X-RAY STRUCTURE OF TURKEY-EGG LYSOZYME COMPLEX WITH TRI-N-ACETYLCHITOTRIOSE - LACK OF BINDING ABILITY AT SUBSITE-A

The turkey-egg lysozyme (TEL) complex with tri-N-acetylchitotriose [(G lcNac)(3)] was co-crystallized from 1.5% TEL and 2 mM (GlcNac)(3) at p H 4.2. The crystal structure was determined by molecular replacement a nd refined to an R value of 0.182 using 10-1.77 Angstrom data. The (Gl cNac)(3) molecule occupies the subsites A, B and C. At the subsites B and CI the sugar residues are bound in a similar manner to that found in the hen-egg lysozyme (HEL) complex. in contrast. the GlcNac residue at the subsite A is exposed to bulk solvent and has no contact with t he protein molecule because the active residue Asp 101 in HEL is repla ced by Gly in TEL. A sulfate ion is bound in the vicinity of subsite B and forms hydrogen bonds with the sugar residue and the guanidino gro up of Arg61, assisting the binding of the sugar residue to subsite B. The active-site cleft of TEL is narrower than that of native TEL, thus attaining the best fit of the (GlcNac)(3) molecule. The lack of bindi ng ability of subsite A is discussed in relation to the catalytic prop erties of TEL. The result suggests that the cleavage pattern of oligos accharide substrates in the catalytic reaction is regulated by the pro tein-sugar interaction at subsite A.