Effect of substituent on the thermodynamics of D-glucopyranoside binding to concanavalin A, pea (Pisum sativum) lectin and lentil (Lens culinaris) lectin

Synthesis and unexpected binding of monofluorinated N,N'-diacetylchitobiose and LacdiNAc to wheat germ agglutinin

Fluorination of carbohydrate ligands of lectins is a useful approach to examine their binding profile, improve their metabolic stability and lipophilicity, and convert them into 19F NMR-active probes. However, monofluorination of monovalent carbohydrate ligands often leads to a decreased or completely lost affinity. By chemical glycosylation, we synthesized the full series of methyl β-glycosides of N,N'-diacetylchitobiose (GlcNAcβ(1-4)GlcNAcβ1-OMe) and LacdiNAc (GalNAcβ(1-4)GlcNAcβ1-OMe) systematically monofluorinated at all hydroxyl positions. A competitive enzyme-linked lectin assay revealed that the fluorination at the 6'-position of chitobioside resulted in an unprecedented increase in affinity to wheat germ agglutinin (WGA) by one order of magnitude. For the first time, we have characterized the binding profile of a previously underexplored WGA ligand LacdiNAc. Surprisingly, 4'-fluoro-LacdiNAc bound WGA even stronger than unmodified LacdiNAc. These observations were interpreted using molecular dynamic calculations along with STD and transferred NOESY NMR techniques, which gave evidence for the strengthening of CH/π interactions after deoxyfluorination of the side chain of the non-reducing GlcNAc. These results highlight the potential of fluorinated glycomimetics as high-affinity ligands of lectins and 19F NMR-active probes.

Fluorinated Carbohydrates as Lectin Ligands: Simultaneous Screening of a Monosaccharide Library and Chemical Mapping by 19F NMR Spectroscopy

Molecular recognition of carbohydrates is a key step in essential biological processes. Carbohydrate receptors can distinguish monosaccharides even if they only differ in a single aspect of the orientation of the hydroxyl groups or harbor subtle chemical modifications. Hydroxyl-by-fluorine substitution has proven its merits for chemically mapping the importance of hydroxyl groups in carbohydrate-receptor interactions. 19F NMR spectroscopy could thus be adapted to allow contact mapping together with screening in compound mixtures. Using a library of fluorinated glucose (Glc), mannose (Man), and galactose (Gal) derived by systematically exchanging every hydroxyl group by a fluorine atom, we developed a strategy combining chemical mapping and 19F NMR T2 filtering-based screening. By testing this strategy on the proof-of-principle level with a library of 13 fluorinated monosaccharides to a set of three carbohydrate receptors of diverse origin, i.e. the human macrophage galactose-type lectin, a plant lectin, Pisum sativum agglutinin, and the bacterial Gal-/Glc-binding protein from Escherichia coli, it became possible to simultaneously define their monosaccharide selectivity and identify the essential hydroxyls for interaction.

Fluorinated Carbohydrates as Lectin Ligands: (19)F-Based Direct STD Monitoring for Detection of Anomeric Selectivity

The characterization of the binding of reducing carbohydrates present as mixtures of anomers in solution to a sugar recepor (lectin) poses severe difficulties. In this situation, NMR spectroscopy enables the observation of signals for each anomer in the mixture by applying approaches based on ligand observation. Saturation transfer difference (STD) NMR allows fast and efficient screening of compound mixtures for reactivity to a receptor. Owing to the exceptionally favorable properties of ¹⁹F in NMR spectroscopy and the often complex ¹H spectra of carbohydrates, ¹⁹F-containing sugars have the potential to be turned into versatile sensors for recognition. Extending the recently established ¹H → ¹H STDre¹⁹F-NMR technique, we here demonstrate its applicability to measure anomeric selectivity of binding in a model system using the plant lectin concanavalin A (ConA) and 2-deoxy-2-fluoro-d-mannose. Indeed, it is also possible to account for the mutual inhibition between the anomers on binding to the lectin by means of a kinetic model. The monitoring of ¹⁹F-NMR signal perturbation disclosed the relative activities of the anomers in solution and thus enabled the calculation of their binding affinity towards ConA. The obtained data show a preference for the α anomer that increases with temperature. This experimental approach can be extended to others systems of biomedical interest by testing human lectins with suitably tailored glycan derivatives.

