Backbone dynamics of azurin in solution: Slow conformational change associated with deprotonation of histidine 35

N-15 relaxation measurements have been performed on the type I blue copper protein azurin from Pseudomonas aeruginosa. The relaxation times show that one loop (residues 103-108) and one turn (residues 74-77) display fast internal motions, The rest of the protein is rigid with an average order parameter S-2 of 0.85 +/- 0.05. The copper binding site shows the same degree of rigidity even though is it composed of several loops and lies outside the beta-sheet sandwich. Substantial exchange broadening was found for a number of residues surrounding the side chain of His-35. The average exchange rate has been determined from NMR exchange spectroscopy experiments and is 45 +/- 6 s(-1) at 41 degrees C. The exchange broadening is caused by the protonation/deprotonation equilibrium of His-35, The NMR results indicate that the two structures of azurin observed by X-ray diffraction of crystals at pH 5.5 and 9.0 [Nar, H., Messerschmidt, A., Huber, R., Van de Kamp, M., Canters, G. W. (1991) J. Mol. Biol, 221, 765-772] are present in solution and that they interconvert slowly.