An X-ray study of the physiological-temperature form of hen egg-white lysozyme at 2 Å resolution

The structure of the high-temperature orthorhombic form of hen egg-white lysozyme has been determined at 2.0 Å resolution. Initial images of the molecule were obtained at 6.0 Å resolution both by double isomorphous replacement and by molecular replacement with use of the known structure of the room-temperature tetragonal lysozyme. The initial model thus obtained ( R = 0.52 at 6.0 Å) was refined first as a rigid body at 6.0 Å and then by restrained least squares at 2.5 Å and later at 2.0 Å resolution. The final model ( R = 0.23 at 2.0 Å) was compared with that of the tetragonal form: the structures are very similar with a root mean square difference in superimposed α-carbon coordinates of 0.46 Å. There are, however, differences which are caused by a crystal contact involving the upper part of this active site in the high-temperature orthorhombic form. Because of this, residues Trp 62 and Pro 70 are much better ordered than in the tetragonal form, where they are exposed to solvent. These differences can partly explain the difficulty of inhibitor-binding in high-temperature orthorhombic crystals, but do not seem to reflect the particular behaviour of lysozyme in solution at high temperature.