Comprehensive list of lectins: origins, natures, and carbohydrate specificities

More than 100 years have passed since the first lectin ricin was discovered. Since then, a wide variety of lectins (lect means "select" in Latin) have been isolated from plants, animals, fungi, bacteria, as well as viruses, and their structures and properties have been characterized. At present, as many as 48 protein scaffolds have been identified as functional lectins from the viewpoint of three-dimensional structures as described in this chapter. In this chapter, representative 53 lectins are selected, and their major properties that include hemagglutinating activity, mitogen activity, blood group specificity, molecular weight, metal requirement, and sugar specificities are summarized as a comprehensive table. The list will provide a practically useful, comprehensive list for not only experienced lectin users but also many other non-expert researchers, who are not familiar to lectins and, therefore, have no access to advanced lectin biotechnologies described in other chapters.

Photogenerated carbohydrate microarrays to study carbohydrate-protein interactions using surface plasmon resonance imaging

A photochemical strategy to generate carbohydrate microarrays on flat sensor surfaces, and to study the protein-binding effects of these arrays by surface plasmon resonance imaging is described. The approach was validated using a panel of carbohydrate-binding proteins. The coupling agents, thiol-functionalized perfluorophenyl azides, allow the covalent attachment of underivatized carbohydrates to gold surfaces by a fast photochemical reaction. Carbohydrate microarrays composed of 3,6-di-O-(α-D-mannopyranosyl)-D-mannopyranose (Man3), 2-O-α-D-mannopyranosyl-D-mannopyranose (Man2), D-mannose (Man), D-glucose (Glc), and D-galactose (Gal) were constructed, and the binding studies were carried out in real-time using surface plasmon resonance imaging. Results showed that the immobilized carbohydrate ligands retained their binding affinities with lectins, the rank order of which was consistent with that of the free ligands in solution. The detection limit of Man3, Man2, Man, and Glc with the lectin Concanavalin A was measured to be 0.29 nM, 0.18 nM, 0.61 nM, and 3.1 nM, respectively. In addition, soybean agglutinin and Griffonia simplicifolia lectin II were tested on the array, and the results were consistent with the binding selectivity of these lectins with the carbohydrate ligands.

Integral geometry analysis of fluorescence micrographs for quantitative relative comparison of protein adsorption onto polymer surfaces

Most methods developed to study protein binding to distinct surfaces can only determine the average amount of adsorbed protein or merely provide (qualitative) information on its spatial distribution. Both these features can be characterized rigorously by integral geometry analysis of fluorescence micrographs. This approach is introduced here to compare the relative protein adsorption onto various polymer surfaces: polystyrene (PS), poly(methyl methacrylate) (PMMA), poly( n-butyl methacrylate) (PnBMA), poly( tert-butyl methacrylate) (PtBMA), and PS(PETA) and cross-linked poly(ethylene oxide) (PEO*(PETA)), admixed with pentaerythritol triacrylate (PETA). The polymeric surfaces were incubated for 15 min in phosphate-buffered saline (pH 7.4) containing 125 mug/mL fluorescently labeled lectins, either lentil lectin (LcH) or concanavalin A (ConA). Fluorescence images were recorded at identical conditions (physiological buffer, same exposure time, magnification, gain). For each image, taken a few times for each polymer, the distribution and average value of the normalized intensity were determined. The results show that the binding of LcH to PS(PETA), PtBMA, PS, PnBMA, PMMA, and PEO*(PETA) can be expressed by the ratio of the following values (mean +/- 95% confidence interval): 0.356 +/- 0.022, 0.298 +/- 0.030, 0.241 +/- 0.014, 0.083 +/- 0.008, 0.039 +/- 0.008, and 0.010 +/- 0.006, respectively. In turn, the relative adsorption of ConA is described by the values 0.252 +/- 0.016, 0.217 +/- 0.014, 0.222 +/- 0.016, 0.046 +/- 0.006, 0.116 +/- 0.008, and 0.006 +/- 0.002, respectively. Low dispersions of fluorescence intensity around average values indicate homogeneous distribution of adsorbed proteins. The introduced approach enables a fast and easy way not only to quantify the relative amount of bound proteins but also to characterize quantitatively the organization of their surface distribution, as demonstrated for patchlike protein adsorption onto the polymer blend surface.

