Metabolic Pathways and Regulation · USMLE Step 1

Which enzyme regulates the reciprocal control between glycolysis and gluconeogenesis via modification of fructose-2,6-bisphosphate levels in liver?

  1. Because both pathways share enzymes and would compete
  2. To prevent a wasteful, ATP-consuming futile cycle
  3. Because gluconeogenesis uses oxygen that glycolysis produces
  4. Because one runs in mitochondria and the other in cytosol
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Correct answer: To prevent a wasteful, ATP-consuming futile cycle

The bifunctional enzyme that acts as PFK-2 and FBPase-2 regulates levels of fructose-2,6-bisphosphate — a potent activator of glycolysis and inhibitor of gluconeogenesis — thus coordinating the two pathways.

Difficulty: Medium Question 9 of 19

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