Biotin-attachment subunit of the 3-methylcrotonyl-CoA carboxylase, an enzyme that catalyzes the conversion of 3-methylcrotonyl-CoA to 3-methylglutaconyl-CoA, a critical step for leucine and isovaleric acid catabolism. 3-methyl-(2E)-butenoyl-CoA + ATP + hydrogencarbonate = 3-methyl-(2E)-glutaconyl-CoA + ADP + H(+) + phosphate Amino-acid degradation; L-leucine degradation; (S)-3-hydroxy-3-methylglutaryl-CoA from 3-isovaleryl-CoA: step 2/3. Probably a dodecamer composed of six biotin-containing alpha subunits (MCCC1) and six beta (MCCC2) subunits (PubMed:17360195). Interacts (via the biotin carboxylation domain) with SIRT4 (PubMed:23438705). Acetylated. The disease is caused by variants affecting the gene represented in this entry. Extended N-terminus.