Hydrogenases and Alternative Energy Strategies

Abstract This chapter considers the hydrogenases, the enzymes that produce and consume molecular hydrogen; how they work and how they might be exploited in a future hydrogen‐based economy. Hydrogenases are enzymes containing iron and in some cases nickel, which efficiently couple the proton‐hydrogen couple to the reduction or oxidation of electron carriers. They have many metabolic functions in the microbial world. Much has been learned from recent research about their active sites and mechanism of action. There are three known types of hydrogenase chemistry: that based on a dinuclear nickel‐iron site (NiFe‐and NiFeSe‐hydrogenases); on more complex iron‐containing clusters (FeFe‐hydrogenases); and an enzyme containing a mononuclear iron site, which catalyzes a specific direct hydrogenation of a cofactor (Hmd). Variations on the NiFe‐hydrogenases are involved in the sensing of H2in cell regulation. Where necessary, hydrogenases have structural features that avoid the inhibitory effects of oxygen, carbon monoxide and other pollutant gases which poison other catalysts such as platinum. The structures of the different types of hydrogenases have been determined and their catalytic mechanisms investigated by spectroscopy and electrochemistry. Genetic and biochemical studies are elucidating the complex metabolic pathways by whereby the different types of hydrogenase catalytic centers are assembled. Three possible energy strategies are suggested by hydrogenase research: (1) hydrogen biotechnology, using microorganisms to produce hydrogen by fermentation of organic wastes, with or without the assistance of photosynthesis; (2) biomimicry, using insights from the structures and mechanisms of hydrogenases to synthesize improved catalysts as a substitute for the limited resources of precious metals; and (3) harnessing hydrogenases on carbon electrodes in electrolysis cells for H2production or fuel cells to produce electricity from hydrogen at low concentrations and with low overpotentials and at rates comparable to those with platinum.