Isothermal Titration Calorimetry and Macromolecular Visualization for the Interaction of Lysozyme and Its Inhibitors

To provide a research-like experience to upper-division undergraduate students in a biochemistry teaching laboratory, isothermal titration calorimetry (ITC) is employed to determine the binding constants of lysozyme and its inhibitors, N-acetyl glucosamine trimer (NAG3) and monomer (NAG). The extremely weak binding of lysozyme/NAG is determined using a competitive binding assay. Such interactions between lysozyme and its inhibitors are visualized with PyMol software, by which the hydrogen bond formation in the complexes is used to explain the binding specificity. The hydrogen bond inventory in the binding interface correlates with the heat enthalpy determined or derived from ITC measurements. A possible explanation for such a correlation is presented and used for an extensive discussion in thermodynamics and ligand–receptor interactions. This laboratory exercise stimulates students' critical thinking about weak/strong binding interactions and the relationship between thermodynamics and structural changes.