Binding of manno-oligosaccharides to concanavalin A. Substitution titration with a fluorescent-indicator ligand

Mannosylated Polymeric Ligands for Targeted Delivery of Antibacterials and Their Adjuvants to Macrophages for the Enhancement of the Drug Efficiency

Bacterial infections and especially resistant strains of pathogens localized in macrophages and granulomas are intractable diseases that pose a threat to millions of people. In this paper, the theoretical and experimental foundations for solving this problem are proposed due to two key aspects. The first is the use of a three-component polymer system for delivering fluoroquinolones to macrophages due to high-affinity interaction with mannose receptors (CD206). Cytometry assay determined that 95.5% macrophage-like cells were FITC-positive after adding high-affine to CD206 trimannoside conjugate HPCD-PEI1.8-triMan, and 61.7% were FITC-positive after adding medium-affine ligand with linear mannose label HPCD-PEI1.8-Man. The second aspect is the use of adjuvants, which are synergists for antibiotics. Using FTIR and NMR spectroscopy, it was shown that molecular containers, namely mannosylated polyethyleneimines (PEIs) and cyclodextrins (CDs), load moxifloxacin (MF) with dissociation constants of the order of 10⁻⁴–10⁻⁶ M; moreover, due to prolonged release and adsorption on the cell membrane, they enhance the effect of MF. Using CLSM, it was shown that eugenol (EG) increases the penetration of doxorubicin (Dox) into cells by an order of magnitude due to the creation of defects in the bacterial wall and the inhibition of efflux proteins. Fluorescence spectroscopy showed that 0.5% EG penetrates into bacteria and inhibits efflux proteins, which makes it possible to increase the maximum concentration of the antibiotic by 60% and maintain it for several hours until the pathogens are completely neutralized. Regulation of efflux is a possible way to overcome multiple drug resistance of both pathogens and cancer cells.

Spectroscopy Approach for Highly-Efficient Screening of Lectin-Ligand Interactions in Application for Mannose Receptor and Molecular Containers for Antibacterial Drugs

Rational search of a ligand for a specific receptor is a cornerstone of a typical drug discovery process. However, to make it more “rational” one would appreciate having detailed information on the functional groups involved in ligand-receptor interaction. Typically, the 3D structure of a ligand-receptor complex can be built on the basis of time-consuming X-ray crystallography data. Here, a combination of FTIR and fluorescence methods, together with appropriate processing, yields valuable information about the functional groups of both the ligand and receptor involved in the interaction, with the simplicity of conventional spectrophotometry. We have synthesized the “molecular containers” based on cyclodextrins, polyethyleneimines (PEI) or spermine with mannose-rich side-chains of different molecular architecture (reticulated, star-shaped and branched) with variable parameters to facilitate delivery to alveolar macrophages. We have shown that synthetic mannose-rich conjugates are highly affine to the model mannose receptor ConA: K_d ≈ 10⁻⁵–10⁻⁷ M vs. natural ligand trimannoside (10⁻⁵ M). Further, it was shown that molecular containers effectively load levofloxacin (dissociation constants are 5·10⁻⁴–5·10⁻⁶ M) and the eugenol adjuvant (up to 15–80 drug molecules for each conjugate molecule) by including them in the cyclodextrins cavities, as well as by interacting with polymer chains. Promising formulations of levofloxacin and its enhancer (eugenol) in star-shaped and polymer conjugates of high capacity were obtained. UV spectroscopy demonstrated a doubling of the release time of levofloxacin into the external solution from the complexes with conjugates, and the effective action time (time of 80% release) was increased from 0.5 to 20–70 h. The synergy effect of antibacterial activity of levofloxacin and its adjuvants eugenol and apiol on Escherichia coli was demonstrated: the minimum effective concentration of the antibiotic was approximately halved.

Computer simulation of the Receptor-Ligand Interactions of Mannose Receptor CD206 in Comparison with the Lectin Concanavalin A Model

