Conformation of the oligosaccharide receptor for E-selectin

A tetrasaccharide related to the blood group oligosaccharides, known as sialyl LewisX, has been proposed as the receptor for the lectin responsible for leukocyte adhesion named alternatively as E-selectin or ELAM-1. The 13C- and 1H-nmr spectra have been completely assigned for a tetrasaccharide model of this receptor, Neu5Ac alpha-(2-->3)-Gal beta-(1-->4)-[Fuc alpha-(1-->3)-]GlcNAc beta-NHAc. Quantitative nuclear Overhauser data (NOESY) have been recorded and analyzed by a complete spin matrix simulation method. Conformational space was exhaustively searched and all conformational models whose simulated NOESY spectra matched the experiment were found. Molecular mechanics and molecular dynamics calculations were carried out to test whether the experimental conformations are low energy and thus likely to represent true single conformations for the tetrasaccharide. It was concluded that while the LewisX trisaccharide portion of the compound adopts a single conformation, there is likely to be some flexibility about the Neu5Ac alpha-(2-->3)-linkage. A model featuring fast exchange between two different conformations of this linkage is found to be consistent with both the nmr experiments and the molecular dynamics simulations.