How oxygen reacts with oxygen-tolerant respiratory [NiFe]-hydrogenases

An oxygen-tolerant respiratory [NiFe]-hydrogenase is proven to be a four-electron hydrogen/oxygen oxidoreductase, catalyzing the reaction 2 H₂ + O₂ = 2 H₂O, equivalent to hydrogen combustion, over a sustained period without inactivating. At least 86% of the H₂O produced by Escherichia coli hydrogenase-1 exposed to a mixture of 90% H₂ and 10% O₂ is accounted for by a direct four-electron pathway, whereas up to 14% arises from slower side reactions proceeding via superoxide and hydrogen peroxide. The direct pathway is assigned to O₂ reduction at the [NiFe] active site, whereas the side reactions are an unavoidable consequence of the presence of low-potential relay centers that release electrons derived from H₂ oxidation. The oxidase activity is too slow to be useful in removing O₂ from the bacterial periplasm; instead, the four-electron reduction of molecular oxygen to harmless water ensures that the active site survives to catalyze sustained hydrogen oxidation.