Computer modeling of complexation of mono- and oligosaccharide ligands with the main (fourth) carbohydrate-binding domain of the mannose receptor CD206 (CRD4), as well as with the model receptor concanavalin A (ConA), was carried out for the first time, using methods of molecular dynamics and neural network analysis. ConA was shown to be a relevant model of CD206 (CRD4) due to similarity of the structural organization of the binding sites and high correlation of the values of free energies of complexation between the literature data and computer modeling (r > 0.9). Role of the main factors affecting affinity of the ligand–receptor interactions is discussed: the number and nature of carbohydrate residues, presence of Me-group in the O1 position, type of the glycoside bond in dimannose. Complexation of ConA and CD206 with ligands is shown to be energetically caused by electrostatic interactions (E) of the charged residues (Asn, Asp, Arg) with oxygen and hydrogen atoms in carbohydrates; contributions of hydrophobic and van der Waals components is lower. Possibility of the additional stabilization of complexes due to the CH–π stacking interactions of Tyr with the Man plane is discussed. The role of calcium and manganese ions in binding ligands has been studied. The values of free energies of complexation calculated in the course of molecular dynamics simulation correlate with experimental data (published for the model ConA): correlation coefficient r = 0.68. The Pafnucy neural network was trained based on the set of PDBbind2020 ligand–receptor complexes, which significantly increased accuracy of the energy predictions to r = 0.8 and 0.82 for CD206 and ConA receptors, respectively. A model of normalization of the complexation energy values for calculating the relevant values of ΔG_bind, K_d is proposed. Based on the developed technique, values of the dissociation constants of a series of CD206 complexes with nine carbohydrate ligands of different structures were determined, which were not previously known. The obtained data open up possibilities for using computer modeling for the development of optimal drug carriers capable of active macrophage targeting, and also determine the limits of applicability of using ConA as a relevant model for studying parameters of the CD206 binding to various carbohydrate ligands.

Concanavalin A: coordination diversity to xenobiotic metal ions and biological consequences

The binding ability of lectins has gained attention owing to the carbohydrate-specific interactions of these proteins. Such interactions can be applied to diverse fields of biotechnology, including the detection, isolation, and concentration of biological target molecules. The physiological aspects of the lectin concanavalin A (ConA) have been intensively studied through structural and functional investigations. X-ray crystallography studies have proven that ConA has two β-sheets and a short α-helix and that it exists in the form of a metalloprotein containing Mn²⁺ and Ca²⁺. These heterometals are coordinated with side chains located in a metal-coordinated domain (MCD), and they affect the structural environment in the carbohydrate-binding domain (CBD), which interacts with carbohydrates through hydrogen bonds. Recent studies have shown that ConA can regulate biophysical interactions with glycoproteins in virus envelopes because it specifically interacts with diverse polysaccharides through its CBD (Tyr, Asn, Asp, and Arg residues positioned next to the MCD). Owing to their protein-protein interaction abilities, ConA can form diverse self-assembled complexes including monomers, dimers, trimers, and tetramers, thus affording unique results in different applications. In this regard, herein, we present a review of the structural modifications in ConA through metal-ion coordination and their effect on complex formation. In recent approaches, ConA has been applied for viral protein detection, on the basis of the interactions of ConA. These aspects indicate that lectins should be thoroughly investigated with respect to their biophysical interactions, for avoiding unexpected changes in their interaction abilities.

Frontal affinity chromatography: An excellent method of analyzing weak biomolecular interactions based on a unique principle

BACKGROUND

Not only strong biomolecular interactions but also weak interactions play important roles in ensuring appropriate operations of many biological systems. Although a thorough investigation of the latter is essential in understanding life science, few suitable research tools are available because of inherent difficulties.

SCOPE OF REVIEW

Frontal affinity chromatography (FAC) is a versatile method that overcomes the inherent difficulties to provide accurate information on weak interactions. Since its concept and merit are not widely recognized, a comprehensive interpretation of FAC is provided in this review to encourage its application among researchers.

MAJOR CONCLUSION

FAC is based on a unique principle of measuring the binding strength by the delayed migration of an analyte through an affinity column. Its utility was elucidated via the lectin-glycan interactions.

GENERAL SIGNIFICANCE

FAC has a great potential as a research tool to solve many difficult problems in general bioscience that are relevant to almost all researchers.

Glyconanomaterials: synthesis, characterization, and ligand presentation

Glyconanomaterials, nanomaterials carrying surface-tethered carbohydrate ligands, have emerged and demonstrated increasing potential in biomedical imaging, therapeutics, and diagnostics. These materials combine the unique properties of nanometer-scale objects with the ability to present multiple copies of carbohydrate ligands, greatly enhancing the weak affinity of individual ligands to their binding partners. Critical to the performance of glyconanomaterials is the proper display of carbohydrate ligands, taking into consideration of the coupling chemistry, the type and length of the spacer linkage, and the ligand density. This article provides an overview of the coupling chemistry for attaching carbohydrate ligands to nanomaterials, and discusses the need for thorough characterization of glyconanomaterials, especially quantitative analyses of the ligand density and binding affinities. Using glyconanoparticles synthesized by a versatile photocoupling chemistry, methods for determining the ligand density by colorimetry and the binding affinity with lectins by a fluorescence competition assay are determined. The results show that the multivalent presentation of carbohydrate ligands significantly enhances the binding affinity by several orders of magnitude in comparison to the free ligands in solution. The effect is sizeable even at low surface ligand density. The type and length of the spacer linkage also affect the binding affinity, with the longer linkage promoting the association of bound ligands with the corresponding lectins.

