Thermodynamic studies of lectin-carbohydrate interactions by isothermal titration calorimetry

Molecular recognition of carbohydrates by acyclic receptors employing noncovalent interactions

Artificial carbohydrate receptors using noncovalent interactions for sugar binding provide valuable model systems to study the underlying principles of carbohydrate-based molecular recognition processes. In addition, well-designed artificial receptors may serve as a basis for the development of saccharide sensors or therapeutics that intervene in biologically important carbohydrate recognitions. Several different strategies have been employed for the design of such synthetic systems. The main focus of this tutorial review is on the carbohydrate binding capabilities of receptors possessing an acyclic structure and employing noncovalent interactions for sugar binding. The acyclic scaffold provides simplicity in the synthetic plan for many modifications of the receptor structure, supplying a base for systematic studies toward recognition motifs for carbohydrates. The review covers both some earlier examples and newer developments in this field.

High affinity interaction between a bivalve C-type lectin and a biantennary complex-type N-glycan revealed by crystallography and microcalorimetry

Codakine is an abundant 14-kDa mannose-binding C-type lectin isolated from the gills of the sea bivalve Codakia orbicularis. Binding studies using inhibition of hemagglutination indicated specificity for mannose and fucose monosaccharides. Further experiments using a glycan array demonstrated, however, a very fine specificity for N-linked biantennary complex-type glycans. An unusually high affinity was measured by titration microcalorimetry performed with a biantennary Asn-linked nonasaccharide. The crystal structure of the native lectin at 1.3A resolution revealed a new type of disulfide-bridged homodimer. Each monomer displays three intramolecular disulfide bridges and contains only one calcium ion located in the canonical binding site that is occupied by a glycerol molecule. The structure of the complex between Asn-linked nonasaccharide and codakine has been solved at 1.7A resolution. All residues could be located in the electron density map, except for the capping beta1-4-linked galactosides. The alpha1-6-linked mannose binds to calcium by coordinating the O3 and O4 hydroxyl groups. The GlcNAc moiety of the alpha1,6 arm engages in several hydrogen bonds with the protein, whereas the GlcNAc on the other antenna is stacked against Trp(108), forming an extended binding site. This is the first structural report for a bivalve lectin.

Stability and structural recovery of the tetramerization domain of p53-R337H mutant induced by a designed templating ligand

Protein p53 is a transcription factor crucial for cell cycle and genome integrity. It is able to induce both cell arrest when DNA is damaged and the expression of DNA repair machinery. When the damage is irreversible, it triggers apoptosis. Indeed, the protein, which is a homotetramer, is mutated in most human cancers. For instance, the inherited mutation p53-R337H results in destabilization of the tetramer and, consequently, leads to an organism prone to tumor setup. We describe herein a rational designed molecule capable of holding together the four monomers of the mutated p53-R337H protein, recovering the tetramer integrity as in the wild-type structure. Two ligand molecules, based on a conical calix[4]arene with four cationic guanidiniomethyl groups at the wider edge (upper rim) and hydrophobic loops at the narrower edge (lower rim), fit nicely and cooperatively into the hydrophobic clefts between two of the monomers at each side of the protein and keep the tetrameric structure, like molecular templates, by both ion-pair and hydrophobic interactions. We found a good agreement between the structure of the complex and the nature of the interactions involved by a combination of theory (molecular dynamics) and experiments (circular dichroism, differential scanning calorimetry and (1)H saturation transfer difference NMR).

