Isothermal Titration Calorimetrie Study of the Association of Hen Egg Lysozyme and the Anti-Lysozyme Antibody HyHEL-5

The thermodynamics of association of hen egg lysozyme and the antibody HyHEL-5 was characterized by isothermal titration calorimetry. The structure of this complex has been determined to 2.8-Å resolution by Sheriff et al. [Sheriff, S., Silverton, E. W., Padlan, E. A., Cohen, G. H., Smith-Gill, S. J., Finzel, B. C., & Davies,D. R. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 8075-8079]. The calorimetric enthalpy of association is negative and declines linearly with temperature in the range 10-37°C (ΔC_p = -340 ± 40 cal mol^-1 K^-1). Entropic contributions calculated using previously determined values of the affinity of association are negative (unfavorable) in this temperature range. This result is consistent with the loss of mobility upon association of the unusually mobile segments of HEL which form the HyHEL-5 epitope. As the affinity of association is approximately constant in this temperature range, an enthalpy-entropy compensation effect is implied. The hydrophobic and vibrational contributions to ΔS and ΔC_P are estimated using the method of Sturtevant [Sturtevant, J. M. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 2236-2240]. The experimental value of ΔC_P is in rather close agreement with the ΔC_P estimated from the polar and nonpolar surface areas buried upon association.