PATHWAYS OF ENERGY-CONSERVATION IN METHANOGENIC ARCHAEA

Methanogenic archaea are strictly anaerobic organisms that derive thei r metabolic energy from the conversion of a restricted number of subst rates to methane, H-2 + CO2 and formate are converted to CH4 via the C O2-reducing pathway, while methanol and methylamines are metabolized b y the methylotrophic pathway. A limited number of methanogenic organis ms utilize acetate by the aceticlastic pathway. Redox reactions involv ed in these processes are partly catalyzed by membrane-bound enzyme sy stems that generate or, in the case of endergonic reactions, use elect rochemical ion gradients. The H-2:heterodisulfide oxidoreductase, the F420H2:heterodisulfide oxido-reductase and the CO:heterodisulfide oxid oreductase, are novel systems that generate a proton motive force by r edox-potential-driven H+ translocation. The methyltetrahydromethanopte rin:coenzyme M methyltransferase is a unique, reversible sodium ion pu mp that couples methyl transfer with the transport of Na+ across the c ytoplasmic membrane. Formylmethanofuran dehydrogenase is a reversible ion pump that catalyzes formylation and deformylation, of methanofuran . In summary, the pathways are coupled to the generation of an electro chemical sodium ion gradient and an electrochemical proton gradient. B oth ion gradients are used directly for ATP synthesis via membrane int egral ATP synthases. The function of the above-mentioned systems and t heir components in the metabolism of methanogens are described in deta il.