Proteinase K is a high-activity serine protease used in protein digestion. Proteinase K activity is stabilized in the presence of calcium salts and increases with protein denaturants such as SDS or urea. Proteinase K can be used on a wide range of substrates during protein digestion and preferentially degrades ester and peptide bonds next to the C-termini of hydrophobic, sulfuric, or aromatic amino acids.