Origin of carbohydrate recognition specificity of human lysozyme revealed by affinity labeling

In order to reveal the origin of carbohydrate recognition specificity of human lysozyme by clarifying the difference in the binding mode of ligands in the active site, the inactivation of human lysozyme by 2',3'-epoxypropyl beta-glycoside derivatives of the disaccharides, N,N'-diacetylchitobiose [GlcNAc-beta-(1-->4)-GlcNAc] and N-acetyllactosamine [Gal-beta-(1-->4)-GlcNAc], was investigated and the three-dimensional structures of the affinity-labeled enzymes were determined by X-ray crystallography at 1.7 A resolution. Under the conditions comprising 2.0 x 10(-3) M labeling reagent and 1.0 x 10(-5) M human lysozyme at pH 5.4, 37 degrees C, the reaction time required to reduce the lytic activity against Micrococcus luteus cells to 50% of its initial activity was lengthened by 3.7 times through the substitution of the nonreducing end sugar residue, GlcNAc to Gal. The refined structure of human lysozyme labeled by 2',3'-epoxypropyl beta-glycoside derivatives of N,N'-diacetylchitobiose (HL/NAG-NAG-EPO complex) indicated that the interaction mode of the N,N'-diacetylchitobiose moiety in substites B and C in this study was essentially the same as in the case of the complex of human lysozyme with the free ligand. On the other hand, the hydrogen-bonding pattern and the stacking interaction at subsite B were remarkably different between the HL/NAG-NAG-EPO complex and human lysozyme labeled by the 2',3'-epoxypropyl beta-glycoside of N-acetyllactosamine (HL/GAL-NAG-EPO complex). The reduced number of possible hydrogen bonds as well as the less favorable stacking between the side chain of Tyr63 in human lysozyme and the galactose residue in the HL/GAL-NAG-EPO complex reasonably explained the less efficient ability of the 2',3'-epoxypropyl beta-glycoside of N-acetyllactosamine as compared to that of N,N'-diacetylchitobiose as an affinity labeling reagent toward human lysozyme.