Modeling the active sites of metalloenzymes. 4. Predictions of the unreadystates of [NiFe] Desulfovibrio gigas hydrogenase from density functional theory

Density functional theory has been used to predict the structures of a vari ety of active site models for the unready states, Ni-A and NI-SU, of the [N iFe] hydrogenase from Desulfovibrio gigas. By comparing available experimen tal results on Ni-A, Ni-SU, and Ni-SI with the computational results on the se model complexes, we have been able to identify the most likely formulas and structures for the active sites of W-A and NI-SU. Ni-A is predicted to be a Ni(III)-Fe(II) species with the bridging hydroxo ligand, rather than t he expected oxo ligand, while Ni-SU is predicted to be a Ni(II)-Fe(II) spec ies with a water molecule coordinated to the Fe center. Both have one of th e terminal S atoms (cysteines) protonated.