ENERGETICS OF SYNTROPHIC FATTY-ACID OXIDATION

Fatty acids are key intermediates in methanogenic degradation of organ ic matter in sediments as well as in anaerobic reactors. Conversion of butyrate or propionate to acetate, (CO,), and hydrogen is endergonic under standard conditions, and becomes possible only at low hydrogen c oncentrations (10(-4)-10(-5) bar). A model of energy sharing between f ermenting and methanogenic bacteria attributes a maximum amount of abo ut 20 kJ per mol reaction to each partner in this syntrophic cooperati on system. This amount corresponds to synthesis of only a fraction (on e-third) of an ATP to be synthesized per reaction. Recent studies on t he biochemistry of syntrophic fatty acid-oxidizing bacteria have revea led that hydrogen release from butyrate by these bacteria is inhibited by a protonophore or the ATPase inhibitor DCCD (N,N'-dicyclohexyl car bodiimide), indicating that a reversed electron transport step is invo lved in butyrate or propionate oxidation. Hydrogenase, butyryl-CoA deh ydrogenase, and succinate dehydrogenase acitivities were found to be p artially associated with the cytoplasmic membrane fraction. Also glyco lic acid is degraded to methane and CO2 by a defined syntrophic cocult ure. Here the most difficult step for hydrogen release is the glycolat e dehydrogenase reaction (E(0) = -92 mV). Glycolate dehydrogenase, hyd rogenase, and ATPase were found to be membrane-bound enzymes. Membrane Vesicles produced hydrogen from glycolate only in the presence of ATP ; protonophores and DCCD inhibited this hydrogen release. This system provides a suitable model to study reversed electron transport in inte rspecies hydrogen transfer between fermenting and methanogenic bacteri a in methanogenic biomass degradation.