This is one of the 2 subunits of the biotin-dependent propionyl-CoA carboxylase (PCC), the enzyme catalyzing the carboxylation of propionyl-CoA/propanoyl-CoA to D-methylmalonyl-CoA/(S)-methylmalonyl-CoA (PubMed:20725044). Within the holoenzyme, the alpha subunit catalyzes the ATP-dependent carboxylation of the biotin carried by the biotin carboxyl carrier (BCC) domain, while the beta subunit then tranfers the carboxyl group from carboxylated biotin to propionyl-CoA (Probable). ATP + hydrogencarbonate + propanoyl-CoA = (S)-methylmalonyl-CoA + ADP + H(+) + phosphate Metabolic intermediate metabolism; propanoyl-CoA degradation; succinyl-CoA from propanoyl-CoA: step 1/3. The holoenzyme is a dodecamer composed of 6 PccA/alpha subunits and 6 PccB/beta subunits. The beta subunit contains the carboxyl transferase (CT) domain. Belongs to the AccD/PCCB family.