MCAT Chemistry Question #3: Answer and Rationale

3. Which experiment can be used to show that Compound 6 is not formed sequentially from either Compound 4 or Compound 5?

A) Conduct the reaction of Compound 4 with Compound 5, and identify the products.

B) Oxidize Compound 4 and Compound 5 with IDO/H2O2, and identify the products.

C) Reduce pure Compound 6 without added catalyst, and identify the products.

D) Conduct the reaction of Compound 2 with H2O2 without added catalyst, and identify the products.

Answer Analysis

The correct answer is B.

Rationale: This question requires you to apply your knowledge of how enzymes catalyze reactions to design and experiment. Based on the compounds in the question, you will need to focus on Reaction 2. The question asks how researchers can be sure that Compound 6 is not formed from either Compound 4 or Compound 5 in a sequential enzyme mechanism. Enzymes are not used up during catalysis and remain unchanged after the chemical reaction. In option A, having both compounds in solution with IDO will make it hard to reliably interpret the results of the experiment. In option C, examining the products of IDO-catalyzed reduction of Compound 6 would not give the necessary direct evidence, as Compound 6 could be sequentially reduced to Compound 3. Option D, involves Compound 2 from Reaction 1, rather than Reaction 2 as discussed in the question. The correct answer, option B, utilizes an experiment that includes Compounds 4 and 5 to determine if either compound is a substrate for IDO-catalyzed conversion to Compound 6.

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