The 12S subunit specifically catalyzes the transfer of the carboxyl group of methylmalonyl CoA to the biotin of the 1.3S subunit forming propanoyl-CoA and carboxylated 1.3S-biotin. (S)-methylmalonyl-CoA + pyruvate = oxaloacetate + propanoyl-CoA Homohexamer (PubMed:8366018, PubMed:12743028). Transcarboxylase is composed of three subunits: 1.3S, 5S, and 12S. The core of the enzyme is composed of six 12S subunits. On each side of the core there are three pairs of 5S subunits. Each 5S dimer is attached to the core by two 1.3S subunits. Thus the total number of chains is 30 (6 + 12 + 12) (Probable).