# Chymotrypsin
**Chymotrypsin** (EC 3.4.21.1) is a pancreatic serine protease, a digestive enzyme that functions in the duodenum, the first section of the small intestine, where it breaks down proteins and polypeptides by hydrolysis[1](https://link.springer.com/article/10.1186/s43094-024-00709-y). It cleaves peptide bonds on the carboxyl side of bulky hydrophobic and aromatic amino acid residues, chiefly phenylalanine, tyrosine, and tryptophan[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/). Its 1967 crystal structure provided the first structural picture of the catalytic triad now recognized as the hallmark of serine proteases[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6309182/).
| Key fact | Detail |
|---|---|
| Classification | Serine protease, EC 3.4.21.1, secreted by the pancreas and active in the duodenum[1](https://link.springer.com/article/10.1186/s43094-024-00709-y) |
| Substrate specificity | Cleaves after aromatic residues: phenylalanine, tyrosine, and tryptophan[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/) |
| Catalytic triad | His-57, Asp-102, and Ser-195, with an oxyanion hole that stabilizes the tetrahedral intermediate[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/) |
| Crystal structure | Solved in 1967 (Matthews et al.); first enzyme in which the serine-histidine-aspartate triad was revealed structurally[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6309182/) • [4](https://journals.iucr.org/a/issues/2013/01/00/wl0020/index.html) |
| Crystallization | Crystallized in the 1930s by John H. Northrop; recognized in the 1946 Nobel Prize in Chemistry[5](https://journals.iucr.org/m/issues/2016/06/00/me0630/) |
| Mature form | Three polypeptide chains linked by disulfide bridges, numbered by the original chymotrypsinogen sequence[6](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/chymotrypsin-trypsin) |
| Laboratory use | One of the proteases suitable for protein fragmentation in mass-spectrometry proteomics[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC4030975/) |
## Catalytic mechanism
Chymotrypsin catalyzes the hydrolysis of peptide (and ester) bonds in two stages. In the first stage, the hydroxyl group of the active-site serine attacks the carbonyl carbon of the scissile bond, a case of nucleophilic catalysis, cleaving the peptide bond of the substrate and forming a covalent acyl-enzyme intermediate[8](https://www.minams.edu.pk/cPanel/ebooks/biochemistry/Marks%E2%80%99%20Basic%20Medical%20Biochemistry%20A%20Clinical%20Approach%2C%202nd%20Edition%20-%20Colleen%20Smith.pdf). The enzyme then positions the catalytic triad to pass a proton through it, and the oxyanion hole stabilizes the tetrahedral intermediate through electrostatic complementarities[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/).
The triad residues act together rather than independently. In the chymotrypsin numbering system, His-57, Asp-102, and Ser-195 form the catalytic triad, and the same aspartate-histidine-serine arrangement recurs in other serine proteases such as the blood-coagulation enzyme thrombin, which works by the same chemical logic[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/) • [8](https://www.minams.edu.pk/cPanel/ebooks/biochemistry/Marks%E2%80%99%20Basic%20Medical%20Biochemistry%20A%20Clinical%20Approach%2C%202nd%20Edition%20-%20Colleen%20Smith.pdf).
## Substrate specificity
Specificity is set by the S1 binding pocket and by more distant structural elements. Residues 189 to 195, 214 to 220, and 225 to 228 form the primary substrate-binding pocket, and residues 185 to 188 and 221 to 224 form two loops, L1 and L2, near it[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/). Chymotrypsin and trypsin have high sequence identity and very similar tertiary structures, yet trypsin favors basic residues such as lysine and arginine while chymotrypsin favors aromatic ones[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/).
The pocket alone does not explain the difference. Residue 189, at the bottom of the S1 pocket, is a negatively charged aspartate in trypsin and a polar serine in chymotrypsin, and this difference was once used to explain the two specificities. But mutating Asp-189 in trypsin to serine did not convert the enzyme to chymotrypsin-like specificity; the mutant simply lost activity, and the corresponding S189D mutation in chymotrypsin did not produce trypsin-like specificity either[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/). When the two surface loops L1 and L2 of trypsin were replaced by those of chymotrypsin in addition to the D189S mutation, the hybrid protein showed an increase in chymotrypsin activity of about 1000-fold over the D189S mutant[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/). Computational analysis found that the two enzymes have distinct dynamic signatures in these loop regions, correlated with motions of residues in the binding pockets, suggesting that cooperative loop motions contribute to activity and specificity[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/).
## Activation from chymotrypsinogen
Chymotrypsin is synthesized as the inactive precursor chymotrypsinogen and activated by proteolytic cleavage. After π-chymotrypsin achieves complete catalytic function, additional cleavages occur: a second π-chymotrypsin molecule strips away the dipeptide at positions 14 and 15, yielding δ-chymotrypsin, and subsequent digestion by chymotrypsin excises the dipeptide at positions 147 and 148, yielding the end product, α-chymotrypsin[6](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/chymotrypsin-trypsin). The mature enzyme contains three polypeptide chains linked by disulfide bridges, so it is not strictly a monomeric enzyme, but the sequential numbering of the original chymotrypsinogen molecule is usually maintained[6](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/chymotrypsin-trypsin).
