Glycerolipids phosphatidic acid

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. 

Acetyl coenzyme A is the precursor of both cholesterol and fatty acids 39 Phosphatidic acid is the precursor of all glycerolipids 42 Sphingolipids are biosynthesized by adding head groups to the ceramide moiety 44... 

Figure 21-3 Major pathways of synthesis of fatty acids and glycerolipids in the green plant Arabidopsis. The major site of fatty acid synthesis is chloroplasts. Most is exported to the cytosol as oleic acid (18 1). After conversion to its coenzyme A derivative it is converted to phosphatidic acid (PA), diacylglycerol (DAG), and the phospholipids phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE). Desaturation also occurs, and some linoleic and linolenic acids are returned to the chloroplasts. See text also. From Sommerville and Browse.106 See also Figs. 21-4 and 21-5. Other abbreviations monogalactosyldiacylglycerol (MGD), digalactosyldiacylglycerol (DGD), sulfolipid (SL), glycerol 3-phosphate (G3P), lysophosphatidic acid (LPA), acyl carrier protein (ACP), cytidine diphosphate-DAG (CDP-DAG).

A recent report suggests that inhibition of PC synthesis constitutes one of the primary events by which C2-ceramide triggers apoptosis (Ramos et al., 2000). Treatment of cerebral granule neurons with C2-ceramide resulted in a rapid (within 1 hour) reduction in PC biosynthesis, whereas only 6 h after exposure to the agonists the first significant drop in cell viability was observed. The authors further showed that addition of exogenous PC resulted in a dose-dependent full prevention of neuronal death. This was specific for PC, because addition of other glycerolipids like, PE, PS, phosphatidylinositol and phosphatidic acid had no effect on C2-ceramide-induced apoptosis. 

In the plastids, acyltransferases provide a direct route for entrance of acyl groups from ACP to membrane lipids. Since this is the standard pathway for phosphatidic acid synthesis in E. coli and cyanobacteria, both the enzymes of phosphatidic acid synthesis in plastids and the glycerolipid backbones they produce are termed prokaryotic . In both chloroplasts and non-green plastids, the glycerol-3-phosphate acyltransferase is a soluble enzyme that, unlike the E. coli enzyme, shows preference for 18 1-ACP over 16 0-ACP. The lysophosphatidic acid acyltransferase, which is a component of the inner envelope of plastids, is extremely selective for 16 0-ACP. The presence of a 16-carbon fatty acid at the... 

In plants, glycerolipid biosynthesis involves a complex web of reactions distributed among multiple compartments [11,12]. As in mammals (Chapters 8 and 10), the synthesis of individual glycerolipids is initiated either by the formation of CDP-diacylglycerol from phosphatidic acid and CTP, or by cleavage of phosphate from phosphatidic acid to produce diacylglycerol. 

In liver and adipose tissne, triacylglycerols are produced by a pathway containing a phosphatidic acid intermediate (Fig. 33.20). Phosphatidic acid is also the precursor of the glycerolipids found in cell membranes and the blood lipoproteins. 

In plant seeds, glycerolipids can be synthesized in two similar pathways, which are known as prokaryotic and eukaryotic systems. These pathways are localized in different subcellular compartments and are characterized by a similar two-stage enzymatic conversion of 5 n-glycero-3-phosphate (G3P) into phosphatidic acid (PA), but different further conversions of PA into structural, storage, or signaling lipids [65]. Despite the closeness of these two biosynthetic pathways, enzymes that catalyze acylation reactions are unique to each system. 

Large quantities of docosahexaenoate are present in all retina glycerolipids, due to the contribution of lipids from the outer segments of photoreceptors, which are known to be enriched in this fatty acid. However, the same holds for diacylglycerols and phosphatidic acid, which likely are not concentrated in these structures. The unusual composition of these intermediates suggests that an important proportion of toad retinal lipid metabolism may be involved in supporting photoreceptor lipid composition. The concentration of both DG and PA is 4-5 fold higher in the toad than in the cattle retina (Aveldaho Bazan, 1974 and unpublished). 

A as the acyl carrier. These observations indicate that the precursor of the glycerolipids is phosphatidic acid which is converted to glyco-, sulfo- and phospholipids in this cyanobacterium. 

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