The delta subunit of malonate decarboxylase serves as an an acyl-carrier protein (ACP) and contains the prosthetic group 2-(5-triphosphoribosyl)-3-dephospho-CoA. Two reactions are involved in the production of the holo-ACP form of this enzyme. The first reaction is catalysed by EC 220.127.116.11, triphosphoribosyl-dephospho-CoA synthase. The resulting prosthetic group is then attached to the ACP subunit via a phosphodiester linkage to a serine residue, thus forming the holo form of the enzyme, in a manner analogous to that of EC 18.104.22.168, citrate lyase holo-[acyl-carrier protein] synthase.
MdcG forms a strong complex with the 2'-(5''-triphosphoribosyl)-3'-dephospho-CoA prosthetic group precursor. This complex is called MdcGi. Upon incubation of MdcGi with apo acyl carrier protein, holo acyl carrier protein is synthesized by forming the phosphodiester bond between the 2'-(5''-phosphoribosyl)-3'-dephospho-CoA prosthetic group and Ser25 of the protein. In absence of the prosthetic group precursor, MdcG catalyzes at a low rate the adenylylation of apo acyl carrier protein using ATP as substrate. The adenylyl ACP thus formed is an unphysiological side product and is not involved in the biosynthesis of holo ACP