NMR investigations of protein-carbohydrate interactions -: Binding studies and refined three-dimensional solution structure of the complex between the B domain of wheat germ agglutinin and N,N′,N"-triacetylchitotriose

The specific interaction of the isolated B domain of wheat germ agglutinin (WGA-B) with N,N',N"-triacetylchitotriose has been analyzed by H-1-NMR spectroscopy. The association constants for the binding of WGA-B to this trisaccharide have been determined from both H-1-NMR titration experiments and microcalorimetry methods. Entropy and enthalpy of binding have been obtained. The driving force for the binding process is provided by a negative Delta H which is partially compensated by negative Delta S. These negative signs indicate that hydrogen bonding and van der Waals forces are the major interactions stabilizing the complex. NOESY NMR experiments in water solution provided 327 protein proton-proton distance constraints. All the experimental constraints were used in a refinement protocol including restrained molecular dynamics in order to determine the refined solution conformation of this protein/carbohydrate complex. With regard to the NMR structure of the free protein, no important changes in the protein NOEs were observed, indicating that carbohydrate-induced conformational changes are small. The average backbone rmsd of the 35 refined structures was 1.05 Angstrom, while the heavy atom rmsd was 2.10 Angstrom. Focusing on the bound ligand, two different orientations of the trisaccharide within WGA-B binding site are possible. It can be deduced that both hydrogen bonds and van der Waals contacts confer stability to both complexes. A comparison of the three-dimensional structure of WGA-B in solution to that reported in the solid state and to those deduced for hevein and pseudohevein in solution has also been performed.