Molecular design of N-linked tetravalent glycosides bearing N-acetylglucosamine, N,N′-diacetylchitobiose and N-acetyllactosamine: Analysis of cross-linking activities with WGA and ECA lectins

Optimization of the conditions for the immobilization of glycopolypeptides on hydrophobic silica particulates and simple purification of lectin using glycopolypeptide-immobilized particulates

Glycopolypeptide-immobilized particulates exhibit high binding selectivities and affinities for several analytes. However, to date, the conditions for the synthesis of glycopolypeptide-immobilized particulates have not been optimized and the application of these particulates as carriers for affinity chromatography has not been reported. Accordingly, herein, as a model compound for determining the optimal conditions for the immobilization of an artificial glycopolymer on hexyl-containing hybrid silica particulates (HSPs), the glycopolypeptide poly [GlcNAcβ1,4GlcNAc-β-NHCO-(CH₂)₅NH-/CH₃(CH₂)₉NH-/γ-PGA] (3) containing multivalent chitobiose moieties and multivalent decyl groups with a γ-polyglutamic acid backbone was synthesized. Immobilization of 3 on HSPs under each condition was evaluated by a lectin-binding assay using wheat germ (Triticum vulgaris) agglutinin (WGA), which is an N-acetylglucosamine-binding lectin. As a result, the optimal immobilization conditions for HSPs at 25 mg/mL were obtained at dimethyl sulfoxide (DMSO) concentration of reaction solvent in the range of 1_(DMSO):9_(water) to 4_(DMSO):6_(water) and a compound 3 concentration in the range of 125 nM-1250 nM. Furthermore, the influence of the alkyl group structure introduced into glycopolypeptide for imparting hydrophobicity to it on the immobilization of glycopolypeptide on HSPs was investigated. As a result of comparing three types, poly [GlcNAcβ1,4GlcNAc-β-NHCO-(CH₂)₅NH-/γ-PGA] (1) with no alkyl group, poly [GlcNAcβ1,4GlcNAc-β-NHCO-(CH₂)₅NH-/CH₃(CH₂)₄NH-/γ-PGA] (2) with a pentyl group, and 3 with a decyl group, 3 showed the best immobilization efficiency on HSPs. Finally, 1 mg 3-immobilized HSPs prepared under the optimum conditions adsorbed approximately 7.5 μg WGA in a structure-specific manner. We also achieved a simple WGA purification from raw wheat germ extract as a practical example using 3-immobilized HSPs. We believe that in the future, these glycopolypeptide-immobilized particulates will be used not only for the purification of plant lectins, but also as specific adsorbents for various lectins-like substances such as in vivo lectins, pathogenic viruses, and toxin proteins.

Functional design of glycan-conjugated molecules using a chemoenzymatic approach

Carbohydrates play important and diverse roles in the fundamental processes of life. We have established a method for accurately and a large-scale synthesis of functional carbohydrates with diverse properties using a unique enzymatic method. Furthermore, various artificial glycan-conjugated molecules have been developed by adding these synthetic carbohydrates to macromolecules and to middle- and low-molecular-weight molecules with different properties. These glycan-conjugated molecules have biological activities comparable to or higher than those of natural compounds and present unique functions. In this review, several synthetic glycan-conjugated molecules are taken as examples to show design, synthesis, and function.

Precipitation-free high-affinity multivalent binding by inline lectin ligands

Multivalent ligand-protein interactions are a key concept in biology mediating, for example, signalling and adhesion. Multivalent ligands often have tremendously increased binding affinities. However, they also can cause crosslinking of receptor molecules leading to precipitation of ligand-receptor complexes. Plaque formation due to precipitation is a known characteristic of numerous fatal diseases limiting a potential medical application of multivalent ligands with a precipitating binding mode. Here, we present a new design of high-potency multivalent ligands featuring an inline arrangement of ligand epitopes with exceptionally high binding affinities in the low nanomolar range. At the same time, we show with a multi-methodological approach that precipitation of the receptor is prevented. We distinguish distinct binding modes of the ligands, in particular we elucidate a unique chelating binding mode, where four receptor binding sites are simultaneously bridged by one multivalent ligand molecule. The new design concept of inline multivalent ligands, which we established for the well-investigated model lectin wheat germ agglutinin, has great potential for the development of high-potency multivalent inhibitors as future therapeutics.

