The recognition of saccharides by proteins has far reaching implications in biology, technology, and drug design. Within the past two decades, researchers have directed considerable effort toward a detailed understanding of these processes. Early crystallographic studies revealed, not surprisingly, that hydrogen-bonding interactions are usually involved in carbohydrate recognition. But less expectedly, researchers observed that despite the highly hydrophilic character of most sugars, aromatic rings of the receptor often play an important role in carbohydrate recognition. With further research, scientists now accept that noncovalent interactions mediated by aromatic rings are pivotal to sugar binding. For example, aromatic residues often stack against the faces of sugar pyranose rings in complexes between proteins and carbohydrates. Such contacts typically involve two or three CH groups of the pyranoses and the π electron density of the aromatic ring (called CH/π bonds), and these interactions can exhibit a variety of geometries, with either parallel or nonparallel arrangements of the aromatic and sugar units. In this Account, we provide an overview of the structural and thermodynamic features of protein-carbohydrate interactions, theoretical and experimental efforts to understand stacking in these complexes, and the implications of this understanding for chemical biology. The interaction energy between different aromatic rings and simple monosaccharides based on quantum mechanical calculations in the gas phase ranges from 3 to 6 kcal/mol range. Experimental values measured in water are somewhat smaller, approximately 1.5 kcal/mol for each interaction between a monosaccharide and an aromatic ring. This difference illustrates the dependence of these intermolecular interactions on their context and shows that this stacking can be modulated by entropic and solvent effects. Despite their relatively modest influence on the stability of carbohydrate/protein complexes, the aromatic platforms play a major role in determining the specificity of the molecular recognition process. The recognition of carbohydrate/aromatic interactions has prompted further analysis of the properties that influence them. Using a variety of experimental and theoretical methods, researchers have worked to quantify carbohydrate/aromatic stacking and identify the features that stabilize these complexes. Researchers have used site-directed mutagenesis, organic synthesis, or both to incorporate modifications in the receptor or ligand and then quantitatively analyzed the structural and thermodynamic features of these interactions. Researchers have also synthesized and characterized artificial receptors and simple model systems, employing a reductionistic chemistry-based strategy. Finally, using quantum mechanics calculations, researchers have examined the magnitude of each property's contribution to the interaction energy.

Different behavior has been observed for the psi torsion angle of the glycosidic linkages of D-GalNAc-Ser and D-GalNAc-Thr motifs, allowing the carbohydrate moiety to adopt a completely different orientation. In addition, the fact that the water pockets found in alpha-D-GalNAc-Thr differ from those obtained for its serine analogue could be related to the different capability that the two model glycopeptides have to structure the surrounding water. This fact could have important biological inferences (i.e., antifreeze activity).

The first detailed structural model for the hevein-chitin complex is presented on the basis of the analysis of NMR data. The resulting model, in combination with ITC and analytical ultracentrifugation data, conclusively shows that recognition of chitin by hevein domains is a dynamic process, which is not exclusively restricted to the binding of the nonreducing end of the polymer as previously thought. This allows chitin to bind with high affinity to a variable number of protein molecules, depending on the polysaccharide chain length. The biological process is multivalent.

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The specific interaction of the isolated B domain of wheat germ agglutinin (WGA-B) with N, N H , N HH-triacetylchitotriose has been analyzed by 1 H-NMR spectroscopy. The association constants for the binding of WGA-B to this trisaccharide have been determined from both 1 H-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 DH which is partially compensated by negative DS. 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 A Ê , while the heavy atom rmsd was 2.10 A Ê. 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. Carbohydrates are one of the most extended families of biomolecules in nature. They play a role in energy storage and as constituents of the structural framework of cells and tissues. In addition, due to their extraordinary capacity to encode information stereochemically these molecules take part in a wide variety of recognition processes of biological significance. Thus, carbohydrate recognition by proteins has been shown to be involved in viral and microbial infection, inflammatory responses, innate immunity, fertilization, tumor spread and growth regulation [1±6]. The elucidation of the biochemical and cell biological processes in the cascades from the initial molecular rendezvous to the triggered response has established a burgeoning research field in glycoscience with obvious perspectives for medical application [7±9]. Detailed information on the three-dimensional structure of protein-carbohydrate complexes has frequently been obtained from X-ray crystallography data [10±14] and modeling [15], as the commonly high molecular mass of lectins has prevented their direct studies by means of NMR spectroscopy. However, in favorable cases, NMR may also provide information about the driving forces behind protein±carbohydrate interactions in solution [16±20]. The hevein domain is one of the most common chitin-binding motifs. Its presence in several lectins [such as hevein, pseudohevein, Urtica dioica agglutinin (UD...

The structural features of MUC1-like glycopeptides bearing the Tn antigen (α-O-GalNAc-Ser/Thr) in complex with an anti MUC-1 antibody are reported at atomic resolution. For the α-O-GalNAc-Ser derivative, the glycosidic linkage adopts a high-energy conformation, barely populated in the free state. This unusual structure (also observed in an α-S-GalNAc-Cys mimic) is stabilized by hydrogen bonds between the peptidic fragment and the sugar. The selection of a particular peptide structure by the antibody is thus propagated to the carbohydrate through carbohydrate/peptide contacts, which force a change in the orientation of the sugar moiety. This seems to be unfeasible in the α-O-GalNAc-Thr glycopeptide owing to the more limited flexibility of the side chain imposed by the methyl group. Our data demonstrate the non-equivalence of Ser and Thr O-glycosylation points in molecular recognition processes. These features provide insight into the occurrence in nature of the APDTRP epitope for anti-MUC1 antibodies.

The three-dimensional structure of hevein, a small protein isolated from the latex of Heveu brusiliensis (rubber tree), in water solution has been obtained by using 'H-NMR spectroscopy and dynamic simulated annealing calculations. The average root-mean-square deviation (rmsd) of the best 20 refined structures generated using DIANA prior to simulated annealing was 0.092 nm for the backbone atoms and 0.163 nm for all heavy atoms (residues 3 -41). The specific interaction of hevein with N-acetylglucosamine-containing oligosaccharides has also been analyzed by 'H-NMR. The association constants, Ka, for the binding of hevein to GlcNAc, chitobiose [GlcNAc-, and GlcNAc-a( 1+6)-Man have been estimated from 'H-NMR titration experiments.Since the measured K, values for chitobiose binding are almost identical with and without calcium ions, it is shown that these cations are not required for sugar binding. The association increases in the order GlcNAc-a(1+6)-Man GlcNAc < chitobiose < chitotriose. The equilibrium thermodynamic parameters entropy and enthalpy of binding, AS' and AHU, for the hevein-chitobiose and hevein-chitotriose associations have been obtained from van't Hoff analysis of the temperature dependence of the K, values between 25-40°C. The driving force for the binding process is provided by a negative AH0 which is partially compensated by a negative AS'. These negative signs seem to indicate that hydrogen bonding and van der Waals forces are the major interactions stabilizing the complex. Protein-carbohydrate nuclear Overhauser enhancements have allowed a three-dimensional model of the hevein-chitobiose complex to be built. From inspection of this model, a hydrogen bond between Serl9 and the non-reducing N-acetyl carbonyl group is suggested, as well as between Tyr3O and HO-3 of the same sugar residue. The N-acetyl methyl group of the non-reducing GlcNAc displays non-polar contacts to the aromatic Tyr30 and Trp21 residues. In addition, the higher affinities deduced for the P-linked oligosaccharides with respect to GlcNAc and GlcNAc-a( 1+6)-Man can be explained by favourable stacking of the second Blinked GlcNAc moiety and TrpZl.

The structural features of the mucin-type simplest model, namely, the glycopeptide alpha-O-GalNAc-l-Ser diamide, have been investigated by combining NMR spectroscopy, molecular dynamics simulations, and DFT calculations. In contrast to previous reports, the study reveals that intramolecular hydrogen bonds between sugar and peptide residues are very weak and, as a consequence, not strong enough to maintain the well-defined conformation of this type of molecule. In fact, the observed conformation of this model glycopeptide can be satisfactorily explained by the presence of water pockets/bridges between the sugar and the peptide moieties. Additionally, DFT calculations reveal that not only the bridging water molecules but also the surrounding water molecules in the first hydration shell are essential to keep the existing conformation.