Structural basis of the preferential binding for globo-series glycosphingolipids displayed by Pseudomonas aeruginosa lectin I

The opportunistic pathogen Pseudomonas aeruginosa contains several carbohydrate-binding proteins, among which is the P. aeruginosa lectin I (PA-IL), which displays affinity for alpha-galactosylated glycans. Glycan arrays were screened and demonstrated stronger binding of PA-IL toward alphaGal1-4betaGal-terminating structures and weaker binding to alphaGal1-3betaGal ones in order to determine which human glycoconjugates could play a role in the carbohydrate-mediated adhesion of the bacteria. This was confirmed in vivo by testing the binding of the lectin to Burkitt lymphoma cells that present large amounts of globotriaosylceramide antigen Gb3/CD77/P(k). Trisaccharide moieties of Gb3 (alphaGal1-4betaGal1-4Glc) and isoglobotriaosylceramide (alphaGal1-3betaGal1-4Glc) were tested by titration microcalorimetry, and both displayed similar affinity to PA-IL in solution. The crystal structure of PA-IL complexed to alphaGal1-3betaGal1-4Glc trisaccharide has been solved at 1.9-A resolution and revealed how the second galactose residue makes specific contacts with the protein surface. Molecular modeling studies were performed in order to compare the binding mode of PA-IL toward alphaGal1-3Gal with that toward alphaGal1-4Gal. Docking studies demonstrated that alphaGal1-4Gal creates another network of contacts for achieving a very similar affinity, and 10-ns molecular dynamics in explicit water allowed for analyzing the flexibility of each disaccharide ligand in the protein binding site. The higher affinity observed for binding to Gb3 epitope, both in vivo and on glycan array, is likely related to the presentation effect of the oligosaccharide on a surface, since only the Gb3 glycosphingolipid geometry is fully compatible with parallel insertion of neighboring trisaccharide heads in two binding sites of the same tetramer of PA-IL.

Synthesis and mycobacterial growth inhibition activities of bivalent and monovalent arabinofuranoside containing alkyl glycosides

Arabinofuranosides constitute one of the important components of cell wall structures of mycobacteria. With this importance of arabinofuranosides in mind, alkyl glycosides bearing arabinofuranoside trisaccharides were prepared, wherein the sugars were presented either in the monovalent or bivalent forms. Following the synthesis, the monovalent and bivalent alkyl glycosides were tested for their activities in a mycobacterial growth assay. The growth of the mycobacterial strain M. smegmatis was assessed in the presence of the alkyl glycosides and it was realized that the alkyl glycosides acted as inhibitors of the mycobacterial growth. The inhibition of the growth, caused by the above alkyl glycosides, was not observed for the arabinofuranose trisaccharide alone, without the alkyl groups, and for an alkyl glycoside bearing maltose as the sugar component.

Structural basis for mannose recognition by a lectin from opportunistic bacteria Burkholderia cenocepacia

Chronic colonization of the lungs by opportunist bacteria such as Pseudomonas aeruginosa and members of the Bcc (Burkholderia cepacia complex) is the major cause of morbidity and mortality among CF (cystic fibrosis) patients. PA-IIL (lecB gene), a soluble lectin from Ps. aeruginosa, has been the subject of much interest because of its very strong affinity for fucose. Orthologues have been identified in the opportunist bacteria Ralstonia solanacearum, Chromobacterium violaceum and Burkholderia of Bcc. The genome of the J2315 strain of B. cenocepacia, responsible for epidemia in CF centres, contains three genes that code for proteins with PA-IIL domains. The shortest gene was cloned in Escherichia coli and pure recombinant protein, BclA (B. cenocepacia lectin A), was obtained. The presence of native BclA in B. cenocepacia extracts was checked using a proteomic approach. The specificity of recombinant BclA was characterized using surface plasmon resonance showing a preference for mannosides and supported with glycan array experiments demonstrating a strict specificity for oligomannose-type N-glycan structures. The interaction thermodynamics of BclA with methyl alpha-D-mannoside demonstrates a dissociation constant (K(d)) of 2.75 x 10(-6) M. The X-ray crystal structure of the complex with methyl alpha-D-mannoside was determined at 1.7 A (1 A=0.1 nm) resolution. The lectin forms homodimers with one binding site per monomer, acting co-operatively with the second dimer site. Each monomer contains two Ca2+ ions and one sugar ligand. Despite strong sequence similarity, the differences between BclA and PA-IIL in their specificity, binding site and oligomerization mode indicate that the proteins should have different roles in the bacteria.

