chymotrypsin

**Synthesis and activation** - Synthesized by pancreatic acinar cells as chymotrypsinogen and stored in zymogen granules. This prevents autodigestion of the pancreas. - Enteropeptidase (enterokinase), a brush-border enzyme of the duodenum, converts trypsinogen to trypsin. - Trypsin then activates chymotrypsinogen to chymotrypsin. Trypsin also activates proelastase, procarboxypeptidase A and B, and prophospholipase A2. Trypsin is therefore the master activator. - Secretion is stimulated by CCK, released from I cells in response to amino acids and fatty acids. Secretin stimulates bicarbonate-rich fluid secretion.

**Mechanism and specificity** - It is a serine protease with a catalytic triad of Ser195, His57, and Asp102, which is a classic biochemistry topic. - It is an endopeptidase. It cleaves on the carboxyl side of aromatic residues: Phe, Tyr, and Trp. It also cleaves, less efficiently, at Leu and Met. - Comparison with other proteases: - Trypsin cleaves after basic residues (Lys, Arg). - Elastase cleaves after small neutral residues (Ala, Gly, Ser). - Carboxypeptidases are exopeptidases that remove C-terminal residues. - Catalysis proceeds through a covalent acyl-enzyme intermediate. The serine hydroxyl acts as a nucleophile, aided by histidine acting as a general base. The oxyanion hole stabilizes the tetrahedral intermediate.

**Clinical significance** - **Chronic pancreatitis and pancreatic insufficiency:** Fecal chymotrypsin is decreased. Fecal elastase-1 is now the preferred noninvasive test. Findings are steatorrhea, malabsorption, and fat-soluble vitamin deficiency (A, D, E, K). - **Acute pancreatitis:** Premature intracellular activation of trypsinogen causes autodigestion. Common causes are gallstones and alcohol, as in the mnemonic "I GET SMASHED." - **Hereditary pancreatitis:** Gain-of-function mutations in PRSS1 (cationic trypsinogen) make trypsin resistant to inactivation. Loss-of-function mutations in SPINK1 (a trypsin inhibitor) also cause it. Both are autosomal dominant. These patients have recurrent pancreatitis and an increased risk of pancreatic cancer. - **Cystic fibrosis:** Thick secretions obstruct pancreatic ducts. This causes insufficiency of all pancreatic enzymes and requires pancreatic enzyme replacement. - **Protease inhibition:** α1-antitrypsin inhibits neutrophil elastase and also other serine proteases such as chymotrypsin. Deficiency causes panacinar emphysema and liver cirrhosis. - **Drug and therapy relevance:** Chymotrypsin is used in oral enzyme preparations, and in ophthalmology (zonulysis in cataract surgery).

**High-yield pearls** - Zymogens are inactive precursors that protect the producing tissue. Trypsin is activated by enteropeptidase, and the other pancreatic zymogens are activated by trypsin. - Chymotrypsin = aromatic residues (Phe, Tyr, Trp). Trypsin = basic residues (Lys, Arg). - Serine proteases include chymotrypsin, trypsin, elastase, thrombin, and other coagulation factors. - Chymotrypsin is a classic example in enzyme kinetics (ping-pong mechanism, biphasic "burst" kinetics).