Characterization of the active site of catalytically inactive forms of [NiFe] hydrogenases by density functional theory.
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The inactive forms, unready (Ni-A, Ni-SU) and ready (Ni-B), of NiFe hydrogenases are modeled by examining the possibility of hydroxo, oxo, hydroperoxo, peroxo, and sulfenate groups in active-site models and comparing predicted IR frequencies and g tensors with those of the enzyme. The best models for Ni-A and Ni-SU have hydroxo (mu-OH) bridges between Fe and Ni and a terminal sulfenate [Ni-S(=O)Cys] group, although a hydroperoxo model for Ni-A is also quite viable, whereas the best model for Ni-B has only a mu-OH bridge. In addition, a mechanism for the activation of unready hydrogenase is proposed on the basis of the relative stabilities of sulfenate models versus peroxide models.