Gamma subunit of the biotin-dependent malonate decarboxylase multienzyme complex (EC 7.2.4.4). The two subunits MADC and MADD are required for the transfer of the malonate carboxy group from the acyl-carrier protein (ACP) to the prosthetic group of the biotin carrier MADF. Required for the regeneration of ACP. malonyl-[ACP] + N(6)-biotinyl-L-lysyl-[protein] = acetyl-[ACP] + N(6)-carboxybiotinyl-L-lysyl-[protein]