Chemistry of lipids

Learning objectives

• Classify lipids based on the physical and chemical properties

• List the components of Phospholipids and Glycolipids

• Describe the Functions of phospholipase A2 and C

• Classify the types of Phospholipids and Glycolipids

• Analyze the Composition of surfactant and its importance

• Describe the structure of lipoproteins and their uses

Definition

• Heterogenous group of compounds • Insoluble in water

• Soluble in non- polar solvents: Chloroform/ Ether.

BIOMEDICAL IMPORTANCE

• Energy source – stored in adipose tissue

• Thermal insulator – maintains body temperature • Electrical insulator – myelin sheath of nerves

• Biomembranes – phospholipids and cholesterol • Steroid hormones – cortisol, aldosterone

• Fat soluble vitamins – vitamin A, D, E, K.

• Emulsifying agents – bile salts

• Essential FA (EFA) – linoleic and linolenic acid

Classification of lipids

• Simple lipids : esters of fatty acids with various alcohols

1. Fats/ oils – esters of fatty acids with glycerol

2. Waxes – esters of fatty acids with higher molecular weight monohydric alcohols.

Fats /oils

• Complex lipids : fatty acids + glycerol with additional groups;

a) Phospholipids : phosphoric acid 1. Glycerophospholipids - Glycerol2. Sphingophospholipids – sphingosine

(sphingomyelin)

b) Glycolipids – fattyacid + sphingosine + carbohydrate ;

C) Sulfolipids and aminolipids;

D) Lipoproteins – HDL, LDL , VLDL;

• Derived lipids : Fatty acids, Glycerol, steroids, alcohols, sterols, fatty aldehydes, ketone bodies, lipid soluble vitamins.

FATTY ACIDS

• Aliphatic carboxylic acids

• Saturated or unsaturated

• Unbranched

• Even numbered (reflects the mode of synthesis)

• Cis double bonds

Classification of fatty acids based on chain length:

• Short-chain fatty acids - fewer than six carbons;

• Medium-chain fatty acids (MCFA) - 6–12 Carbons;

• Long-chain fatty acids (LCFA) longer than 12 Carbons;

• Very-Long-chain fatty acids (VLCFA) longer than 22 Carbons;

• Eicosanoids –derived and modified from 20 C Arachidonic acid.

Trans fat• Trans fat : Not digested by

LIPASE

• Deposition in the blood vessels.

• Partial hydrogenation of unsaturated oils – leads to trans fats.

• Free radical generation.

Nomenclature

• Saturated – anoic . Ex : octanoic acid. no double bonds – mostly solids

• Unsaturated – enoic . Ex: octadecaenoic acid double bonds – mostly liquids

• PUFA – Essential fatty acids.

Nomenclature of fatty acids:

1. Delta (∆) naming; carbon ∆1 is carboxyl, ∆ 2 is called α carbon, ∆ 3 is b carbon, etc.

2. # of carbons: # of double bonds (carbons at which double bonds occur)i.e. oleic acid is 18:1(9)

3. w carbon numbering - terminal methyl group is always the omega carbon one. Number in the bracket is first double bond from omega i.e. (3).w

Essential fatty acids

• ω-3 fatty acids: α-Linolenic acid or ALA (18:3) 9, 12, 15;

• ω-6 fatty acids: Linoleic acid or LA (18:2)9,12.

Why essential? • Lack the ability to introduce double bonds in

fatty acids beyond carbon ∆ 9 and 10.

• Δ9desaturase, Δ6desaturase, Δ5 desaturase, and Δ4 desaturase are present in the body.

OTHER FATTY ACIDS

• CYCLIC FATTY ACIDS: 1. Chaulmoogric acid 2. Hydnocarpic acid • BRANCHED CHAIN FATTY ACIDS: 1. Phytanic acid in butter 2. Sebaceous glands

Refsum disease: deficiency of enzyme phytanic acid oxidase leading to accumulation of branched chain phytanic acid.

PHYSICAL PROPERTIES

1. Melting point: Directly proportional to (a) chain length (b) saturation 2. Solubility: Inversely proportional to (a) chain length (b) saturation3. Membrane fluidity: Inversely proportional to (a) chain length (b) saturation – particle fraction