Chymotrypsin
| Chymotrypsin | |||||||||
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ExPASy NiceZyme view | | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| Gene Ontology | AmiGO / QuickGO | ||||||||
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| Chymotrypsin C | |||||||||
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| Identifiers | |||||||||
ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
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Chymotrypsin (
Structurally, it is the archetypal structure for its
Activation
Chymotrypsin is
Mechanism of action and kinetics
 | This article is missing information about which specific protein this residue numbering is based on (use hatnote). (June 2022) |
In vivo, chymotrypsin is a proteolytic enzyme (serine protease) acting in the digestive systems of many organisms. It facilitates the cleavage of peptide bonds by a hydrolysis reaction, which despite being thermodynamically favorable, occurs extremely slowly in the absence of a catalyst. The main substrates of chymotrypsin are peptide bonds in which the amino acid N-terminal to the bond is a tryptophan, tyrosine, phenylalanine, or leucine. Like many proteases, chymotrypsin also hydrolyses 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. Along with histidine 57 and aspartic acid 102, this serine residue constitutes the catalytic triad of the active site. These findings rely on inhibition assays and the study of the kinetics of cleavage of the aforementioned substrate, exploiting the fact that the enzyme-substrate intermediate p-nitrophenolate has a yellow colour, enabling measurement of its concentration by measuring light absorbance at 410 nm.
Chymotrypsin catalysis of the hydrolysis of a protein substrate (in red) is performed in two steps. First, the nucleophilicity of Ser-195 is enhanced by general-base catalysis in which the proton of the serine hydroxyl group is transferred to the imidazole moiety of His-57 during its attack on the electron-deficient carbonyl carbon of the protein-substrate main chain (k1 step). This occurs via the concerted action of the three-amino-acid residues in the catalytic triad. The buildup of negative charge on the resultant tetrahedral intermediate is stabilized in the enzyme's active site's oxyanion hole, by formation of two hydrogen bonds to adjacent main-chain amide-hydrogens.
The His-57 imidazolium moiety formed in the k1 step is a general acid catalyst for the k-1 reaction. However, evidence for similar general-acid catalysis of the k2 reaction (Tet2)[8] has been controverted;[9] apparently water provides a proton to the amine leaving group.
Breakdown of Tet1 (via k3) generates an acyl enzyme, which is hydrolyzed with His-57 acting as a general base (kH2O) in formation of a tetrahedral intermediate, that breaks down to regenerate the serine hydroxyl moiety, as well as the protein fragment with the newly formed carboxyl terminus.
Uses
Medical uses
Chymotrypsin has been used during cataract surgery.[10] It was marketed under the brand name Zolyse.[11]
Isozymes
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See also
- Trypsin
- PA clanof proteases
References
- PMID 5442169.
- ISBN 978-0-12-181881-4.
- ISBN 978-0-12-815499-1, retrieved 2023-03-18
- PMID 3555886.
- S2CID 6877875.
- ISBN 978-0-12-815499-1, retrieved 2023-03-18
- ISBN 9781839471605.
- PMID 5133099.
- PMID 4694533.
- PMID 14436866.
- ^ "Final List of Withdrawn Applications for Biological Products That Were Removed From the Orange Book on 3-23-20 | FDA". www.fda.gov. Retrieved 5 April 2024.
Further reading
- Stryer L, Berg JM, Tymoczko JL (2002). Biochemistry. San Francisco: W.H. Freeman. ISBN 0-7167-4684-0.
- Grisham CM, Reginald H (2005). Biochemistry. Australia: Thomson Brooks/Cole. ISBN 0-534-49033-6.
External links
- The MEROPS online database for peptidases and their inhibitors: S01.001 Archived 2019-09-15 at the Wayback Machine
- Chymotrypsin at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
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