Involved in degradation of the branched-chain amino acids, phytol and lysine for the supply of carbon and electrons to the ETF/ETFQO complex during dark-induced sugar starvation. 3-methylbutanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = 3-methyl-(2E)-butenoyl-CoA + reduced [electron-transfer flavoprotein] Amino-acid degradation; L-leucine degradation; (S)-3-hydroxy-3-methylglutaryl-CoA from 3-isovaleryl-CoA: step 1/3. Homodimer. Expressed in leaves, stems and flowers. Not detected in roots. No visible phenotype under normal growth conditions. Belongs to the acyl-CoA dehydrogenase family.