PNA-encoded synthesis (PES) of a 10 000-member hetero-glycoconjugate library and microarray analysis of diverse lectins

Identification of selective and synthetically tractable ligands to glycan-binding proteins is important in glycoscience. Carbohydrate arrays have had a tremendous impact on profiling glycan-binding proteins and as analytical tools. We report a highly miniaturized synthetic format to access nucleic-acid-encoded hetero-glycoconjugate libraries with an unprecedented diversity in the combinations of glycans, linkers, and capping groups. Novel information about plant and bacterial lectin specificity was obtained by microarray profiling, and we show that a ligand identified on the array can be converted to a high-affinity soluble ligand by straightforward chemistry.

Expeditive synthesis of trithiotriazine-cored glycoclusters and inhibition of Pseudomonas aeruginosa biofilm formation

Readily accessible, low-valency glycoclusters based on a triazine core bearing D-galactose and L-fucose epitopes are able to inhibit biofilm formation by Pseudomonas aeruginosa. These multivalent ligands are simple to synthesize, are highly soluble, and can be either homofunctional or heterofunctional. The galactose-decorated cluster shows good affinity for Pseudomonas aeruginosa lectin lecA. They are convenient biological probes for investigating the roles of lecA and lecB in biofilm formation.

Structures of a human blood group glycosyltransferase in complex with a photo-activatable UDP-Gal derivative reveal two different binding conformations

Glycosyltransferases (GTs) catalyse the sequential addition of monosaccharides to specific acceptor molecules and play major roles in key biological processes. GTs are classified into two main families depending on the inverted or retained stereochemistry of the glycosidic bond formed during the reaction. While the mechanism of inverting enzymes is well characterized, the precise nature of retaining GTs is still a matter of much debate. In an attempt to clarify this issue, studies were initiated to identify reaction-intermediate states by using a crystallographic approach based on caged substrates. In this paper, two distinct structures of AA(Gly)B, a dual-specificity blood group synthase, are described in complex with a UDP-galactose derivative in which the O6'' atom is protected by a 2-nitrobenzyl group. The distinct conformations of the caged substrate in both structures of the enzyme illustrate the highly dynamic nature of its active site. An attempt was also made to photolyse the caged compound at low temperature, which unfortunately is not possible without damaging the uracil group as well. These results pave the way for kinetic crystallography experiments aiming at trapping and characterizing reaction-intermediate states in the mechanism of enzymatic glycosyl transfer.

Membrane deformation by neolectins with engineered glycolipid binding sites

Lectins are glycan-binding proteins that are involved in the recognition of glycoconjugates at the cell surface. When binding to glycolipids, multivalent lectins can affect their distribution and alter membrane shapes. Neolectins have now been designed with controlled number and position of binding sites to decipher the role of multivalency on avidity to a glycosylated surface and on membrane dynamics of glycolipids. A monomeric hexavalent neolectin has been first engineered from a trimeric hexavalent bacterial lectin, From this neolectin template, 13 different neolectins with a valency ranging from 0 to 6 were designed, produced, and analyzed for their ability to bind fucose in solution, to attach to a glycosylated surface and to invaginate glycolipid-containing giant liposomes. Whereas the avidity only depends on the presence of at least two binding sites, the ability to bend and invaginate membranes critically depends on the distance between two adjacent binding sites.

A LecA ligand identified from a galactoside-conjugate array inhibits host cell invasion by Pseudomonas aeruginosa

Lectin LecA is a virulence factor of Pseudomonas aeruginosa involved in lung injury, mortality, and cellular invasion. Ligands competing with human glycoconjugates for LecA binding are thus promising candidates to counteract P. aeruginosa infections. We have identified a novel divalent ligand from a focused galactoside(Gal)-conjugate array which binds to LecA with very high affinity (Kd = 82 nM). Crystal structures of LecA complexed with the ligand together with modeling studies confirmed its ability to chelate two binding sites of LecA. The ligand lowers cellular invasiveness of P. aeruginosa up to 90 % when applied in the range of 0.05-5 μM. Hence, this ligand might lead to the development of drugs against P. aeruginosa infection.

