(2E)-butenoyl-CoA + reduced [electron-transfer flavoprotein] => butanoyl-CoA + oxidized [electron-transfer flavoprotein] + H(+)
(2e)-butenoyl-coa + reduced [electron-transfer flavoprotein] => butanoyl-coa + oxidized [electron-transfer flavoprotein] + h(+)
(2E)-butenoyl-CoA + reduced [electron-transfer flavoprotein] => butanoyl-CoA + oxidized [electron-transfer flavoprotein] + H(+)
(2e)-butenoyl-coa + reduced [electron-transfer flavoprotein] => butanoyl-coa + oxidized [electron-transfer flavoprotein] + h(+)