General · EdgepediaSeptember 17, 20265 min read

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 hydrolysis¹. It cleaves peptide bonds on the carboxyl side of bulky hydrophobic and aromatic amino acid residues, chiefly phenylalanine, tyrosine, and tryptophan². Its 1967 crystal structure provided the first structural picture of the catalytic triad now recognized as the hallmark of serine proteases³.

Key fact	Detail
Classification	Serine protease, EC 3.4.21.1, secreted by the pancreas and active in the duodenum¹
Substrate specificity	Cleaves after aromatic residues: phenylalanine, tyrosine, and tryptophan²
Catalytic triad	His-57, Asp-102, and Ser-195, with an oxyanion hole that stabilizes the tetrahedral intermediate²
Crystal structure	Solved in 1967 (Matthews et al.); first enzyme in which the serine-histidine-aspartate triad was revealed structurally³^•⁴
Crystallization	Crystallized in the 1930s by John H. Northrop; recognized in the 1946 Nobel Prize in Chemistry⁵
Mature form	Three polypeptide chains linked by disulfide bridges, numbered by the original chymotrypsinogen sequence⁶
Laboratory use	One of the proteases suitable for protein fragmentation in mass-spectrometry proteomics⁷

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 intermediate⁸. The enzyme then positions the catalytic triad to pass a proton through it, and the oxyanion hole stabilizes the tetrahedral intermediate through electrostatic complementarities².

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 logic²^•⁸.

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 it². 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 ones².

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 either². 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 mutant². 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 specificity².

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-chymotrypsin⁶. 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 maintained⁶.

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 proteins⁹. Northrop shared the 1946 Nobel Prize in Chemistry with James B. Sumner and Wendell M. Stanley for preparing enzymes and virus proteins in pure form⁵.

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 proteins³. 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 enzymes⁴^•¹⁰.

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 released⁷. 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 analysis¹¹.

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 healing¹².

References

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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