Tetravalent glycocyclopeptide with nanomolar affinity to wheat germ agglutinin

A series of tetravalent glycocyclopeptides functionalized with GlcNAc was synthesized using copper(i)-catalysed alkyne-azide cycloaddition, oxime ligation and thiol-ene coupling. The binding ability of these compounds towards wheat germ agglutinin was studied by a competitive ELLA test and ITC experiments. While all compounds were able to inhibit WGA binding to GlcNAc-polymer coated surfaces at low concentrations, derivative 17 having an aliphatic spacer and thioether linkage was 4.9 × 10(6) times more potent on a per sugar basis than GlcNAc. This remarkably strong effect was confirmed by ITC experiments as these revealed an association constant of 9 nM for this compound, therefore presenting a gain of 200,000 times over GlcNAc. These results for compound 17 represent the highest binding properties reported for WGA.

Aromatic thioglycoside inhibitors against the virulence factor LecA from Pseudomonas aeruginosa

Three small families of hydrolytically stable thioaryl glycosides were prepared as inhibitors of the LecA (PA-IL) virulence factor corresponding to the carbohydrate binding lectin from the bacterial pathogen Pseudomonas aeruginosa. The monosaccharidic arylthio β-d-galactopyranosides served as a common template for the major series that was also substituted at the O-3 position. Arylthio disaccharides from lactose and from melibiose constituted the other two series members. In spite of the fact that the natural ligand for LecA is a glycolipid of the globotriaosylceramide having an α-d-galactopyranoside epitope, this study illustrated that the β-d-galactopyranoside configuration having a hydrophobic aglycon could override the requirement toward the anomeric configuration of the natural sugar. The enzyme linked lectin assay together with isothermal titration microcalorimetry established that naphthyl 1-thio-β-d-galactopyranoside () gave the best inhibition with an IC50 twenty-three times better than that of the reference methyl α-d-galactopyranoside. In addition it showed a KD of 6.3 μM which was ten times better than that of the reference compound. The X-ray crystal structure of LecA with was also obtained.

Monitoring lectin interactions with carbohydrates

Protein-carbohydrate interactions are often involved in the first step of infection and Pseudomonas aeruginosa produces several proteins that are able to bind specifically to glycan epitopes present on host epithelia. The experimental approaches for studying protein-carbohydrate interaction have been inspired, with some adaptations, from those commonly used for protein-protein or protein-ligand interactions. A range of methods are described herein for detecting lectin activity, screening for monosaccharide or oligosaccharide specificity, determining the affinity of binding together with thermodynamics and kinetics parameters, and producing crystal of lectin-carbohydrate complexes for further structural studies.

A soluble fucose-specific lectin from Aspergillus fumigatus conidia--structure, specificity and possible role in fungal pathogenicity

Aspergillus fumigatus is an important allergen and opportunistic pathogen. Similarly to many other pathogens, it is able to produce lectins that may be involved in the host-pathogen interaction. We focused on the lectin AFL, which was prepared in recombinant form and characterized. Its binding properties were studied using hemagglutination and glycan array analysis. We determined the specificity of the lectin towards l-fucose and fucosylated oligosaccharides, including α1-6 linked core-fucose, which is an important marker for cancerogenesis. Other biologically relevant saccharides such as sialic acid, d-mannose or d-galactose were not bound. Blood group epitopes of the ABH and Lewis systems were recognized, Le(Y) being the preferred ligand among others. To provide a correlation between the observed functional characteristics and structural basis, AFL was crystallized in a complex with methyl-α,L-selenofucoside and its structure was solved using the SAD method. Six binding sites, each with different compositions, were identified per monomer and significant differences from the homologous AAL lectin were found. Structure-derived peptides were utilized to prepare anti-AFL polyclonal antibodies, which suggested the presence of AFL on the Aspergillus' conidia, confirming its expression in vivo. Stimulation of human bronchial cells by AFL led to IL-8 production in a dose-dependent manner. AFL thus probably contributes to the inflammatory response observed upon the exposure of a patient to A. fumigatus. The combination of affinity to human epithelial epitopes, production by conidia and pro-inflammatory activity is remarkable and shows that AFL might be an important virulence factor involved in an early stage of A. fumigatus infection.

