Involved in the catabolism of amino acids. Uses isovaleryl-CoA as substrate. Minor activity detected with 2-methylpalmitoyl-CoA or 2-methylbutanoyl-CoA, but no activity with short- and medium-straight chain acyl-CoA esters or with 2-methylhexanoyl-CoA. 3-methylbutanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = 3-methyl-(2E)-butenoyl-CoA + reduced [electron-transfer flavoprotein] kcat is 0.98 sec(-1) per tetramer for Isovaleryl-CoA. Amino-acid degradation; L-leucine degradation; (S)-3-hydroxy-3-methylglutaryl-CoA from 3-isovaleryl-CoA: step 1/3. Homotetramer. Expressed in flowers and tubers. Was previously thought to be duplicated, but PubMed:15574432 showed that the protein encoded by the second gene (2MBCD) is a 2-methylbutanoyl-CoA dehydrogenase. Belongs to the acyl-CoA dehydrogenase family.