The serine protease enterokinase is the physiological activator of trypsinogen and has a specificity for the sequence (Asp)4-Lys-Ile. The enzyme consists of two subunits linked by a disulfide bond. The heavy chain achors enterokinase in the intestinal brush border membrane and the light chain is the catalytic subunit, which has the same mechanism of action as trypsin and chymotrypsin. Many properties of enterokinase resemble blood-clotting enzymes, suggesting that enterokinase lies on the same phylogenetic branch as the blood-clotting proteins.