Regioselectivity of H Cluster Oxidation

The H-2-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 O-2-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 O-2(-) and HO2-. After protonation of O-2(-), 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 O-2-induced inhibition is accompanied by distortions of the [Fe4S4] moiety and account for the irreversibility of this process.