MCAT Biochemistry Question 128: Answer and Explanation

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Question: 128

8. An investigator is measuring the activity of various enzymes involved in reactions of intermediary metabolism. One of the enzymes has greatly decreased activity compared to reference values. The buffer of the assay contains citrate. Which of the following enzymes will most likely be directly affected by the use of citrate?

  • A. Fructose-2,6-bisphosphatase
  • B. Isocitrate dehydrogenase
  • C. Phosphofructokinase-1
  • D. Pyruvate carboxylase

Correct Answer: C

Explanation:

Citrate is produced by citrate synthase from acetyl-CoA and oxaloacetate. This reaction takes place in the mitochondria. When the citric acid cycle slows down, citrate accumulates. In the cytosol, it acts as a negative allosteric regulator of phosphofructokinase-1, the enzyme that catalyzes the rate-limiting step of glycolysis.

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