The Zn-site in bovine copper, zinc superoxide dismutase studied by 111Cd PAC

The active site in bovine copper, zinc superoxide dismutase (Cu2, Zn2SOD) has been studied by 111Cd time differential Perturbed Angular Correlation (PAC) on enzyme with Zn2+ replaced by excited 111Cd2+. The PAC spectra obtained for both the oxidized and the reduced form of Cu2Cd2SOD show no asymmetry between the two Zn-sites in the dimeric enzyme. The spectra further reveal that a significant change has taken place at the Zn-site in the reduced form compared to the oxidized form. Semi-empirical calculations based on the Angular Overlap Model (AOM) and coordinates from the crystal structure of the native enzyme are in agreement with the experimental PAC data of the oxidized enzyme. The results indicate that Cd2+ coordinates in the same manner as Zn2+ and that the crystal structure of SOD is valid for the enzyme in solution. The PAC spectrum of the reduced enzyme can be explained by extending the AOM calculations to the enzyme in the reduced form and assuming that the imidazol ring of His61 is no longer bridging the copper and cadmium ions in the reduced state.