Biochemistry. Classification of Lipids METABOLISM OF LIPIDS. Principal Investigator

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1 Paper : 05 Metabolism of Lipids Module: 04 Principal Investigator Paper Coordinator and Content Writer Content Reviewer Dr. Sunil Kumar Khare, Professor, Department of Chemistry, IIT-Delhi Dr. Suaib Luqman, Scientist (CSIR-CIMAP) & Assistant Professor (AcSIR) CSIR-CIMAP, Lucknow Dr. Vijaya Khader Dr. MC Varadaraj Prof. Prashant Mishra, Professor, Department of Biochemical Engineering and Biotechnology, IIT-Delhi 1

2 DESCRIPTION OF MODULE Subject Name Paper Name Module Name/Title 05 Metabolism of Lipids 04 Lipids-Classification 2

3 1. Objectives To understand the classification of lipids How many classes of lipids are present What are the significance of each class 2. Concept Map Natural Lipids Phospho Lipids Conn & Stumpf (1966) Glyco Lipids Terpenoids 3

4 3. Description Lipids can be broadly classified into following major classes: (a) Saponifiable lipids and (b) Non-saponifiable lipids. This classification is based on the basics of their reaction with sodium or potassium hydroxide. 4

5 a. The saponifiable lipid has one or more ester groups allocating it to endure hydrolysis in the charisma of an acid, base or enzyme. b. The non-saponifiable lipid includes phospholipids, prostaglandins, sphingolipids, steroids, terpenes, triglycerides and waxes which cannot be wrecked up into smaller molecules by hydrolysis. 5

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7 On the basis of polarity, lipids are divided into two classes: (1) Polar and (2) Non-Polar 1. Polar (Glycerophospholipids & Sphingolipids): Some of the characteristic of polar lipids are as under. a. It forms bilayer impulsively in water. Phosphatidyl choline (PC, a purified polar lipid from membranes) restrain up to 30 combinations of fatty acids on its chains. b. The bilayers of lipid have four states: Crystal, Gel, Liquid and Liquid crystal. It exhibits a sharp Tm defined from the gel to liquid crystal transition. The Tm is very soaring for saturated chains and very short for polyunsaturated chains. c. In the liquid crystal state, biological membranes are always just over the Tm. Examples are fish 7

8 and alcoholics. d. There are three sub-classes of polar lipids: anionic, uncharged and zwitter-ionic. No cationic lipids are reported in nature. Anionic Lipids Examples of the anionic lipids are Cardiolipin (CL), Phosphotidyl glycerol (PG), Phosphotidyl inositol (PI), Phosphotidyl serine (PS) and many others. i. Agreement of X-ray structures of crystals with NMR conformations. ii. Vile interactions improved by screening of cations from solution and H-bonds. iii. ph at the surface lower that of water (2-3 units if not in salt) and the ph difference wanes off at high ionic strength. iv. The anionic lipids constantly involved in fusion and the surface can bind calcium and hydrogen which eases fusion. v. Cardiolipin acts as an acid-anion provokes hydrogen ion to operate as an attractant. Fusion arises because of acid-anion formation and negative charges become striking by trapping protons. vi. The entire membranes have a net negative charge on their exterior that attracts cations (H + & Na ++ ). vii. Similarly, all biological membranes have a proton gradient sustained by a proton pump (except presence of sodium gradient in animal plasma membrane). 8

9 Uncharged Lipids Examples of the uncharged lipids are Cerebrosides, Gangliosides and Glycolipids. The surface of gangliosides & sphingolipids are anionic but have H-bond lateral interactions. Glycolipids have H-bonds amid neighbours with each lipid has three neighbours. Zwitter-Ionic Lipids Examples of the zwitter-ionic lipids are Phosphotidyl choline (PC), Phosphatidyl ethanolamine (PE), Sphingomyelin (SM). i. Position wise phosphate is on top of the glycerol and glycerol is upright to the membrane. ii. Primary ester is underneath the glycerol and secondary ester is parallel to the membrane before (at C2) the chain plunges. iii. Choline/Ethanolamine (base) gyrates at an angle of ~15º to the plane. iv. Each headgroup of zwitter ionic lipids interact with three neighbours ionically (+,-) 2. Non-polar (Triglycerides) A lot of lipids, on the other hand, are non-polar, implicating that the charge allocation is uniformly distributed. Infact, non-polar molecules do not dissolve well in water. Polar and non-polar molecules tend to deter each other in the similar way as the oil and water do not mix and will split from each other even if they are shaken robustly in an endeavor to mix them. This peculiarity between non-polar and polar molecules has imperative effect for living things, which are poised of both molecules. 9

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11 On the basis of function, lipids can be classified into following: 1. Storage lipids (Fatty acids, Triacylglycerols, Sterols) 2. Structural lipids in membranes (Phospholipids, Saccharolipids) 3. Lipids as signals, cofactors and pigments (Lipoproteins, Proteolipids, Polyketides) In the beginning it was understood that lipids are oily substance utilized for two major reasons: (a) dole out as energy souce and (b) structural blocks of membranes. George & Mildred Burr (1929) on the other hand drive out this allegory and established linoleic acid as an indispensable dietary constituent that play an imperative function in many processes in the body. Bergström et al., (1964) also revealed arachidonate (EFA) as the antecedent of the prostaglandins and explained its effect on inflammation and allied disease giving impetus to lipids that gained new-fangled significance among biochemists. Amphipathic nature of membrane lipids and orientation of their hydrophilic and hydrophobic regions articulate their stuffing into the bilayer. Among structural membrane lipids, three types have been described: (a) Glycerophospholipids (two fatty acids connected to glycerol are present in hydrophobic regions), (b) Sphingolipids (sphingosine: a fatty amine linked to single fatty acid) and (c) sterols (compounds with four fused hydrocarbon rings arranged in an inflexible arrangement). Glycerophospholipids and sphingolipids include charged alcohols at their polar ends and a few in addition contain phosphate moeity. In these amphipathic compounds, the hydrophilic moieties could be more complex or as elementary as a single -OH group at one end of the sterol ring system. Among these membrane lipids classes, colossal miscellany results from various combinations of tails and polar heads of fatty acid. 11

12 The products of hydrolysis have also been considered for classifying the lipids into following category. 1. Simple Lipids: On hydrolysis yield alcohol (monohydric or trihydric) and fatty acids Simple glyceride: Include identical FAs Mixed glyceride: Include dissimilar FAs Oils: Glycerol with UFAs Fats: Glycerol with SFAs Waxes: Mono or dihydric alcohol with FAs 2. Complex Lipids: (Compound lipids): On hydrolysis yield fatty acids, glycerol and phosphoric acid along with serine, sphingosine, ethanolamine and various sugars. Phospholipid: Glycerol + fatty acids + phosphoric acid + nitrogenous base 12

13 e.g. Lecithin: Glycerol + fatty acids + phosphoric acid + choline Cephalin: Glycerol + fatty acids + phosphoric acid + ethanolamine Glycolipids: Glycerol + fatty acid + carbohydrates (on hydrolysis) They are sub-classified as sulpholipids, galactosyl diglyceride and cerebrosides. Sphingophosphoiplds: Sphingosine + fatty acids + phosphoric acid + choline 3. Derived Lipids: Are the hydrolytic commodities of complex and simple lipids. Examples are Alcohols (Sterol and glycerol), fatty acids, terpenoids etc. 4. Summary In this lecture we learnt about: The Different types of classifications of the lipids 13