Protein-carbohydrate interaction. On the mode of bonding of aromatic moieties to concanavalin A, the phytohemagglutinin of the jack bean

Sequence-Defined Introduction of Hydrophobic Motifs and Effects in Lectin Binding of Precision Glycomacromolecules

This study investigates the influence of an increasingly hydrophobic backbone of multivalent glycomimetics based on sequence-defined oligo(amidoamines) on their resulting affinity toward bacterial lectins. Glycomacromolecules are obtained by stepwise assembly of tailor-made building blocks on solid support, using both hydrophobic aliphatic and aromatic building blocks to enable a gradual change in hydrophobicity of the backbone. Their binding behavior toward model lectin Concanavalin A (ConA) is evaluated using isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR) showing higher affinities for glycomacromolecules with higher content of hydrophobic and aromatic moieties in the backbone. Finally, glycomacromolecules are tested in a bacterial adhesion inhibition study against Escherichia coli where more hydrophobic backbones yield higher inhibitory potentials most likely due to additional secondary interactions with hydrophobic regions of the protein receptor as well as a change in conformation exposing carbohydrate ligands for increased binding. Overall, the results highlight the influence and thereby importance of the polymer backbone itself on the resulting properties of polymeric biomimetics.

Streptococcus pneumoniae fructose-1,6-bisphosphate aldolase, a protein vaccine candidate, elicits Th1/Th2/Th17-type cytokine responses in mice

Streptococcus pneumoniae (S. pneumoniae) is a major pathogen worldwide. The currently available polysaccharide-based vaccines significantly reduce morbidity and mortality. However, the inherent disadvantages of the currently available polysaccharide-based vaccines have motivated the search for other bacterial immunogens capable of eliciting a protective immune response against S. pneumoniae. Fructose-1,6-bisphosphate aldolase (FBA) is a glycolytic enzyme, which was found to localize to the bacterial surface, where it functions as an adhesin. Previously, immunizing mice with recombinant FBA (rFBA) in the presence of alum elicited a protective immune response against a lethal challenge with S. pneumoniae. Thus, the aim of the present study was to determine the cytokine responses that are indicative of protective immunity following immunization with rFBA. The protective effects against pneumococcal challenge in mice immunized with rFBA with complete Freund's adjuvant (CFA) in the initial immunization and with incomplete Freund's adjuvant (IFA) in booster immunizations surpassed the protective effects observed following immunization with either rFBA + alum or pVACfba. CD4+ T-cells obtained from the rFBA/CFA/IFA/IFA-immunized mice co-cultured with rFBA-pulsed antigen-presenting cells (APCs), exhibited a significantly greater proliferative ability than CD4+ T-cells obtained from the adjuvant-immunized mice co-cultured with rFBA‑pulsed APCs. The levels of the Th1-type cytokines, interferon (IFN)-γ, interleukin (IL)-2, tumor necrosis factor (TNF)-α and IL-12, the Th2-type cytokines, IL-4, IL-5 and IL-10, and the Th17-type cytokine, IL-17A, significantly increased within 72 h of the initiation of co-culture with CD4+ T-cells obtained from the rFBA‑immunized mice, in comparison with the co-cultures with CD4+ T-cells obtained from the adjuvant-immunized mice. Immunizing mice with rFBA resulted in an IgG1/IgG2 ratio of 41, indicating a Th2 response with substantial Th1 involvement. In addition, rabbit and mouse anti-rFBA antisera significantly protected the mice against a lethal S. pneumoniae challenge in comparison with preimmune sera. Our results emphasize the mixed involvement of the Th1, Th2 and Th17 arms of the immune system in response to immunization with pneumococcal rFBA, a potential vaccine candidate.

Non-carbohydrate inhibitors of the lectin DC-SIGN

The C-type lectin dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) is found on the surface of dendritic cells. It can mediate adhesion between dendritic cells and T lymphocytes and facilitate antigen capture and presentation. Many pathogens can exploit DC-SIGN binding for nefarious purposes. For example, DC-SIGN can facilitate the dissemination of viruses, like HIV-1. Alternatively, some microbes (e.g., Mycobacterium tuberculosis) use their ability to interact with DC-SIGN to evade immune detection. The diverse roles attributed to DC-SIGN provide impetus to identify ligands that can be used to explore its different functions. Such compounds also could serve as therapeutic leads. Most of the DC-SIGN ligands studied previously are mannose- or fucose-derived monosaccharides or oligosaccharides with inhibitory constants in the range of 0.1-10 mM. To identify monovalent ligands with more powerful DC-SIGN blocking properties, we devised a high-throughput fluorescence-based competition assay. This assay afforded potent non-carbohydrate, small molecule inhibitors (IC50 values of 1.6-10 microM). These compounds block not only DC-SIGN-carbohydrate interactions but also DC-SIGN-mediated cell adhesion. Thus, we anticipate that these non-carbohydrate inhibitors can be used to illuminate the role of DC-SIGN in pathogenesis and immune function.