A photochemically initiated chemistry for coupling underivatized carbohydrates to gold nanoparticles

The sensitive optoelectronic properties of metal nanoparticles make nanoparticle-based materials a powerful tool to study fundamental biorecognition processes. Here we present a new and versatile method for coupling underivatized carbohydrates to gold nanoparticles (Au NPs) via the photochemically induced reaction of perfluorophenylazide (PFPA). A one-pot procedure was developed where Au NPs were synthesized and functionalized with PFPA by a ligand-exchange reaction. Carbohydrates were subsequently immobilized on the NPs by a fast light activation. The coupling reaction was efficient, resulting in high coupling yield as well as high ligand surface coverage. A colorimetric system based on the carbohydrate-modified Au NPs was used for the sensitive detection of carbohydrate-protein interactions. Binding and cross-reactivity studies were carried out between carbohydrate-functionalized Au NPs and lectins. Results showed that the surface-bound carbohydrates not only retained their binding affinities towards the corresponding lectin, but also exhibited affinity ranking consistent with that of the free ligands in solution.

A novel application of thermo-responsive polymer to affinity precipitation of polysaccharide

The lectin receptors concanavalin A (ConA) and wheat germ lectin (WGL) were successfully conjugated to a thermo-responsive hydrogel polymer (poly-N-isopropylacrylamide) (PNIPAAm) as an affinity ligand. The coupling efficiencies of the ConA and WGL to PNIPAAm were 5.1% and 44%, respectively. These lectin receptor-polymer conjugates were then tested for their efficiency in purification of various polysaccharides or polysaccharide-containing compounds such as beta-glucan. Results indicated that these conjugates separated various polysaccharides from a complex mixture. The use of a thermo-responsive polymer in low-temperature purification of potentially heat-labile glycoproteins is advantageous. Additionally, other affinity ligands could be coupled to the polymer for separation of the respective bioactive molecules.

Binding specificity of mannose-specific carbohydrate-binding protein from the cell surface of Trypanosoma cruzi

The sugar binding specificity of the recently described mannose-specific carbohydrate-binding proteins (CBP) isolated to homogeneity from both the epimastigote and trypomastigote stages of the pathogenic protozoa Trypanosoma cruzi has been studied by quantitative hapten inhibition of the biotinylated CBPs to immobilized thyroglobulin using model oligosaccharides. The results clearly show a differential specificity toward high-mannose glycans between the CBPs from the two developmental stages. Thus, the isolated CBP from epimastigotes exhibited stronger affinity for higher mannose oligomers containing the Manalpha1-2Manalpha1-6Manalpha1-6 structure. Its affinity decreased, as did the number of mannose residues on the oligomer or removal of the terminal Manalpha1-2-linked mannose. By contrast the CBP isolated from the trypomastigote stage showed about 400-fold lower avidity than the epimastigote form, and contrary to it, it was slightly more specific toward Man5GlcNAc than Man9GlcNAc. Analysis of the interaction of epimastigote-Man-CBP with its ligands by UV difference spectroscopy indicates the existence of an extended binding site in that protein with a large enthalpic contribution to the binding. The thermodynamic parameters of binding were obtained by isothermal titration calorimetry and been found that the DeltaH values to be in good agreement with the van't Hoff values. The binding reactions are mainly enthalpically driven and exhibit enthalpy-enthropy compensation. In addition, analysis of the high-mannose glycans from different parts of the digestive tract of the reduviid insect vector of T. cruzi suggest a role of the CBP in the retention of the epimastigote stage in the anterior portion of the gut.

Structural basis of functional mimicry between carbohydrate and peptide ligands of con A

Crystallographic studies have shown independent binding sites for sugar and peptide ligands of concanavalin A, although they were considered functional mimics based on biochemical experiments. The topological correlation of 12-residue peptide with different carbohydrate ligands of concanavalin A showed similarity between trimannose and the YPY region of the peptide establishing structural mimicry. Molecular docking of trimannose and the YPY motif on the reciprocal binding sites revealed equivalent interactions and energetics implying that the peptide-binding sites may constitute additional sugar-binding subsites of concanavalin A. The binding of a mannose-rich neoglycoprotein with significantly higher affinity compared with that of the methyl alpha-d-mannopyranoside is consistent with this interpretation.

