X-RAY STRUCTURE OF A MONOCLINIC FORM OF HEN EGG-WHITE LYSOZYME CRYSTALLIZED AT 313-K - COMPARISON OF 2 INDEPENDENT MOLECULES

A monoclinic crystal of hen egg lysozyme (HEL, E.C. 3.2.1.17) was obtained at 313 K from a 10%(w/v) NaCl solution at pH 7.6 containing 5%(v/v) 1-propanol. Cell dimensions were a = 27.23, b = 63.66, c = 59.12 angstrom and beta = 92.9-degrees, and the space group was P2(1). The unit cell contains four molecules (V(m) = 1.79 angstrom3 Da-1). The structure was solved by the isomorphous replacement method with anomalous scattering followed by phase improvement by the solvent-flattening method. The refinement of the structure was carried out by the simulated-annealing method. The conventional R value was 0.187 for 18 260 reflections [\F(o)\ > 3sigma(F)] in the resolution range 10-1.72 angstrom. The r.m.s. deviations from the ideal bond distances and angles were 0.015 angstrom and 3.0-degrees, respectively. The two molecules in the asymmetric unit are related by a translation of half a lattice unit along the a and c axes. The r.m.s. difference of equivalent Ca atoms between the two molecules was 0.64 angstrom and the largest difference was 3.57 angstrom for Gly71. A significant structural change was observed in the regions of residues 45-50, 65-73 and 100-104. The residues 45-50, which connect two beta-strands, are shifted parallel to the beta-sheet plane between the two molecules. The residues 100-104 belong to the substrate-binding site (subsite A) and the high flexibility of this region may be responsible for the binding of the substrate and the release of reaction products.