The conformational of the T-antigen disaccharide bound to Maclura pomifera agglutinin in aqueous solution

The complex of Maclura pomifera agglutinin with the T-antigen disaccharide (β-D-Gal-(1→3)-α-D-GalNAc-(1→O)-Me) was investigated by NMR spectroscopy in aqueous solution. Intramolecular transferred nuclear Overhauser 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 difference experiments and intermolecular transferred NOE effects. It is demonstrated that structural differences exist for the ligand-lectin complex in aqueous solution as compared with the previously published crystal structure (Lee, 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). In 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 crystal. The NMR data are in accord with two conformational families at the β-(1→3)glycosidic linkage in the solution complex with interglycosidic angles φ/ψ = 45/-65°and -65/-18°. These differ from the bound conformation of the ligand in the crystal (φ/ψ = 39/-8°) and are not highly populated by the ligand in the free state. The reason for the structural differences at the β-(1→3)glycosidic linkage are hydrogen bonds that stabilize the relative orientation of the monosaccharide units in the crystal. Our results demonstrate that the crystallization of a protein-carbohydrate complex can interfere with the delicate process of carbohydrate recognition in solution.