Mapping of heparin/heparan sulfate binding sites on αvβ3 integrin by molecular docking

Heparin/heparan sulfate interact with growth factors, chemokines, extracellular proteins, and receptors. Integrins are αβ heterodimers that serve as receptors for extracellular proteins, regulate cell behavior, and participate in extracellular matrix assembly. Heparin binds to RGD-dependent integrins (αIIbβ3, α5β1, αvβ3, and αvβ5) and to RGD-independent integrins (α4β1, αXβ2, and αMβ2), but their binding sites have not been located on integrins. We report the mapping of heparin binding sites on the ectodomain of αvβ3 integrin by molecular modeling. The surface of the ectodomain was scanned with small rigid probes mimicking the sulfated domains of heparan sulfate. Docking results were clustered into binding spots. The best results were selected for further docking simulations with heparin hexasaccharide. Six potential binding spots containing lysine and/or arginine residues were identified on the ectodomain of αvβ3 integrin. Heparin would mostly bind to the top of the genu domain, the Calf-I domain of the α subunit, and the top of the β subunit of RGD-dependent integrins. Three spots were close enough from each other on the integrin surface to form an extended binding site that could interact with heparin/heparan sulfate chains. Because heparin does not bind to the same integrin site as protein ligands, no steric hindrance prevents the formation of ternary complexes comprising the integrin, its protein ligand, and heparin/heparan sulfate. The basic amino acid residues predicted to interact with heparin are conserved in the sequences of RGD-dependent but not of RGD-independent integrins suggesting that heparin/heparan sulfate could bind to different sites on these two integrin subfamilies.

Secondary sugar binding site identified for LecA lectin from Pseudomonas aeruginosa

The galactose-specific lectin LecA from Pseudomonas aeruginosa is a target for the development of new anti-infectious compounds. Sugar based molecules with anti-adhesive properties present great potential in the fight against bacterial infection and biofilm formation. LecA is specific for oligosaccharides with terminal α-galactoside residues and displays strong affinity for melibiose (αGal1-6Glc) with a Kd of 38.8 µM. The crystal structure of LecA/melibiose complex shows classical calcium-bridged binding of αGal in the primary binding site but also revealed a secondary sugar binding site with glucose bound. This sugar binding site is in close proximity to the galactose binding one, is independent of calcium and mainly involves interactions with a symmetry-related protein. This discovery would help to the design of new potent inhibitors targeting both binding sites.

Lipo-chitooligosaccharidic symbiotic signals are recognized by LysM receptor-like kinase LYR3 in the legume Medicago truncatula

While chitooligosaccharides (COs) derived from fungal chitin are potent elicitors of defense reactions, structurally related signals produced by certain bacteria and fungi, called lipo-chitooligosaccharides (LCOs), play important roles in the establishment of symbioses with plants. Understanding how plants distinguish between friend and foe through the perception of these signals is a major challenge. We report the synthesis of a range of COs and LCOs, including photoactivatable probes, to characterize a membrane protein from the legume Medicago truncatula. By coupling photoaffinity labeling experiments with proteomics and transcriptomics, we identified the likely LCO-binding protein as LYR3, a lysin motif receptor-like kinase (LysM-RLK). LYR3, expressed heterologously, exhibits high-affinity binding to LCOs but not COs. Homology modeling, based on the Arabidopsis CO-binding LysM-RLK AtCERK1, suggests that LYR3 could accommodate the LCO in a conserved binding site. The identification of LYR3 opens up ways for the molecular characterization of LCO/CO discrimination.

Reduction of lectin valency drastically changes glycolipid dynamics in membranes but not surface avidity

Multivalency is proposed to play a role in the strong avidity of lectins for glycosylated cell surfaces and also in their ability to affect membrane dynamics by clustering glycosphingolipids. Lectins with modified valency were designed from the β-propeller fold of Ralstonia solanacearum lectin (RSL) that presents six fucose binding sites. After identification of key amino acids by molecular dynamics calculations, two mutants with reduced valency were produced. Isothermal titration calorimetry confirmed the loss of three high affinity binding sites for both mutants. Crystal structures indicated that residual low affinity binding occurred in W76A but not in R17A. The trivalent R17A mutant presented unchanged avidity toward fucosylated surfaces, when compared to hexavalent RSL. However, R17A is not able anymore to induce formation of membrane invaginations on giant unilamellar vesicules, indicating the crucial role of number of binding sites for clustering of glycolipids. In the human lung epithelial cell line H1299, wt-RSL is internalized within seconds whereas the kinetics of R17A uptake is largely delayed. Neolectins with tailored valency are promising tools to study membrane dynamics.

