NMR investigations of protein-carbohydrate interactions: refined three-dimensional structure of the complex between hevein and methyl -chitobioside

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Abstract
The specific interaction of hevein with GlcNAc-containing oligosaccharides has been analyzed by 1H-NMR spectroscopy. The association constants for the binding of hevein to a variety of ligands have been estimated from 1H-NMR titration experiments. The association constants increase in the order GlcNAc-α(1→6)-Man < GlcNAc < benzyl-β-GlcNAc < p-nitrophenyl-β-GlcNAc < chitobiose < p-nitrophenyl-β-chitobioside < methyl-β-chitobioside < chitotriose. Entropy and enthalpy of binding for different complexes have been obtained from van't Hoff analysis. The driving force for the binding process is provided by a negative ΔH0 which is partially compensated by negative ΔS0. 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 475 accurate protein proton-proton distance constraints after employing the MARDIGRAS program. In addition, 15 unambiguous protein/carbohydrate NOEs were detected. All the experimental constraints were used in a refinement protocol including restrained molecular dynamics in order to determine the highly refined solution conformation of this protein-carbohydrate complex. With regard to the NMR structure of the free protein, no important changes in the protein nOe's were observed, indicating that carbohydrate-induced conformational changes are small. The average backbone rmsd of the 20 refined structures was 0.055 nm, while the heavy atom rmsd was 0.116 nm. It can be deduced that both hydrogen bonds and van der Waals contacts confer stability to the complex. A comparison of the three-dimensional structure of he-vein in solution to those reported for wheat germ agglutinin (WGA) and hevein itself in the solid state has also been performed. The polypeptide conformation has also been compared to the NMR-derived structure of a smaller antifungical peptide, Ac-AMP2.