Phospholipid synthesis in Escherichia coli. Characteristics of fatty acid transfer from acyl-acyl carrier protein to sn-glycerol 3-phosphate.

Two kinetically distinguishable sn-glycerol 3-phosphate (glycerol-P) acryltransferase activities were detected in Escherichia coli inner membranes using acyl-acyl carrier protein (ACP) substrates. The first system was characterized as having a Michaelis constant (Km) for glycerol-P of 90 microM and utilized palmitoyl-ACP to form primarily 1-acylglycerol-P. Palmitoyl-CoA and cis-vaccenoyl-ACP were also utilized by this system but, with these substrates, significantly more phosphatidic acid was formed as compared to palmitoyl-ACP. Although palmitoyl-ACP and palmitoyl-CoA had kinetically indistinguishable glycerol-P sites, distinct acyl donor binding sites were inferred from kinetic experiments using acyl carrier protein as an acyltransferase inhibitor. A second enzyme system, characterized as having a Km for glycerol-P of 700 microM, was found using palmitoyl-ACP as a substrate. This acyltransferase had a slightly higher pH optimum than the low Km acyltransferase activity, and phosphatidic acid was the major product. Two degradative reactions were identified in this system. One reaction yielded diacylglycerol when palmitoyl-ACP was the substrate. The other degradative reaction produced glycerol. Glycerol was formed in all incubations but was most pronounced when palmitoyl-ACP was the substrate.