Structural basis for chitotetraose coordination by CGL3, a novel galectin-related protein from Coprinopsis cinerea

Recent advances in genome sequencing efforts have revealed an abundance of novel putative lectins. Among these, many galectin-related proteins, characterized by many conserved residues but intriguingly lacking critical amino acids, have been found in all corners of the eukaryotic superkingdom. Here we present a structural and biochemical analysis of one representative, the galectin-related lectin CGL3 found in the inky cap mushroom Coprinopsis cinerea. This protein contains all but one conserved residues known to be involved in beta-galactoside binding in galectins. A Trp residue strictly conserved among galectins is changed to an Arg in CGL3 (R81). Accordingly, the galectin-related protein is not able to bind lactose. Screening of a glycan array revealed that CGL3 displays preference for oligomers of beta1-4-linked N-acetyl-glucosamines (chitooligosaccharides) and GalNAc beta 1-4GlcNAc (LacdiNAc). Carbohydrate-binding affinity of this novel lectin was quantified using isothermal titration calorimetry, and its mode of chitooligosaccharide coordination not involving any aromatic amino acid residues was studied by X-ray crystallography. Structural information was used to alter the carbohydrate-binding specificity and substrate affinity of CGL3. The importance of residue R81 in determining the carbohydrate-binding specificity was demonstrated by replacing this Arg with a Trp residue (R81W). This single-amino-acid change led to a lectin that failed to bind chitooligosaccharides but gained lactose binding. Our results demonstrate that, similar to the legume lectin fold, the galectin fold represents a conserved structural framework upon which dramatically altered specificities can be grafted by few alterations in the binding site and that, in consequence, many metazoan galectin-related proteins may represent lectins with novel carbohydrate-binding specificities.

Structural and thermodynamic analyses of solute-binding Protein from Bifidobacterium longum specific for core 1 disaccharide and lacto-N-biose I

Recently, a gene cluster involving a phosphorylase specific for lacto-N-biose I (LNB; Galbeta1-3GlcNAc) and galacto-N-biose (GNB; Galbeta1-3GalNAc) has been found in Bifidobacterium longum. We showed that the solute-binding protein of a putative ATP-binding cassette-type transporter encoded in the cluster crystallizes only in the presence of LNB or GNB, and therefore we named it GNB/LNB-binding protein (GL-BP). Isothermal titration calorimetry measurements revealed that GL-BP specifically binds LNB and GNB with K(d) values of 0.087 and 0.010 microm, respectively, and the binding process is enthalpy-driven. The crystal structures of GL-BP complexed with LNB, GNB, and lacto-N-tetraose (Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc) were determined. The interactions between GL-BP and the disaccharide ligands mainly occurred through water-mediated hydrogen bonds. In comparison with the LNB complex, one additional hydrogen bond was found in the GNB complex. These structural characteristics of ligand binding are in agreement with the thermodynamic properties. The overall structure of GL-BP was similar to that of maltose-binding protein; however, the mode of ligand binding and the thermodynamic properties of these proteins were significantly different.

Insights into the structural basis of the pH-dependent dimer-tetramer equilibrium through crystallographic analysis of recombinant Diocleinae lectins

The structural ground underlying the pH-dependency of the dimer-tetramer transition of Diocleinae lectins was investigated by equilibrium sedimentation and X-ray crystal structure determination of wild-type and site-directed mutants of recombinant lectins. Synthetic genes coding for the full-length alpha-chains of the seed lectins of Dioclea guianensis (termed r-alphaDguia) and Dioclea grandiflora (termed r-alphaDGL) were designed and expressed in Escherichia coli. This pioneering approach, which will be described in detail in the present paper, yielded recombinant lectins displaying carbohydrate-binding activity, dimer-tetramer equilibria and crystal structures indistinguishable from their natural homologues. Conversion of the pH-stable tetrameric r-alphaDGL into a structure exhibiting pH-dependent dimer-tetramer transition was accomplished through mutations that abolished the interdimeric interactions at the central cavity of the tetrameric lectins. Both the central and the peripheral interacting regions bear structural information for formation of the canonical legume lectin tetramer. We hypothesize that the strength of the ionic contacts at these sites may be modulated by the pH, leading to dissociation of those lectin structures that are not locked into a pH-stable tetramer through interdimeric contacts networking the central cavity loops.