Hyaluronan-based Glycoclusters as Probes for Chemical Glycobiology

A strategy is described for the synthesis of β-(1,3)-GlcA-GlcNAc dimeric and tetrameric glycoclusters through the conjugation of disaccharide groups onto a diaminodiamide aromatic scaffold by reductive amination.

Lectins: tools for the molecular understanding of the glycocode

Recent progress in glycobiology has revealed that cell surface oligosaccharides play an essential role in recognition events. More precisely, these saccharides may be complexed by lectins, carbohydrate-binding proteins other than enzymes and antibodies, able to recognise sugars in a highly specific manner. The ubiquity of lectin-carbohydrate interactions opens enormous potential for their exploitation in medicine. Therefore, extraordinary effort is made into the identification of new lectins as well as into the achievement of a deep understanding of their functions and of the precise mechanism of their association with specific ligands. In this review, a summary of the main features of lectins, particularly those found in legumes, will be presented with a focus on the mechanism of carbohydrate-binding. An overview of lectin-carbohydrate interactions will also be given, together with an insight into their energetics. In addition, therapeutic applications of lectins will be discussed.

Synthesis and biological properties of mannosylated starburst poly(amidoamine) dendrimers

Starburst PAMAM dendrimers ending with mannopyranoside residues were readily synthesized in large scale and good yields from commercially available dendrimers bearing high-density amine functionality on their surface and p-isothiocyanatophenyl 2,3,4,6-tetra-O-acetyl-alpha-D-mannopyranoside. The first four generations of this novel class of monodispersed neoglycoconjugates having up to 32 mannoside units were evaluated as ligands for the phytohemagglutinins from concanavalin A (Con A) and Pisum sativum (pea lectin) using enzyme-linked lectin assay (ELLA) and turbidimetric analyses. The binding properties of these glycodendrimers, together with reference monosaccharides, were determined using yeast mannan as a coating antigen and peroxidase-labeled lectins. These mannosylated dendrimers were demonstrated to be potent inhibitors with IC50 values 400 times better than those of monomeric methyl alpha-D-mannopyranoside taken as a standard. Their lipophilic character was shown to be sufficient for their direct use as coating antigens in microtiter plate assays. Moreover, their ability to bind and form insoluble carbohydrate-lectin complexes was also demonstrated by radial double immunodiffusion and turbidimetric analyses. Furthermore, the ability of these ligands to selectively precipitate a mannose-binding protein (Con A) from a crude lectin mixture was also demonstrated using polyacrylamide gel electrophoresis (SDS-PAGE). These multivalent neoglycoconjugates were shown to constitute novel biochromatography materials of high affinity for the easy isolation of carbohydrate-binding proteins.

Modes of association of concanavalin A with alpha-D-glycosides

Complexes of Con A with alpha-D-glycosides were studied using 1H-NMR, ESR and fluorescence methods. Correlation times, tau c, for the interaction of the aglycon protons with the manganese ion, present at the S1 site of the protein, were calculated from T1 measurements at two frequencies. The protons of aromatic aglycons have tau c values comparable to the rotational correlation time of the protein molecule, whereas those of non-aromatic aglycons have tau cs 10 to 100 times lower. The correlation times were combined with the experimentally acquired paramagnetic contributions to proton relaxation due to the presence of the manganese ion to yield manganese-proton distances. These distances show that aromatic aglycons have additional favorable contacts with the protein which stabilize the lectin-saccharide interaction. The results are compared to the crystal structure of the methyl alpha-D-glycopyranoside complex with Con A and to models earlier proposed for the binding of monosaccharides to Con A.

Synthesis of high-affinity, hydrophobic monosaccharide derivatives and study of their interaction with concanavalin A, the pea, the lentil, and fava bean lectins

Concanavalin A (Con A) and agglutinins from the pea (PSA), lentil (LCH), and fava bean (VFA) constitute a group of D-mannose/D-glucose binding legume lectins. In addition to their sugar binding specificity, these lectins also contain sites that bind hydrophobic ligands. The present study explores a class of nonpolar binding sites reportedly present adjacent to the carbohydrate binding site in PSA, LCH, and VFA. A series of 2-O- and 3-O-substituted nitrobenzoyl and nitrobenzyl derivatives of methyl alpha-D-glucopyranoside and methyl alpha-D-mannopyranoside were synthesized. Evaluation of their binding to Con A, PSA, LCH, and VFA was carried out by the technique of hapten inhibition of precipitation reaction. The hapten inhibition assay results reveal that the presence of a methyl or methylene group at the O-2 or O-3 position of the sugar is essential for hydrophobic interaction with PSA, LCH, and VFA. The substitution of methyl by nitrobenzyl leads to enhanced binding (1.7-16.7 times for the 2-O-substituted compounds and 7.9-40.5 times for the 3-O-substituted compounds) with the m-nitrobenzyl group contributing to maximum binding. A hydrophobic interaction is also involved between Con A and 2-O-nitrobenzyl derivatives, resulting in enhanced binding, but the corresponding 3-O-isomers bind poorly due probably to steric reasons. These results may be rationalized on the basis of the recently published X-ray data of Con A and VFA. The nitrobenzyl derivatives, after transformation to their azido analogs, have potential applications in the photoaffinity labeling of these lectins.

