Chymotrypsin is a digestive enzyme that can perform proteolysis. Chymotrypsin preferentially cleaves peptide amide bonds where the carboxyl side of the amide bond (the P1 position) is a tyrosine, tryptophan, or phenylalanine. These amino acids contain an aromatic ring in their sidechain that fits into a 'hydrophobic pocket' (the S1 position) of the enzyme. The hydrophobic and shape complementarity between the peptide substrate P1 sidechain and the enzyme S1 binding cavity accounts for the substrate specificity of this enzyme. Chymotrypsin also hydrolyzes other amide bonds in peptides at slower rates, particularly those containing leucine at the P1 position.
Activation of chymotrypsin
Chymotrypsin is synthesized in the pancreas by protein biosynthesis as a precursor called chymotrypsinogen that is enzymatically inactive. On cleavage by trypsin into two parts that are still connected via an S-S bond, cleaved chymotrypsinogen molecules can activate each other by removing two small peptides in a trans-proteolysis. The resulting molecule is active chymotrypsin, a three-polypeptide molecule interconnected via disulfide bonds.
Action and kinetics of chymotrypsin
In vivo, chymotrypsin is a proteolytic enzyme (Serine protease) acting in the digestive systems of mammals and other organisms. It facilitates the cleavage of peptide bonds by a hydrolysis reaction, a process that, albeit thermodynamically favourable, occurs extremely slowly in the absence of a catalyst. The main substrates of chymotrypsin include tryptophan, tyrosine, phenylalanine, leucine, and methionine, which are cleaved at the carboxyl terminal. Like many proteases, chymotrypsin will also hydrolyse amide bonds in vitro, a virtue that enabled the use of substrate analogs such as N-acetyl-L-phenylalanine p-nitrophenyl amide for enzyme assays.
Chymotrypsin cleaves peptide bonds by attacking the unreactive carbonyl group with a powerful nucleophile, the serine 195 residue located in the active site of the enzyme, which briefly becomes covalently bonded to the substrate, forming an enzyme-substrate intermediate.
Full article ▸