Methanogenesis involves direct hydride transfer from H₂ to an organic substrate

Certain anaerobic microorganisms evolved a mechanism to use H₂ as a reductant in their energy metabolisms. For these purposes, the microorganisms developed H₂-activating enzymes, which are aspirational catalysts in a sustainable hydrogen economy. In the case of the hydrogenotrophic pathway performed by methanogenic archaea, 8e^- are extracted from 4H₂ and used as reducing equivalents to convert CO₂ into CH₄. Under standard cultivation conditions, these archaea express [NiFe]-hydrogenases, which are Ni-dependent and Fe-dependent enzymes and heterolytically cleave H₂ into 2H⁺ and 2e^-, the latter being supplied into the central metabolism. Under Ni-limiting conditions, F₄₂₀-reducing [NiFe]-hydrogenases are downregulated and their functions are predominantly taken over by an upregulated [Fe]-hydrogenase. Unique in biology, this Fe-dependent hydrogenase cleaves H₂ and directly transfers H^- to an imidazolium-containing substrate. [Fe]-hydrogenase activates H₂ at an Fe cofactor ligated by two CO molecules, an acyl group, a pyridinol N atom and a cysteine thiolate as the central constituent. This Fe centre has inspired chemists to not only design synthetic mimics to catalytically cleave H₂ in solution but also for incorporation into apo-[Fe]-hydrogenase to give semi-synthetic proteins. This Perspective describes the enzymes involved in hydrogenotrophic methanogenesis, with a focus on those performing the reduction steps. Of these, we describe [Fe]-hydrogenases in detail and cover recent progress in their synthetic modelling.