Molecular dynamics simulations of pea (Pisum sativum) lectin structure with octyl glucoside detergents: the ligand interactions and dynamics

The mitogenic pea (Pisum sativum) lectin is a legume protein of non-immunoglobulin nature capable of specific recognition of glucose derivatives without altering its structure. Molecular dynamics simulations were performed in a realistic environment to investigate the structure and interaction properties of pea lectin with various concentrations of n-octyl-beta-d-glucopyranoside (OG) detergent monomers distributed inside explicit solvent cell. In addition, the diffusion coefficients of the ligands (OG, Ca2+, Mn2+, and Cl-) and the water molecules were also reported. The structural flexibility of the lectin was conserved in all simulations. The self-assembly of OG monomers into a small micelle at the hydrophobic site of the lectin was noticed in the simulation with 20 OG monomers. The interaction energy analysis concludes that the lectin was appropriately termed an adaptive structure. One or rarely two binding sites were observed at an instant in each simulation that were electrostatically favoured for the OG to interact with the surface amino acid residues. Enhanced binding of OG to the pea lectin was quantified in the system containing only Ca2+ divalent ions. Interestingly, no binding was observed in the simulation without divalent ions. Furthermore, the lectin-ligand complex was stabilized by multiple hydrogen bonds and at least one water bridge. Finally, the work was also in accordance with the published work elsewhere that the simulations performed with different initial conditions and using higher nonbonded cutoffs for the van der Waals and electrostatic interactions provide more accurate information and clues than the single large simulation of the biomolecular system of interest.

Leveraging protein purification strategies in proteomics

The proteomic studies, although, tend to be analytical in nature, yet many strategies of preparative protein purification can be usefully employed in such studies. This review points out the importance of purification techniques which are capable of dealing with samples which are suspensions rather than clear solution, e.g. aqueous two phase partitioning, three phase partitioning, expanded bed chromatography, etc. The review also outlines the potential of non-chromatographic techniques in dealing with fractionation of proteomes. Separation protocols which can deal with post-translationally modified (PTM) proteins are also considered.

Sulfonamide drugs binding to the colchicine site of tubulin: thermodynamic analysis of the drug-tubulin interactions by isothermal titration calorimetry

