The biotinyl 1.3S subunit serves as a carboxyl carrier between the substrate-binding sites on the 12S and 5S subunits. (S)-methylmalonyl-CoA + pyruvate = oxaloacetate + propanoyl-CoA 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).