paper : 05 metabolism of lipids module: 04 classification...

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Biochemistry METABOLISM OF LIPIDS

Classification of Lipids

Dr. Vijaya Khader Dr. MC Varadaraj

Paper : 05 Metabolism of Lipids

Module: 04 Classification of Lipids

Principal Investigator

Paper Coordinator and

Content Writer

Dr. Sunil Kumar Khare, Professor,

Department of Chemistry, IIT-Delhi

Dr. Suaib Luqman, Scientist (CSIR-CIMAP)

& Assistant Professor (AcSIR)

CSIR-CIMAP, Lucknow

Content Reviewer Prof. Prashant Mishra, Professor,

Department of Biochemical Engineering

and Biotechnology, IIT-Delhi

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DESCRIPTION OF MODULE

Subject Name Biochemistry

Paper Name 05 Metabolism of Lipids

Module Name/Title 04 Lipids-Classification

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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

Glyco Lipids Terpenoids

Conn & Stumpf

(1966)

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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.

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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.

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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

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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).

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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.

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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.

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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

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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

paper : 05 metabolism of lipids module: 04 classification...

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