Chemistry of Lipids

Lecture No. 3

What are Lipids?

Lipids or Fats

• Lipids are heterogenous group of compounds related either actually or potentially to the fatty acids.

• They are water-insoluble organic substances found in cells, which are soluble in organic, non-polar solvents such as chloroform, ether, acetone, benzene, n-hexane, TCA, CCl, etc.

• They are widely distributed in nature.

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, of

the leaves of higher plants, of the exoskeleton of insects and 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 the

organism and the environment to prevent infection and excessive loss or gain of water

Classification of LipidsA. Classification by White – based on the presence or absence of glycerol1. Fatty acids2. Lipids containing glycerola) Neutral fats (triacylglycerols)b) Phosphoglycerides or phospholipidsc) Cardiolipind) Plasmalogens3. Lipids not containing glycerola) Sphingolipidsb) Alipathic alcohol and waxesc) Terpenesd) Steroidse) Prostaglandins4. Lipids combined with other classes of compoundsa) lipoproteins and proteolipidsb) Glycolipids

Classification by Bloors

1.Simple Lipidsa) Fats and Oilsb) Waxes2. Compound Lipidsa) Phospholipidsb) Glycolipids3. Derived Lipidsa) Fatty acidsb) Alcoholc0 Sterols

Fatty Acids

• Fatty acids are the most abundant occurring lipids. They are the building blocks of the several classes of lipids and neutral fats, phosphoglycerides, glycolipids, cholesterol esters and some waxes. They are the products 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

Fatty Acids

Saturated fatty acids

• Saturated fatty acids belong to acetic acid series with general formula CnH2nO2 -. They contain up to 8 C and are liquid at room temperature. Longer chains of saturated FA are solids. The systematic name ends in “anoic”

• Examples: Lauric or duodecanoicMyristic or tetradecanoicPalmitic or hexadecanoic Stearic or octadecanoicArachidic or didecanoic

Triacylglycerols (TAG)

• Storage of fatty acids in organisms is largely in the form of TAG or fats.

• These substances are triesters of fatty acids and glycerol.

• The long chains of fatty acids are extraordinarily for energy storage, because they contain C in a fully reduced form and will therefore yield a maximum amount of energy on oxidation.

Unsaturated fatty acids

• Unsaturated fatty acids contain double bond and their systematic names end in “enoic.” They are unstable and reactive due to the presence of the double bond – therefore they are capable of taking up molecules of H2O, H, I2, O2, B2 at each double bond

• Double bonds in naturally occurring fatty acids are always in a cis- form as opposed to trans- configuration.

• The presence of the double bond gives rise to the possibility of geometric isomerism

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 all naturally existing fats

Indispensable fatty acids

• The body has the ability to desaturate fatty acids but more unsaturated fatty acids 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.

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

Glycerophospholipids or Phosphoglycerides

• They are the major class of naturally occurring phospholipids, lipids with phosphate-containing head groups.

• These compounds make up a significant fraction of the membrane lipids throughout the bacterial, plant and animal kingdoms.

• Examples are: phosphotidyl choline, phosphotidylethanolamine, phosphotidylserine, phosphotidylinositol

Cardiolipin & Plasmalogens

• Cardiolipin consists of 2 molecules of phosphatidic acid connected by a glycerol bridge

• It is of immunological importance• Plasmalogens make up 10% of the

phospholipids of the brain and muscles.• They are phospholipids where alkyl ether is

attached at alpha position of glycerol.• Three principal classes of plasmalogens are:

1. phosphotidalcholine2. phosphotidalserine3. phosphotidal ethanolamine

Plasmalogens

Sphingolipids• They are comlex lipids containing sphingosine or a

related base, as their backbone• All sphingolipids contain 3 characteristics:

1. 1 molecule of fatty acid2. 1 molecule of sphingosine or 1 of its derivatives3. a polar head group • The greatest concentration of sphingolipids is found

in the CNS, particularly in the white matter, although all human tissues contain some.

• Kinds of sphingolipids are:1. Shingomyelin2. Glycosphingolipida) Gangliosideb) Cerebroside

Sphingomyelin

Ceramide

Chemical Reaction of Lipids

Steroids• Steroids are found in association with fats

and separated from the fats after saponification in the “unsaponifiable residue.”

• They all possess similar cyclic nucleus resembling 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 naturally occurring steroids.

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

Lanosterol and Ergosterol

Cholesterol

• Cholesterol is widely distributed in cells particularly in the nervous tissue

• It is the major component of the plasma membrane.

• It is the parent compound of all steroids synthesized in the body.

• It occurs in animal fats, but not in plant fats.

Cholesterol

Estrogen

• Estrogen differs from all steroids in that ring A is aromatic

• Hydroxyl group at position 3 is phenolic rather than alcoholic

• Because of this fact, estrogens behave like weak acids and are extractable from benzene solution with aqueous alkali

Androgen or Testosterone• Androgen or male sex hormone is C19

steroid and belongs to the class of steroids devoid of C side chain at C17

• Synthesized from the testis• Has a OH in C17

TestosteroneIt is also called androgen.It is C19 steroid.This is a steroid without a carbon side chain at position 17.

Progesterone

• It is also an adrenal cortex hormone and a C21 steroid

• But it has CH3

instead of the CH2OH of corticosterone

Cortisosterone

It is a C21 steroidContains 2 carbon atoms in the side chain at position 17

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

Bile Acids

• Cholic acid Deoxycholic acid

Terpenes

Terpenes

Examples of Terpenes

Monoterpene - limoneneDiterpene – phytolTriterpene – squaleneTetraterpene – α,ß,γ-carotenePolyterpene – natural rubber, gutta perchae

Vitamin D

• Vitamin D2 Vitamin D3

Vitamin K

Vitamin K2

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

Thromboxane A2

Leukotrienes

Micelle

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 the properties of bisolubility.

• Bile acids are strong emulsifying agents that help dispense fatty acids into micelles, which may be absorbed or more rapidly broken down by enzymes.

Lipids in Cell Membranes

Chemical tests for Lipids

• Liebermann-Burchard test - for cholesterol• Acrolein test – for glycerol• Saponication – fats react with alkali to produce

metallic salt of fatty acid or soap• Halogenation – when halogens are added to the

double bond of unsaturated FA, the reaction proceeds spontaneously in a given suitable solvent.

• Hydrogenation – when unsaturated FA are hydrogenated in the presence of catalyst like Pt, Pa, or Ni

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