Linear and cyclic oligo-β-(1→6)-D-glucosamines: Synthesis, conformations, and applications for design of a vaccine and oligodentate glycoconjugates

Poly-β-(1→6)-N-acetyl-D-glucosamine is an exopolysaccharide secreted by numerous pathogenic bacteria, includingStaphylococcus aureus,Escherichia coli,Yersinia pestis,Bordetella pertussis,Acinetobacter baumannii,Burkholderiaspp., and others. A convergent approach was developed for the synthesis of oligosaccharide fragments consisting of 5, 7, 9, and 11 glucosamine orN-acetylglucosamine units and for the preparation of five nona-β-(1→6)-D-glucosamines with variousN-acetylation patterns. Penta- and nona-β‑(1→6)-D-glucosamines conjugated to protein carriers through a specially developed sulfhydryl linker proved to be highly immunogenic in mice and rabbits and elicited antibodies that mediated opsonic killing of multiple strains ofS. aureus(including methicillin-resistantS. aureus, MRSA) andE. coli, and protected againstS. aureusskin abscesses and lethalE. coliandB. cenocepaciaperitonitis. These findings provide a basis for the construction of a unique semisynthetic vaccine against multiple bacterial targets. Conformational studies by means of special NMR experiments and computer modeling revealed that the oligo-β-(1→6)-D-glucosamine chain exists mostly in a helix-like conformation, where the terminal monosaccharides are arranged close to each other. Owing to this feature, oligoglucosamines consisting of 2 to 7 residues easily form products of cycloglycosylation. Cyclooligo-β-(1→6)-D-glucosamines represent a new family of functionalized cyclic oligosaccharides. Owing to their molecular architectonics, these compounds are convenient scaffolds for the design of conjugates with defined valency, symmetry, flexibility, and function.

High affinity glycodendrimers for the lectin LecB from Pseudomonas aeruginosa

Following an iterative oxime ligation procedure, cyclopeptide (R) and lysine-based dendron (D) were combined in all possible arrangements and successively functionalized with α-fucose and β-fucose to provide a new series of hexadecavalent glycosylated scaffolds (i.e., scaffolds RD16, RR16, DR16, and DD16). These compounds and smaller analogs (tetra- and hexavalent scaffolds R4 and R6) were used to evaluate the influence of the ligand valency and architecture, and of the anomer configuration in the binding to the αFuc-specific lectin LecB from Pseudomonas aeruginosa . Competitive enzyme-linked lectin assays (ELLA) revealed that only the RD16 architecture displaying αFuc (9A) reaches strong binding improvement (IC50 of 0.6 nM) over αMeFuc, and increases the α-selectivity of LecB. Dissociation constant of 28 nM was measured by isothermal titration micorcalorimetry (ITC) for 9A, which represents the highest affinity ligand ever reported for LecB. ITC and molecular modeling suggested that the high affinity observed might be due to an aggregative chelate binding involving four sugar head groups and two lectins. Interestingly, unprecedented binding effects were observed with β-fucosylated conjugates, albeit being less active than the corresponding ligands of the αFuc series. In particular, the more flexible lysine-based dendritic structures (15B and 18B) showed a slight inhibitory enhancement in comparison with those having cyclopeptide core.

Deciphering the glycan preference of bacterial lectins by glycan array and molecular docking with validation by microcalorimetry and crystallography