Structural and functional studies of Peptide-carbohydrate mimicry

Certain peptides act as molecular mimics of carbohydrates in that they are specifically recognizedby carbohydrate-binding proteins. Peptides that bind to anti-carbohydrate antibodies, carbohydrate-processingenzymes, and lectins have been identified. These peptides are potentially useful as vaccines andtherapeutics; for example, immunologically functional peptide molecular mimics (mimotopes) can strengthenor modify immune responses induced by carbohydrate antigens. However, peptides that bind specificallyto carbohydrate-binding proteins may not necessarily show the corresponding biological activity, andfurther selection based on biochemical studies is always required. The degree of structural mimicryrequired to generate the desired biological activity is therefore an interesting question. This reviewwill discuss recent structural studies of peptide-carbohydrate mimicry employing NMR spectroscopy,X-ray crystallography, and molecular modeling, as well as relevant biochemical data. These studiesprovide insights into the basis of mimicry at the molecular level. Comparisons with other carbohydrate-mimeticcompounds, namely proteins and glycopeptides, will be drawn. Finally, implications for the designof new therapeutic compounds will also be presented.

Recommendations on measurement and analysis of results obtained on biological substances using isothermal titration calorimetry (IUPAC Technical Report)

Isothermal titration calorimetry (ITC) is widely used to determine the thermodynamics of biological interactions including protein-protein, small molecule-protein, protein-DNA, small molecule-DNA, and antigen-antibody interactions. An ITC measurement consists of monitoring the transfer of heat between an analyte solution in a sample vessel and a reference solution in a reference vessel upon injection of a small aliquot of titrant solution into the sample vessel at a fixed ITC operating temperature. A binding isotherm is generated from the heat-transferred-per-injection data and values for the binding constants, the apparent binding enthalpies, and the apparent ratio of the amount of titrant to analyte for the binding reaction are then determined from fits of a binding model, whether it is a single site, identical multi-site, or an interacting multi-site binding model, to the binding isotherm. Prior to the fitting procedure, corrections should be made for contributions from extraneous heat of mixing determined separately from injections of the titrant into just the dialysate/buffer solution. Ultra-high binding constants, which cannot be directly determined from an ITC measurement, can be determined by a displacement ITC method where injections of the tight-binding titrant into a solution of a weaker-binding titrant-analyte complex displaces the weaker-binding titrant from the complex. The Michaelis and catalytic constants can be determined for an enzyme reaction from injections of a substrate or enzyme titrant into an enzyme or substrate analyte solution. Several binding reactions are suggested to check the operating performance of the ITC. The reporting of ITC results must be specific with regard to the composition of the titrant and the analyte solutions, the temperature, and the model used in the analysis.

Iterative Synthesis of Spacered Glycodendrons as Oligomannoside Mimetics and Evaluation of Their Antiadhesive Properties

Dendrimer chemistry is an attractive concept for mimicry of the highly branched character of the bioactive carbohydrates found as part of a cell's sugar coat, called the glycocalyx. Glycodendrimers have thus been used to study biological processes occurring on cell surfaces, such as bacterial adhesion. This paper details a new approach in glycodendrimer synthesis, in which a 3,6-diallylated carbohydrate is utilised as core molecule, hydroboration-oxidation is the activating step, and glycosylation with branched and unbranched sugar trichloroacetimidates is used for dendritic growth. To obtain pure dendritic pseudo-tri- and -heptasaccharides in good yields, radical addition of mercaptoethanol to peripheral double bonds was also evaluated with great success. A collection of six new hyperbranched glycodendrons was tested for their potential as inhibitors of type 1 fimbriae-mediated bacterial adhesion in an ELISA and the results were interpreted with regard to sugar valency and spacer characteristics.

