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Major Roles of Biological of Lipids

Biological molecules that are insoluble in aqueous solutions and soluble in organic solvents are classified as lipids. The lipids of physiological importance for humans have four major functions:
  • 1. They serve as structural components of biological membranes.
  • 2. They provide energy reserves, predominantly in the form of triacylglycerols.
  • 3. Both lipids and lipid derivatives serve as vitamins and hormones.
  • 4. Lipophilic bile acids aid in lipid solubilization.
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Fatty Acids

Fatty acids fill two major roles in the body:
  • as the components of more complex membrane lipids.
  • as the major components of stored fat in the form of triacylglycerols.

Fatty acids are long-chain hydrocarbon molecules containing a carboxylic acid moiety at one end. The numbering of carbons in fatty acids begins with the carbon of the carboxylate group. At physiological pH, the carboxyl group is readily ionized, rendering a negative charge onto fatty acids in bodily fluids.
Fatty acids that contain no carbon-carbon double bonds are termed saturated fatty acids; those that contain double bonds are unsaturated fatty acids. The numeric designations used for fatty acids come from the number of carbon atoms, followed by the number of sites of unsaturation (eg, palmitic acid is a 16-carbon fatty acid with no unsaturation and is designated by 16:0). The site of unsaturation in a fatty acid is indicated by the symbol D and the number of the first carbon of the double bond (e.g. palmitoleic acid is a 16-carbon fatty acid with one site of unsaturation between carbons 9 and 10, and is designated by 16:1D9).
Saturated fatty acids of less than eight carbon atoms are liquid at physiological temperature, whereas those containing more than ten are solid. The presence of double bonds in fatty acids significantly lowers the melting point relative to a saturated fatty acid.
The majority of body fatty acids are acquired in the diet. However, the lipid biosynthetic capacity of the body (fatty acid synthase and other fatty acid modifying enzymes) can supply the body with all the various fatty acid structures needed. Two key exceptions to this are the highly unsaturated fatty acids know as linoleic acid and linolenic acid, containing unsaturation sites beyond carbons 9 and 10. These two fatty acids cannot be synthesized from precursors in the body, and are thus considered the essential fatty acids; essential in the sense that they must be provided in the diet. Since plants are capable of synthesizing linoleic and linolenic acid humans can aquire these fats by consuming a variety of plants or else by eating the meat of animals that have consumed these plant fats.
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Physiologically Relevant Fatty Acids

Numerical Symbol Common Name Structure Comments
14:0 Myristic acid CH3(CH2)12COOH Often found attached to the N-term. of plasma membrane-associated cytoplasmic proteins
16:0 Palmitic acid CH3(CH2)14COOH End product of mammalian fatty acid synthesis
16:1D9 Palmitoleic acid CH3(CH2)5C=C(CH2)7COOH  
18:0 Stearic acid CH3(CH2)16COOH  
18:1D9 Oleic acid CH3(CH2)7C=C(CH2)7COOH  
18:2D9,12 Linoleic acid CH3(CH2)4C=CCH2C=C(CH2)7COOH Essential fatty acid
18:3D9,12,15 Linolenic acid CH3CH2C=CCH2C=CCH2C=C(CH2)7COOH Essential fatty acid
20:4D5,8,11,14 Arachidonic acid CH3(CH2)3(CH2C=C)4(CH2)3COOH Precursor for eicosanoid synthesis

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Basic Structure of Triacylglycerides

Triacylglycerides are composed of a glycerol backbone to which 3 fatty acids are esterified.

Basic composition of a triacylglyceride. The glycerol backbone is in blue.

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Basic Structure of Phospholipids

The basic structure of phospolipids is very similar to that of the triacylglycerides except that C-3 (sn3)of the glycerol backbone is esterified to phosphoric acid. The building block of the phospholipids is phosphatidic acid which results when the X substitution in the basic structure shown in the Figure below is a hydrogen atom. Substitutions include ethanolamine (phosphatidylethanolamine), choline (phosphatidylcholine, also called lecithins), serine (phosphatidylserine), glycerol (phosphatidylglycerol), myo-inositol (phosphatidylinositol, these compounds can have a variety in the numbers of inositol alcohols that are phosphorylated generating polyphosphatidylinositols), and phosphatidylglycerol (diphosphatidylglycerol more commonly known as cardiolipins).

Basic composition of a phospholipid. X can be a number of different substituents.

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Basic Structure of Plasmalogens

Plasmalogens are complex membrane lipids that resemble phospholipids, principally phosphatidylcholine. The major difference is that the fatty acid at C-1 (sn1) of glycerol contains either an O-alkyl or O-alkenyl ether species. A basic O-alkenyl ether species is shown in the Figure below. One of the most potent biological molecules is platelet activating factor (PAF) which is a choline plasmalogen in which the C-2 (sn2) position of glycerol is esterified with an acetyl group insted of a long chain fatty acid.

Top: basic composition of O-alkenyl plasmalogens.
Bottom: structure of PAF.

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Basic Structure of Sphingolipids

Sphingolipids are composed of a backbone of sphingosine which is derived itself from glycerol. Sphingosine is N-acetylated by a variety of fatty acids generating a family of molecules referred to as ceramides. Sphingolipids predominate in the myelin sheath of nerve fibers. Sphingomyelin is an abundant sphingolipid generated by transfer of the phosphocholine moiety of phosphatidylcholine to a ceramide, thus sphingomyelin is a unique form of a phospholipid.
The other major class of sphingolipids (besides the sphingomyelins) are the glycosphingolipids generated by substitution of carbohydrates to the sn1 carbon of the glycerol backbone of a ceramide. There are 4 major classes of glycosphingolipids:
  • Cerebrosides: contain a single moiety, principally galactose.
  • Sulfatides: sulfuric acid esters of galactocerebrosides.
  • Globosides: contain 2 or more sugars.
  • Gangliosides: similar to globosides except also contain sialic acid.

Top: Sphingosine
the atoms in red are derived from glycerol.
Bottom: Basic composition of a ceramide
n indicates any fatty acid may be N-acetylated at this position.

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This article has been modified by Dr. M. Javed Abbas.
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20:36 21/12/2002