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Ni-containing superoxide dismutase (NiSOD) catalyzes disproportionation of superoxide anion to dioxygen and hydrogen peroxide.Recently,X-ray structures of NiSOD in the Ni(Ⅱ) and Ni(Ⅲ) states were determined.The active center in the Ni(Ⅱ) state was coordinated with amino nitrogen of terminal histidine,amidato nitrogen of petide backbone,and two thiolato sulfurs of cystein residues in the equatorial plane,and that in the Ni(Ⅲ) state was additionally ligated by imidazole nitrogen of terminal hystidine at the axial position.