A facilitated electron transfer of copper--zinc superoxide dismutase (SOD) based on a cysteine-bridged SOD electrode.

The direct electrochemical redox reaction of bovine erythrocyte copper--zinc superoxide dismutase (Cu(2)Zn(2)SOD) was clearly observed at a gold electrode modified with a self-assembled monolayer (SAM) of cysteine in phosphate buffer solution containing SOD, although its reaction could not be observed at the bare electrode. In this case, SOD was found to be stably confined on the SAM of cysteine and the redox response could be observed even when the cysteine-SAM electrode used in the SOD solution was transferred to the pure electrolyte solution containing no SOD, suggesting the permanent binding of SOD via the SAM of cysteine on the electrode surface. The electrode reaction of the SOD confined on the cysteine-SAM electrode was found to be quasi-reversible with the formal potential of 65 +/- 3 mV vs. Ag/AgCl and its kinetic parameters were estimated: the electron transfer rate constant k(s) is 1.2 +/- 0.2 s(-1) and the anodic (alpha(a)) and cathodic (alpha(c)) transfer coefficients are 0.39 +/- 0.02 and 0.61 +/- 0.02, respectively. The assignment of the redox peak of SOD at the cysteine-SAM modified electrode could be sufficiently carried out using the native SOD (Cu(2)Zn(2)SOD), its Cu- or Zn-free derivatives (E(2)Zn(2)SOD and Cu(2)E(2)SOD, E designates an empty site) and the SOD reconstituted from E(2)Zn(2)SOD and Cu(2+). The Cu complex moiety, the active site for the enzymatic dismutation of the superoxide ion, was characterized to be also the electroactive site of SOD. In addition, we found that the SOD confined on the electrode can be expected to possess its inherent enzymatic activity for dismutation of the superoxide ion.