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A common mechanism of hapten binding to immunoglobulins and their heterologous chain recombinants.

Kinetics and thermodynamics of binding of the hapten beta-D-(1-6)-galactotriose to the homogeneous IgA T-601 and to heterologous recombinants of heavy and light chains prepared from mouse myeloma IgA's X-24, J-539, and T-601, which all have the same galactan specificity, have been studied by the chemical relaxation method. All the immunoglobulin-hapten systems investigated were found to exhibit two relaxation times. The reciprocal value of the fast time increased linearly, while that of the slow time leveled off with increasing hapten concentration. This behavior indicates the presence of a fast bimolecular association and a slower monomolecular step. The data obtained for homologous and hybrid immunoglobulins were all found to fit a mechanism where the proteins exist in two conformations and hapten binding shifts their equilibrium to the higher affinity conformer. Furthermore, the kinetic and thermodynamic parameters for the hapten binding by the hybrids were found to be similar to those of their parent proteins. These results strongly suggest that this conformational transition is an inherent property of the tertiary domain structure of the antibody, probably involving changes in the interactions between heavy- and light-chain domains.