ISOTHERMAL TITRATION CALORIMETRIC STUDY OF THE ASSOCIATION OF HEN EGG LYSOZYME AND THE ANTILYSOZYME 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-angstrom 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-degrees-C (DELTAC(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 DELTAS and DELTAC(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 DELTAC(p) is in rather close agreement with the DELTAC(p) estimated from the polar and nonpolar surface areas buried upon association.