Methanogens and the diversity of archaebacteria.

The bioenergetics of methanogenesis.

Methanogenic archaea are strictly anaerobic organisms that derive their metabolic energy from the conversion of a restricted number of substrates to methane. H2+CO2 and formate are converted to CH4 via the CO2-reducing pathway, while methanol and methylamines are metabolized by the methylotrophic pathway. A limited number of methanogenic organisms utilize acetate by the aceticlastic pathway. Redox reactions involved in these processes are partly catalyzed by membrane-bound enzyme systems that generate or, in the case of endergonic reactions, use electrochemical ion gradients. The H2:heterodisulfide oxidoreductase, the F420H2:heterodisulfide oxidoreductase and the CO:heterodisulfide oxidoreductase, are novel systems that generate a proton motive force by redox-potential-driven H+ translocation. The methyltetrahydromethanopterin:coenzyme M methyltransferase is a unique, reversible sodium ion pump that couples methyl transfer with the transport of Na+ across the cytoplasmic 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 electrochemical sodium ion gradient and an electrochemical proton gradient. Both ion gradients are used directly for ATP synthesis via membrane integral ATP synthases. The function of the above-mentioned systems and their components in the metabolism of methanogens are described in detail.

Pathways of energy conservation in methanogenic archaea

Nutritional requirements of the methanogenic archaebacteria

This chapter concentrates on the reactions and enzymes involved in methanogenesis from CO2 and H2. The coenzymes and electron carriers involved are described only as far as necessary for the understanding of their functions; they are dealt with extensively in other chapters. The bioenergetics of CO2 reduction to CH4 are only briefly discussed. For details the reader is referred to Chapter 8. We have tried to cover the complete literature dealing with the enzymology of methanogenesis from CO2 and H2. However, mainly work is cited which was performed with purified or at least partially purified preparations following the famous dictum of Efraim Racker: don’t waste clean thinking on dirty enzymes.

M. Bruschi

Papers

Electron transport components from methanogenic bacteria: The ferredoxin from Methanosarcinabarkeri (strain Fusaro)