Reverse methanogenesis

Hallam SJ, N Putnam, CM Preston, JC Detter, D Rokhsar, PM Richardson, EF DeLong (2004) Reverse methanogenesis testing the hypothesis with environmental genomics. Science 305 1457-1462. 

A modification of the reverse methanogenesis hypothesis was offered by Valentine and Reeburg (2000). This reaction would yield twice the amount of energy normally associated with anaerobic CH4 oxidation (reaction (6)), making it more thermodynamically favorable ... 

This enzyme catalyzes reduction of CO2 to formyl-MF (Reaction 48) during methanogenesis from CO2, and the oxidation of formyl-MF (the reverse of Reaction 48) during methylotrophic methanogenesis [184] ... 

Several portions of the pathway are still unclear. How the methyl of methanol enters the reversed methanogenic pathway is unknown, since some suggest it may not proceed via methyl-CoM nonetheless, evidence is clear that most of the reversed H2-CO2 path is used. Also, our understanding of methyl-transfer reactions at several portions of the pathways remains incomplete, e.g. the way in which methanol is reduced to methane, the enzymes involved in methyl transfer in CO2 methanogenesis, and routes of nonmethanol methyl-substrate entry into the path. In several cases, the source of electrons for a reductive step is unknown, e.g. the heterodisulfide reductase and formyl-methanofuran dehydrogenase steps. 

Fig. 12. Proposed role of methyl-tetrahydromethanopterin (CH3-H4MPT) coenzymeM (H-S-CoM) methyltransferase in methanogenesis from CO2, acetate and methanol methyltransferase as a reversible Na -translocating membrane protein. During CH4 formation from CO2 and from acetate, methyltransferase is involved in primary Na" extrusion generating A/INa during methanol disproportionation to CH4 and CO2 the enzyme is involved in A/iNa -driven methanol oxidation.

Chlorobiaceae photosynthesis is via reversal of the citric acid cycle tchemotrophes include sulphur and ammonium oxidizing bacteria (see Box 1.7) teach additional heterotrophic step involves a similar fractionation methanogenesis generally leads to a fractionation of c.40%o in freshwater conditions but c.70%o under marine conditions. 

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