THERMODYNAMICS OF THE INTERACTION OF HUMAN-IMMUNOGLOBULIN-E WITH ITS HIGH-AFFINITY RECEPTOR FC-EPSILON-RI

We have employed isothermal titration calorimetry (ITC) and circular d ichroism (CD) spectroscopy to characterize the binding of soluble frag ments of IgE (IgE-Fc and Fc epsilon 3-4) to a soluble fragment of the high-affinity receptor Fc epsilon RI alpha-chain (sFc epsilon RI alpha ). The thermodynamic parameters for the interaction of IgE-Fc and Fc e psilon 3-4 with sFc epsilon RI alpha, determined using ITC, confirm th e earlier conclusion that the C epsilon 2 domain is not involved in th e interaction and that the stoichiometry of both complexes is 1:1, For both IgE-Fe and Fc epsilon 3-4, the value of Delta H degrees is -36.9 +/- 4.6 kcal mol-l at 37.3 degrees C and Delta C(p)degrees is -820 +/ - 120 cal mol(-1) K-1. The temperature at which Delta S degrees is zer o is 284 +/- 1 K, indicating that the entropy contribution to the ther modynamics of association is unfavorable at physiological temperature. Of particular interest is the large value of Delta Cp degrees. The la rge surface area of IgE and Fc epsilon RI alpha that is implicated in complex formation from previous mutagenesis studies on the two protein s may account in part for the magnitude of Delta C(p)degrees. Addition al contributions may arise from hydration within the binding site and changes in tertiary structure of the individual components of the comp lex. However, the CD spectra of IgE, IgE-Fc, and Fc epsilon 3-4 comple xes with sFc epsilon RI alpha are merely the sum of the spectra of the ir individual components, indicating that the secondary structure of t he immunoglobulin domain folds are preserved on complex formation. Thu s, any change in tertiary structure must be limited to the relative di sposition of the immunoglobulin domains C epsilon 3 and C epsilon 4 in IgE and the two immunoglobulin-like domains in the alpha-chain of Fc epsilon RI.