Structure of Azurin I from the Denitrifying Bacterium Alcaligenes xylosoxidans NCIMB 11015 at 2.45 Å Resolution

Azurin I from Alcaligenes xylosoxidans NCIMB 11015 (AzN-I) was crystallized by using PEG 4000 as a precipitant. The crystals belong to the monoclinic crystal system and have a space group C2 with the unit-cell parameters of a = 130.67, b = 54.26, c = 74.55 Å, and β = 95.99°. The structure of AzN-I has been solved by the molecular replacement method. Azurin II from the same bacterium (AzN-II) was chosen as the initial structural model. The final crystallographic R value is 17.3% and free R value is 23.6% for 10 958 reflections at a resolution of 2.45 Å. The root-mean-square deviations for main-chain atoms range between 0.19 and 0.26 Å among the four independent molecules in the asymmetric unit. The Cu atom is coordinated to Nδ of His46 and His117 at 2.0 (1) and 1.9 (1) Å, and to Sγ of Cys112 at 2.2 (1) Å, while the carbonyl O atom of Gly45 and Sδ of Met121 coordinate axially to Cu atom at 2.5 (1) and 3.1 (1) Å, respectively. The Cu—N and Cu—S distances of AzN-I are quite similar to those of AzN-II, however, the Cu—O (Gly45) bond length in AzN-I is 0.25 Å shorter than its counterpart in AzN-II. The results have been used to discuss the differences in the spectra of these two proteins.