Calorimetric study of carbohydrate binding to concanavalin A

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.

Isothermal titration calorimetry reveals differential binding thermodynamics of variable region-identical antibodies differing in constant region for a univalent ligand

The classical view of immunoglobulin molecules posits two functional domains defined by the variable (V) and constant (C) regions, which are responsible for antigen binding and antibody effector functions, respectively. These two domains are thought to function independently. However, several lines of evidence strongly suggest that C region domains can affect the specificity and affinity of an antibody for its antigen (Ag), independent of avidity-type effects. In this study, we used isothermal titration calorimetry to investigate the thermodynamic properties of the interactions of four V region-identical monoclonal antibodies with a univalent peptide antigen. Comparison of the binding of IgG1, IgG2a, IgG2b, and IgG3 with a 12-mer peptide mimetic of Cryptococcus neoformans polysaccharide revealed a stoichiometry of 1.9-2.0 with significant differences in thermodynamic binding parameters. Binding of this peptide to the antibodies was dominated by favorable entropy. The interaction of these antibodies with biotinylated peptides manifested greater enthalpy than for native peptides indicating that biotin labeling affected the types of Ag-Ab complexes formed. Our results provide unambiguous thermodynamic evidence for the notion that the C region can affect the interaction of the V region with an Ag.

Energetics of carbohydrate binding to Momordica charantia (bitter gourd) lectin: an isothermal titration calorimetric study

Physico-chemical and carbohydrate binding studies have been carried out on the Momordica charantia (bitter gourd) seed lectin (MCL). The lectin activity is maximal in the pH range 7.4-11.0, but decreases steeply below pH 7.0. The lectin activity is mostly unaffected in the temperature range 4-50 degrees C, but a sharp decrease is seen between 50 and 60 degrees C, which could be correlated to changes in the structure of the protein as seen by circular dichroism and fluorescence spectroscopy. Isothermal titration calorimetric studies show that the tetrameric MCL binds two sugar molecules and the binding constants (Kb), determined at 288.15 K, for various saccharides were found to vary between 7.3 x 10(3) and 1.52 x 10(4)M(-1). The binding reactions for all the saccharides investigated were essentially enthalpy driven, with the binding enthalpies (DeltaHb) at 288.15 K being in the range of -50.99 and -43.39 kJ mol(-1), whereas the contribution to the binding reaction from the entropy of binding was negative, with values of binding entropy (DeltaSb) ranging between -99.2 and -72.0 J mol(-1)K(-1) at 288.15 K. Changes in heat capacity (DeltaCp) for the binding of disaccharides, lactose and lactulose, were significantly larger in magnitude than those obtained for the monosaccharides, methyl-beta-D-galactopyranoside, and methyl-alpha-D-galactopyranoside, and could be correlated reasonably well with the surface areas of these ligands. Enthalpy-entropy compensation was observed for all the sugars studied, suggesting that water structure plays an important role in the overall binding reaction. CD spectroscopy indicates that carbohydrate binding does not lead to significant changes in the secondary and tertiary structures of MCL, suggesting that the carbohydrate binding sites on this lectin are mostly preformed.

A predominantly hydrophobic recognition of H-antigenic sugars by winged bean acidic lectin: a thermodynamic study

The thermodynamics of binding of winged bean (Psophocarpus tetragonolobus) acidic agglutinin to the H-antigenic oligosaccharide (Fucalpha1-2Galbeta1-4GlcNAc-oMe) and its deoxy and methoxy congeners were determined by isothermal titration calorimetry. We report a relatively hydrophobically driven binding of winged bean acidic agglutinin to the congeners of the above sugar. This conclusion is arrived, from the binding parameters of the fucosyl congeners, the nature of the enthalpy-entropy compensation plots and the temperature dependence of binding enthalpies of some of the congeners. Thus, the binding site of winged bean acidic agglutinin must be quite extended to accommodate the trisaccharide, with non-polar loci that recognize the fucosyl moiety of the H-antigenic determinant.

Thermodynamics of binding of the core trimannoside of asparagine-linked carbohydrates and deoxy analogs to Dioclea grandiflora lectin

The Man/Glc-specific seed lectin from Dioclea grandiflora (DGL) is a member of the Diocleinae subtribe that includes the jack bean lectin concanavalin A (ConA). Both DGL and ConA bind with high affinity to the "core" trimannoside moiety, 3, 6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside, which is present in asparagine-linked carbohydrates. Recent hemagglutination inhibition studies suggest that DGL and ConA recognize similar epitopes of the trisaccharide but possess different binding specificities for complex carbohydrates (Gupta, D., Oscarson, S., Raju, T. S., Stanley, P., Toone, E. J., and Brewer, C. F. (1996) Eur. J. Biochem. 242, 320-326). In the present study, we have used isothermal titration microcalorimetry to determine the thermodynamics of binding of DGL to a complete set of monodeoxy analogs of the core trimannoside as well as a tetradeoxy analog. The thermodynamic data indicate that DGL recognizes the 2-, 3-, 4-, and 6-hydroxyl groups of the alpha(1,6) Man residue, the 3- and 4-hydroxyl group of the alpha(1, 3) Man residue, and the 2- and 4-hydroxyl groups of the central Man residue of the trimannoside. The thermodynamic data for the tetradeoxy analog lacking the 3- and 4-hydroxyl group of the alpha(1, 3) Man residue, and the 2- and 4-hydroxyl groups of the central Man residue of the trimannoside are consistent with the involvement of these hydroxyl groups in binding. While the overall pattern of data for DGL binding to the deoxy analogs is similar to that for ConA (Gupta, D., Dam, T. K., Oscarson, S., and Brewer, C. F. (1997) J. Biol. Chem. 272, 6388-6392), differences exist in the data for certain monodeoxy analogs binding to the two lectins. Differences are also observed in the thermodynamics of binding of DGL and ConA to a biantennary complex carbohydrate. In the following paper (Rozwarski, D. A., Swami, B. M., Brewer, C. F., and Sacchettini, J. C. (1998) J. Biol. Chem. 273, 32818-32825), the x-ray crystal structure of DGL complexed to the core trimannoside is presented, and a comparison is made of the thermodynamic binding data for DGL and ConA as well as the structures of their respective trimannoside complexes.