Interactions of wheat-germ agglutinin with GlcNAcβ1,6Gal sequence

The interactions of wheat-germ agglutinin (WGA) with the GlcNAcbetal,6Gal sequence, a characteristic component or branched poly-N-acetyllactosaminoglycans, were investigated using isothermal titration calorimetry and X-ray crystallography, GlcNAcbetal,6Gal exhibited an affinity greater than GlcNAcbetal,4GlcNAc to all WGA isolectins, whereas Galbetal,6GlcNAc showed Much less affinity than GlcNAcbetal,4GlcNAc. X-ray structural analyses of the glut araldehyde-c ross I inked WGA isolectin 3 crystals in complex with GlcNAcbetal,6Gal, GlcNAcbetal,4GlcNAc and GlcNAcbetal,6Galbetal4Glc were performed at 2.4, 2.2 atid 2.2 A resolution, respectively. In spite of different glycosidic linkages, GlcNAcbetal,6Gal and GlcNAcbetal,4GlcNAc exhibited basically similar binding modes to each other, in contact with side chains of two aromatic residues, Tyr64 and His66. However, the conformations of the ligands in the two primary binding sites were not always identical. GlcNAcbetal,6Gal showed more extensive variation in the parameters defining the glycosidic linkage structure compared to GlcNAcbetal,4GlcNAc, demonstrating large conformational flexibility of the former ligand in the interaction with WGA. The difference in the ligand binding conformation was accompanied by alterations of the side chain conformation of the amino acid residues involved in the interactions. The hydrogen bond between Ser62 and the non-reducing end GlcNAc was always observed regardless of the ligand type, indicating the key role of this interaction. In addition to the hydrogen bonding and van der Waals interactions, CH--pi interactions involving Tyr64, His66 and Tyr73 are suggested to play an essential role in determining the ligand binding conformation in all complexes. One of the GlcNAcbetal,6Gal ligands had no crystal packing contact with another WGA molecule. therefore the conformation might be more relevant to the interaction mode in solution. (C) 2002 Elsevier Science B.V. All rights reserved.