Question 1

Which of the following best describes the catalytic triad in chymotrypsin and its role in catalysis?

Accepted Answer

It is a group of three residues—His 57, Ser 195, and Asp 102—that work together to generate a powerful nucleophile from serine for peptide bond cleavage.

Question 2

During the chymotrypsin mechanism, which residue acts as a general base to activate the nucleophile for the initial attack on the substrate?

Accepted Answer

His 57

Question 3

What is the function of the oxyanion hole in the chymotrypsin active site?

Accepted Answer

It stabilizes the negatively charged transition state during formation of the tetrahedral intermediate.

Question 4

Which of the following best explains the pH dependence of chymotrypsin's $$k_{cat}$$ and $$K_m$$ values?

Accepted Answer

At low pH, $$k_{cat}$$ decreases due to protonation of His 57, while at high pH, $$K_m$$ increases due to deprotonation of the N-terminus, weakening substrate binding.

Question 5

Which experimental observation provided evidence for the formation of an acyl-enzyme intermediate in chymotrypsin catalysis?

Accepted Answer

A rapid burst of product formation followed by a slower steady-state rate in pre-steady-state kinetics.

Question 6

In the chymotrypsin mechanism, which step involves the formation of a covalent bond between the enzyme and the substrate?

Accepted Answer

Collapse of the first tetrahedral intermediate to form the acyl-enzyme intermediate.

Question 7

Which of the following is true regarding the substrate specificity of chymotrypsin?

Accepted Answer

It cleaves peptide bonds after aromatic residues such as Trp, Tyr, and Phe.

Question 8

Which group acts as the nucleophile during the deacylation phase of chymotrypsin catalysis?

Accepted Answer

A water molecule activated by His 57

Question 9

What is the effect of diisopropylfluorophosphate (DIFP) on chymotrypsin?

Accepted Answer

It irreversibly inactivates chymotrypsin by covalently modifying Ser 195.

Question 10

Which of the following best describes the role of His 57 during the catalytic cycle of chymotrypsin?

Accepted Answer

It acts as both a general base and a general acid at different steps of the mechanism.

Question 11

Which of the following steps in the chymotrypsin mechanism is directly responsible for the release of the first product (P1)?

Accepted Answer

Collapse of the first tetrahedral intermediate to form the acyl-enzyme intermediate.

Question 12

Which of the following is a correct representation of the overall reaction catalyzed by chymotrypsin for peptide bond hydrolysis?

Accepted Answer

$$\mathrm{R_1$-CONH-R_2$} + H_2O \rightarrow \mathrm{R_1$-COOH} + \mathrm{H_2N-R_2$}$$

Question 13

Which feature of chymotrypsin's active site is primarily responsible for recognizing aromatic residues in substrates?

Accepted Answer

The hydrophobic pocket

Question 14

During the catalytic cycle, how is the serine hydroxyl group regenerated after product release?

Accepted Answer

By His 57 donating a proton to the leaving group, restoring the serine hydroxyl.

Question 15

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

Accepted Answer

It is stabilized by hydrogen bonds in the oxyanion hole.

Question 16

Which of the following best explains why peptide bond hydrolysis by chymotrypsin is much faster than the uncatalyzed reaction?

Accepted Answer

Chymotrypsin stabilizes the transition state and provides a nucleophile via its catalytic triad.

Question 17

In a colorimetric assay for chymotrypsin activity using p-nitrophenylacetate, what is the observed product and its significance?

Accepted Answer

Nitrophenolate ion, which is bright yellow and indicates enzyme activity.

Question 18

Which of the following steps in the chymotrypsin mechanism involves His 57 acting as a general acid?

Accepted Answer

During the formation of the acyl-enzyme intermediate and during the collapse of the second tetrahedral intermediate.

Question 19

Which of the following is a correct sequence of events in the double-displacement mechanism of chymotrypsin?

Accepted Answer

Substrate binds, Ser 195 attacks, tetrahedral intermediate forms, acyl-enzyme intermediate forms, first product released, water attacks, second tetrahedral intermediate forms, second product released.

Enzyme Mechanisms: Chymotrypsin as a Model Protease

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