Transferred nuclear Overhauser enhancement (NOE) and rotating-frame NOE experiments reflect the size of the bound segment of the Forssman pentasaccharide in the binding site of Dolichos biflorus lectin

A complex between the Forssman pentasaccharide alpha-D-GalNAc-(1-->3)-beta-D-GalNAc-(1-->3)-alpha-D-Gal- (1-->4)-beta-D-Gal-(1-->4)-D-Glc and the seed lectin from Dolichos biflorus was studied using transfer-NOESY and transfer rotating frame NOE spectroscopy (ROESY) experiments. The evolution of transferred NOEs and ROEs as a function of the pentasaccharide/lectin ratio was different for the non-reducing disaccharide moiety alpha-D-GalNAc-(1-->3)-beta-D-GalNAc compared to the rest of the molecule, which reflects distinct relaxation properties and effects of exchange broadening of the corresponding ligand resonances. Significantly, several intermolecular transferred NOEs were observed between protons of the nonreducing disaccharide moiety alpha-D-GalNAc-(1-->3)-beta-D-GalNAc and aliphatic as well as aromatic amino acid side chain protons in the binding pocket of the lectin. It is concluded that the non-reducing disaccharide fragment is buried in the lectin-binding pocket, whereas the reducing trisaccharide portion alpha-D-Gal(1-->4)-beta-D-Gal-(1-->4)-D-Glc has no immediate contacts with the protein. The experimental transfer NOE data were qualitatively compared to theoretical proton-proton distances from a model that was based on a previous homology modeling study of a complex between the disaccharide fragment alpha-D-GalNAc-(1-->3)-beta-D-GalNAc and D. biflorus lectin. It was found that all intermolecular transferred NOEs matched short interatomic distances between ligand protons and aliphatic or aromatic amino acid side chain protons predicted by the theoretical model.