Oxidoreductase involved in syntrophic growth of S.wolfei with butyrate. Is presumed to link the electron flow from butyryl-CoA dehydrogenases to the membrane, in conjunction with the electron transfer flavoprotein EtfAB. May transfer electrons to the menaquinone pool of the membrane. Binds 2 [4Fe-4S] clusters per subunit. Lipid metabolism; butanoate metabolism. Might constitute a membrane-associated complex with EtfA (Swol_0697), EtfB (Swol_0696), and the butyryl-CoA dehydrogenase Swol_1933 and/or Swol_2052. Highly expressed during syntrophic growth with butyrate (at protein level) (PubMed:19648244, PubMed:23468890). Seems to be constitutively expressed (PubMed:23468890).