The discovery of several sulfonamide drugs paved the way toward the synthesis of 6 (N-[2-[(4-hydroxyphenyl)amino]-3-pyridinyl]-4-methoxybenzenesulfonamide, E7010) and 7 (N-(3-fluoro-4-methoxyphenyl)pentafluorobenzenesulfonamide, T138067), both of which inhibit tubulin polymerization and are under clinical development. A series of diarylsulfonamides containing an indole scaffold was also found to have antimitotic properties, but their mode of interactions with tubulin has remained unidentified so far. In this study, we demonstrate that these sulfonamide drugs bind to the colchicine site of tubulin in a reversible manner. They quenched intrinsic tryptophan fluorescence of tubulin presumably due to drug-induced conformational changes in the protein, but were unable to modulate GTPase activity of tubulin in contrast to colchicine that enhances the same enzymatic activity. Further investigation using isothermal titration calorimetry (ITC) revealed that 5 (N-(5-chloro-7-indolyl)-4-methoxybenzenesulfonamide) afforded a large positive value of heat capacity change (DeltaC(p)() = +264 cal mol(-1) K(-1)) on binding to tubulin, suggesting a substantial conformational transition in the protein along with partial enthalpy-entropy compensation. On the other hand, the 2-chloro regioisomer 2 gave a large negative value of DeltaC(p)() (-589 cal mol(-1) K(-1)) along with complete enthalpy-entropy compensation. This thermodynamic profile was thought to be attributable to a prominent contribution of van der Waals interaction and hydrogen bonding between specific groups in the drug-tubulin complex. These results indicate that a mere alteration in the position of a single substituent chlorine on the indole scaffold has a great influence on the drug-tubulin binding thermodynamics.

Thermodynamic analysis of porphyrin binding to Momordica charantia (bitter gourd) lectin

Owing to the use of porphyrins in photodynamic therapy for the treatment of malignant tumors, and the preferential interaction of lectins with tumor cells, studies on lectin-porphyrin interaction are of significant interest. In this study, the interaction of several free-base and metalloporphyrins with Momordica charantia (bitter gourd) lectin (MCL) was investigated by absorption spectroscopy. Difference absorption spectra revealed that significant changes occur in the Soret band region of the porphyrins on binding to MCL. These changes were monitored to obtain association constants (Ka) and stoichiometry of binding. The tetrameric MCL binds four porphyrin molecules, and the stoichiometry was unaffected by the presence of the specific sugar, lactose. In addition, the agglutination activity of MCL was unaffected by the presence of the porphyrins used in this study, clearly indicating that porphyrin and carbohydrate ligands bind at different sites. Both cationic and anionic porphyrins bind to the lectin with comparable affinity (Ka =10(3)-10(5) m(-1)). The thermodynamic parameters associated with the interaction of several porphyrins, obtained from the temperature dependence of the Ka values, were found to be in the range: DeltaH degrees = -98.1 to -54.4 kJ.mol(-1) and DeltaS degrees =-243.9 to -90.8 J.mol(-1).K(-1). These results indicate that porphyrin binding to MCL is governed by enthalpic forces and that the contribution from binding entropy is negative. Enthalpy-entropy compensation was observed in the interaction of different porphyrins with MCL, underscoring the role of water structure in the overall binding process. Analysis of CD spectra of MCL indicates that this protein contains about 13%alpha-helix, 36%beta-sheet, 21%beta-turn, and the rest unordered structures. Binding of porphyrins does not significantly alter the secondary and tertiary structures of MCL.

Thermodynamic analysis of saccharide binding to snake gourd (Trichosanthes anguina) seed lectin. Fluorescence and absorption spectroscopic studies

The interaction of different saccharides with the snake gourd (Trichosanthes anguina) seed lectin (SGSL) was investigated by fluorescence spectroscopy. Binding of 4-methylumbelliferyl-beta-D-galactopyranoside (MeUmb beta Gal) to SGSL resulted in a significant increase in the fluorescence emission intensity of the sugar at 376 nm, and this change was used to estimate the association constants for the binding interaction. Interestingly, the increase in emission intensity changed with a change in temperature, increasing from 19.2% at 20 degrees C to 80.2% at 40 degrees C. At 20 degrees C the association constant, K(a), for the MeUmb beta Gal-SGSL interaction was found by fluorescence titration to be 5.8 x 10(4) M(-1). From the temperature dependence of the association constants, the changes in enthalpy (Delta H) and entropy (Delta S) associated with binding of MeUmb beta Gal to SGSL were estimated to be -80.85 kJ.mol(-1) and -184.0 J.mol(-1).K(-1), respectively. Binding of unlabeled sugars was investigated by monitoring the decrease in fluorescence intensity when they were added to a mixture of SGSL and MeUmb beta Gal. The Ka values for different sugars were determined at several temperatures, and Delta H and Delta S were determined from the van't Hoff plots. Enthalpy-entropy compensation was noticed in all cases. The results indicate that saccharide binding to SGSL is enthalpy-driven and the negative contribution from entropy is, in general, quite high.

