Structural Basis of Hydrogenotrophic Methanogenesis.

Most methanogenic archaea use the rudimentary hydrogenotrophic pathway-from CO and H 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 C species to the C carriers methanofuran, tetrahydromethanopterin, and coenzyme M at different oxidation states. The four reduction reactions from CO to methane involve one molybdopterin-based two-electron reduction, two coenzyme F-based hydride transfers, and one coenzyme F-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 CO reduction and fixation. Here, we review the knowledge about the structure-based catalytic mechanism of each enzyme of hydrogenotrophic methanogenesis.