This is one of the 2 subunits of the biotin-dependent propionyl-CoA carboxylase (PCC), a mitochondrial enzyme involved in the catabolism of odd chain fatty acids, branched-chain amino acids isoleucine, threonine, methionine, and valine and other metabolites. Propionyl-CoA carboxylase catalyzes the carboxylation of propionyl-CoA/propanoyl-CoA to D-methylmalonyl-CoA/(S)-methylmalonyl-CoA (By similarity). 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 also significantly acts on butyryl-CoA/butanoyl-CoA, which is converted to ethylmalonyl-CoA/(2S)-ethylmalonyl-CoA (By similarity). Other alternative minor substrates include (2E)-butenoyl-CoA/crotonoyl-CoA (By similarity). ATP + hydrogencarbonate + propanoyl-CoA = (S)-methylmalonyl-CoA + ADP + H(+) + phosphate ATP + butanoyl-CoA + hydrogencarbonate = (2S)-ethylmalonyl-CoA + ADP + H(+) + phosphate Binds 2 magnesium or manganese ions per subunit. Metabolic intermediate metabolism; propanoyl-CoA degradation; succinyl-CoA from propanoyl-CoA: step 1/3. The holoenzyme is a dodecamer composed of 6 alpha subunits and 6 beta subunits (By similarity). Interacts with sir-2.2 and sir-2.3 (PubMed:23438705). 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. The biotin cofactor is covalently attached to the C-terminal biotinyl-binding domain and is required for the catalytic activity.