Chymotrypsin (E.C. 3.4.21.1) is a mammalian digestive serine proteinase, of the trypsin family, that is synthesized in the acinar cells of the pancreas in the form of an inactive precursor, chymotrypsinogen. This zymogen is stored, along with other digestive enzymes and enzyme precursors, in pancreatic granules, whose contents are released into the duodenum when the pancreas is stimulated by the hormones cholecystokinin and acetylcholine, which are secreted in response to eating a meal.
Chymotrypsinogen is a single polypeptide chain of 245 amino acid residues that becomes enzymatically active on proteolytic cleavage of the peptide bond that connects Arg15 and Ile16. This cleavage is catalyzed by another pancreatic enzyme, trypsin, which in turn is generated from its zymogen precursor, trypsinogen, by enterokinase, an intestinal serine proteinase. Cleavage of the Argl5—lie 16 bond results in a reorganization of the protein structure, alignment of the catalytic triad, and generation of a substrate binding site (1). Other proteolytic cleavages accompany activation of chymotrypsinogen, but only the one cited is crucial for generating enzymatic activity.
The biological function of chymotrypsin is to digest dietary proteins in the small intestine. It catalyzes the cleavage of peptide bonds in which the carbonyl group is supplied primarily by aromatic or bulky hydrophobic amino acids (tyrosine, tryptophan, phenylalanine, leucine, isoleucine, methionine). These residues become the C-termini of the product peptides and are subsequently removed by digestion with carboxypeptidase A. The catalytic mechanism of chymotrypsin and other serine proteinases is described elsewhere (see Serine Proteinase).
Numerous chymotrypsin-like enzymes generally referred to as chymases have been identified in various animal tissues, particularly in mast cells, neutrophils, and lymphocytes (2). The biological functions of these enzymes are not well understood, but a chymase isolated from human heart is a major angiotensin II -forming enzyme (3). The prostate-specific antigen, which is used in the diagnosis of prostate cancer, is a serine proteinase with limited chymotrypsin-like activity (4).
Being a serine proteinase, chymotrypsin is inhibited by diisopropylfluorophosphate (DIFP) and also by numerous, naturally occurring serine proteinase inhibitors and serpins (5), as well as many small synthetic peptide analogues or active site reagents (6).