Synthesis and application of alpha-D-mannosyl clusters as photoaffinity ligands for mannose-binding proteins: concanavalin A as a model receptor

Mono-, di-, and tri-antennary alpha-D-mannopyranosyl derivatives were synthesized as oligosaccharide mimics. The compounds vary in the length of the acyclic aglyconic spacers linking, in the case of the di- and tri-antennary derivatives, the glycosyl endgroups. By haemagglutination assay (Coombs test) the affinity of the alpha-D-mannopyranosyl ligands against Con A was determined in comparison with methyl alpha-D-mannopyranoside. Affinity enhancement and strong cross-linking capacity were found with the di- and tri-antennary compounds where alpha-D-mannopyranosyl endgroups are separated by spacers of 5-37 atoms in length. The optimal ligand had Ki of 5.1 microM in comparison with 3.12 microM for methyl alpha-D-mannopyranoside. The monoantennary compound and, to certain extents, one di- and short tri-antennary ligands with short spacer lengths did not differ significantly in affinity from methyl alpha-D-mannopyranoside. A triantennary, radiolabelled ligand equipped with a photolabile diazirino group was used to covalently modify Con A by photoaffinity labelling. A significant degree of covalent cross-linking of Con A monomers was observed.

Thermodynamics of monosaccharide and disaccharide binding to Erythrina corallodendron lectin

Isothermal titration calorimetry measurements of the binding of 2'-fucosyllactose, lactose, N-acetyllactosamine, galactopyranose, 2-acetamido-2-deoxygalactopyranoside, methyl alpha-N-dansylgalactosaminide (Me-alpha-DNS-GalN), methyl alpha-D-galactopyranoside, methyl beta-D-galactopyranoside, and fucose to Erythrina corallodendron lectin (ECorL), a dimer with one binding site per subunit, were performed at 283-286 and 297-299 K. The site binding enthalpies, DeltaHb, with the exception of Me-alpha-DNS-GalN, are the same at both temperatures and range from -47.1 +/- 1.0 kJ mol-1 for N-acetyllactosamine to -4.4 +/- 0.3 kJ mol-1 for fucose, and the site binding constants range from 3.82 +/- 0.9 x 10(5)M-1 for Me-alpha-DNS-GalN at 283.2 K to 0.46 +/- 0.05 x 10(3) M-1 for fucose at 297.2 K. The binding reactions are mainly enthalpically driven except for fucose and exhibit enthalpy-entropy compensation. The binding enthalpies of the disaccharides are about twice the binding enthalpies of the monosaccharides in contrast to concanavalin A where the binding enthalpies do not double for the disaccharides. Differential scanning calorimetry measurements show that denaturation of the ECorL dimer results in dissociation into its monomer subunits. The binding constants from the increase in denaturation temperature of ECorL in the presence of saccharides are in agreement with values from isothermal titration calorimetry results. The thermal denaturation of ECorL occurs around 333 K, well below the 344-360 K denaturation temperature of other legume lectins of similar size and tertiary structure, undoubtedly due to the difference in its quaternary structure relative to other legume lectins. This is also apparent from the independent unfolding of its two domains.

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.

Interactions of concanavalin A with asparagine-linked glycopeptides. Structure/activity relationships of the binding and precipitation of oligomannose and bisected hybrid-type glycopeptides with concanavalin A

We have recently demonstrated that certain oligomannose and bisected hybrid-type glycopeptides are bivalent for concanavalin A (ConA) binding and that they can precipitate the lectin [Bhattacharyya, L., Ceccarini, C., Lorenzoni, P & Brewer, C. F. (1987) J. Biol. Chem. 262, 1288-1293]. Two protein-binding sites on each glycopeptide were identified: one on the alpha(1-6) arm of the core beta-mannose residue which binds with high affinity (primary site); the other on the alpha(1-3) arm of the core beta-mannose residue which binds with lower affinity (secondary site). In the present study, we have investigated the relationship between the structures of the primary sites of oligomannose-type glycopeptides and their affinities for ConA. Two mechanisms of binding at the primary sites of oligomannose-type glycopeptides have been identified which account for the 3000-fold increase in affinity of a Man9 glycopeptide relative to that of methyl alpha-D-mannopyranoside. Changes in the structures and affinities of both the primary and secondary sites are observed to influence the precipitation activities of the glycopeptides. These findings have important consequences for the specificity of ConA binding in solutions containing mixtures of the carbohydrates.

