Configurational effects in antibody–antigen interactions studied by microcalorimetry

In this paper we study the binding of two monoclonal antibodies, E3 and E8, to cytochrome c using high‐sensitivity isothermal titration calorimetry. We combine the calorimetric results with empirical calculations which relate changes in heat capacity to changes in entropy which arise from the hydrophobic effect. The change in heat capacity for binding E3 is −350 ± 60 cal K⁻¹ mol⁻¹ while for E8 it is −165 ± 40 cal K⁻¹ mol⁻¹. This result indicates that the hydrophobic effect makes a much larger contribution for E3 than for E8. Since the total entropy change at 25°C is very similar for both antibodies, it follows that the configurational entropy cost for binding E3 is much larger than for binding E8 (−77 ± 15 vs. −34 ± 11 cal K⁻¹ mol⁻¹). These results illustrate a case of entropy compensation in which the cost of restricting conformational degrees of freedom is to a large extent compensated by solvent release. We also show that the thermodynamic data can be used to make estimates of the surface area changes that occur upon binding. The results of the present study are consistent with previous hydrogen–deuterium exchange data, detected using 2D NMR, on the two antibody–antigen interactions. The NMR study indicated that protection from exchange is limited to the binding epitope for E8, but extends beyond the epitope for E3. These results were interpreted as suggesting that a larger surface area was buried on cytochrome c upon binding to E3 than to E8, and that larger changes in configurational entropy occur upon binding of E3 than E8. These findings are confirmed by the present study using isothermal titration calorimetry. © 1995 Wiley‐Liss, Inc.