Carbohydrate binding properties of the stinging nettle (Urtica dioica) rhizome lectin

The interaction of the stinging nettle rhizome lectin (UDA) with carbohydrates was studied by using the techniques of quantitative precipitation, hapten inhibition, equilibrium dialysis, and uv difference spectroscopy. The Carbohydrate binding site of UDA was determined to be complementary to an N,N',N"-triacetylchitotriose unit and proposed to consist of three subsites, each of which has a slightly different binding specificity. UDA also has a hydrophobic interacting region adjacent to the carbohydrate binding site. Equilibrium dialysis and uv difference spectroscopy revealed that UDA has two carbohydrate binding sites per molecule consisting of a single polypeptide chain. These binding sites either have intrinsically different affinities for ligand molecules, or they may display negative cooperativity toward ligand binding.

Protection of sodium dodecyl sulfate-induced aggregation of concanalvalin A by saccharide ligands

Concanavalin A is visibly aggregated by low concentrations of sodium dodecyl sulfate, maximum aggregation being obtained at pH 4.6. Other denaturants, such as urea, guanidine hydrochloride, Triton X-100, cetyltrimethylammonium bromide, Tween 80, and Brij 35 are ineffective in promoting visible aggregation. The sodium dodecyl sulfate-induced aggregation of concanavalin A requires the presence of an intact, saccharide-ligand binding-site. Rapid and complete reversal of the detergent effect was achieved by use of saccharides which bind to the lectin. Such compounds as tryptophan and o-nitrophenyl beta-D-galactopyranoside did not inhibit the aggregation of concanavalin A by sodium dodecyl sulfate, suggesting that the detergent does not bind the hydrophobic pocket on the surface of the protein. The results suggest that concanavalin A may have an additional, ligand-binding site which is metal-dependent and which can be modified by the addition of a saccharide ligand.

Studies on the combining sites of concanavalin A

The initial event in the biological activity of concanavalin A (Con A) involves binding of the protein to cell surface receptors. The nature and mechanism whereby such binding may occur is described in terms of cell surface carbohydrates and the demonstrated specificity of the protein. Although considerable latitude is tolerated at the C-2 position of the alpha-D-hexopyranose ring system, the carbohydrate binding site of Con A appears to be complemnetary to alpha-D-mannopyranosyl residues. Hapten inhibition studies indicate that each of the hydroxyl groups of this sugar is probably involved in the binding mechanism. Of the common sugars present on cell surfaces (D-glucose, D-mannose and N-acetyl-D-glucosamine), it is probably alpha-D-mannopyranosyl residues which react with Con A. Since the latter units are primary receptors for Con A. Evidence supporting this view includes hapten inhibition studies with model oligosaccarides and preciptin studies with macromolecules containing internal 2-o-substituted alpha-D-mannopyranosyl residues. The binding to Con A of a series of oligosaccharides containing alpha-(1leads to2)-linked D-mannosyl units appears to increase up to the tetraose and then decreases; several possible explanations are considered. Acetylated Con A, although retaining its specificity, is about 50% as active as the native protein. Some biological properties of the modified protein are described. Data suggesting that Con A behaves differently in the solution phase than in the crystalline state are presented in terms of UV difference displacement studies. It is suggested that the so-called carbohydrate binding site reportedly identified in Con A crystals may not be correct.

The covalent and three-dimensional structure of concanavalin A

The tentative amino-acid sequence and three-dimensional structure of the lectin concanavalin A have been determined. The amino-acid sequence, which was determined chemically, contains 238 residues. The sequences of three short stretches were assigned on the basis of x-ray crystallographic data. Interpretation of an electron density map at 2-A resolution indicates that the predominant structural element is extended polypeptide chain arranged in two anti-parallel pleated sheets or beta-structures. Residues not included in the beta-structures are arranged in regions of random coil. One of the pleated sheets contributes extensively to the interactions among the monomers to form both dimers and tetramers. The positions at which Mn(2+), Ca(2+), and saccharide are bound to the protein, and the point of cleavage for the formation of the naturally occurring fragments A(1) and A(2), have been tentatively assigned. Both metal-binding sites are at least 20-A removed from the position at which saccharides are bound. The saccharide-binding site is a deep pocket of approximately 6A x 7.5A x 18A, the inner portion of which is occupied by hydrophobic residues.