Fragment-based ligand discovery

An example of isothermal titration calorimetry (ITC). The heat evolved or absorbed as a compound binds to a protein is a calorimetric property. ITC is limited by high solubility requirements for ligand and protein and by the amount of heat generated, but typically can measure K_D values in the range of 10nM to 500μM. Shown in the data here is the titration of the tripeptide K-cyclohexylalanine-K to the protein OppA. The upper curve is the direct measurement of heat generated as ligand is added to protein; the bottom curve plots these values to provide a K_D, which combined with the total heat generated (area under the curve) allows enthalpy and entropy to be estimated.