Involved in syntrophic growth of S.wolfei with butyrate, as part of the butyrate oxidation pathway. Catalyzes the oxidation of butanoyl-CoA to crotonyl-CoA. Probably passes the electrons released by this reaction on to electron-transfer flavoproteins (EtfAB) to finally generate hydrogen and/or formate. butanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = (2E)-butenoyl-CoA + reduced [electron-transfer flavoprotein] a short-chain 2,3-saturated fatty acyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = a short-chain (2E)-enoyl-CoA + reduced [electron-transfer flavoprotein] Lipid metabolism; butanoate metabolism. Highly expressed during syntrophic growth with butyrate (at protein level) (PubMed:19648244, PubMed:23468890). Seems to be constitutively expressed (PubMed:23468890). Belongs to the acyl-CoA dehydrogenase family.