Chymotrypsinogen A
Chymotrypsin is one of the well known serine proteases found in the human digestive system, and is able to catalyse the breakdown of peptide substrates to amino acids, showing a preference for cleavage after aromatic residues such as phenylalanine. The enzyme shares significant sequence and structural homology to many mammalian serine proteases, including trypsin and elastase, as well as being part of a wider family involving proteases from diverse organisms including bacteria.
Reference Protein and Structure
- Sequence
-
P00766
(3.4.21.1)
(Sequence Homologues)
(PDB Homologues)
- Biological species
-
Bos taurus (Cattle)
- PDB
-
1yph
- High resolution structure of bovine alpha-chymotrypsin
(1.34 Å)
- Catalytic CATH Domains
-
2.40.10.10
(see all for 1yph)
Enzyme Reaction (EC:3.4.21.1)
Enzyme Mechanism
Introduction
The general mechanism for chymotrypsin is the classic serine protease mechanism.
Hydrolytic proteolysis by alpha-chymotrypsin begins with initial nucleophilic attack on the peptide bond by Ser 195, activated by deprotonation by His 57. This leads to the formation of a tetrahedral intermediate, stabilised by the amide groups of Ser 195 and Gly 193. Subsequent collapse of this intermediate, assisted by protonation of the leaving group by His 57 and Asp 102 leads to an acyl enzyme intermediate. Activation of a water molecule by His 57 and Asp 102 facilitates hydrolysis of this intermediate, resulting in the reformation of the catalytically active serine residue and the release of the product facilitated by protonation with His 57.
Catalytic Residues Roles
| UniProt | PDB* (1yph) | ||
| His57 | His57(42)C(B) | Activates Ser 195 by general base catalysis and facilitates collapse of the intermediate by general acid catalysis. Also activates a water molecule by general base catalysis and allows release of the products by general acid catalysis. | proton shuttle (general acid/base), electrostatic stabiliser |
| Asp102 | Asp102(87)C(B) | Modifies the pKa of His 57 to allow it to act as a general acid base at physiological pH. | modifies pKa |
| Gly193 (main-N) | Gly193(45)E(C) (main-N) | Forms part of the oxyanion hole to stabilise the tetrahedral intermediate and transition states. | electrostatic stabiliser |
| Ser195 | Ser195(47)E(C) | Acts as the nucleophile in attack of the substrate and formation of the acyl-enzyme intermediate; also stabilises the tetrahedral transition states by formation of the oxyanion hole through the backbone amide group. | covalent catalysis |
| Gly196 (main-N) | Gly196(48)E(C) (main-N) | Forms electrostatic interactions with the oxyanion to stabilise the tetrahedral intermediate. | electrostatic stabiliser |
Chemical Components
References
- Brothers HM 2nd et al. (1990), Biochemistry, 29, 7468-7474. Catalytic activity of the serine proteases .alpha.-chymotrypsin and .alpha.-lytic protease tagged at the active site with a (terpyridine)platinum(II) chromophore. DOI:10.1021/bi00484a016. PMID:2223778.
- Wells GB et al. (1994), J Biol Chem, 269, 4577-4586. Structure at the active site of an acylenzyme of alpha-chymotrypsin and implications for the catalytic mechanism. An electron nuclear double resonance study. PMID:8308029.
- Frigerio F et al. (1992), J Mol Biol, 225, 107-123. Crystal and molecular structure of the bovine alpha-chymotrypsin-eglin c complex at 2.0 A resolution. DOI:10.2210/pdb1acb/pdb. PMID:1583684.
Catalytic Residues Roles
| Residue | Roles |
|---|---|
| Gly193(45)E(C) (main-N) | electrostatic stabiliser |
| Asp102(87)C(B) | modifies pKa |
| Ser195(47)E(C) | covalent catalysis |
| His57(42)C(B) | proton shuttle (general acid/base), electrostatic stabiliser |
| Gly196(48)E(C) (main-N) | electrostatic stabiliser |