Observations on the binding of four anti-carbohydrate monoclonal antibodies to their homologous ligands

Abstract

The binding of four monoclonal immunoglobulins, two with specificity for β(1→6)-linked D-galactopyranans (IgA X24 and IgA J539) and two with specificity for the chain terminus of α(1→6)-linked D-glucopyranans (IgA W3129 and IgA 16.4.12E), was measured with a number of their homologous oligosaccharide ligands at different temperatures. The results show a linear relationship between lnK(a) and 1/T, where K(a) is the affinity constant and T is the absolute temperature. The unitary free energy of binding, ΔG(u), is virtually independent of T, and the ΔS(u) is small when compared with ΔG(u). The enthalpy changes derived from van't Hoff plots are large and negative, indicating an exothermic binding effect, whereas the entropy changes are small and negative, indicating minor overall hydrophobic contributions. Measurements of the free energies of binding, in low and high salt buffers, of methyl β-D-galactopyranoside and the methyl glycoside of β(1→6)-D-galactopyranotetraose with anti-galactan IgA X24 indicate that the monosaccharide has no hydrophobic interaction with the highest affinity subsite of IgA, whereas the tetraoside might have a modest hydrophobic interaction with the three other hapten-binding subsites of IgA. The standard entropy change of binding of the two groups (galactosyl and glucosyl) of oligosaccharides to the two respective sets (anti-galactan and anti-dextran) of antibodies shows a distinct, differing correlation with the hapten chain length within each set. This correlation agrees with the type of association previously established between the antibodies and either the interior determinants of the antigen (in the case of the anti-galactans) or the chain terminus (in the case of the anti-dextrans).