The enoyl-[acyl-carrier-protein] reductase (FabI) of Escherichia coli, which catalyzes a key regulatory step in fatty acid biosynthesis, accepts NADH and NADPH as cofactors and is inhibited by palmitoyl-CoA.

Reduction of enoyl-acyl-carrier-protein (ACP) substrates by enoyl-ACP reductase is a key regulatory step in fatty acid elongation of Escherichia coli. Two enoyl-ACP reductase activities have been described in E. coli, one specific for NADH, the other for NADPH as cofactor. Because of their distinct enzymatic properties, these activities were ascribed to two different proteins. The NADH-dependent enoyl-ACP reductase of E. coli has previously been identified as the FabI protein, which is the target of a group of antibacterial compounds, the diazaborines. We now demonstrate that both enoyl-ACP reductase activities reside in FabI. In crude cell extracts of FabI-overproducing strains, both NADH-dependent and NADPH-dependent enoyl-ACP reductase activities are increased. Mutations in the fabI gene that lead either to temperature-sensitive growth or diazaborine resistance result in the reduction of both activities. When FabI is purified in pH 6.5 buffers, the protein exhibits NADH-dependent and NADPH-dependent reductase activities. Both enzymatic activities are inhibited by diazaborine. The NADPH-dependent enoyl-ACP reductase activity, however, turned out to be approximately eight times more resistant to diazaborine. The difference in sensitivity indicates that binding of either NADPH or NADH to FabI results in distinct changes in the configuration of the protein or, alternatively, it is different due to the different charge of the cofactors. These effects might be responsible for the differences in the enzymatic properties. Both reductase activities of the FabI protein are inhibited by physiologically relevant concentrations of palmitoyl-CoA, which might be important in regulating endogenous fatty acid biosynthesis in E. coli in the presence of exogenous fatty acids.