Heparin-mimetic sulfated peptides with modulated affinities for heparin-binding peptides and growth factors

Heterogeneity in the composition and in the polydispersity of heparin has motivated the development of homogeneous heparin mimics, and peptides of appropriate sequence and chemical function have therefore recently emerged as potential replacements for heparin in selected applications. Here, we report the assessment of the binding affinities of multiple sulfated peptides (SPs) for a set of heparin-binding peptides (HBPs) and for vascular endothelial growth factor isoform 165 (VEGF165); these binding partners have application in the selective immobilization of proteins and in hydrogel formation through non-covalent interactions. Sulfated peptides were produced via solid-phase methods, and their affinity for the HBPs and VEGF165 was assessed via affinity liquid chromatography (ALC), surface plasmon resonance (SPR), and in selected cases, isothermal titration calorimetry (ITC). The shortest peptide, SP(a), showed the highest affinity binding of HBPs and VEGF165 in both ALC and SPR measurements, with slight exceptions. Of the investigated HBPs, a peptide based on the heparin-binding domain of human platelet factor 4 showed greatest binding affinities toward all of the SPs, consistent with its stronger binding to heparin. The affinity between SP(a) and PF4(ZIP) was indicated via SPR (K(D)=5.27 microM) and confirmed via ITC (K(D)=8.09 microM). The binding by SP(a) of both VEGF and HBPs suggests its use as a binding partner to multiple species, and the use of these interactions in assembly of materials. Given that the peptide sequences can be varied to control binding affinity and selectivity, opportunities are also suggested for the production of a wider array of matrices with selective binding and release properties useful for biomaterials applications.

A fluorescent lectin array using supramolecular hydrogel for simple detection and pattern profiling for various glycoconjugates

Because sugar and its derivatives play important roles in various biological phenomena, the rapid and high-throughput analysis of various glycoconjugates is keenly desirable. We describe herein the construction of a novel fluorescent lectin array for saccharide detection using a supramolecular hydrogel matrix. In this array, the fluorescent lectins were noncovalently fixed under semi-wet conditions to suppress the protein denaturation. It is demonstrated by fluorescence titration and fluorescence lifetime experiments that the immobilized lectins act as a molecular recognition scaffold in the hydrogel matrix, similar to that in aqueous solution. That is, a bimolecular fluorescence quenching and recovery (BFQR) method can successfully operate under both conditions. This enables one to fluorescently read-out a series of saccharides on the basis of the recognition selectivity and affinity of the immobilized lectins without tedious washing processes and without labeling the target saccharides. Simple and high-throughput sensing and profiling were carried out using the present lectin array for diverse glycoconjugates, which not only included a simple glucose, but also oligosaccharides, and glycoproteins, and, furthermore, the pattern recognition and profiling of several types of cell lysates were also accomplished.

Probing multivalent carbohydrate-lectin interactions by an enzyme-linked lectin assay employing covalently immobilized carbohydrates

We report here the synthesis of a series of mono- to trivalent N-acetylglucosamine (GlcNAc) derivatives as ligands for the plant lectin wheat germ agglutinin (WGA). Their WGA binding potencies were determined by an established enzyme-linked lectin assay (ELLA) employing microtiter plates with non-covalently immobilized porcine stomach mucin (PSM) as reference ligand and an ELLA with a new GlcNAc derivative covalently immobilized via a thiourea linkage. Comparison of both assays revealed that the type of presentation of GlcNAc residues on the microtiter plates either as part of a glycoprotein or as a covalently immobilized monosaccharide derivative strongly influences the outcome of the assay. Although the apparent dissociation constants K(D)(ELLA) for the interaction of peroxidase-labeled WGA with the microtiter plates are comparable for both surfaces, IC(50) values obtained with the PSM-free ELLA were substantially lower. Even more strikingly, this ELLA displayed a better differentiation between ligands of different valency leading to significantly higher relative inhibitory potencies of multivalent ligands compared to monovalent. Additionally, problems associated with the use of PSM, such as maximum inhibition at considerably less than 100% and poor reproducibility of IC(50) values could be overcome with this type of ELLA.

