Microcalorimetry of protein-protein interactions

Isothermal titration microcalorimetry (ITC) is a powerful and increasingly popular technique for studying protein-protein interactions in solution at concentrations in the few milligrams per millilitre range. For heterogeneous interactions (involving complexation between different proteins or subunits) conventional ITC procedures are appropriate and are a straightforward way to establish stoichiometry and enthalpies of binding, even in tight-binding situations where binding constants may be too high to determine. For homogeneous interactions (involving dimerization or higher oligomers of identical proteins or subunits) ITC dilution techniques have been developed. Examples of both kinds of experiment will be described, including illustrative data from our own and collaborative work on peptide-antibiotic complexes, protein and subunit interactions in multi-enzyme complexes and cell surface receptors, and dissociation of insulin dimers.