Classification of Lipids

Suaib Luqman

Objectives

v To understand the classification of lipids
v How many classes of lipids are present
v What are the significance of each class

Concept Map

The non-saponifiable lipid includes phospholipids, prostaglandins, sphingolipids, steroids, terpenes, triglycerides and waxes which cannot be wrecked up into smaller molecules by hydrolysis.

Polar (Glycerophospholipids & Sphingolipids): Some of the characteristic of polar lipids are as under.

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.
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.
1. 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.
2. - 1. Agreement of X-ray structures of crystals with NMR conformations.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.The entire membranes have a net negative charge on their exterior that attracts cations (H+ & Na++).In the liquid crystal state, biological membranes are always just over the Tm. Examples are fish and alcoholics.
      1. 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.
    1. Vile interactions improved by screening of cations from solution and H-bonds.
  1. The anionic lipids constantly involved in fusion and the surface can bind calcium and hydrogen which eases fusion.
- Similarly, all biological membranes have a proton gradient sustained by a proton pump (except presence of sodium gradient in animal plasma membrane).

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).

Position wise phosphate is on top of the glycerol and glycerol is upright to the membrane.
Primary ester is underneath the glycerol and secondary ester is parallel to the membrane before (at C2) the chain plunges.

Choline/Ethanolamine (base) gyrates at an angle of ~15º to the plane.

Each headgroup of zwitter ionic lipids interact with three neighbours ionically (+,-)
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.

On the basis of function, lipids can be classified into following:

Storage lipids (Fatty acids, Triacylglycerols, Sterols)
Structural lipids in membranes (Phospholipids, Saccharolipids)
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.

The products of hydrolysis have also been considered for classifying the lipids into following category.

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

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

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

Derived Lipids: Are the hydrolytic commodities of complex and simple lipids.

Examples are Alcohols (Sterol and glycerol), fatty acids, terpenoids etc.

Summary

In this lecture we learnt about:

The Different types of classifications of the lipids

you can view video on Classification of Lipids

Weblinks

Books

Christie W.W. in “Lipid Analysis” Pergamon Press, Oxford, 1982;
Pomeranz Y. and Meloan C.L. in “Food Analysis; Theory and Practice” 4th ed., AVI, Westport,Connecticut, 1994;

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008.
Arienti G. Le basi molecolari della nutrizione. Seconda edizione. Piccin, 2003
Bender D.A. “Benders’ dictionary of nutrition and food technology”. 2006, 8th Edition. Woodhead Publishing. Oxford
Bergstroem S., Danielsson H., Klenberg D. and Samuelsson B. The enzymatic conversion of essential fatty acids into prostaglandins. J Biol Chem 1964;239:PC4006-PC4008 [Full Text]
Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008
Cozzani I. e Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006
Giampietro M. L’alimentazione per l’esercizio fisico e lo sport. Il Pensiero Scientifico Editore. Prima edizione 2005
Mahan L.K., Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000
Rosenthal M.D., Glew R.H. Mediacal biochemistry. Human metabolism in health and disease. John Wiley & Sons, Inc. 2009
Shils M.E., Olson J.A., Shike M., Ross A.C.: “Modern nutrition in health and disease” 9th ed. 1999
Stipanuk M.H.. Biochemical and physiological aspects of human nutrition. W.B. Saunders Company-An imprint of Elsevier Science, 2000
Biochemistry by Thomas Briggs, Albert M. Chandler. 1995. https://books.google.co.in/books?isbn=0387943986