Synthesis and conformational behavior of the difluoromethylene linked C-glycoside analog of beta-galactopyranosyl-(1<-->1)-alpha-mannopyranoside

C-Glycosides in which the pseudoglycosidic substituent is a methylene group have been advertised as hydrolytically stable mimetics of their parent O-glycosides. While this substitution assures greater stability, the lower polarity and increased conformational flexibility in the intersaccharide linker brought about by this change may compromise biological mimicry. In this regard, C-glycosides, in which the pseudoanomeric methylene is replaced with a difluoromethylene group, are interesting because the CF2 group is more of an isopolar replacement for oxygen than CH2. In addition, the CF2 residue is expected to instill conformational bias into the intersaccharide torsions. Herein is described the synthesis and conformational behavior of the difluoromethylene linked C-glycoside of beta-D-galactopyranosyl-(1<-->1)-alpha-D-mannopyranoside. The synthesis centers on the formation of the galactose residue via an oxocarbenium ion-enol ether cyclization. Conformational analysis, using a combination of molecular mechanics, dynamics, and NMR spectroscopy, suggests that the difluoro-C-glycoside populates the non-exo-Gal/exo-Man conformer to a major extent (ca 50%), with a minor contribution ( approximately 15%) from the exo-Gal/exo-Man conformer that corresponds to the ground sate of the parent O-glycoside.

Isothermal titration calorimetry at very low c

In the study of 1:1 binding, M + X right arrow over left arrow MX, isothermal titration calorimetry (ITC) can be used successfully at values of c=K[M](0) well below the value 1.0 that is often considered its lower limit. However, analysis of low-c ITC data may require freezing the stoichiometry parameter n, and that is thought to be prohibitive for biological systems, where n can be poorly known. Here it is noted that the least-squares estimates of the binding constant K are virtually independent of errors in n at low c, permitting reliable determination of K and, from its temperature dependence, DeltaH degrees and n, down to c=10(-4) or lower, ligand solubility permitting.

Photoderivatized polymer thin films at quartz crystal microbalance surfaces: sensors for carbohydrate-protein interactions

Photoderivatized polymer-coated gold surfaces have been developed following a perfluorophenylazide-based double ligation strategy. Gold-plated quartz crystal microbalance (QCM) crystals were initially covalently functionalized with a monolayer of poly(ethylene glycol) (PEG), using photo- or thermolytic nitrene formation and insertion. The polymer surfaces were subsequently used as substrates for photoinsertion of carbohydrate-derivatized photoprobes, yielding different recognition motifs for selective protein binding. The resulting robust and biocompatible sensor surfaces were applied to a flow-through QCM instrument for monitoring lectin-carbohydrate interactions in real time. The results clearly show the predicted lectin selectivity, demonstrating the applicability of the approach.

Mannosylated G(0) Dendrimers with Nanomolar Affinities toEscherichia coli FimH

Pentaerythritol and bis-pentaerythritol scaffolds were used for the preparation of first generation glycodendrimers bearing aryl alpha-D-mannopyranoside residues assembled using single-step Sonogashira and click chemistry. The carbohydrate precursors were built with either para-iodophenyl, propargyl, or 2-azidoethyl aglycones whereas the pentaerythritol moieties were built with terminal azide or propargyl groups, respectively. Cross-linking abilities of this series of glycodendrimers were first evaluated with the lectin from Canavalia ensiformis (Concanavalin A). Surface plasmon resonance measurements showed these two families of mannosylated clusters as the best ligands known to date toward Escherichia coli K12 FimH with subnanomolar affinities. Tetramer 4 had a K(d) of 0.45 nM. These clusters were 1000 times more potent than mannose for their capacity to inhibit the binding of E. coli to erythrocytes in vitro.