Isothermal titration calorimetric studies on the binding of deoxytrimannoside derivatives with artocarpin: implications for a deep-seated combining site in lectins

The carbohydrate binding specificity of the seed lectin from Artocarpus integrifolia, artocarpin, has been elucidated by the enzyme-linked lectin absorbent assay [Misquith, S., et al (1994) J. Biol. Chem. 269, 30393-30401], wherein it was demonstrated to be a Man/Glc specific lectin with high affinity for the trisaccharide present in the core of all N-linked oligosaccharide chains of glycoproteins. As a consequence of this characterization, the binding epitopes of this trisaccharide, 3, 6-di(alpha-D-mannopyranosyl)-D-mannose, for artocarpin were investigated by isothermal titration calorimetry using its monodeoxy as well as Glc and Gal analogues. The thermodynamic data presented here implicate 2-, 3-, 4-, and 6-hydroxyl groups of the alpha(1-3) Man and alpha(1-6) Man residues, and the 2- and 4-OH groups of the central Man residue, in binding to artocarpin. Nevertheless, alpha(1-3) Man is the primary contributor to the binding affinity, unlike other Man/Glc binding lectins which exhibit a preference for alpha(1-6) Man. In addition, unlike the binding reactions of most lectins reported so far, the interaction of mannotriose involves all of its hydroxyl groups with the combining site of the lectin. Moreover, the free energy and enthalpy contributions to binding of individual hydroxyl groups of the trimannoside estimated from the corresponding monodeoxy analogues show nonlinearity, suggesting differential contributions of the solvent and protein to the thermodynamics of binding of the analogues. Thus, this study not only provides evidence for the extended site recognition of artocarpin for the trimannoside epitope but also suggests that its combining site is best described as a deep cleft as opposed to shallow indentations implicated in other lectins.

Kinetic analysis of the binding of Ulex europeas agglutinin 1 (UEA 1) to H-antigenic fucolipid

The kinetics and thermodynamics of Ulex europeas agglutinin 1-H-antigenic determinant was studied by monitoring the binding of the lectin to H-antigenic fucolipid incorporated in liposomes tethered to the sensor surface by surface plasmon resonance. The second-order rate constants for the interactions are several order of magnitude slower than the diffusion controlled reactions. Analyses of activation parameters reveals that these slow second-order rate constants are a consequence of an unfavorable activation enthalpy term.

Thermodynamic studies of saccharide binding to artocarpin, a B-cell mitogen, reveals the extended nature of its interaction with mannotriose [3,6-Di-O-(alpha-D-mannopyranosyl)-D-mannose]

The thermodynamics of binding of various saccharides to artocarpin, from Artocarpus integrifolia seeds, a homotetrameric lectin (M(r) 65, 000) with one binding site per subunit, was determined by isothermal titration calorimetry measurements at 280 and 293 K. The binding enthalpies, DeltaH(b), are the same at both temperatures, and the values range from -10.94 to -47.11 kJ mol(-1). The affinities of artocarpin as obtained from isothermal titration calorimetry are in reasonable agreement with the results obtained by enzyme-linked lectin absorbent essay, which is based on the minimum amount of ligand required to inhibit horseradish peroxidase binding to artocarpin in enzyme-linked lectin absorbent essay (Misquith, S., Rani, P. G., and Surolia, A. (1994) J. Biol. Chem. 269, 30393-30401). The interactions are mainly enthalpically driven and exhibit enthalpy-entropy compensation. The order of binding affinity of artocarpin is as follows: mannotriose>Manalpha3Man>GlcNAc(2)Man(3)>MealphaMan>Man>M analpha6Man> Manalpha2Man>MealphaGlc>Glc, i.e. 7>4>2>1.4>1>0.4>0.3>0.24>0.11. The DeltaH for the interaction of Manalpha3Man, Manalpha6Man, and MealphaMan are similar and 20 kJ mol(-1) lower than that of mannotriose. This indicates that, while Manalpha3Man and Manalpha6Man interact with the lectin exclusively through their nonreducing end monosaccharide with the subsites specific for the alpha1,3 and alpha1,6 arms, the mannotriose interacts with the lectin simultaneously through all three of its mannopyranosyl residues. This study thus underscores the distinction in the recognition of this common oligosaccharide motif in comparison with that displayed by other lectins with related specificity.

