MECHANISM OF H-H ACTIVATION BY NICKEL-IRON HYDROGENASE

DFT quantum chemical methods are used to probe the mechanism of the ni ckel-iron hydrogenases. Starting from the experimental X-ray structure , all plausible oxidation states and spin states were investigated, Th e structure and reactivity pattern of the NiFe cluster are best reprod uced by assuming a NiFe(II,III) oxidation state assignment of the rest ing state of the cluster. In our proposed mechanism of H-2 oxidation b y the enzyme, H-2 first binds to Fe in the form of a molecular hydroge n complex, which then undergoes heterolytic splitting. This process is spin-dependent and does not occur for the high-spin sextet state. In the key step, hydride transfer to iron and proton transfer to the adja cent cysteinethiolate ligand is accompanied by decoordination of the p rotonated cysteinethiol from Ni while remaining bound to iron. Simulta neously, the cyanide ligand on iron binds with the nickel atom in a ra re bridging binding mode. After the H-2 dissociation, the hydride boun d to Fe can then be transferred to Ni which should be a necessary prel iminary for subsequent hydrogen atom or electron transport. The transi tion state for hydrogen splitting was located, and the resulting calcu lated energy barrier is in remarkably good agreement with the experime ntal value.