Evidence of his61 imidazolate bridge rupture in reduced crystalline Cu,Zn superoxide dismutase

X-ray absorption spectroscopy has been carried out on the copper K edge in oxidized and reduced bovine Cu,Zn SOD in solution and in crystalline state. The results indicate that the copper coordination geometry is unaffected by the solution or by the crystalline state of the protein, in both oxidation states. Moreover the two oxidation states of the active copper ion are reflected under, all the experimental conditions, by distinct coordination spheres around the catalytic metal, which is four-coordinated and three-coordinated in the Cu(II) and in the Cu(I) enzyme, respectively.