Most methanogenic archaea use the rudimentary hydrogenotrophic pathway—from CO₂ and H₂ to methane—as the terminal step of microbial biomass degradation in anoxic habitats. The barely exergonic process that just conserves sufficient energy for a modest lifestyle involves chemically challenging reactions catalyzed by complex enzyme machineries with unique metal-containing cofactors. The basic strategy of the methanogenic energy metabolism is to covalently bind C₁ species to the C₁ carriers methanofuran, tetrahydromethanopterin, and coenzyme M at different oxidation states. The four reduction reactions from CO₂ to methane involve one molybdopterin-based two-electron reduction, two coenzyme F₄₂₀–based hydride transfers, and one coenzyme F₄₃₀–based radical process. For energy conservation, one ion-gradient-forming methyl transfer reaction is sufficient, albeit supported by a sophisticated energy-coupling process termed flavin-based electron bifurcation for driving the endergonic CO₂ reduction and fixation. Here, we review the knowledge about the structure-based catalytic mechanism of each enzyme of hydrogenotrophic methanogenesis.

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