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 (PubMed:9428714). The alpha subunit catalyzes the Na(+)-independent carboxyltransfer from methylmalonyl-CoA to the prosthetic biotin group located on the gamma subunit (By similarity). (S)-methylmalonyl-CoA + H(+)(out) + Na(+)(in) = CO2 + Na(+)(out) + propanoyl-CoA The methylmalonyl-CoA decarboxylase is composed of four subunits: the carboxyltransferase alpha subunit (MmdA), the tunnel beta subunit (MmdB), the biotin-containing gamma subunit (MmdC) and the delta subunit (MmdD). Belongs to the AccD/PCCB family.