Zinc-substituted pseudoazurin solved by S/Zn-SAD phasing

The copper(II) centre of the blue copper protein pseudoazurin from Alcaligenes faecalis has been substituted by zinc(II) via denaturing the protein, chelation and removal of copper and refolding the apoprotein, followed by the addition of an aqueous solution of ZnCl₂. Vapour-diffusion experiments produced colourless hexagonal crystals (space group P6₅), which when cryocooled had unit-cell parameters a = b = 49.01, c = 98.08 Å. Diffraction data collected at 100 K using a copper sealed tube were phased by the weak anomalous signal of five S atoms and one Zn atom. The structure was fitted manually and refined to 1.6 Å resolution. The zinc-substituted protein exhibits similar overall geometry to the native structure with copper. Zn²⁺ binds more strongly to its four ligand atoms (His40 N^δ1, Cys78 S^γ, His81 N^δ1 and Met86 S^δ) and retains the tetrahedral arrangement, although the structure is less distorted than the native copper protein.