In enzymology, a [acyl-carrier-protein] S-malonyltransferase () is an enzyme that catalyzes the chemical reaction malonyl-CoA + acyl carrier protein CoA + malonyl-[acyl-carrier-protein] Thus, the two substrates of this enzyme are malonyl-CoA and acyl carrier protein, whereas its two products are CoA and malonyl-acyl-carrier-protein. This enzyme belongs to the family of transferases, specifically those acyltransferases transferring groups other than aminoacyl groups. This enzyme participates in fatty acid biosynthesis. Nomenclature The systematic name of this enzyme class is malonyl-CoA:[acyl-carrier-protein] S-malonyltransferase. Other names in common use include malonyl coenzyme A-acyl carrier protein transacylase, [acyl carrier protein]malonyltransferase, FabD, malonyl transacylase, malonyl transferase, malonyl-CoA-acyl carrier protein transacylase, malonyl-CoA:ACP transacylase, malonyl-CoA:AcpM transacylase, malonyl-CoA:acyl carrier protein transacylase, MAT, and MCAT. Structure Crystal Structures of FabD from E.coli and Streptomyces coelicolor are known and provide great insight into the catalytic mechanism of FabD. In E.Coli, FabD primarily involved in FAS pathway. However, in Streptomyces coelicolor, FabD is involved in FAS and polyketide synthase pathways. In both cases, the structures and active sites are very similar. The protein has an α/β type architecture, but the fold is unique. the active site inferred from the location of the catalytic Ser92 contains a typical nucleophilic elbow as observed in α/ β hydrolases. Serine 92 is hydrogen bonded to His 201 in a fashion similar to various serine hydrolases. however, instead of the carboxylic acid typically found in catalytic triads, the main chain carbonyl of Gln 250 serves as a hydrogen bond acceptor in an interaction with His 201. Two other residues, Arg-117 and Glu-11 are also located in the active site, but their function is not clear. Function The fatty acid synthetic pathway is the principal route for the production of membrane phospholipid acyl chains in bacterial and plants. The reaction sequence is carried out by a series of individual soluble proteins that are each encoded by a discrete gene, and the pathway intermediates are shuttled between the enzymes. Malony-CoA:ACP Transacylase (FabD) is one such individual soluble protein and catalyzes the following reaction: malonyl-CoA + acyl carrier protein CoA + malonyl-[acyl-carrier-protein] The transfer of malonate to acyl-carrier-protein (ACP) converts the acyl groups into thioester forms which are characteristic of acyl intermediates in fatty acid synthesis and which are strictly required for the condensation reactions catalyzed by β-ketoacyl-ACP synthetase. Mechanism Malonyl-CoA:ACP Transacylase uses a ping-pong kinetic mechanism with a bound malonyl ester as the acyl intermediate attached to a serine residue residing