1H and 15N nuclear magnetic resonance assignments, secondary structure in solution, and solvent exchange properties of azurin from Alcaligenes denitrificans

Complete sequential 1H and 15N resonance assignments for the reduced Cu(I) form of the blue copper protein azurin (M(r) = 14,000, 129 residues) from Alcaligenes denitrificans have been obtained at pH 5.5 and 32 degrees C using homo- and heteronuclear two-dimensional and heteronuclear three-dimensional NMR spectroscopy. Comparison of the resonance assignments for the backbone protons with those of Pseudomonas aeruginosa azurin, which is 68% homologous in its amino acid sequence and has a very similar three-dimensional structure, showed a high similarity in chemical shift positions. After adjustment for random coil contributions the mean difference in NH chemical shifts is 0.00 ppm (root mean square width = 0.30 ppm), whereas for C alpha protons the mean difference is 0.09 ppm (root mean square width = 0.23 ppm). Characteristic NOE connectivities and 3JHN alpha values were used to determine the secondary structure of azurin in solution. Two beta-sheets, one helix, and nine tight and four helical turns were identified, and some long-range NOE contacts were found that connect the helix with the beta-sheets. The secondary structure obtained is in agreement with the structure derived from X-ray diffraction data [Baker, E. N. (1988) J. Mol. Biol. 203, 1071-1095]. Studies of the hydration of the protein in the vicinity of the copper ligand residue His117 revealed that the solvent-exposed N epsilon 2 of His117 is in slow exchange with the bulk solvent. However, no evidence was obtained for the presence of a long-lived water molecule at the position corresponding to a well-defined water molecule observed in the crystal structures of A. denitrificans and Ps. aeruginosa azurin.