Cristallographic studies of reduced Cu(I)-Zn(II) bovine superoxide dismutase and its adducts with small inorganic anions

The dimeric enzyme Cu, Zn superoxide dismutase that catalyzes the dismutation of the superoxide anion into mole- cular oxygen and hydrogen peroxide may exist in an oxidized and in reduced form . During the catalytic cycle the copper ion undergoes alternate oxidation and reduction reactions. The oxidized enzyme from different sources has been well characterized through X-ray and other physico-chemical studies . In the oxidized state, the copper ion is coordinated to four histidines, one of which is bridging copper and zinc. In the commonly accepted mechanism, the bridging histidine protonates upon copper reduction and leaves the cuprous ion tricoordinate. UV and visible absorption spectroscopy, NMR and EXAFS data are available for the reduced Cu(I) Zn SOD, which provide evidence for the breaking of the Cu-His61 and simultaneous protonation of Ne2 atom. We have solved and refined at 2.0 resolution the X-ray structure of Cu(I) Zn SOD and of its complexes with the anionic inhibitors azide and thiocyanide. We have found that the imidazolate bridge is maintained in all cases in the crystalline state. We have also located the anion bridging site in the cavity of the reduced enzyme and found relevant structural differences between the two monomers. The implications of these results on the catalytic mechanism and the possibility to propose different reaction pathways will be discussed also in the light of the structure of similar complexes in the oxidized enzyme.