Carboxyltransferase subunit of the sodium ion pump methylmalonyl-CoA decarboxylase, which converts the chemical energy of a decarboxylation reaction into an electrochemical gradient of Na(+) ions across the cytoplasmic membrane, thereby creating a sodium ion motive force that is used for ATP synthesis. The alpha subunit catalyzes the Na(+)-independent carboxyltransfer from methylmalonyl-CoA to the prosthetic biotin group located on the gamma subunit (PubMed:7070502, PubMed:6852015, PubMed:3609308, PubMed:2920730, PubMed:1991479, PubMed:7601825). Can also convert malonyl-CoA into acetyl-CoA (PubMed:6852015, PubMed:2920730). (S)-methylmalonyl-CoA + H(+)(out) + Na(+)(in) = CO2 + Na(+)(out) + propanoyl-CoA Completely inhibited by avidin. Optimum pH is 6.4-7.0. The methylmalonyl-CoA decarboxylase is composed of five subunits: the carboxyltransferase alpha subunit (MmdA), the tunnel beta subunit (MmdB), the biotin-containing gamma subunit (MmdC), and the delta (MmdD) and epsilon (MmdE) subunits (PubMed:8227015, PubMed:7601825). Interacts with the gamma subunit (PubMed:7601825). Belongs to the AccD/PCCB family.