The methylcrotonoyl-CoA carboxylase complex is a biotin-dependent carboxylase that catalyzes the fourth step in the leucine catabolic pathway. It carboxylates 3-methylcrotonyl-CoA at the 4-carbon to form 3-methylglutaconyl-CoA. It is a heteromeric mitochondrial enzyme composed of multiple copies of biotin-containing alpha subunits and smaller beta subunits. (Adapted from PMID: 11181649.)
