Electron Transfer Between Copper-Zinc Superoxide Dismutase and Hexacyanoferrate(II).

The electron-transfer rate between oxidized copper, zinc superoxide dismutase and Fe(CN)64- has been measuredfor several mutants of human SOD and compared to the rate obtained with the native enzyme. Among the results, the following are meaningful: (i) substitution of the positive Arg at position 143 with a neutral group decreases the electron transfer rate, (ii) mutation of Thr-137 has little influence, and (iii) substitution of Glu-133 with the neutral Gln increases the rate. Such pattern is similar to that of activity towards superoxide dismutation and to that of affinity for the anion N3-. It is proposed that Fe(CN)6 4- approaches the copper ion at the entrance of the cavity. 13C NMR measurements on Co(CN)6 3- allow an estimate of the distance between the terminal N and copper in wild type SOD of 3.9 A. Such distance is consistent with direct nitrogen to copper electron transfer. No reduction occurs in the case of zinc-depleted derivatives