The crystal structures of Man(alpha1-3)Man(alpha1-O)Me and Man(alpha1-6)Man(alpha1-O)Me in complex with concanavalin A

The crystal structures of concanavalin A in complex with Man(alpha1-6)Man(alpha1-O)Me and Man(alpha1-3)Man(alpha1-O)Me were determined at resolutions of 2.0 and 2.8 A, respectively. In both structures, the O-1-linked mannose binds in the conserved monosaccharide-binding site. The O-3-linked mannose of Man(alpha1-3)Man(alpha1-O)Me binds in the hydrophobic subsite formed by Tyr-12, Tyr-100, and Leu-99. The shielding of a hydrophobic surface is consistent with the associated large heat capacity change. The O-6-linked mannose of Man(alpha1-6)Man(alpha1-O)Me binds in the same subsite formed by Tyr-12 and Asp-16 as the reducing mannose of the highly specific trimannose Man(alpha1-3)[Man(alpha1-6)]Man(alpha1-O)Me. However, it is much less tightly bound. Its O-2 hydroxyl makes no hydrogen bond with the conserved water 1. Water 1 is present in all the sugar-containing concanavalin A structures and increases the complementarity between the protein-binding surface and the sugar, but is not necessarily a hydrogen-bonding partner. A water analysis of the carbohydrate-binding site revealed a conserved water molecule replacing O-4 on the alpha1-3-linked arm of the trimannose. No such water is found for the reducing or O-6-linked mannose. Our data indicate that the central mannose of Man(alpha1-3)[Man(alpha1-6)]Man(alpha1-O)Me primarily functions as a hinge between the two outer subsites.

Structural bases of lectin-carbohydrate affinities: comparison with protein-folding energetics

We have made a comparative structure based analysis of the thermodynamics of lectin-carbohydrate (L-C) binding and protein folding. Examination of the total change in accessible surface area in those processes revealed a much larger decrease in free energy per unit of area buried in the case of L-C associations. According to our analysis, this larger stabilization of L-C interactions arises from a more favorable enthalpy of burying a unit of polar surface area, and from higher proportions of polar areas. Hydrogen bonds present at 14 L-C interfaces were identified, and their overall characteristics were compared to those reported before for hydrogen bonds in protein structures. Three major factors might explain why polar-polar interactions are stronger in L-C binding than in protein folding: (1) higher surface density of hydrogen bonds; (2) better hydrogen-bonding geometry; (3) larger proportion of hydrogen bonds involving charged groups. Theoretically, the binding entropy can be partitioned into three main contributions: entropy changes due to surface desolvation, entropy losses arising from freezing rotatable bonds, and entropic effects that result from restricting translation and overall rotation motions. These contributions were estimated from structural information and added up to give calculated binding entropies. Good correlation between experimental and calculated values was observed when solvation effects were treated according to a parametrization developed by other authors from protein folding studies. Finally, our structural parametrization gave calculated free energies that deviate from experimental values by 1.1 kcal/mol on the average; this amounts to an uncertainty of one order of magnitude in the binding constant.

