The pH dependence of the electron self-exchange rate of azurin from Pseudomonas aeruginosa as studied by 1H-NMR.

The electron self-exchange rate of azurin from Pseudomonas aeruginosa has been measured by proton NMR as a function of temperature under various conditions of pH, buffer and ionic strength. The rate does not appear very sensitive to variations in the latter parameters. Qualitative thermodynamic compensation is observed for the entropy and enthalpy of activation, with a compensation temperature of 309 +/- 7 K, and an average exchange rate of 1.3 X 10(6) M-1 s-1. The observed high entropy of activation contributes significantly to the high exchange rate. The data are analyzed by considering an encounter complex in which two azurin molecules associate along their hydrophobic patches. Copper-to-copper electron transfer over a distance of 1.5 nm in the complex is facilitated by the favourable disposition of the His-117 ligands.