Effect of pH and Ligand Binding on the Structure of the Cu Site of the Met121Glu Mutant of Azurin from Pseudomonas aeruginosa

A pH-dependent X-ray absorption fine structure (XAFS) study has been undertaken to provide a structural interpretation of the spectroscopic properties of the Met121Glu mutant of azurin from Pseudomonas aeruginosa (Azp). Ligand binding studies have been carried out to investigate the effect of the cavity formed at the Cu site as a result of the mutation. The optical spectrum at pH 4 exhibits an intense band at ∼600 nm and a weaker band at ∼450 nm, typical for the blue copper proteins. As the pH is increased, these bands decrease in intensity and shift to 570 and 413 nm, respectively, with the latter becoming the more intense of the two [Karlsson, B. G., et al. (1991) Protein Eng. 4 (3), 343−349]. These changes are accompanied by a change in the EPR spectrum from a rhombic type 1 Cu spectrum at pH 4 to a spectrum with the rhombic splitting decreasing to zero and the hyperfine coupling increasing from 25 to 83 G. X-ray absorption at the Cu K-edge shows that this change results from the lengthening of the Cu−...