Thermodynamic analysis of a hydrophobic binding site: probing the PDZ domain with nonproteinogenic peptide ligands.

[structure: see text] Isothermal titration calorimetry (ITC) is used to study the thermodynamic consequences of systematically modifying the hydrophobic character of a single residue in a series of protein-binding ligands. By substituting standard and nonproteinogenic aliphatic amino acids for the C-terminal valine of the hexapeptide KKETEV, binding to the third PDZ domain (PDZ3) of the PSD-95 protein is characterized by distinct changes in the Gibbs free energy (DeltaG), enthalpy (DeltaH), and entropy (TDeltaS) parameters. One notable observation is that peptide binding affinity can be improved with a nonstandard residue.