Determination of lectin-sugar dissociation constants by agarose affinity electrophoresis

Agarose crossed affinity electrophoresis (aff-EP) was employed for the determination of lectin-sugar dissociation constants (Ki). In the first dimension of the aff-EP increasing amounts of sugar (alpha-methyl-D-mannoside) were added to a given concentration of lectin (concanavalin A). Then the electrophoresis was run with alpha 1-acid glycoprotein, alpha 1-antitrypsin and alpha-fetoprotein as markers of lectin-sugar interactions. Mathematical equations for determination of the mechanisms and constants of lectin-sugar-glycoprotein interactions were developed. The mean value of the concanavalin A-alpha-methyl-D-mannoside dissociation constant calculated according to the introduced equations was 0.28 mM. In this system it was also possible to determine lectin-glycoprotein dissociation constants (K). The observed influence of the sugar on lectin-glycoprotein binding might be due to hydrophobic interactions since the addition of nonionic detergent caused reversal of this phenomenon.

Interaction of rice (Oryza sativa) lectin with N-acetylglucosaminides. Fluorescence studies

The interaction of lectin isolated from rice (Oryza sativa) embryos with N-acetylglucosaminides was studied by equilibrium dialysis and fluorescence. Equilibrium dialysis with 4-methylumbelliferyl-(GlcNac)2 showed that rice lectin (Mr 38000) contains four equivalent saccharide-binding sites. Addition of the N-acetylglucosaminides GlcNac, (GlcNac)2 and (GlcNac)3 enhanced the intrinsic fluorescence of rice lectin and this was accompanied by a 10nm blue-shift of its maximum fluorescence with (GlcNac)2 and (GlcNac)3. These changes in intensity allowed determination of the association constants, which increased with the number of saccharide units: at 20 degrees C, Ka = (1.3 +/- 0.1) X 10(3), (5.1 +/- 0.4) X 10(4) and (2.6 +/- 0.1) X 10(5) M-1 for GlcNac, (GlcNac)2 and (GlcNac)3 respectively. The binding enthalpy, delta H0, for the three glucosaminides were very low and ranged from -12.1 to -20.6 kJ X mol-1. The results are compared with those obtained with wheat-germ agglutinin, another GlcNac-specific gramineaous lectin.

Binding of simple carbohydrates and some N-acetyllactosamine-containing oligosaccharides to Erythrina cristagalli agglutinin as followed with a fluorescent indicator ligand

Erythrina cristagalli agglutinin, a dimeric lectin [J.L. Iglesias, et al. (1982) Eur. J. Biochem. 123, 247-252] was shown by equilibrium dialysis to be bivalent for 4-methylumbelliferyl-beta-D-galactoside. Upon binding to the lectin, this ligand showed a difference absorption spectrum with two maxima (at 322 and 336 nm) of equal intensity (delta epsilon = 1.2 X 10(3) M-1 cm-1). A similar spectrum with a comparable value of delta epsilon was obtained with 4-methylumbelliferyl-N-acetyl-beta-D-galactosaminide. Binding of methyl-alpha-D-galactoside, lactose, and N-acetyllactosamine all produced small but equally intense protein difference spectra with a maximum (delta epsilon = 2.8 X 10(2) M-1 cm-1) at 291.6 nm. Upon binding of N-dansyl-D-galactosamine to the lectin, there was a fivefold increase in fluorescence intensity of this ligand. The association constant for N-dansyl-D-galactosamine was caused by a very favorable delta S degree of the dansyl group without affecting the strictly carbohydrate-specific character of binding. N-Dansyl-D-galactosamine was employed as a fluorescent indicator ligand in substitution titrations. This involved the use of simple carbohydrates, N-acetyllactosamine, and oligosaccharides which occur in the carbohydrate units of N-glycoproteins; the latter were Gal(beta 1----4)GlcNAc(beta 1----2)Man, Gal(beta 1----4)GlcNAc(beta 1----6)Man, and Gal(beta 1----4)GlcNAc(beta 1----6)[Gal(beta 1----4)GlcNAc(beta 1----2)]Man. The titrations were performed at two temperatures to determine the thermodynamic parameters. In the series N-acetyl-D-galactosamine, methyl-alpha-D-galactoside, and lactose, -delta H degrees increased from 24 to 41 kJ mol-1; it increased further for N-acetyllactosamine and then remained unchanged for the N-acetyllactosamine-containing oligosaccharides (55 +/- 1 kJ mol-1. This indicated that the site specifically accommodated the disaccharide structure with an important contribution of the 2-acetamido group in the penultimate sugar. Beyond this, no additional contacts seemed to be formed. This conclusion also followed from considerations of delta S degrees values which became more unfavorable in the above series (-23 to -101 +/- 4 J mol-1 K-1); the most negative value of delta S degrees was observed with N-acetyllactosamine and the three N-acetyllactosamine-containing oligosaccharides.