Insight into Erythrina Lectins: Properties, Structure and Proposed Physiological Significance

The genus Erythrina, collectively known as “coral tree”, are pantropical plants, comprising of more than 112 species. Since the early 1980s, seven of these have been found to possess hemagglutinating activity, although not yet characterized. However, around two dozen galactose-binding lectins have been isolated and fully characterized with respect to their sugar specificity, glycoconjugates agglutination, dependence of activity on metal ions, primary and secondary structures and stability. Three lectins have been fully sequenced and the crystal structures of the two proteins have been solved with and without the haptenic sugar. Lectins isolation and characterization from most of these species usually originated from the seeds, although the proteins from other vegetative tissues have also been reported. The main objective of this review is to summarize the physicochemical and biological properties of the reported purified Erythrina lectins to date. Structural comparisons, based on available lectins sequences, are also made to relate the intrinsic physical and chemical properties of these proteins. Particular attention is also given to the proposed biological significance of the lectins from the genus Erythrina.

Synthesis of β-galactosylamides as ligands of the peanut lectin. Insights into the recognition process

The synthesis of mono and divalent β-galactosylamides linked to a hydroxylated chain having a C2 symmetry axis derived from l-tartaric anhydride is reported. Reference compounds devoid of hydroxyl groups in the linker were also prepared from β-galactosylamine and succinic anhydride. After functionalization with an alkynyl residue, the resulting building blocks were grafted onto different azide-equipped scaffolds through the copper catalyzed azide-alkyne cycloaddition. Thus, a family of structurally related mono and divalent β-N-galactopyranosylamides was obtained and fully characterized. The binding affinities of the ligands towards the model lectin PNA were measured by the enzyme-linked lectin assay (ELLA). The IC₅₀ values were significantly higher than that of galactose but the presence of hydroxyl groups in the aglycone chain improved lectin recognition. Docking and molecular dynamics experiments were in accordance with the hypothesis that a hydroxyl group properly disposed in the linker could mimic the Glc O3 in the recognition process. On the other hand, divalent presentation of the ligands led to lectin affinity enhancements.

Synthesis of tetravalent LacNAc-glycoclusters as high-affinity cross-linker against Erythrina cristagalli agglutinin

Four kinds of tetravalent double-headed glycoclusters [(LacNAc)4-DHGs] were designed with linkers of varying lengths consisting of alkanedioic carboxyamido groups (C6, C12, C18 and C24) between two bi-antennary LacNAc-glycosides. These glycoclusters served as high-affinity cross-linking ligands for the LacNAc-binding lectin Erythrina cristagalli agglutinin (ECA). The binding activity and cross-linking between each ligand and ECA were characterized by a hemagglutination inhibition (HI) assay, isothermal titration calorimetry (ITC), a quantitative precipitation assay and dynamic light scattering (DLS). For the precipitation assay and DLS measurement, the synthesized (LacNAc)4-DHGs were found to be capable of binding and precipitating the ECA as multivalent ligands. ITC analysis indicated the binding of (LacNAc)4-DHGs was driven by a favorable enthalpy change. Furthermore, the entropy penalty from binding (LacNAc)4-DHGs clearly decreased in a spacer length-dependent manner. The binding affinities of flexible (LacNAc)4-DHGs (C18 and C24) with long spacers were found to be more favorable than those of the clusters having short spacers (C6 and C12). These results were supported by molecular dynamics simulations with explicit water molecules for the tetravalent glycoclusters with ECA. We concluded that the subtle modification in the epitope-presenting scaffolds exerts the significant effect in the recognition efficiency involved in the LacNAc moieties by ECA.

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.

Community-based network study of protein-carbohydrate interactions in plant lectins using glycan array data

Lectins play major roles in biological processes such as immune recognition and regulation, inflammatory responses, cytokine signaling, and cell adhesion. Recently, glycan microarrays have shown to play key roles in understanding glycobiology, allowing us to study the relationship between the specificities of glycan binding proteins and their natural ligands at the omics scale. However, one of the drawbacks in utilizing glycan microarray data is the lack of systematic analysis tools to extract information. In this work, we attempt to group various lectins and their interacting carbohydrates by using community-based analysis of a lectin-carbohydrate network. The network consists of 1119 nodes and 16769 edges and we have identified 3 lectins having large degrees of connectivity playing the roles of hubs. The community based network analysis provides an easy way to obtain a general picture of the lectin-glycan interaction and many statistically significant functional groups.