Recent advances in glycobiology revealed the essential role of lectins for deciphering the glycocode by specific recognition of carbohydrates. Integrated multiscale approaches are needed for characterizing lectin specificity: combining on one hand high-throughput analysis by glycan array experiments and systematic molecular docking of oligosaccharide libraries and on the other hand detailed analysis of the lectin/oligosaccharide interaction by x-ray crystallography, microcalorimetry and free energy calculations. The lectins LecB from Pseudomonas aeruginosa and BambL from Burkholderia ambifaria are part of the virulence factors used by the pathogenic bacteria to invade the targeted host. These two lectins are not related but both recognize fucosylated oligosaccharides such as the histo-blood group oligosaccharides of the ABH(O) and Lewis epitopes. The specificities were characterized using semi-quantitative data from glycan array and analyzed by molecular docking with the Glide software. Reliable prediction of protein/oligosaccharide structures could be obtained as validated by existing crystal structures of complexes. Additionally, the crystal structure of BambL/Lewis x was determined at 1.6 Å resolution, which confirms that Lewis x has to adopt a high-energy conformation so as to bind to this lectin. Free energies of binding were calculated using a procedure combining the Glide docking protocol followed by free energy rescoring with the Prime/Molecular Mechanics Generalized Born Surface Area (MM-GBSA) method. The calculated data were in reasonable agreement with experimental free energies of binding obtained by titration microcalorimetry. The established predictive protocol is proposed to rationalize large sets of data such as glycan arrays and to help in lead discovery projects based on such high throughput technology.

Discovery of two classes of potent glycomimetic inhibitors of Pseudomonas aeruginosa LecB with distinct binding modes

The treatment of infections due to the opportunistic pathogen Pseudomonas aeruginosa is often difficult, as a consequence of bacterial biofilm formation. Such a protective environment shields the bacterium from host defense and antibiotic treatment and secures its survival. One crucial factor for maintenance of the biofilm architecture is the carbohydrate-binding lectin LecB. Here, we report the identification of potent mannose-based LecB inhibitors from a screening of four series of mannosides in a novel competitive binding assay for LecB. Cinnamide and sulfonamide derivatives are inhibitors of bacterial adhesion with up to a 20-fold increase in affinity to LecB compared to the natural ligand methyl mannoside. Because many lectins of the host require terminal saccharides (e.g., fucosides), such capped structures as reported here may offer a beneficial selectivity profile for the pathogenic lectin. Both classes of compounds show distinct binding modes at the protein, offering the advantage of a simultaneous development of two new lead structures as anti-pseudomonadal drugs with an anti-virulence mode of action.

Insights into the mechanism by which interferon-γ basic amino acid clusters mediate protein binding to heparan sulfate

The extensive functional repertoire of heparin and heparan sulfate, which relies on their ability to interact with a large number of proteins, has recently emerged. To understand the forces that drive such interactions the binding of heparin to interferon-γ (IFNγ), used as a model system, was investigated. NMR-based titration experiments demonstrated the involvement of two adjacent cationic domains (D1: KTGKRKR and D2: RGRR), both of which are present within the carboxy-terminal sequence of the cytokine. Kinetic analysis showed that these two domains contribute differently to the interaction: D1 is required to form a complex and constitutes the actual binding site, whereas D2, although unable to associate with heparin by itself, increased the association rate of the binding. These data are consistent with the view that D2, through nonspecific electrostatic forces, places the two molecules in favorable orientations for productive binding within the encounter complex. This mechanism was supported by electrostatic potential analysis and thermodynamic investigations. They showed that D1 association to heparin is driven by both favorable enthalpic and entropic contributions, as expected for a binding sequence, but that D2 gives rise to entropic penalty, which opposes binding in a thermodynamic sense. The binding mechanism described herein, by which the D2 domain kinetically drives the interaction, has important functional consequences and gives a structural framework to better understand how specific are the interactions between proteins and heparin.

Multivalent glycoconjugates as anti-pathogenic agents

Multivalency plays a major role in biological processes and particularly in the relationship between pathogenic microorganisms and their host that involves protein-glycan recognition. These interactions occur during the first steps of infection, for specific recognition between host and bacteria, but also at different stages of the immune response. The search for high-affinity ligands for studying such interactions involves the combination of carbohydrate head groups with different scaffolds and linkers generating multivalent glycocompounds with controlled spatial and topology parameters. By interfering with pathogen adhesion, such glycocompounds including glycopolymers, glycoclusters, glycodendrimers and glyconanoparticles have the potential to improve or replace antibiotic treatments that are now subverted by resistance. Multivalent glycoconjugates have also been used for stimulating the innate and adaptive immune systems, for example with carbohydrate-based vaccines. Bacteria present on their surfaces natural multivalent glycoconjugates such as lipopolysaccharides and S-layers that can also be exploited or targeted in anti-infectious strategies.