Question

What kind of enzyme is chymotrypsin? What are the specific substrates of chymotrypsin?

• What features of chymotrypsin make it specific to particular substrates ?

• Can you describe the main features of the reaction mechanism?

• What is a catalytic triad and what is its role in the active site of chymotrypsin?

• What is an acyl-enzyme intermediate?

• What is conformational selection vs. “induced fit”?

Answer 1

Chymotrypsin is a type of digestive enzyme which belongs to the category of serine proteases. It performs the C - terminal hydrolysis of aromatic amino acids. Some aromatic amino acids are phenylalanine, tryptophan, and tyrosine. Thus chymotrypsin performs the cleavage of C-terminal of amino acids like phenylalanine, tyrosine and tryptophan. The specific substarates of chymotrypsin is hence aromatic amino acids like phenylalanine, tyrosine and tryptophan.

Chymotrypsin has a hydrophobic pocket in its active site. Thus only those parts of the protein that are hydrophobic will be enetering this pocket. Aromatics are hydrophobic and hence chymotrypsin selectively cleaves aromatic amino acids.

The main feature of the reaction includes the nucleophilic attack by serine on the carbonyl carbon of the substrate. This results the breaking of peptide bonds. In order to transform serine into a good nucleophile, two other amino acids namely histidine and aspartate work together. All the three aminoacids collectively is referred as the catalytic triad.

The catalytic activity of chymotrypsin is mainly based on active site serine residue. The reaction mechanism involves the catalytic triad. Herein, three amino acids namely serine, histidine and aspartate forms the catalytic triad. They work together to break peptide bonds. Histidine and aspartate work together to convert serine from a poor togood nucleophile. This results in the nucleophilic attack of serine on the carbonyl carbon of the substrate which finally will lead to the breakage of peptide bonds.

Conformational selection and induced fit are two models for explaining enzyme activity.

For an enzyme there exist both active and inactive forms. In conformational selection model, the active and inactive forms are in equilibrium with each other. The addition of a ligand results in equilibrium shift towards the active form. Thus in this model it says that conformational changes occur before the addition of ligand.

In induced fit model, enzyme is present only in inactive form. After the addition of a ligand, certain coformational changes are induced into the enzyme to convert it to the active form. Thus conformational change occurs after addition of ligands.