## Structural history
The enzyme entered protein science early. During the 1930s and 1940s, chymotrypsin and its precursor chymotrypsinogen were crystallized alongside trypsin, pepsin, and their precursors by John Howard Northrop, work that helped establish that enzymes are proteins[9](https://www.nature.com/articles/nsb0599_411b). Northrop shared the 1946 [Nobel Prize in Chemistry](https://www.edgepedia.org/nobel-prize-in-chemistry) with [James B. Sumner](https://www.edgepedia.org/james-b-sumner) and [Wendell M. Stanley](https://www.edgepedia.org/wendell-m-stanley) for preparing enzymes and virus proteins in pure form[5](https://journals.iucr.org/m/issues/2016/06/00/me0630/).
The crystal structure followed in 1967. David Blow and colleagues at the MRC Laboratory of Molecular Biology in Cambridge determined the structure of chymotrypsin, the first enzyme in which the canonical catalytic triad of a serine, a histidine, and an aspartate was revealed structurally, illustrating how the protease hydrolyzes peptide bonds in target proteins[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6309182/). Early enzyme structures of this period, typified by lysozyme in 1965 and chymotrypsin in 1967, provided clear pictures of the catalytic process, and the chymotrypsin structure paved the way to uncovering the classic catalytic triad and oxyanion hole shared by proteases, esterases, and other hydrolytic enzymes[4](https://journals.iucr.org/a/issues/2013/01/00/wl0020/index.html) • [10](https://pmc.ncbi.nlm.nih.gov/articles/PMC7756376/).
## Applications
In the laboratory, chymotrypsin is one of the standard proteases for fragmenting proteins before mass-spectrometric analysis. Trypsin is the most widely used digestion enzyme for this purpose, but chymotrypsin, pepsin, papain, elastase, and several others are also suitable, and the choice of protease determines the number and length of the peptide fragments released[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC4030975/). Using an alternative protease or multiple proteases increases protein sequence coverage, the number of post-translational modifications identified, and the number of splice junctions covered in proteomic analysis[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898146/pdf/nihms-1850565.pdf).
Chymotrypsin also has commercial and therapeutic uses. A 2025 industry filing reports that the chymotrypsin market in China grew from RMB0.7 billion in 2019 to RMB1.8 billion in 2024, with a projection of RMB2.0 billion by 2025, driven in part by expanding applications in inflammation resolution and wound healing[12](https://www1.hkexnews.hk/listedco/listconews/sehk/2025/1202/2025120200034.pdf).
## References
1. [Trypsin, chymotrypsin and elastase in health and disease, Future Journal of Pharmaceutical Sciences (2024)](https://link.springer.com/article/10.1186/s43094-024-00709-y)
2. [Specificity of Trypsin and Chymotrypsin: Loop-Motion-Controlled Dynamic Correlation as a Determinant, Biophysical Journal (2005)](https://pmc.ncbi.nlm.nih.gov/articles/PMC1366603/)
3. [A brief history of macromolecular crystallography, illustrated by a family tree and its Nobel fruits, Acta Crystallographica Section F (2018)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6309182/)
4. [Evolution of diffraction methods for solving crystal structures, Acta Crystallographica Section A (2013)](https://journals.iucr.org/a/issues/2013/01/00/wl0020/index.html)
5. [Impact and influence of crystallography across the sciences, IUCrJ (2016)](https://journals.iucr.org/m/issues/2016/06/00/me0630/)
6. [Chymotrypsin Trypsin, ScienceDirect Topics overview](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/chymotrypsin-trypsin)
7. [Research Applications of Proteolytic Enzymes in Molecular Biology, PLOS ONE (2014)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4030975/)
8. [Marks' Basic Medical Biochemistry, 2nd edition, Chapter 8: Enzymes as Catalysts](https://www.minams.edu.pk/cPanel/ebooks/biochemistry/Marks%E2%80%99%20Basic%20Medical%20Biochemistry%20A%20Clinical%20Approach%2C%202nd%20Edition%20-%20Colleen%20Smith.pdf)
9. [Early crystals, Nature Structural & Molecular Biology (1999)](https://www.nature.com/articles/nsb0599_411b)
10. [Looking Back: A Short History of the Discovery of Enzymes and How They Became Powerful Chemical Tools, ChemCatChem (2020)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7756376/)
11. [Overview and Considerations in Bottom-Up Proteomics, Analytical Chemistry (2022)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898146/pdf/nihms-1850565.pdf)
12. [Industry Overview, HKEX filing (2025)](https://www1.hkexnews.hk/listedco/listconews/sehk/2025/1202/2025120200034.pdf)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Enzyme classes and activities › Proteolytic and peptidase enzymes › Proteases by catalytic mechanism › Serine proteases › Trypsin family and trypsinogens › Chymotrypsin and chymotrypsinogens*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 11, 2026; Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license