FUNCTION Catalyzes the conversion of isovaleryl-CoA/3-methylbutanoyl-CoA to 3-methylbut-2-enoyl-CoA as an intermediate step in the leucine (Leu) catabolic pathway (PubMed:7640268). To a lesser extent, is also able to catalyze the oxidation of other saturated short-chain acyl-CoA thioesters as pentanoyl-CoA, hexenoyl-CoA and butenoyl-CoA (PubMed:7640268).CATALYTIC ACTIVITY 3-methylbutanoyl-CoA + oxidized [electron-transfer flavoprotein] + H(+) = 3-methylbut-2-enoyl-CoA + reduced [electron-transfer flavoprotein]CATALYTIC ACTIVITY pentanoyl-CoA + oxidized [electron-transfer flavoprotein] + H(+) = (2E)-pentenoyl-CoA + reduced [electron-transfer flavoprotein]CATALYTIC ACTIVITY hexanoyl-CoA + oxidized [electron-transfer flavoprotein] + H(+) = (2E)-hexenoyl-CoA + reduced [electron-transfer flavoprotein]CATALYTIC ACTIVITY butanoyl-CoA + oxidized [electron-transfer flavoprotein] + H(+) = (2E)-butenoyl-CoA + reduced [electron-transfer flavoprotein]COFACTOR Amino-acid degradation; L-leucine degradation; (S)-3-hydroxy-3-methylglutaryl-CoA from 3-isovaleryl-CoA: step 1/3.SUBUNIT Homotetramer.INTERACTION The disease is caused by variants affecting the gene represented in this entry.SIMILARITY Belongs to the acyl-CoA dehydrogenase family.CAUTION It is uncertain whether Met-1 or Met-4 is the initiator.SEQUENCE CAUTION Truncated N-terminus.SEQUENCE CAUTION Truncated N-terminus.SEQUENCE CAUTION Truncated N-terminus.SEQUENCE CAUTION Truncated N-terminus.