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:9683657). 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 carboxylates acetyl-CoA, butyryl-CoA and succinyl-CoA (PubMed:9683657). ATP + hydrogencarbonate + propanoyl-CoA = (S)-methylmalonyl-CoA + ADP + H(+) + phosphate Binds 2 magnesium or manganese ions per subunit. Optimum pH is 7.0-7.5. Optimum temperature is 25-30 degrees Celsius. Metabolic intermediate metabolism; propanoyl-CoA degradation; succinyl-CoA from propanoyl-CoA: step 1/3. Dodecamer composed of six biotin-containing alpha subunits and six beta subunits. 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. During development the activity increased gradually with the maximum during the sporulation stage.