Glycerophospholipids structure

FIGURE 8.6 Structures of several glycerophospholipids and space-filling models of phosphatidylcholine, phosphatidylglycerol, and phosphatidylinositol. 

Figure 4.22 (a) Stearic and oleic acid (b) glycerol and a triglyceride (c) the general structure of a glycerophospholipid and (d) the glycerophospholipid l-stearoyl-2-oleoyl-3-phosphatidylcholine. 

Glycerophospholipids are used for membrane synthesis and for producing a hydrophilic surface layer on lipoproteins such as VLDL. In cell membranes, they also serve as a reservoir of second messengers such as diacylglycerol, inositol 1,4,5-triphosphate, and arachidonic acid. Their structure is similar to triglycerides, except that the last fatty acid is replaced by phosphate and a water-soluble group such as choline (phosphatidylcholine, lecithin) or inositol (phosphatidyl-inositol). 

The simplest of the glycerophospholipids is phosphatidic acid, in which phosphate is linked to the third hydroxyl function, forming a phosphate ester. More complex glycerophospholipids are derivatives of phosphatidic acid in which one of several groups is attached commonly choline, ethanolamine, serine, or myo-inositol. Structures are collected in table 19.1. 

Fatty acids are key constituents of several structural classes of lipids triglycerides, glycerophospholipids, and glycolipids. 

A bilayer formed from complex lipids, largely glycerophospholipids, forms the core structure of biological membranes. This bilayer forms a barrier to penetration of exogenous molecules into the cellular interior. Proteins penetrate into or through this bilayer. 

Phosphatidic acid the structural backbone of the glycerophospholipids two molecules of fatty acids are esterified to a molecule of glyceryl phosphate. 

Figure 5.1 The structure of a glycerophospholipid. A simple diagram showing the charges on the head group. In this struction, palmitic and oleic acids, provide the hydrophobic component of the phospholipids and choline (and four bases) and the phosphate group provide the hydrophilic head. The unsaturated fatty acid, oleic acid, provides a kink in the structure and therefore some flexibility in the membrane structure which allows for fluidity. The more unsaturated the fatty acid, the larger is the kink and hence more fluidity in the membrane. Cholesterol molecules can fill the gaps left by the kink and hence reduce flexibility. Hydroxyl groups on the bases marked are those that form phosphoester links. Choline and inositol may sometimes be deficient in the diet so that they are, possibly, essential micronutrients (Chapter 15).

Figure 4-1. Structures of the membrane bilayer and an amphipathic phospholipid. The head group attachment, X, may be H as in phosphatidic acid or one of several substituents linked via phosphoesters in the glycerophospholipids. The nonpolar tail is depicted as composed of saturated fatty acids in this molecule. The overall length of the hydrocarbon chain of the fatty acids may vary from 14 to 20.

Bacteria also contain a very rich variety of glycolipids with unusual structures. Lipid A13 is the site of attachement of the 0-specific chain of Gram (-) bacteria, which constitutes the antigenic lipopolysaccharide [87]. Other members of this family can be quoted, for example glycosyl glycerophospholipids in which the carbohydrate and glycerol moieties are linked by a phosphodiester bond (e.g. GPI anchor 14) [88] or carbohydrate esters (e.g. cord-factor of mycobacteria 15). 

More complex structures, often related to natural products are prepared by organic synthesis. Among them can be mentioned (f )-3-hydroxytetradecanoic acid (the double-tail hydrophobic moiety of lipid A), sphingosine derivatives related to the ceramides or 1,2- and l,3-dialkyl(acyl)glycerols related to glyco-glycerolipids, glycerophospholipids, and GPI anchors of membrane proteins. The preparations of the above derivatives were reported several years ago but some improvements have been published more recently. 

FIGURE 10-13 The similarities in shape and in molecular structure of phosphatidylcholine (a glycerophospholipid) and sphingomyelin (a sphingolipid) are clear when their space-filling and structural formulas are drawn as here. 

Glycerophospholipids differ in the structure of their head group common gycerophospholipids are phosphatidylethanolamine and phosphatidylcholine. The polar heads of the gycerophospholipids carry electric charges at pH near 7. 

A. Structures of some glycerophospholipids. B. Phosphatidic acid. = phosphate, PO4-2. 

Structure and function of cardiolipin Cardiolipin contains two molecules of phosphatidic acid esterified through their phosphate groups to an additional molecule of glycerol. This is the only human glycerophospholipid that is antigenic. It is an important component of the inner mitochondrial membrane. 

Nakagawa Y., Kurihara K., Sugiura T., and Waku K. (1985). Heterogeneity in the metabolism of the arachidonoyl molecular species of glycerophospholipids of rabbit alveolar macrophages. The relationship between metabolic activities and chemical structures of the arachidonoyl molecular species. Eur. J. Biochem. 153 263-268. 

Fig. 1. Structures of membrane glycerophospholipids. R1 and Ft2 represent hydrocarbon chains of fatty acids.

Jackson S, Wang H, Woods A (2007) In situ structure characterization of glycerophospholipids and sulfatides in brain tissue using MALDI-MS/MS. J Am Soc Mass Spectrom 18 17-26. doi 10.1016/j.jasms.2006.08.015... 

Cephalins are glycerophospholipids present in foods. They differ from lecithins by having ethanolamine or serine instead of choline in their structure. Could you differentiate between lecithins and cephalins on the basis of the three tests to be performed in this experiment ... 

Besides the lecithin and other glycerophospholipids, two more classes of complex lipds are given in your textbook (a) sphingolipids and (b) glycolipids. (Look up their structures in your textbook.) Would any of these compounds give you a positive test with molybdate solution ... 

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