Isothermal Titration Calorimetry of Protein–Protein Interactions

The interaction of biologicalmacromolecules, whether protein–DNA, antibody–antigen, hormone–receptor, etc., illustrates the complexity and diversity of molecular recognition. The importance of such interactions in the immune response, signal transduction cascades, and gene expression cannot be overstated. It is of great interest to determine the nature of the forces that stabilize the interaction. The thermodynamics of association are characterized by the stoichiometry of the interaction (n), the association constant (Ka), the free energy (delta Gb), enthalpy (delta Hb), entropy (delta Sb), and heat capacity of binding (delta Cp). In combination with structural information, the energetics of binding can provide a complete dissection of the interaction and aid in identifying the most important regions of the interface and the energetic contributions. Various indirect methods (ELISA, RIA, surface plasmon resonance, etc.) are routinely used to characterize biologically important interactions. Here we describe the use of isothermal titration calorimetry (ITC) in the study of protein–protein interactions. ITC is the most quantitative means available for measuring the thermodynamic properties of a protein–protein interaction. ITC measures the binding equilibrium directly by determining the heat evolved on association of a ligand with its binding partner. In a single experiment, the values of the binding constant (Ka), the stoichiometry (n), and the enthalpy of binding (delta Hb) are determined. The free energy and entropy of binding are determined from the association constant. The temperature dependence of the delta Hb parameter, measured by performing the titration at varying temperatures, describes the delta Cp term. As a practical application of the method, we describe the use of ITC to study the interaction between cytochrome c and two monoclonal antibodies.