Determination of Ligand Binding Affinity and Specificity of Purified START Domains by Thermal Shift Assays Using Circular Dichroism.

The use of direct calorimetric methods such as isothermal titration calorimetry for measuring the affinity and specificity of protein-ligand interactions requires large amounts of proteins and ligands. When material is scarce and/or in the absence of calorimeters, thermal Shift Assays (TSA) using Circular Dichroism (CD) or other spectroscopic methods offers an alternative and quantitative method for the determination of apparent or indirect thermodynamical parameters describing the affinity of ligands for proteins. Indeed, the binding constants of ligands (K) and other parameters such as the enthalpy and Gibbs free energy of binding may be estimated from the changes in the stability curves ΔG(T) of a protein in the presence of a ligand. Here we describe the application of two different procedures proposed by Layton and Hellinga et al. (Biochemistry 49:10831-10841, 2010) to evaluate the apparent K of testosterone to the START (StAR-related lipid transfer domain) domains.