Electrostatic recognition in redox copper proteins: a 1H NMR study of the protonation behavior of His 19 in oxidized and reduced Cu,Zn superoxide dismutase

The pK shift of the His 19 residue in bovine Cu,Zn superoxide dismutase was carefully measured by nuclear magnetic resonance spectroscopy as a function of the change of the copper oxidation state. The measured pK values were identical within experimental error, at variance with results obtained with other copper proteins under similar conditions. The DelPhi program, based on a macroscopic dielectric model for the electrostatic interactions in proteins, was used to calculate pK shifts, as a function of charge perturbation introduced by metal oxidation, between the oxidized and the reduced enzyme, which are assumed to have the same structure on the basis of previous spectroscopic data. A nice fit with the experimental pK values was obtained protonating the imidazole of the copper-zinc bridging His 61, which is known to release the copper in the reduced enzyme. Protonation of His 61 in the reduced state gives rise to an electrostatic potential distribution around the protein almost identical to that observed in the oxidized one. These results suggest that a major role for the bridging histidine in Cu,Zn superoxide dismutase is to provide identical electrostatic steering of the substrate in the two oxidation states of the enzyme by redox-linked protonation-deprotonation processes. This property is discussed in comparison with the effect of copper reduction on the distribution of the electric field in small blue copper proteins.