Two types of malonate decarboxylase are currently known, both of which form multienzyme complexes. This enzyme is a cytosolic protein that is biotin-independent. The other type is a biotin-dependent, Na+-translocating enzyme that includes both soluble and membrane-bound components (cf. EC 7.2.4.4, biotin-dependent malonate decarboxylase). As free malonate is chemically rather inert, it has to be activated prior to decarboxylation. In both enzymes, this is achieved by exchanging malonate with an acetyl group bound to an acyl-carrier protiein (ACP), to form malonyl-ACP and acetate, with subsequent decarboxylation regenerating the acetyl-ACP. The ACP subunit of both enzymes differs from that found in fatty-acid biosynthesis by having phosphopantethine attached to a serine side-chain as 2-(5-triphosphoribosyl)-3-dephospho-CoA rather than as phosphopantetheine 4'-phosphate. The individual enzymes involved in carrying out the reaction of this enzyme complex are EC 2.3.1.187 (acetyl-S-ACP:malonate ACP transferase), EC 2.3.1.39 ([acyl-carrier-protein] S-malonyltransferase) and EC 4.1.1.87 (malonyl-S-ACP decarboxylase). The carboxy group is lost with retention of configuration [6].
Identification of the active site of phosphoribosyl-dephospho-coenzyme A transferase and relationship of the enzyme to an ancient class of nucleotidyltransferases.
