Spectroscopic and computational studies of Ni superoxide dismutase: electronic structure contributions to enzymatic...

Ni-containing superoxide dismutase (NiSOD) is the most recently discovered member of the class of metalloenzymes that detoxify the superoxide radical in aerobic organisms. In this study, we have employed a variety of spectroscopic and computational methods to probe the electronic structure of the NiSOD active site in both its oxidized (NiSOD(ox), possessing a low-spin (S = (1)/(2)) Ni(3+) center) and reduced (NiSOD(red), containing a diamagnetic Ni(2+) center) states. Our experimentally validated computed electronic-structure description for NiSOD(ox) reveals strong sigma-bonding interactions between Ni and the equatorial S/N ligands, which give rise to intense charge-transfer transitions in the near-UV region of the absorption spectrum. Resonance Raman (rR) spectra obtained with laser excitation in this region exhibit two features at 349 and 365 cm(-)(1) that are assigned to Ni-S(Cys) stretching modes. The NiSOD(red) active site also exhibits a high degree of metal-ligand bond covalency as well as filled/filled pi-interactions between Ni and S/N orbitals, which serve to adjust the redox potential of the Ni(2+) center. Comparison of our computational results for NiSOD(red) with those obtained in parallel studies of synthetic [NiS(2)N(2)] complexes reveals that the presence of an anionic N-donor ligand is crucial for promoting metal-based (versus S-based) oxidation of the active site. The implications of our electronic-structure descriptions with respect to the function of NiSOD are discussed, and a comparison of M-S(Cys) bonding in NiSOD and other metalloenzymes with sulfur ligation is provided.