Temperature-induced ultraviolet difference absorption spectrometry for determination of enthalpy changes. Binding of 4-methylumbelliferyl glycosides to four lectins

Raising the temperature in a single mixture of a lectin and a chromophoric glycoside allows determination of the binding enthalpy. This is made possible by continuously monitoring the displacement of the complex from its equilibrium concentration with a sensitive difference absorption spectrophotometer. The method is illustrated with the following lectins: concanavalin A, soybean agglutinin, peanut agglutinin and Erythrina cristagalli agglutinin. The ligands are 4-methylumbelliferyl glycosides. The binding enthalpies found range from -60 kJ X mol-1 for the Gal beta 1----3GalNAc-beta glycoside and peanut agglutinin to -30 kJ X mol-1 for a monosaccharide glycoside and the other lectins.

Calorimetric study of carbohydrate binding to concanavalin A

Flow microcalorimetry has been used to examine the delta H of binding of two types of saccharides, a series of simple monosaccharides and a series of alpha-(1----4)-linked glucosides, to the lectin Concanavalin A. It has been found that the delta H decreases with any change in the stereochemistry of a hydroxyl group relative to methyl alpha-D-mannopyranoside. The data have allowed the calculation of the relative contribution of two of the hydroxyl groups. The delta H's of binding for the alpha-(1----4)-linked glucosides are approximately 31 kJ/mol, and the apparent association constants vary insignificantly with increasing length. This result indicates that only one glucose residue binds to concanavalin A by hydrogen bonds, and that the additional glucose residues have no interaction either by hydrogen bonds or by nonspecific hydrophobic interactions. This result confirms the absence of an extended binding site for alpha-(1----4)-linked glucopyranosides, in contrast to that proposed for alpha-(1----2)-linked mannopyranosides which show an increase in apparent association constants with increasing length.

4-Methylumbelliferyl-glycosides as fluorescence probes of sugar-binding sites on lectin molecules: spectral properties and dependence of fluorescence on polarity and viscosity

The spectral properties of 4-methylumbelliferyl-glycosides (MeUmb-glycosides) were investigated in order to assess their usefulness as probes of the microenvironment of sugar binding sites on lectin molecules. It was shown that the abnormally high values for fluorescence polarization of free MeUmb-glycosides (from 0.07 to 0.251) were due neither to their molecular size nor to the blockade of their movement, but to the short lifetimes (less than 0.55 ns) of the excited state of these compounds. Working essentially with two MeUmb-monosaccharides and one MeUmb-disaccharide (MeUmb-alpha-D-galactopyranoside, MeUmb-beta-D-galactopyranoside, and MeUmb-2-acetamido-2-deoxy-3-O-(beta-D-galactopyranosyl)-beta-D- galactopyranoside) which were solubilized in various solvents, it was demonstrated that solvent polarity and viscosity definitely affected the fluorescence intensity of MeUmb-glycosides. A low-polarity medium reduced this intensity, and high viscosity enhanced it. The implications of these findings are discussed in relation to the variations in the fluorescence intensity of MeUmb-glycosides when these compounds were bound to lectins.

Salt-induced inhibition of the precipitin reaction of concanavalin A with polysaccharides and glycoprotein

The time course of the precipitin reactions of concanavalin A with glycogen, dextran and ovalbumin was investigated by a light-scattering method near 30 degrees C in 10 mM-Tris/HCl buffer, pH 7.4, containing neutral salts, i.e. NaCl, KCl, NaBr, KI and NaClO4. With 0.8 microM-lectin and 0.36 mg of glycogen/ml, the half-life, t 1/2, of the precipitin reaction was independent of salt concentration between 0.1 M and 1.5 M, and was the same (175s) in the presence of NaCl, KCl, NaBr and KI but was significantly (27%) higher in NaClO4. In contrast, the five salts caused significant to marked enhancement in t 1/2 for the reactions of concanavalin A with dextran and ovalbumin. Likewise, whereas the turbidity produced in 1 h as a result of lectin-glycogen precipitation remained unchanged, those measured for the binding of dextran and ovalbumin were decreased in the presence of three salts. The increase in t 1/2 and decrease in turbidity were found to be higher with NaClO4, followed by KI; NaBr produced moderate and NaCl (or KCl) small but generally significant inhibition of the precipitin reactions with dextran and ovalbumin. The results showed that the lectin-ligand precipitin reactions involve salt-sensitive polar interactions that are less pronounced with compactly folded ligands such as glycogen.

