The conformation of the T-antigen disaccharide found to Maclura pomifera agglutinin in aqueous solution

The complex of Maclura pomifera agglutinin with the T-antigen disaccharide (beta -D-Gal-(1-->3)-alpha -D-GalNAc-(1-->O)- Me) was investigated by NMR s pectroscopy in aqueous solution Intramolecular transferred nuclear Overhaus er enhancement (NOE) effects between the monosaccharide moieties were used to derive the Ligand conformation in the lectin-bound state. Ligand protons in contact with the protein were identified by saturation transfer differe nce experiments and intermolecular transferred NOE effects. It is demonstra ted that structural differences exist for the ligand-lectin complex in aque ous solution as compared with the previously published crystal structure (L ee,X, Thompson, A, Zhiming, Z., Ton-that, H., Biesterfeldt, J., Ogata, C., Xu, L., Johnston, R. A Z., and Young, N. M. (1998) J. Biol: Chem 273, 6312- 6318). Tn order to accommodate the O-methyl group of the disaccharide, the amino acid side chain of Tyr-122 has to rotate from its position in the cry stal The NMR data are in accord with two conformational families at the bet a-(1-->3)glycosidic linkage in the solution complex with interglycosidic an gles phi/psi = 45/-65 degrees and -65/ -18 degrees. These differ from the b ound conformation of the ligand in the crystal (phi/psi = 39/-8 degrees) an d are not highly populated by the ligand in the free state. The reason for the structural, differences at the beta-(1-->)glycosidic linkage are hydrog en bonds that stabilize the relative orientation of the monosaccharide unit s in the crystal. Our results demonstrate that the crystallization of a pro tein-carbohydrate complex car interfere with the delicate process of carboh ydrate recognition in solution.