X-ray structure of monoclinic turkey egg lysozyme at 1.3 Å resolution

Monoclinic crystals of turkey egg lysozyme (TEL, E.C. 3.2.1.17) were obtained from 2.2 M ammonium sulfate solution at pH 4.2. They belong to space group P2₁ with unit-cell dimensions a = 38.07, b = 33.20, c = 46.12 Å and β = 110.1°, and contain one molecule in the asymmetric unit (V_m = 1.91 ų Da⁻¹). The three-dimensional structure of TEL was solved by the method of multiple isomorphous replacement with anomalous scattering. Area detector data to 1.5 Å resolution from native and heavy-atom derivatives were used for the structure determination. The structure was refined by the simulated-annealing method with diffraction data of 10–1.30 Å resolution. The conventional R factor was 0.189. The root-mean-square deviations from ideal bond distances and angles were 0.016 Å and 2.9°, respectively. The backbone structure of TEL is very similar to that of hen egg lysozyme (HEL) and the difference in seven amino-acid residues does not affect the basic folding of the polypeptide chain. Except for the region from Gly101 to Gly104, the geometry of the active-site cleft is conserved between TEL and HEL. The Gly101 residue is located at the end of the sugar-binding site and the structural change in this region between TEL and HEL is considered to be responsible for the difference in their enzymatic properties.