Metal ion binding and conformational transitions in concanavalin A: a structure-function study

The affinity of the lectin Concanavalin A (Con A) for saccharides, and its requirement for metal ions such as Mn2+ and Ca2+, have been known for about 50 years. However the relationship between metal ion binding and the saccharide binding activity of Con A has only recently been examined in detail. Brown et al. (Biochemistry 16, 3883 (1977)) showed that Con A exists as a mixture of two conformational states: a "locked" form and an "unlocked" form. The unlocked form of the protein weakly binds metal ions and saccharide, and is the predominate conformation of demetallized Con A (apo-Con A) at equilibrium. The locked form binds two metal ions per monomer with the resulting complex(es) possessing full saccharide binding activity. Brown and coworkers measured the kinetics of the transition of the unlocked form to the fully metallized locked conformation containing Mn2+ and Ca2+. They also demonstrated that Mn2+ alone could form a locked ternary complex with Con A, and that rapid removal of the ions resulted in a metastable form of apo-Con A in the locked conformation which slowly (hours at 25 degrees C) reverted back to (predominantly) the unlocked conformation. The ability to form either conformation in the absence or presence of metal ions has thus allowed us to explore the relationship between metal ion binding and conformational transitions in Con A as determinants of the saccharide binding activity of the lectin. Based on the kinetics of the transition of unlocked apo-Con A to fully metallized locked Con A, and X-ray crystallographic data, it appears that the transition between the two conformations of Con A involves a cis-trans isomerization of an Ala-Asp peptide bond in the backbone of the protein, near one of the two metal ion binding sites. The relatively large activation energy for the transition (approximately 22 kcal M-1) results in relatively slow interconversions between the conformations (from minutes to days), whereas the equilibria with metal ions and saccharide are rapid. Thus, many metastable complexes can be formed and a variety of transition pathways between the two conformations studied. We have identified and characterized binary, ternary, and quaternary complexes of both conformations of Con A containing Mn2+ and saccharide, and have determined both metal ion and saccharide dissociation constants for all of them, as well as equilibrium and kinetic values for the conformational transitions between them.(ABSTRACT TRUNCATED AT 400 WORDS)

Manganese, calcium, and saccharide binding to concanavalin A, as studied by ultrafiltration

The binding of the ligands Mn2+, Ca2+, and methyl alpha-D-glucopyranoside to concanavalin A, purified as described (A.J. Sophianopoulos and J.A. Sophianopoulos (1981) Prep. Biochem. 11, 413-435), was studied by ultrafiltration in 0.2 M NaCl, pH 5.2 and pH 6.5 to 7, and at 23 to 25 degrees C. The association constant (Ka) of methyl alpha-D-glucopyranoside to concanavalin A was (2 +/- 0.2) X 10(3) M-1, both at pH 5.2 and 7. At pH 5.2 and in the absence of Ca2+, the Ka of Mn2+ to concanavalin A was (5 +/- 1) X 10(3) M-1, and in the presence of 1 mM Ca2+, the Ka was (9.1 +/- 2.1) X 10(5) M-1. At pH 6.5 Mn2+ bound to concanavalin A with a Ka of (7.3 +/- 1.8) X 10(5) M-1, and the binding affinity was virtually independent of the presence of Ca2+. Experiments of binding of 4-methylumbelliferyl alpha-D-mannopyranoside to concanavalin A indicated that at pH 5.2, binding of a single Mn2+ per concanavalin A monomer was sufficient to induce a fully active saccharide binding site. Ca2+ is not necessary for such activation, but rather it increases the affinity of concanavalin A for binding Mn2+.

Release of cell-associated concanavalin A by methyl alpha-D-mannopyranoside reveals three binding states of concanavalin-A receptors on mouse fibroblasts

Based on the partial reversibility of concanavalin A binding by saturating concentrations of methyl alpha-D-mannopyranoside (MeManp) three states of cellular association could be characterized: type I, most rapidly established and most tightly bound, not released by MeManp at 0 degrees C or at 37 degrees C; type II, most loosely bound, released by MeManp at 0 degrees C, therefore not critically dependent on the temperature at which the release reaction is performed; type III, intermediate strength of binding, released by MeManp only at 37 degrees C, thus reflecting the temperature-sensitive nature of these cell complexes. A similar temperature dependence was found for cell-bound concanavalin A when it was displaced by an excess of the same lectin. The types of binding are seen irrespective of the temperature at which the cellular association was established. About 10% of the concanavalin A molecules bind to receptor structures in a saturable way and represent the type I association. Type I association is clearly distinct from types II and III. Type II seems to be the precursor of type III. Most of the type II associations can be converted into type III associations since ConA-cell complexes originally sensitive to the action of MeManp at low temperature gradually do become resistant after prolonged association at the same temperature. The temperature dependence in binding to cells was, however, not related to receptor mobility since glutaraldehyde-treated cells had most of the release properties that were observed in untreated cells and which did not markedly differ between 3T3 cells and their SV40-transformed counterparts. In contrast to cellular binding, dissociation of concanavalin A from Sephadex beads by saturating concentrations of MeManp was complete irrespective of temperature.

