Mechanisms of Electron Transfer in Catalysis by Copper Zinc Superoxide

Understanding the origins of oxidation-reduction catalysis by metalloenzymes requires detailed information concerning the mech-anisms of electron transfer (ET). Copper zinc superoxide dismutase (SOD) is perhaps the most thoroughly studied member of a family of exceptionally well-tuned catalysts for the dismutation of super-oxide: 2O2¥- + 2H+ f O2 + H2O2.1 Under physiological conditions, the rate of the enzymatic reaction is 104 times faster than that of the spontaneous disproportionation of O2¥- in solution. The origin of enzyme catalysis with respect to the near barrierless nature of ET has not been understood, yet this enhanced reactivity is the basis of SOD’s protective function against physiological oxidative damage.1 While there is a wealth of structural and mechanistic data on SOD, there is little evidence to suggest whether ET occurs by an inner-sphere or outer-sphere mechanism.2 In inner-sphere ET,