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2016 Lipids lecture(1) by Prof.Dr.Moaed Al-Gazally Chemistry of Lipids Lipids constitute a heterogeneous group of compounds of biochemical importance. Lipids may be defined as compounds which are relatively insoluble in water, but freely soluble in nonpolar organic solvents like benzene, chloroform, ether, hot alcohol, acetone, etc. Functions of Lipids 1. Storage form of energy (triglycerides). 2.Structural components of biomembranes (phospholipids and cholesterol). 3. Metabolic regulators (steroid hormones and prostaglandins). 4. Act as surfactants, detergents and emulsifying agents (amphipathic lipids). 2016 Lipids lecture(1) by Prof.Dr.Moaed Al-Gazally 5. Act as electric insulators in neurons. 6.Provide insulation against changes in external temperature (subcutaneous fat) 7. Give shape and contour to the body. 8. Protect internal organs by providing a cushioning effect (pads of fat) 9. Help in absorption of fat soluble vitamins (A, D, E and K) 10. Improve taste and palatability of food. Clinical Applications 1. Excessive fat deposits cause obesity. Truncal obesity is a risk factor for heart attack. 2.Abnormality in cholesterol and lipoprotein metabolism leads to atherosclerosis and cardiovascular diseases . 3. In diabetes mellitus, the metabolisms of fatty acids and lipoproteins are deranged, leading to ketosis. Calcification 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 . a. Triacylglycerol or Triglycerides or neutral fat b. Waxes 2016 Lipids lecture(1) by Prof.Dr.Moaed Al-Gazally 2. Compound lipids : They are fatty acids esterified with alcohol; but in addition they contain other groups. Depending on these extra groups, they are subclassified in to A) Phospholipids, containing phosphoric acid: 1. Nitrogen containing glycerophosphatides: i. Lecithin (phosphatidyl choline) ii. Cephalin (phosphatidyl ethanolamine) iii. Phosphatidyl serine 2. Non-nitrogen glycerophosphatides i. Phosphatidyl inositol ii. Phosphatidyl glycerol iii. Diphosphatidyl glycerol (cardiolipin) 3. Plasmalogens, having long chain alcohol i. Choline plasmalogen ii. Ethanolamine plasmalogen 4. Phospho sphingosides, with sphingosine Sphingomyelin. B) Non-phosphorylated lipids 1. Glycosphingolipids (carbohydrate) i. Cerebrosides (ceramide monohexosides) oligosaccharides) ii. Globosides (ceramide iii. Gangliosides (ceramide + oligosaccharides + N-acetyl neuraminic acid) 2. Sulpholipids or sulfatides i. Sulphated cerebrosides ii. Sulphated globosides iii. Sulphated gangliosides 3. Derived lipids: They are compounds which are derived from lipids or precursors of lipids, e.g. fatty acids, steroids. Proteolipids and lipoproteins. 4. Lipids complexes to other compounds: Proteolipids and lipoproteins. 2016 Lipids lecture(1) by Prof.Dr.Moaed Al-Gazally Fatty acids Fatty acids, are included in the group of derived lipids. It is the most common component of lipids in the body. They are generally found in ester linkage in different classes of lipids. In the human body free fatty acids are formed only during metabolism. Fatty acids are aliphatic carboxylic acids and have the general formula, R—CO—OH, where COOH (carboxylic group) represents the functional group. Depending on the R group (the hydrocarbon chain), the physical properties of fatty acids may vary. Classification of fatty acids 1. Depending on total number of carbon atoms: a. Even chain :They have carbon atoms 2,4,6 and similar series. Most of the naturally occurring lipids contain even chain fatty acids. b. Odd chain :They have carbon atoms 3, 5, 7, etc. Odd numbered fatty acids are seen in microbial cell walls. They are also present in milk. 2. Depending on length of hydrocarbon chain: a. Short chain with 2 to 6 carbon atoms. b.Medium chain with 8 to 14 carbon atoms.(Capric acid 10 C) c. Long chain with 16 to 22 carbon atoms.(Linoleic acid 16 C). 2016 Lipids lecture(1) by Prof.Dr.Moaed Al-Gazally d. Very long chain fatty acids (more than 24 carbon) 3. Depending on nature of hydrocarbon chain: a. Saturated fatty acids . b. Unsaturated fatty acids which may be sub classified into Monounsaturated (mono-enoic) having single double bond or Polyunsaturated (poly-enoic) with 2 or more double bonds . c. Branched chain fatty acids d. Hydroxy fatty acids. Saturated fatty acids Franz Soxhlet, a German chemist, invented the Soxhlet apparatus in 1879. i. They have the general formula CH3-(CH2)n- COOH. For example, Acetic acid CH3—COOH Butyric acid CH3(CH2)2—COOH Palmitic acid CH3—(CH2)14—COOH Stearic acid CH3—(CH2)16—COOH 2016 Lipids lecture(1) by Prof.Dr.Moaed Al-Gazally ii. They are named by adding the suffix ‘anoic' after the hydrocarbon. iii. The two carbon acetic acid and 4 carbon butyric acid are important metabolic intermediates. iv. The C16 (palmitic acid) and C18 (stearic acid) are most abundant in body fat. v. Each animal species will have characteristic pattern of fatty acid composition. Thus human body fat contains 50% oleic acid, 25% palmitic acid 10% linoleic and 5% stearic acid. vi. The carbon atoms of fatty acids are numbered as C1, C2, etc (Delta system) starting from the COOH group. Or, starting from the methyl end, the carbon atoms may be numbered as omega system (ω)-1,2,3,etc. 6 5 4 3 2 1 CH3 — CH2 — CH2 — CH2— CH2 — COOH ω1 ω2 ω3 ω4 ω5