pK values of amino acid side chains play an important role in defining the pH-dependent characteristics of a protein. The pH-dependence of the activity displayed by enzymes and the pH-dependence of protein stability, for example, are properties that are determined by the pK values of amino acid side chains.
The pK values of an amino acid side chain in solution is typically inferred from the pK values of model compounds (compounds that are similar to the side chains of amino acids). See Amino acid for the pK values of all amino acid side chains inferred in such a way. There are also numerous experimental studies that have yielded such values, for example by use of NMR spectroscopy.
The table below lists the model pK values that are often used in a protein pK calculation, and contains a third column based on protein studies.