Binding studies of alpha-GalNAc-specific lectins to the alpha-GalNAc (Tn-antigen) form of porcine submaxillary mucin and its smaller fragments

Isothermal titration microcalorimetry (ITC) and hemagglutination inhibition measurements demonstrate that a chemically and enzymatically prepared form of porcine submaxillary mucin that possesses a molecular mass of approximately 10(6) daltons and approximately 2300 alpha-GalNAc residues (Tn-PSM) binds to the soybean agglutinin (SBA) with a K(d) of 0.2 nm, which is approximately 10(6)-fold enhanced affinity relative to GalNAcalpha1-O-Ser (Tn), the pancarcinoma carbohydrate antigen. The enzymatically derived 81 amino acid tandem repeat domain of Tn-PSM containing approximately 23 alpha-GalNAc residues binds with approximately 10(3)-fold enhanced affinity, while the enzymatically derived 38/40 amino acid cleavage product(s) of Tn-PSM containing approximately 11-12 alpha-GalNAc residues shows approximately 10(2)-fold enhanced affinity. A natural carbohydrate decorated form of PSM (Fd-PSM) containing 40% of the core 1 blood group type A tetrasaccharide, and 58% peptide-linked GalNAcalpha1-O-Ser/Thr residues, with 45% of the peptide-linked alpha-GalNAc residues linked alpha-(2,6) to N-glycolylneuraminic acid, shows approximately 10(4) enhanced affinity for SBA. Vatairea macrocarpa lectin (VML), which is also a GalNAc binding lectin, displays a similar pattern of binding to the four forms of PSM, although there are quantitative differences in its affinities as compared with SBA. The higher affinities of SBA and VML for Tn-PSM relative to Fd-PSM indicate the importance of carbohydrate composition and epitope density of mucins on their affinities for lectins. The higher affinities of SBA and VML for Tn-PSM relative to its two shorter chain analogs demonstrate that the length of a mucin polypeptide and hence total carbohydrate valence determines the affinities of the three Tn-PSM analogs. The results suggest a binding model in which lectin molecules "bind and jump" from alpha-GalNAc residue to alpha-GalNAc residue along the polypeptide chain of Tn-PSM before dissociating. The complete thermodynamic binding parameters for these mucins including their binding stoichiometries are presented. The results have important implications for the biological activities of mucins including those expressing the Tn cancer antigen.

Specific recognition of saturated and 4,5-unsaturated hexuronate sugars by a periplasmic binding protein involved in pectin catabolism

The process of pectin depolymerization by pectate lyases and glycoside hydrolases produced by pectinolytic organisms, particularly the phytopathogens from the genus Erwinia, is reasonably well understood. Indeed each extracellular and intracellular catabolic stage has been identified using either genetic, bioinformatic or biochemical approaches. Nevertheless, the molecular details of many of these stages remain unknown. In particular, the mechanism and ligand binding profiles for the transport of pectin degradation products between cellular compartments remain entirely uninvestigated. Here we present the structure of TogB, a 45.7 kDa periplasmic binding protein from Yersinia enterocolitica. This protein is a component of the TogMNAB ABC transporter involved in the periplasmic transport of oligogalacturonides. In addition to the unliganded complex (at 2.2 A), we have also determined the structures of TogB in complex with digalacturonic acid (at 2.2 A), trigalacturonic acid (at 1.8 A) and 4,5-unsaturated digalacutronic acid (at 2.3 A). The molecular determinants of oligogalacturonide binding include a novel salt-bridge between the non-reducing sugar uronate group, selectivity for the unsaturated ligand, and the overall sugar configuration. Complementing this are UV difference and isothermal titration calorimetry experiments that highlight the thermodynamic basis of ligand specificity. The ligand binding profiles of the TogMNAB transporter complex nicely complement pectate lyase-mediated pectin degradation, which is a significant component of pectin depolymerization reactions.

