MCAT Chemistry Question #2: Answer and Rationale

2. The following kinetic parameters were obtained for the IDO-catalyzed oxidation of Compound 3 by H2O2 in the presence of L-Trp.

Based on this data, what effect does L-Trp have on the reaction?

A) L-Trp oxidizes Compound 3 directly.

B) L-Trp is oxidized instead of Compound 3.

C) L-Trp does not interact with the enzyme.

D) L-Trp inhibits the enzyme.

Answer Analysis

The correct answer is D.

Rationale: This question requires you to combine knowledge of enzyme kinetics with interpretation of data. Kcat is representative of the rate of product turnover. The decreasing values of kcat in the presence of higher concentrations of L-Trp indicates that the enzyme produces less product in the presence of L-Trp. Combining this trend in the data with a knowledge of enzyme kinetics, it can be concluded that L-Trp is inhibiting the reaction.

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