Methanogenesis involves direct hydride transfer from H2 to an organic substrate.

ABSTRACT

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.