The conformational behaviour and P-selectin inhibition of fluorine-containing sialyl LeX glycomimetics

A combination of experimental J/NOE NMR data with molecular mechanics and dynamics calculations has been used to examine the conformational behaviour and assign the configuration of synthetically prepared epimeric 3-carboxymethyl-O-Gal-(1-->1)-alpha-Man-fluoro-C-glycosides. It is shown that the population distributions around the glycosidic linkages strongly depend on the configuration at the fluorinated carbon of the pseudoacetal residue. It is also shown that these compounds resemble the inhibition ability of sialyl LeX towards P-selectin.

Exploration of oligosaccharide-protein interactions in glycoprotein quality control by synthetic approaches

High-mannose-type oligosaccharides, which are cotranslationally introduced to nascent polypeptides, play important roles in glycoprotein quality control. This process is highly complex, involving a number of lectins, chaperones, and glycan-processing enzymes. For example, calnexin and calreticulin (CRT) are molecular chaperones that recognize monoglucosylated forms of high-mannose-type glycans. UDP-glucose : glycoprotein glucosyltransferase (UGGT) only glucosylates high-mannose-type glycans attached to partially folded proteins. Fbs1 is a component of ubiquitin ligase that recognizes sugar chains. Although recent studies have clarified the properties of these proteins, most of them used oligosaccharides derived from natural sources, which contain structural heterogeneity. In order to gain a more precise understanding, we started our program to comprehensively synthesize high-mannose-type glycans associated with a protein quality control system. Additionally, investigation of artificial glycoproteins led us to the discovery of the first nonpeptidic substrate of UGGT. These synthetic oligosaccharide probes have allowed us to conduct quantitative evaluations of the activity and specificity of CRT, Fbs1, and UGGT.

On the importance of carbohydrate-aromatic interactions for the molecular recognition of oligosaccharides by proteins: NMR studies of the structure and binding affinity of AcAMP2-like peptides with non-natural naphthyl and fluoroaromatic residues

The specific interaction of a variety of modified hevein domains to chitooligosaccharides has been studied by NMR spectroscopy in order to assess the importance of aromatic-carbohydrate interactions for the molecular recognition of neutral sugars. These mutant AcAMP2-like peptides, which have 4-fluoro-phenylalanine, tryptophan, or 2-naphthylalanine at the key interacting positions, have been prepared by solid-phase synthesis. Their three-dimensional structures, when bound to the chitin-derived trisaccharide, have been deduced by NMR spectroscopy. By using DYANA and restrained molecular dynamics simulations with the AMBER 5.0 force field, the three-dimensional structures of the protein-sugar complexes have been obtained. The thermodynamic analysis of the interactions that occur upon complex formation have also been carried out. Regarding binding affinity, the obtained data have permitted the deduction that the larger the aromatic group, the higher the association constant and the binding enthalpy. In all cases, entropy opposes binding. In contrast, deactivation of the aromatic rings by attaching fluorine atoms decreases the binding affinity, with a concomitant decrease in enthalpy. The role of the chemical nature of the aromatic ring for establishing sugar contacts has been thus evaluated.

The Interaction of a Carbohydrate-binding Module from a Clostridium perfringens N-Acetyl-β-hexosaminidase with Its Carbohydrate Receptor

Clostridium perfringens is a notable colonizer of the human gastrointestinal tract. This bacterium is quite remarkable for a human pathogen by the number of glycoside hydrolases found in its genome. The modularity of these enzymes is striking as is the frequent occurrence of modules having amino acid sequence identity with family 32 carbohydrate-binding modules (CBMs), often referred to as F5/8 domains. Here we report the properties of family 32 CBMs from a C. perfringens N-acetyl-beta-hexosaminidase. Macroarray, UV difference, and isothermal titration calorimetry binding studies indicate a preference for the disaccharide LacNAc (beta-d-galactosyl-1,4-beta-d-N-acetylglucosamine). The molecular details of the interaction of this CBM with galactose, LacNAc, and the type II blood group H-trisaccharide are revealed by x-ray crystallographic studies at resolutions of 1.49, 2.4, and 2.3 A, respectively.