Factors affecting the acyl selectivities of acyltransferases in Escherichia coli.

In Escherichia coli the synthesis of phosphatidic acid from glycerophosphate involves first the intermediate formation of 1-acyl glycerophosphate. This reaction is catalyzed by the membrane-bound acyl-CoA(acyl-ACP):glycerophosphate acyltransferase. The 1-acylglycerophosphate is then converted to phosphatidic acid by acyl-CoA(acyl-ACP):1-acylglycerophosphate acyltransferase (Okuyama, H., and Wakil, S.J. (1973) J. Biol. Chem. 248, 5197-5205). In vitro both acyltransferases utilize various saturated and unsaturated acyl-CoAs at comparable rates, resulting in the incorporation of both saturated and unsaturated fatty acids into position 1 as well as position 2 of the glycerophosphate moiety. Thus, the specificities of acyltransferase systems as compared with regard to the maximal velocities for various acyl-AoAs, do not explain the positional distribution of the individual fatty acid in phospholipid molecules. The selectivities of the acyltransferases for acyl-CoAs are variable depending upon the acceptor concentration. In the presence of both palmitoyl-CoA and oleoyl-CoA and at low concentrations of the acceptors comparable to those found in vivo, the acylation at position 1 of glycerophosphate by acyl-CoA:glycerophosphate acyltransferase showed higher preference for palmitate, whereas the acylation at position 2 by acyl-CoA:1-acylglycerophosphate acyltransferase showed higher selectivity for oleate. In the presence of saturating amounts of the acceptors, the acylation at position 1 or position 2 was less selective for the acyl-CoAs. The ratios of saturated acyl-CoA to unsaturated acyl-CoA also affect the ratios of the fatty acids incorporated in vitro into positions 1 and 2 of phosphatidic acid; relatively more palmitate was incorporated when the proportion of palmitoyl-CoA among the acyl donors was higher and vice versa. Thus, highly selective positioning of various acyl-CoAs observed at lower concentrations of the acceptors in phosphatidic acid synthesis in vitro helps to explain the selective distribution of saturated and unsaturated fatty acids at positions 1 and 2 of glycerophospholipids in the membranes. Another factor, the availability of acyl donors, affects the proportions of different molecular species of phosphatidic acid, which at least partly explains the variability of molecular species of phospholipids found in vivo.