Effect of metal ion substitutions in concanavalin A on the binding of carbohydrates and on thermal stability

Isothermal titration calorimetry (ITC) measurements were performed on the binding of alpha methyl-D-mannopyranoside, D-mannopyranose, alpha methyl-D-glucopyranoside, and D-glucopyranose (Glu) to cobalt, nickel, and cadmium substituted concanavalin A (Con A) derivatives at pH = 6.9 and at 25 degrees C. The metal substituted Con A derivatives consisted of Co2+, Ni2+, and Cd2+ substituted for the Mn2+ ion in the S1 site of Con A which is about 12.8 A away from the center of the carbohydrate binding site of Con A. The thermodynamic quantities determined from the ITC measurements were the same for most of the binding reactions indicating that the structure of the binding site in solution is the same for all the Con A derivatives in solution and that the presence of different 2+ metal ions in the S1 site has little effect on the binding reactions. Differential scanning calorimetry scans of solutions of the metal ion derivatives of Con A show that the thermodynamics of the unfolding transition for the cobalt and nickel substituted derivatives are the same as for Con A: they dissociate from tetramers into monomers as they unfold around 85 degrees C. The cadmium substituted Con A derivative, however, exhibits an additional transition around 93 degrees C which also appears following the addition of Cd2+ to the Con A solutions. This transition results from the unfolding of a species of Con A with Cd2+ substituted into a third binding site at the monomeric interface of the Con A tetramer. The higher stability of this species is not only exemplified by the higher thermal transition temperature but also by the lack of dissociation as it unfolds. Cd2+ is released from the S3 site upon decreasing the pH from 6.9 to 6.4. ITC measurements on the binding reaction of Cd2+ to Con A show that the binding enthalpy is 40.2 +/- 0.4 kJ mol-1 at 23.4 +/- 0.2 degrees C and the binding reaction exhibits a large heat capacity change of 1.43 +/- 0.41 kJ mol-1 K-1.

Effect of substituents on the thermodynamics of D-galactopyranoside binding to winged bean (Psophocarpus tetragonolobus) basic lectin

Isothermal titration calorimetric measurements of the binding of deoxy, fluorodeoxy, and methoxy derivatives of D-galactopyranoside (alpha-D-Gal) to the basic lectin from winged bean Psophocarpus tetragonolobus, WBA I, have been carried out. Each of the ligands binding to WBA I displayed the same stoichiometry of one per subunit (29 kDa) of WBA I. The binding enthalpies for various derivatives are essentially independent of temperature and show complementary changes with respect to binding entropies. Replacement of the hydroxyl group by fluorine or hydrogen on C3 and C4 of the galactopyranoside eliminates binding to the lectin, consistent with C3-OH and C4-OH acting as hydrogen bond donors. The affinity for C2 derivatives of galactose decreases in the order GalNAc > 2MeOGal > 2FGal congruent with Gal > 2HGal, which suggests that both polar and nonpolar residues surround the C2 locus of galactose, consistent with the observed high affinity of WBA I toward GalNAc where the acetamido group at C2 position is probably stabilized by both nonpolar interactions with the methyl group and polar interactions with the carbonyl group. The binding of C6 derivatives follows the order Gal > 6FGal > D-Fuc >> 6MeOGal congruent with L-Ara, indicating the presence of favourable polar interactions with a hydrogen bond donor in the vicinity. On the basis of these results the hydrogen bond donor-acceptor relationship of the complexation of methyl-alpha-D-galactopyranoside with the primary combining site of WBA I is proposed.

Characterization of binding interactions by isothermal titration calorimetry

Isothermal titration calorimetry is a high-accuracy method for measuring binding affinities. Titration calorimetry is a universal method that has broad impact throughout biotechnology. In recent years, microcalorimeters that are capable of characterizing binding interactions of biological macromolecules have become commercially available. Results from these studies are providing new insight into the molecular nature of macromolecular interactions.