ISOTHERMAL TITRATION CALORIMETRIC STUDY OF THE ASSOCIATION OF HEN EGGLYSOZYME 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 st ructure of this complex has been determined to 2.8-angstrom resolution by Sheriff et al. [Sheriff, S., Silverton, E. W., Padlan, E. A., Cohe n, G. H., Smith-Gill, S.J., Finzel, B.C., & Davies, D.R.(1987) Proc. N atl. Acad. Sci. U.S.A. 84, 8075-8079]. The calorimetric enthalpy of as sociation is negative and declines linearly with temperature in the ra nge 10-37-degrees-C (DELTAC(p) = -340 +/- 40 cal mol-1 K-1). Entropic contributions calculated using previously determined values of the aff inity of association are negative (unfavorable) in this temperature ra nge. This result is consistent with the loss of mobility upon associat ion of the unusually mobile segments of HEL which form the HyHEL-5 epi tope. 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. (197 7) Proc. Natl. Acad. Sci. U.S.A. 74, 2236-2240]. The experimental valu e of DELTAC(p) is in rather close agreement with the DELTAC(p) estimat ed from the polar and nonpolar surface areas buried upon association.