High Enzyme Activity of a Binuclear Nickel Complex Formed with the Binding Loops of the NiSOD Enzyme*

Detailed equilibrium, spectroscopic and superoxide dismutase (SOD) activity studies are reported on a nickel complex formed with a new metallopeptide bearing two nickel binding loops of NiSOD. The metallopeptide exhibits unique nickel binding ability and the binuclear complex is a major species with 2×(NH₂ ,N_amide ,S^- ,S^- ) donor set even in an equimolar solution of the metal ion and the ligand. Nickel(III) species were generated by oxidizing the Ni^II complexes with KO₂ and the coordination modes were identified by EPR spectroscopy. The binuclear complex formed with the binding motifs exhibits superior SOD activity, in this respect it is an excellent model of the native NiSOD enzyme. A detailed kinetic model is postulated that incorporates spontaneous decomposition of the superoxide ion, the dismutation cycle and fast redox degradation of the binuclear complex. The latter process leads to the elimination of the SOD activity. A unique feature of this system is that the Ni^III form of the catalyst rapidly accumulates in the dismutation cycle and simultaneously the Ni^II form becomes a minor species.