Glycerol—325----------------------------phosphatides

Phosphatidic acids not only are intennediates in the biosynthesis of triacylglycerols but also are biosynthetic precursors of other members of a group of compounds called phosphoglycerides or glycerol phosphatides. Phosphorus-containing derivatives of lipids are known as phospholipids, and phosphoglycerides are one type of phospholipid. 

A second esterification of this type leads to a phosphatidate (enzyme l-acylglycerol-3-phosphate acyltransferase 2.3.1.51). Unsaturated acyl residues, particularly oleic acid, are usually incorporated at C-2 of the glycerol. Phosphatidates (anions of phosphatidic acids) are the key molecules in the biosynthesis of fats, phospholipids, and glycolipids. 

Baer, E. and Buchnea, D. (1959) Synthesis of L-a-(dioleoyl)-cephalin, with a comment on the stereochemical designation of glycerol phosphatides,./. Am. Chem. 

Wilson and Rinne (1976) have supplied soybean cotyledons with labeled acetate and labeled glycerol. Phosphatidic acid and DG turned over rapidly, as might be expected (half-times of 6 min). Half-times of 30-60 min were calculated for PC, PE, and Pi, but inspection of the data shows relatively little change in specific activity after 60 min. Moore (1977) examined phospholipid turnover in soybean suspension cultures and found much longer half-times than Wilson and Rinne (1976). Phosphatidylcholine labeled with choline had a half-life of 36 h, and PE labeled with ethanolamine decayed in three phases with half-lives of 12, 34, and 136 h. 

Baer, E. (1957) The synthesis of glycerol phosphatides in phosphoric esters and related compounds Chem. Soc. (Lond.) Spec. Publ. No. 8,103-113. 

Triacylglycerols arise not by acylation of glycerol itself but by a sequence of steps m which the first stage is acyl transfer to l glycerol 3 phosphate (from reduction of dihy droxyacetone 3 phosphate formed as described m Section 25 21) The product of this stage IS called a phosphatidic acid... 

Fig. 1. Chemical stmcture of phosphatidylcholine (PC) (1) and other related phosphohpids. R C O represents fatty acid residues. The choline fragment may be replaced by other moieties such as ethanolamine (2) to give phosphatidylethanolamine (PE), inositol (3) to give phosphatidylinositol (PI), serine (4), or glycerol (5). IfH replaces choline, the compound is phosphatidic acid (6). The corresponding lUPAC-lUB names ate (1), l,2-diacyl-t -glyceto(3)phosphocholine (2), l,2-diacyl-t -glyceto(3)phosphoethanolamine (3), 1,2-diacyl-t -glyceto(3)phosphoinositol (4), 1,2-diacyl-t -glyceto(3)phospho-L-serine and (5), l,2-diacyl-t -glyceto(3)phospho(3)-t -glycetol.

Phosphatidic acid, the parent compound for the glycerol-based phospholipids (Figure 8.4), consists of 5w-glycerol-3-phosphate, with fatty acids esterified at the T and 2-positions. Phosphatidic acid is found in small amounts in most natural systems and is an important intermediate in the biosynthesis of the more common glycerophospholipids (Figure 8.6). In these compounds, a... 

FIGURE 25.18 Synthesis of glycerolipids in eukaryotes begins with the formation of phosphatidic acid, which may be formed from dihydroxyace-tone phosphate or glycerol as shown. 

Phosphatidic acid is glycerol esterified at the sn-1 and sn-2 positions to two fatty acids and at the sn-3 position to phosphoric acid. It is a product of phospholipase D action that is also an intermediate in the biosynthesis of phosphatidylseiine and phosphatidylinositol. 

Triacylglycerols and some phosphoglycerols are synthesized by progressive acylation of glycerol 3-phosphate. The pathway bifurcates at phosphatidate, forming inositol phospholipids and cardiolipin on the one hand and triacylglycerol and choline and ethanolamine phospholipids on the other. 

Of the two major phosphohpid classes present in membranes, phosphoglycerides are the more common and consist of a glycerol backbone to which are attached two fatty acids in ester linkage and a phosphorylated alcohol (Figure 41-2). The fatty acid constiments are usually even-numbered carbon molecules, most commonly containing 16 or 18 carbons. They are unbranched and can be saturated or unsamrated. The simplest phosphoglyceride is phosphatidic acid, which is... 

Glycerol phosphate comes from glycerol (not in adipose) or from dihydroxyacetone phosphate (in liver and adipose). Nitrogen-containing phospholipids are made from diglyceride. Other phospholipids are made from phosphatidic acid. 

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