X-RAY STRUCTURE OF TURKEY EGG LYSOZYME COMPLEX WITH DI-N-ACETYL-CHITOBIOSE - RECOGNITION AND BINDING OF ALPHA-ANOMERIC FORM

The crystalline complex of turkey-egg lysozyme (TEL) with di-N-acetylc hitobiose (NAG2) was prepared by a soaking method and the structure wa s determined by X-ray analysis at 1.55 Angstrom resolution. The struct ure was refined to an R value of 0.175 by simulated annealing and ener gy minimization. The alpha-anomer of NAG2 is located at subsite D with the orientation perpendicular to the direction of the active-site cle ft. The anomeric residue is deeply inserted into the cleft and the O1- H hydroxyl group is hydrogen bonded to the carboxyl group of Glu35 whi ch is a catalytic residue. The other sugar residue protrudes outside t he cleft and is in van der Waals contact with the beta-sheet region co mprising of residues 43-53. The binding of NAG2 makes the active-site cleft 0.3-0.5 Angstrom narrower and supresses the thermal motion of tw o lobes constructing the cleft. The NAG2 molecule is bound in a manner not assumed in the catalytic action of the enzyme and the geometry of binding indicates that the alpha-anomer blocks the active center and acts only as an inhibitor.