Acyl Phosphatidylglycerol

The fragmentation pattern of acyl PG is very similar to that of A-acyl PE, but is more complicated than that of A-acyl PE due to the presence of the additional loss of the FA substituent of the third acyl chain as a FA acid [42, 57], which are as follows  

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Phosphatidylglycerol is also the main precursor of bis(monoacylglycero)-phosphate although diphosphatidylglycerol can also function thus. Acyl-phosphatidylglycerol is also found in small amounts in bacteria. Both these derivatives of phosphatidylglycerol have their acyl groups donated from other phospholipids rather than from acyl-CoA. 

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).

Glycerophospholipids contain a glycerol skeleton to which two fatty acids are esterified saturated fatty acids occupy mostly sn-position 1, whereas unsaturated fatty acids are mainly present on sn-position 2. The third hydroxyl is linked to a phosphate group to which an organic base is mostly esterified (Fig. 1). The most important components of soybean lecithin are phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). Phosphatidic acid (PA) may become important due to the presence of phospholipase D this enzyme slowly converts PC into PA in vegetable lecithins. Phosphatidylserine (PS), phosphatidylglycerol (PG), and lyso-phosphatidylcholine (LPC) are known as minor components lysophospholipids contain only one acyl group per molecule. Besides, ether phospholipids occur in which one or both fatty acyl... 

Lipid synthesis is unique in that it is almost exclusively localized to the surface of membrane structures. The reason for this restriction is the amphipathic nature of the lipid molecules. Phospholipids are biosynthesized by acylation of either glycerol-3-phosphate or dihydroxyacetone phosphate to form phosphatidic acid. This central intermediate can be converted into phospholipids by two different pathways. In one of these, phosphatidic acid reacts with CTP to yield CDP-diacylglycerol, which in bacteria is converted to phosphatidylserine, phosphatidylglycerol, or diphos-... 

Takayama, K., Qureshi, N., Mascagni, P., Nashed, M.A., Anderson, L., Raetz, C.R.H. Fatty acyl derivatives of glucosamine 1-phosphate in Escherichia coli and their relation to lipid A. Complete structure of A diacyl GlcN-1-P found in a phosphatidylglycerol-deficient mutant. J Biol Chem 258 (1983) 7379-7385. 

Many monolayer studies have been made with phospholipids and sterols which have helped to clarify the details of the molecular properties of both types of molecule required to bring about the so-called condensation effect [21-23,27]. After various speculations and suggestions, it was shown that a cis double bond at the 9,10 position of the acyl chain was not necessary for condensation, and that trans isomers and even fully saturated phospholipid could bring about this condensation effect [24]. The condensation effects are not restricted to particular phospholipid classes but have been demonstrated to occur with phosphatidylcholines and phosphatidylethanolamines as well as with phosphatidic acid [25], sphingomyelin, phosphatidylglycerol and phospholipid derivatives [26]. 

Fig. 9. Phospholipid turnover. The 1,2-diacylglyceroI kinase cycle involves the (1) transfer of the sn-1 -glycerol phosphate moiety from phosphatidylglycerol to MDO by the enzyme MdoB. (2) Diacylglycerol kinase converts the diacylglycerol to phosphatidic acid, which can regenerate the phosphatidylglycerol (see Fig. 6). Phosphatidylethanolamine cycling involves (3) the transfer of an acyl chain to membrane lipoprotein and (4) re-esterification of the 1-position by 2-acylgiycerophosphoethanolamine (Aas).

Wang, P. Y., Chen, J. W. and Hwang, F. (1993) Anisodamine causes acyl chain interdigitation in phosphatidylglycerol. FEBSLett. 332, 193-196. 

In view of the fact that leaf lipids are dominated by those of the chloroplasts, it is worth emphasizing certain features of the latter s membranes. In contrast to mammalian and most bacterial membranes phospholipids are minor components. Phos-phatidylglycerol is the only phosphoglyceride of importance. About 75% of the acyl lipid content is glycosylglyceride (Table 3.215). In addition, the fatty acid composition of chloroplast acyl lipids is unusually rich in polyunsaturates. a-Linolenic acid, the main fatty acid, is particularly enriched in the two galactosylglycerides. Phosphatidylglycerol also con-... 

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