FUNCTION Carboxyltransferase 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.CATALYTIC ACTIVITY 3-methylbut-2-enoyl-CoA + hydrogencarbonate + ATP = 3-methyl-(2E)-glutaconyl-CoA + ADP + phosphate + H(+)BIOPHYSICOCHEMICAL PROPERTIES kcat is 4.0 sec(-1).PATHWAY Amino-acid degradation; L-leucine degradation; (S)-3-hydroxy-3-methylglutaryl-CoA from 3-isovaleryl-CoA: step 2/3.SUBUNIT Probably a dodecamer composed of six biotin-containing alpha subunits (MCCC1) and six beta (MCCC2) subunits.INTERACTION The disease is caused by variants affecting the gene represented in this entry.SIMILARITY Belongs to the AccD/PCCB family.