Structural Lipids in Membranes:- Glycerophospholipids Are Derivatives of Phosphatidic Acid
Glycerophospholipids, also called phosphoglycerides, are membrane lipids in which two fatty acids are attached in ester linkage to the first and second carbons of glycerol, and a highly polar or charged group is attached through a phosphodiester linkage to the third carbon. Glycerol is prochiral; it has no asymmetric carbons, but attachment of phosphate at one end converts it into a chiral compound, which can be correctly named either L-glycerol 3-phosphate, D-glycerol 1-phosphate, or sn-glycerol 3-phosphate (Fig. 10–7). Glycerophospholipids are named as derivatives of the parent compound, phosphatidic acid (Fig. 10–8), according to the polar alcohol in the head group. Phosphatidylcholine and phosphatidylethanolamine have choline and ethanolamine in their polar head groups, for example. In all these compounds, the head group is joined to glycerol through a phosphodiester bond, in which the phosphate group bears a negative charge at neutral pH. The polar alcohol may be negatively charged (as in phosphatidylinositol 4,5-bisphosphate), neutral (phosphatidylserine), or positively charged (phosphatidylcholine, phosphatidylethanolamine). As we shall see in Chapter 11, these charges contribute greatly to the surface properties of membranes. The fatty acids in glycerophospholipids can be any of a wide variety, so a given phospholipid (phosphatidylcholine, for example) may consist of a number of molecular species, each with its unique complement of fatty acids. The distribution of molecular species is specific for different organisms, different tissues of the same organism, and different glycerophospholipids in the same cell or tissue. In general, glycerophospholipids contain a C16 or C18 saturated fatty acid at C-1 and a C18 to C20 unsaturated fatty acid at C-2. With few exceptions, the biological significance of the variation in fatty acids and head groups is not yet understood.
FIGURE 10–7 L-Glycerol 3-phosphate, the backbone of phospho lipids. Glycerol itself is not chiral, as it has a plane of symmetry through C-2. However, glycerol can be converted to a chiral compound by adding a substituent such as phosphate to either of the OCH2OH groups; that is, glycerol is prochiral. One unambiguous nomenclature for glycerol phosphate is the DL system (described on p. 77), in which the isomers are named according to their stereochemical relationships to glyceraldehyde isomers. By this system, the stereoisomer of glycerol phosphate found in most lipids is correctly named either L-glycerol 3-phosphate or D-glycerol 1-phosphate. Another way to specify stereoisomers is the stereospecific numbering (sn) system, in which C-1 is, by definition, that group of the prochiral compound that occupies the pro-S position. The common form of glycerol phosphate in phospholipids is, by this system, sn-glycerol 3-phosphate.
