Evidence of reversed electron transport in syntrophic butyrate or benzoate oxidation by Syntrophomonas wolfei and Syntrophus buswellii

Syntrophomonas wolfei and Syntrophus buswellii were grown with butyrate or benzoate in a defined binary coculture with Methanospirillum hungatei. Both strains also grew independent of the partner bacteria with crotonate as substrate. Localization of enzymes involved in butyrate oxidation by S. wolfei revealed that ATP synthase, hydrogenase, and butyryl-CoA dehydrogenase were at least partially membrane-associated whereas 3-hydroxybutyryl-CoA dehydrogenase and crotonase were entirely cytoplasmic. Inhibition experiments with copper chloride indicated that hydrogenase faced the outer surface of the cytoplasmic membrane. Suspensions of butyrate-or benzoate-grown cells of either strain accumulated hydrogen during oxidation of butyrate or benzoate to a low concentration that was thermodynamically in equilibrium with calculated reaction energetics. The protonophore carbonylcyanide m-chlorophenyl-hydrazone (CCCP) and the proton-translocating ATPase inhibitor N,N′dicyclohexylcarbodiimide (DCCD) both specifically inhibited hydrogen formation from butyrate or benzoate at low concentrations, whereas hydrogen formation from crotonate was not affected. A menaquinone was extracted from cells of S. wolfei and S. buswellii grown syntrophically in a binary methanogenic culture. The results indicate that a proton-potential-driven process is involved in hydrogen release from butyrate or benzoate oxidation.