Structure of hexagonal turkey egg-white lysozyme at 1.65A resolution

The structure of hexagonal turkey egg-white lysozyme (TEWL) has been determined and refined at 1.65 A resolution. The crystals were grown from a 150 mM potassium thiocyanate solution at pH 4.5 and belong to space group P6(1)22 with unit-cell dimensions a = b = 70.96, c = 83.01 A alpha = beta = 90, gamma = 120 degrees. 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 A and 2.2 degrees, 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.