This is one of the 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. 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 transfers the carboxyl group from carboxylated biotin to propionyl-CoA (By similarity). Propionyl-CoA carboxylase is involved in propionate utilization and in the production of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV), which is a water-insoluble biopolymer used as intracellular energy reserve material when cells grow under conditions of nutrient limitation (PubMed:25398867). Propionyl-CoA carboxylase is also able to catalyze the carboxylation of acetyl-CoA (PubMed:25398867). ATP + hydrogencarbonate + propanoyl-CoA = (S)-methylmalonyl-CoA + ADP + H(+) + phosphate ATP + hydrogencarbonate + N(6)-biotinyl-L-lysyl-[protein] = ADP + H(+) + N(6)-carboxybiotinyl-L-lysyl-[protein] + phosphate Binds 2 magnesium or manganese ions per subunit. Metabolic intermediate metabolism; propanoyl-CoA degradation; succinyl-CoA from propanoyl-CoA: step 1/3. The propionyl coenzyme A carboxylase (PCC) complex is composed of three subunits: PccA (biotin carboxylase and biotin-carboxyl carrier), PccB (carboxyltransferase) and PccX. Consists of an N-terminal biotin carboxylation/carboxylase (BC) domain that catalyzes the transient carboxylation of the biotin covalently attached to the C-terminal biotinyl-binding/biotin carboxyl carrier (BCC) domain.