Thermodynamics of the Interaction of Human Immunoglobulin E with Its High-Affinity Receptor FcεRI

We have employed isothermal titration calorimetry (ITC) and circular dichroism (CD) spectroscopy to characterize the binding of soluble fragments of IgE (IgE-Fc and Fcε3-4) to a soluble fragment of the high-affinity receptor FcεRI α-chain (sFcεRIα). The thermodynamic parameters for the interaction of IgE-Fc and Fcε3-4 with sFcεRIα, determined using ITC, confirm the earlier conclusion that the Cε2 domain is not involved in the interaction and that the stoichiometry of both complexes is 1:1. For both IgE-Fc and Fcε3-4, the value of ΔH° is −36.9 ± 4.6 kcal mol-1 at 37.3 °C and ΔC p ° is −820 ± 120 cal mol-1 K-1. The temperature at which ΔS° is zero is 284 ± 1 K, indicating that the entropy contribution to the thermodynamics of association is unfavorable at physiological temperature. Of particular interest is the large value of ΔC p °. The large surface area of IgE and FcεRIα that is implicated in complex formation from previous mutagenesis studies on the two proteins may account in part for the magnitude of ΔC p °. Additional contributions may arise from hydration within the binding site and changes in tertiary structure of the individual components of the complex. However, the CD spectra of IgE, IgE-Fc, and Fcε3-4 complexes with sFcεRIα are merely the sum of the spectra of their individual components, indicating that the secondary structure of the immunoglobulin domain folds are preserved on complex formation. Thus, any change in tertiary structure must be limited to the relative disposition of the immunoglobulin domains Cε3 and Cε4 in IgE and the two immunoglobulin-like domains in the α-chain of FcεRI.