MCAT Biochemistry Question 126: Answer and Explanation

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Question: 126

6. When fatty acid β-oxidation predominates in the liver, mitochondrial pyruvate is most likely to be:

  • A. carboxylated to phosphoenolpyruvate for entry into gluconeogenesis.
  • B. oxidatively decarboxylated to acetyl-CoA for oxidation in the citric acid cycle.
  • C. carboxylated to oxaloacetate for entry into gluconeogenesis.
  • D. reduced to lactate in the process of fermentation.

Correct Answer: C

Explanation:

Pyruvate is converted primarily into three main intermediates: acetyl-CoA (choice (B)) for the citric acid cycle (via pyruvate dehydrogenase), lactate (choice (D)) during fermentation (via lactate dehydrogenase), or oxaloacetate (choice (C)) for gluconeogenesis (via pyruvate carboxylase). High levels of acetyl-CoA, which is produced during β-oxidation, will inhibit pyruvate dehydrogenase and shift the citric acid cycle to run in the reverse direction, producing oxaloacetate for gluconeogenesis. Acetyl-CoA also stimulates pyruvate carboxylase directly.

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