THE ROLE OF NICKEL IN HYDROGENASES - IMPLICATIONS FOR A HETERODINUCLEAR ACTIVE-SITE

Hydrogenases (H(2)ases) are enzymes that catalyze the reversible two-e lectron redox chemistry of H-2. Most (but not all) H(2)ases are now kn own to contain Ni, which is intimately associated with unusual epr sig nals that are characteristic of the enzymes. A recent crystal structur e of the H(2)ase from Desulfovibrio gigas reveals that the Ni in these enzymes is not an isolated metal center, but part of a Ni,Fe dinuclea r cluster. The structure of the biological Ni,Fe cluster is compared w ith those of model complexes, and the redox chemistry of the model sys tems is examined for mechanistic insights into the role played by the active site cluster. Coupled with biophysical studies designed to prob e the role of the Ni center, these studies fail to provide support for schemes that involve Ni-centered redox chemistry or utilize Ni as a b inding site for the substrate (H-2) or for inhibitors like CO. Instead , they point to the involvement of the cysteinate ligands and/or the F e center in the catalysis of H-2 redox chemistry.