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The structure of hexagonal turkey egg-white lysozyme (TEWL) has been determined and refined at 1.65 Å resolution. The crystals were grown from a 150 mM potassium thiocyanate solution at pH 4.5 and belong to space group P6122 with unit-cell dimensions a = b = 70.96, c = 83.01 Å α = β = 90, γ = 120°. The crystals were isomorphous with those of hexagonal pH 8.0 TEWL. The coordinates of PDB entry code 3LZ2 were therefore used as the initial model and subjected to rigid-body refinement, simulated annealing and least-squares refinement to a final residual of 0.20. The root-mean-square deviations from the ideal bond distances and angles were 0.016 Å and 2.2°, respectively. During the refinement, 86 water molecules and one thiocyanate ion were located in the structure. The thiocyanate ion lies close to the interface between two symmetry-related molecules. The S atom of the ion forms two direct intermolecular contacts with Argl4 and interacts indirectly via a network of water molecules to Arg5 of a symmetry-related molecule. The structure provides direct evidence for the mode of thiocyanate binding to arginine residues and suggests a possible mechanism for the efficiency of thiocyanate in crystallizing basic proteins.
