Structural basis of electron transfer modulation in the purple Cu(A) center

The X-ray structure of an engineered purple Cu(A) center in azurin from Pseudomonas aeruginosa has been determined and refined at 1.65 Å resolution. Two independent purple Cu(A) azurin molecules are in the asymmetric unit of a new P2₁ crystal, and they have nearly identical conformations (rmsd of 0.27 for backbone atoms). The purple Cu(A) azurin was produced by the loop- engineering strategy, and the resulting overall structure is unperturbed. The insertion of a slightly larger Cu-binding loop into azurin causes the two structural domains of azurin to move away from each other. The high- resolution structure reveals the detailed environment of the delocalized mixed-valence [Cu(1.5) ··· Cu(1.5)] binuclear purple Cu(A) center, which serves as a useful reference model for other native proteins, and provides a firm basis for understanding results from spectroscopic and functional studies of this class of copper center in biology. The two independent Cu-Cu distances of 2.42 and 2.35 Å (with respective concomitant adjustments of ligand-Cu distances) are consistent with that (2.39 Å) obtained from X-ray absorption spectroscopy with the same molecule, and are among the shortest Cu-Cu bonds observed to date in proteins or inorganic complexes. A comparison of the purple Cu(A) azurin structure with those of other Cu(A) centers reveals an important relationship between the angular position of the two His imidazole rings with respect to the Cu₂S₂(Cys) core plane and the distance between the Cu and the axial ligand. This relationship strongly suggests that the fine structural variation of different Cu(A) centers can be correlated with the angular positions of the two histidine rings because, from these positions, one can predict the relative axial ligand interactions, which are responsible for modulating the Cu-Cu distance and the electron transfer properties of the Cu(A) centers.