malonylCoAACP

Malonyl-CoA-ACP transacylase, also called FabD, is an enzyme that catalyzes the transfer of the malonyl group from malonyl-CoA to the 4'-phosphopantetheine arm of the acyl carrier protein (ACP) used in fatty acid biosynthesis. This reaction produces malonyl-ACP, the key substrate for the elongation steps carried out by the type II fatty acid synthase (FAS II) system found in bacteria and plant plastids.

FabD typically operates through a ping-pong mechanism characteristic of acyltransferases, using an active-site serine to form a transient malonyl-enzyme intermediate, which then transfers the malonyl group to ACP and releases CoA. The enzyme otherwise acts as a soluble cytosolic protein, with molecular features that are conserved across diverse bacterial species, and homologues are present in plant chloroplasts.

In the Escherichia coli fatty acid synthesis operon, fabD encodes this enzyme. Across bacteria, FabD is generally around 250–320 amino acids in length and shares structural motifs of the α/β hydrolase superfamily, though specific details vary. Its activity is coordinated with the availability of ACP and other FAS II enzymes such as ketoacyl-ACP synthases to ensure efficient production of saturated and unsaturated fatty acids.

Significance: Malonyl-CoA-ACP transacylase is essential for continuous fatty acid production in organisms employing FAS II and is a focal point of investigations into antibiotic development, metabolic engineering, and plant lipid biosynthesis. See also fatty acid biosynthesis, acyl carrier protein, and FabD.