Superoxide dismutase, a study of the electronic properties of the copper and zinc by x-ray absorption spectroscopy

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Abstract
The X-ray absorption for Cu and Zn in oxidized and reduced [bovine] superoxide dismutase, as well as in various model compounds, was studied. Upon reduction of the protein, the added electron affects the Cu site almost exclusively, while the Zn remains virtually unchanged. Reduction decreases the charge on the Cu atom [toward Cu(I)] and changes the configuration of the Cu site so that it becomes less symmetric. An analysis of the Cu absorption observed with the oxidized enzyme and a comparison with that for Cu(II)(imid)4 suggests that the Cu is not simply ligated to 4 imidazoles. The addition of H2O2 to superoxide dismutase reduces the Cu to Cu(I), while O2 addition to the peroxide-reduced protein restores the Cu to Cu(II).