Mechanisms of electron transfer in catalysis by copper zinc superoxide dismutase.

Activated oxygen intermediates during copper zinc superoxide dismutase (SOD) catalysis were investigated using an isotope fractionation technique and natural abundance reagents. Competitive oxygen kinetic isotope effects (KIEs) are reported for the enzyme-catalyzed disproportionation of superoxide as well as the stoichiometric reaction of reduced SOD with molecular oxygen. Analysis within the context of quantum mechanical electron transfer theory provides evidence against an outer-sphere mechanism for O2*- oxidation. A CuII-O2-I intermediate is, therefore, proposed. The SOD-catalyzed oxidation of O2*- is characterized by an inverse (<1) KIE which is similar to those determined for the analogous reactions of synthetic copper compounds. An inverse kinetic isotope effect upon the enzymatic reduction of O2*- is also observed and proposed to arise from rate-determining proton transfer which leads to the formation of HO2* in the SOD active site.