Part of an electron transfer flavoprotein involved in syntrophic growth of S.wolfei with butyrate. Probably receives electrons from butyryl-CoA dehydrogenases, and transfers them to the membrane-bound quinone oxidoreductase Swol_0698. Binds 1 FAD per subunit. Lipid metabolism; butanoate metabolism. Heterodimer of an alpha and a beta subunit. Highly expressed during syntrophic growth with butyrate (at protein level) (PubMed:19648244, PubMed:23468890). Seems to be constitutively expressed (PubMed:23468890). Belongs to the ETF alpha-subunit/FixB family.