Q: Which of the following equations correctly represents the overall hydrolysis of a peptide bond by chymotrypsin?

$$\mathrm{R-C(=O)-NH-R'} + H_2O \xrightarrow{\text{chymotrypsin}} \mathrm{R-C(=O)-OH} + \mathrm{H_2N-R'}$$

Question 1

Which amino acid residue in chymotrypsin acts as the primary nucleophile during peptide bond hydrolysis?

Accepted Answer

Ser195

Question 2

Chymotrypsin exhibits burst kinetics when hydrolyzing p-nitrophenyl acetate. What does the initial 'burst' phase indicate about the reaction mechanism?

Accepted Answer

A long-lived acyl-enzyme intermediate is formed after rapid product release.

Question 3

Which of the following best describes the role of the oxyanion hole in chymotrypsin catalysis?

Accepted Answer

It stabilizes the negative charge on the tetrahedral intermediate.

Question 4

The catalytic triad of chymotrypsin consists of which three amino acids?

Accepted Answer

Ser195, His57, Asp102

Question 5

Which step in the chymotrypsin mechanism involves water acting as a nucleophile?

Accepted Answer

Hydrolysis of the acyl-enzyme intermediate to release the N-terminal product

Question 6

What is the effect of pH on the catalytic efficiency ($$k_{cat}/K_M) of $$chymotrypsin?

Accepted Answer

It reaches a maximum at pH 8 and decreases above and below this value.

Question 7

Which reagent is used to covalently label Ser195 in chymotrypsin, confirming its role in catalysis?

Accepted Answer

Diisopropyl fluorophosphate (DIFP)

Question 8

Trypsin and chymotrypsin are both serine proteases. What is the main difference in their substrate specificity?

Accepted Answer

Chymotrypsin cleaves after aromatic residues; trypsin after basic residues (Lys, Arg).

Question 9

Which of the following best explains why serine alone is a poor nucleophile in peptide bond hydrolysis?

Accepted Answer

Serine's hydroxyl group is not sufficiently deprotonated under physiological conditions.

Question 10

In the chymotrypsin mechanism, what is the function of His57?

Accepted Answer

It acts as a general base to deprotonate Ser195 and water.

Question 11

Which of the following equations correctly represents the overall hydrolysis of a peptide bond by chymotrypsin?

Accepted Answer

$$\mathrm{R-C(=O)-NH-R'} + H_2O \xrightarrow{	ext{chymotrypsin}} \mathrm{R-C(=O)-OH} + \mathrm{H_2N-R'}$$

Question 12

Which of the following statements about the acyl-enzyme intermediate in chymotrypsin catalysis is correct?

Accepted Answer

It is a covalent complex between the enzyme and the substrate's acyl group.

Question 13

Which of the following best describes the effect of Asp102 in the catalytic triad of chymotrypsin?

Accepted Answer

Asp102 increases the basicity of His57 by stabilizing its positive charge.

Question 14

A patient has a mutation that replaces Ser195 with alanine in chymotrypsin. What is the most likely effect on enzyme activity?

Accepted Answer

Enzyme activity will be abolished because alanine cannot act as a nucleophile.

Question 15

Which of the following is a common feature of all serine proteases?

Accepted Answer

They all have a catalytic triad involving serine, histidine, and aspartate.

Question 16

During peptide bond hydrolysis by chymotrypsin, which group is the leaving group after the first tetrahedral intermediate collapses?

Accepted Answer

The C-terminal fragment of the substrate

Question 17

Which of the following best explains why water alone is a poor nucleophile in peptide bond hydrolysis?

Accepted Answer

Water is not sufficiently activated (deprotonated) under physiological conditions.

Question 18

Which class of proteases does chymotrypsin belong to according to the EC classification?

Accepted Answer

3.4.21 Serine proteases

Question 19

Which of the following statements about the stabilization of the tetrahedral intermediate in chymotrypsin catalysis is correct?

Accepted Answer

It is stabilized by hydrogen bonds from the amide dipoles in the oxyanion hole.

Peptide Bond Hydrolysis by Chymotrypsin: Mechanism and Enzyme Catalysis

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