Short and branched chain specific acyl-CoA dehydrogenase that catalyzes the removal of one hydrogen from C-2 and C-3 of the fatty acyl-CoA thioester, resulting in the formation of trans-2-enoyl-CoA. Among the different mitochondrial acyl-CoA dehydrogenases, acts specifically on short and branched chain acyl-CoA derivatives such as (S)-2-methylbutyryl-CoA as well as short straight chain acyl-CoAs such as butyryl-CoA (By similarity). Plays an important role in the metabolism of L-isoleucine by catalyzing the dehydrogenation of 2-methylbutyryl-CoA, one of the steps of the L-isoleucine catabolic pathway (By similarity). Can also act on valproyl-CoA, a metabolite of the valproic acid drug (By similarity). 2-methylbutanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = (2E)-2-methylbut-2-enoyl-CoA + reduced [electron-transfer flavoprotein] (2S)-2-methylbutanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = (2E)-2-methylbut-2-enoyl-CoA + reduced [electron-transfer flavoprotein] (2R)-2-methylbutanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = ethylacryloyl-CoA + reduced [electron-transfer flavoprotein] butanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = (2E)-butenoyl-CoA + reduced [electron-transfer flavoprotein] 2-methylpropanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = (2E)-2-methylpropenoyl-CoA + reduced [electron-transfer flavoprotein] H(+) + hexanoyl-CoA + oxidized [electron-transfer flavoprotein] = (2E)-hexenoyl-CoA + reduced [electron-transfer flavoprotein] H(+) + oxidized [electron-transfer flavoprotein] + valproyl-CoA = (2E)-2-propylpent-2-enoyl-CoA + reduced [electron-transfer flavoprotein] Lipid metabolism; mitochondrial fatty acid beta-oxidation. Amino-acid degradation; L-isoleucine degradation. Homotetramer. Belongs to the acyl-CoA dehydrogenase family.