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Abstract

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.