Structure of Chymotrypsin:
A representation of primary structure, showing disulfide bonds and the amino
acid residues crucial to catalysis. The protein consists of 3 polypeptide chains
linked by disulfide bonds. The active-site amino acid residues are grouped
together in the 3D structure.
Refer to ‘Example of an enzyme mechanism’ slide; picture on the left
A depiction of the enzyme emphasizing its surface. The hydrophobic pocket in
which the aromatic amino acid side chain of the substrate is bound is shown in
blue. Key active site residues, including Ser 195, His , Asp102, are red.
Refer to ‘Chymotrypsin is a protease’ slide; picture on the left
The polypeptide backbone as a ribbon structure. Disulfide bonds are blue, the
three chains are coloured.
Refer to ‘Example of an enzyme mechanism’ slide; picture on the right
A close up of the active site with a substrate (white and blue) bound. They
hydroxyl of Ser attacks the carbonyl group of the substrate (oxygen are red);
the developing negative charge on the oxygen is stabilized by the oxyanion hole
(amide nitrogens from Ser 195and Gly 193, in purple) The aromatic amino acid side
chain of the substrate (yellow) sits in the hydrophobic pocket. The amid nitrogen
of the peptide bond to be cleaved (protruding toward the viewer and projecting
the path of the rest of the substrate polypeptide chain_ is shown in white.
The Chymotrypsin Mechanism Involves Acylation and Deacylation of a Ser
Bovine pancreatic chymotrypsin is a protease, an enzyme that catalyses the
hydrolytic cleavage of peptide bonds. This protease is specific for peptide bonds
adjacent to aromatic amino acid residues (Trp, Phe, Tyr). Chymotrypsin
enhances the rate of peptide bond hydrolysis by a factor of at least 10 . It does
not catalyse a direct attack of water on the peptide bond; instead, a transient
covalent acyl-enzyme intermediate is formed. The reaction thus has two distinct
phases. In the acylation phase, the peptide bond is cleaved and an ester linkage is
formed between the peptide carbonyl carbon and the enzyme. In the deacylation
phase, the ester linkage is hydrolysed and the nonacylated enzyme regenerated.
Kinetic and structural analyses have revealed that the change in Kcat reflects the
ionization state of His . The declin