Reactivation of the Ready and Unready Oxidized States of [NiFe]-Hydrogenases: Mechanistic Insights from DFT Calculations.

The apparently simple dihydrogen formation from protons and electrons (2H + 2e ⇄ H) is one of the most challenging reactions in nature. It is catalyzed by metalloenzymes of amazing complexity, called hydrogenases. A better understanding of the chemistry of these enzymes, especially that of the [NiFe]-hydrogenases subgroup, has important implications for production of H as alternative sustainable fuel. In this work, reactivation mechanism of the oxidized and inactive Ni-B and Ni-A states of the [NiFe]-hydrogenases active site has been investigated using density functional theory. Results obtained from this study show that one-electron reduction and protonation of the active site promote the removal of the bridging hydroxide ligand contained in Ni-B and Ni-A. However, this process is sufficient to activate only the Ni-B state. H binding to the active site is required to convert Ni-A to the active Ni-SI state. Here, we also propose a reasonable structure for the spectroscopically well-characterized Ni-SI and Ni-SU species, formed respectively from the one-electron reduction of Ni-B and Ni-A. Ni-SI, depending on the pH at which the reaction occurs, features a bridging hydroxide ligand or a water molecule terminally coordinated to the Ni atom, whereas in Ni-SU a water molecule is terminally coordinated to the Fe atom, and the Cys64 residue is oxidized to sulfenate. The sulfenate oxygen atom in the Ni-A state affects the stereoelectronic properties of the binuclear cluster by modifying the coordination geometry of Ni, and consequently, by switching the regiochemistry of HO and H binding from the Ni to the Fe atom. This effect is predicted to be at the origin of the different reactivation kinetics of the oxidized and inactive Ni-B and Ni-A states.