Time resolved thermodynamics of ligand binding to heme proteins

Understanding the thermodynamics of ligand binding in metallo-proteins is of critical importance in probing the underlying energetic basis for the reaction mechanism. However, many kinetic events occurring subsequent to ligand binding/release are on a time scale outside of that accessible using traditional calorimetric techniques thus making the construction of thermodynamic profiles for early kinetic events difficult. Photothermal methods have enjoyed considerable success in probing photo-triggered reactions on timescales ranging from ∼ns to >ms. These techniques, including photoacoustic calorimetry (PAC) and photothermal beam deflection (PBD), have been applied to obtain both molar volume and enthalpy changes for a wide range of biological processes including ligand binding to metallo-proteins. Here we review the progress made to date in utilizing photothermal methods (PAC and PBD) to probe the thermodynamics of small molecule binding to heme proteins ranging from simple globin-type proteins to the more complex heme/copper oxidases.