

eicosatetraenoic acid20:4,5,8,11,14


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Glycerophospholipids or

Phosphoglycerides

They are the major class of naturally

occurring phospholipids, lipids with

phosphate-containing head groups.

These compounds make up a significantfraction of the membrane lipids throughout

the bacterial, plant and animal kingdoms.

Examples are: phosphotidyl choline,

phosphotidylethanolamine,

phosphotidylserine, phosphotidylinositol


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Cardiolipin & Plasmalogens

Cardiolipin consists of 2 molecules ofphosphatidic acid connected by a glycerol bridge

It is of immunological importance

Plasmalogens make up 10% of thephospholipids of the brain and muscles.

They are phospholipids where alkyl ether isattached at alpha position of glycerol.

Three principal classes of plasmalogens are:

1. phosphotidalcholine2. phosphotidalserine

3. phosphotidal ethanolamine


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Plasmalogens


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Sphingolipids

They are comlex lipids containing sphingosine or arelated base, as their backbone

All sphingolipids contain 3 characteristics:

1. 1 molecule of fatty acid

2. 1 molecule of sphingosine or 1 of its

derivatives3. a polar head group

The greatest concentration of sphingolipids is foundin the CNS, particularly in the white matter, althoughall human tissues contain some.

Kinds of sphingolipids are:1. Shingomyelin

2. Glycosphingolipid

a) Ganglioside

b) Cerebroside


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Sphingomyelin


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Ceramide


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Chemical Reaction of Lipids


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Steroids

Steroids are found in association with fatsand separated from the fats aftersaponification in the unsaponifiableresidue.

They all possess similar cyclic nucleusresembling phenanthrene (rings A, B, and C),to which a cyclopentane ring is attached

Steroid nucleus is

cyclopentanoperhydrophenanthrene There is an oxygenated substituent on C3

which is shared by almost all naturallyoccurring steroids.


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Biologically Important Steroids

Sterols Cholesterol, ergosterol,

lanosterol

Bile Acids cholic acid, glycholic acid, Adrenocortical Hormones

costicosterone, cortisol, aldosterone

Sex hormones androgen, estrogen

Vitamins Vitamin D, K


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Lanosterol and Ergosterol


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Cholesterol

Cholesterol is widely distributed in

cells particularly in the nervous tissue

It is the major component of the plasmamembrane.

It is the parent compound of all

steroids synthesized in the body.

It occurs in animal fats, but not in plant

fats.


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Cholesterol


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Estrogen

Estrogen differs from all steroids in that

ring A is aromatic

Hydroxyl group at position 3 is phenolicrather than alcoholic

Because of this fact, estrogens behave

like weak acids and are extractable from

benzene solution with aqueous alkali


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Androgen or Testosterone

Androgen or male sex hormone is C19steroid and belongs to the class of steroids

devoid of C side chain at C17

Synthesized from the testis Has a OH in C17


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Testosterone

It is also called

androgen.

It is C19 steroid.

This is a steroidwithout a carbon

side chain at

position 17.


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Progesterone

It is also an

adrenal cortex

hormone and a

C21 steroid

But it has CH3instead of the

CH2OH ofcorticosterone


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Cortisosterone

It is a C21 steroid

Contains 2

carbon atomsin the side

chain at

position 17


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Bile Acids

Bile acids or salts are steroids in nature.

Four different salts occur in human bile:

Glycocholic acid, deoxycholic acid, cholic

acid and lithocholic acid

They are strong emulsifying agent that

help disperse fatty materials

They stimulate intestinal motility


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Bile Acids

Cholic acid Deoxycholic acid


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Terpenes


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Terpenes


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Examples of Terpenes

Monoterpene -limonene

Diterpene phytol

Triterpene squaleneTetraterpene ,,-

carotene

Polyterpene natural rubber,gutta perchae


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Vitamin D

Vitamin D2 Vitamin D3


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Vitamin K


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Vitamin K2


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Prostaglandins

They are family of fatty acids which have

hormonal or regulatory nature.

The best known prostaglandins are PGE1

PGF1a PGF2a

The prostaglandins differ from each other

with respect to their biological activity,

although all show at least some activity in

lowering blood pressure and inducing

muscles to contract


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Thromboxane A2


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Leukotrienes


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Micelle


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Micelles

When fatty acids are placed in water, they

appear to dissolve, they are not, in true

solution, but present as micelles.

Micelles or microscopic fat cells have theproperties of bisolubility.

Bile acids are strong emulsifying agents

that help dispense fatty acids intomicelles, which may be absorbed or more

rapidly broken down by enzymes.


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Lipids in Cell Membranes


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Chemical tests for Lipids

Liebermann-Burchard test - for cholesterol

Acrolein test for glycerol

Saponication fats react with alkali to producemetallic salt of fatty acid or soap

Halogenation when halogens are added tothe double bond of unsaturated FA, the reactionproceeds spontaneously in a given suitablesolvent.

Hydrogenation when unsaturated FA arehydrogenated in the presence of catalyst like Pt,Pa, or Ni


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Biological Significance of Lipids

The current biological significance of

lipids can be seen in the following:

1. Obesity

2. Arteriosclerosis

Role of polyunsaturated fatty acids

(PUFA)in nutrition and health

biochemistry lecture 3

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