“Onion peel” dendrimers: a straightforward synthetic approach towards highly diversified architectures

We report herein a novel “onion peel strategy” for the divergent construction of glycodendrimers using different building blocks at each layer of the dendritic growth.

High-affinity multivalent wheat germ agglutinin ligands by one-pot click reaction

A series of six mono-, di-, and trivalent N,N’-diacetylchitobiose derivatives was conveniently prepared by employing a one-pot procedure for Cu(II)-catalyzed diazo transfer and Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) starting from commercially available amines. These glycoclusters were probed for their binding potencies to the plant lectin wheat germ agglutinin (WGA) from Triticum vulgaris by an enzyme-linked lectin assay (ELLA) employing covalently immobilized N-acetylglucosamine (GlcNAc) as a reference ligand. IC₅₀ values were in the low micromolar/high nanomolar range, depending on the linker between the two disaccharides. Binding enhancements β up to 1000 for the divalent ligands and 2800 for a trivalent WGA ligand, compared to N,N’-diacetylchitobiose as the corresponding monovalent ligand, were observed. Molecular modeling studies, in which the chitobiose moieties were fitted into crystallographically determined binding sites of WGA, correlate the binding enhancements of the multivalent ligands with their ability to bind to the protein in a chelating mode. The best WGA ligand is a trivalent cluster with an IC₅₀ value of 220 nM. Calculated per mol of contained chitobiose, this is the best WGA ligand known so far.

Design and synthesis of high-avidity tetravalent glycoclusters as probes for Sambucus sieboldiana agglutinin and characterization of their binding properties

We designed and synthesized three tetravalent sialo-glycoclusters that had different separations between the terminal sialic acids and the linking carboxy groups of the ethylene glycol bis(β-aminoethyl ether)-N,N,N',N'-tetraacetate scaffold to serve as ligands for the sialic acid-binding lectin Sambucus sieboldiana agglutinin (SSA). The interaction between each glycocluster and SSA was characterized by hemagglutination inhibition, quantitative precipitation, and double-diffusion assays. For the precipitation assays, the precipitin curves indicated that the ligands and SSA bound in either a 1:1 or a 1:2 ratio, i.e., stoichiometrically. The strong interactions of these sialo-glycoclusters with SSA could be ascribed to a combination of multivalency and spacer effects. We also assessed the nature of the ligand-SSA complexes by isothermal titration calorimetry and dynamic light scattering. The results of those experiments indicated that formation of intermolecular complexes occurred at less than stoichiometric ratios of ligand to SSA concentrations and that, as the concentrations of the ligands increased, larger cross-linked aggregates formed. Large aggregates that were present concurrently with visible precipitation and with a particle size centered at ~600 to 800 nm were identified by dynamic light scattering.

Differential expression and characterization of a member of the mucin-associated surface protein family secreted by Trypanosoma cruzi

We describe the characterization, purification, expression, and location of a 52-kDa protein secreted during interaction between the metacyclic form of Trypanosoma cruzi and its target host cell. The protein, which we have named MASP52, belongs to the family of mucin-associated surface proteins (MASPs). The highest levels of expression of both the protein and mRNA occur during the metacyclic and bloodstream trypomastigote stages, the forms that infect the vertebrate host cells. The protein is located in the plasma membrane and in the flagellar pockets of the epimastigote, metacyclic, and trypomastigote forms and is secreted into the medium at the point of contact between the parasite and the cell membrane, as well as into the host-cell cytosol during the amastigote stage. IgG antibodies specific against a synthetic peptide corresponding to the catalytic zone of MASP52 significantly reduce the parasite's capacity to infect the host cells. Furthermore, when the protein is adsorbed onto inert particles of bentonite and incubated with a nonphagocytic cell culture, the particles are able to induce endocytosis in the cells, which seems to demonstrate that MASP52 plays a role in a process whereby the trypomastigote forms of the parasite invade the host cell.