Protein function is known to depend on the acidity of the medium. Such a pH dependence is caused by the (de)protonation of amino acid residues, whereby a proton is added or removed from an amino acid side chain. As this process is pH-dependent, at certain pH levels, the event will be more or less favorable and, by definition, at the pK a , it will be equally probable (i.e., ∆G prot = 0). Knowledge of the residue pK a values in a protein is essential for understanding function. It not only allows for a rationalization of protein properties (e.g., stability, solubility, etc.) and interactions at a specific pH, but in the context of enzymatic and redox reactions, pK a values can provide insight into how favorable a proton transfer will be under certain conditions. As alluded to, the pK a is fundamentally a free energy relationship; for an isolated, protonatable group, the value is proportional to the free energy of protonation: