Regioselectivity of H Cluster Oxidation

The H₂-evolving potential of [FeFe] hydrogenases is severely limited by the oxygen sensitivity of this class of enzymes. Recent experimental studies on hydrogenase from C. reinhardtii point to O₂-induced structural changes in the [Fe₄S₄] subsite of the H cluster. Here, we investigate the mechanistic basis of this observation by means of density functional theory. Unexpectedly, we find that the isolated H cluster shows a pathological catalytic activity for the formation of reactive oxygen species such as O₂^– and HO₂^–. After protonation of O₂^–, an OOH radical may coordinate to the Fe atoms of the cubane, whereas H₂O₂ specifically reacts with the S atoms of the cubane-coordinating cysteine residues. Both pathways are accompanied by significant structural distortions that compromise cluster integrity and thus catalytic activity. These results explain the experimental observation that O₂-induced inhibition is accompanied by distortions of the [Fe₄S₄] moiety and account for the irreversibility of this process