Thermodynamics of binding between saccharides and Helix pomatia A hemagglutinin

Binding constants for the interactions between Helix pomatia A hemagglutinin and the following saccharides were estimated at pH 7.0 and 25 degree C, using the circular dichroism method: N-acetyl-D-galactosamine, 5700 M-1; N-acetyl-D-glucosamine, 1000 M-1; melibiose, 86 M-1; raffinose, 350 M-1. The binding of N-acetyl-D-galactosamine to Helix pomatia A hemagglutinin was investigated in detail, using the circular dichroism and fluorescence methods, over the temperature range 5-50 degree C and pH range 7.0-2.5. The thermodynamic parameters, delta H degree (kcal . mol-1), delta G degree (kcal . mol-1), and delta S degree (e.u.), for this binding reaction at 25 degrees C were estimated as follows: -11.3, -5.24, -20.3 at pH 7.0; -9.1, -5.71, -11.2 at pH 4.5; -38.3, -3.19, -118 at pH 2.5. The negative values of delta H degrees and delta S degrees at pH 2.5 were especially large. This may be related to the restoration of the Helix pomatia A hemagglutinin molecule or its binding site from an unstable configuration caused by lowering the pH of the reaction medium to the original configuration of the presence of N-acetyl-D-galactosamine.

Binding of disaccharides by peanut agglutinin as studied by ultraviolet difference spectroscopy

The binding of the disaccharides methyl beta-D-lactoside and 2-acetamido-2-deoxy-3-O-(beta-D-galactopyranosyl)-beta-D-galactopyranose [beta-D-Gal-(l leads to 3)-D-GalNAc] to peanut agglutinin was studied by ultraviolet difference spectroscopy. The magnitude of the difference spectra varied with the concentration of the carbohydrates; association constants and thermodynamic parameters were determined from titration experiments at different temperatures. The enthalpy and entropy changes for binding of methyl beta-D-lactoside were found to be delta H degree = -65 +/- 4 kJ mol-1, delta S degree = -156 +/- 14 J mol-1 K-1. For beta-D-Gal-(1 leads to 3)-D-GalNAc the observed thermodynamic parameters were delta H degree = -78 +/- 5 kJ mol-1,, delta S degree = -177 +/- 16 J mol-1 K-1. For both disaccharides, the enthalpy change upon binding to the lectin is much larger than found for the binding site on peanut agglutinin. The observed parameters are compared with those found for the binding of monosaccharides and oligosaccharides to other lectins and to lysozyme. Molecular models of the minimum energy conformers of beta-D-Gal(1 leads to 3)-D-GalNAc and methyl beta-D-lactoside are used to interpret the interaction of these, and structurally related ligands, with the peanut agglutinin binding site.

Kinetics of binding of methyl alpha- and beta-D-galactopyranoside to peanut agglutinin: a carbon-13 nuclear magnetic resonance study

The binding kinetics of methyl alpha- and methyl beta-D-galactopyranoside to the anti-T lectin from peanuts were studied by 13C NMR, employing methyl galactopyranosides specifically enriched in 13C at C-1. Association and dissociation rate constants, as well as their activation parameters, are reported. The association rate constants, 4.6 X 10(4) M-1 s-1 for the alpha-galactopyranoside and 3.6 X 10(4) M-1 s-1 for the beta-galactopyranoside, are several orders of magnitude below those expected for a diffusion-controlled process. For both anomers, the association rate constant was temperature independent, implying that the association process occurs without a significant activation enthalpy. However, a considerable association activation entropy was found for both ligands. The dissociation rate constants were in the range of 9-46 s-1 within a temperature range of 5-35 degrees C for the alpha-galactopyranoside, and in the range of 9-39 s-1 within a temperature range of 5-25 degrees C for the beta-galactopyranoside. A considerable dissociation activation enthalpy of ca. 10 kcal mol-1 was found for both anomers. A two-step binding model, consistent with the present NMR data and with previous UV and CD spectroscopic data, is presented.