MCAT Biochemistry Question 124: Answer and Explanation
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Question: 124
4. After an overnight fast, which of the following enzymes would be expected to have little, if any, physiological activity?
- A. Malate dehydrogenase
- B. Glucokinase
- C. α-Ketoglutarate dehydrogenase
- D. Phosphofructokinase-1
Correct Answer: B
Explanation:
After an overnight fast, the liver is producing glucose and glucokinase activity would be insignificant. Glucokinase is used to trap extra glucose in liver cells as part of a storage mechanism; with low blood glucose, liver cells would be generating new glucose, not storing it. It is also in the pancreas, where it serves as a glucose sensor; if glucose levels are low, it has little activity in this tissue as well. Malate dehydrogenase, choice (A), and α-ketoglutarate dehydrogenase, choice (C), are citric acid cycle enzymes. Phosphofructokinase-1, choice (B), is a glycolytic enzyme. Other enzymes used in glycolysis, the citric acid cycle, or gluconeogenesis, such as phosphofructokinase-1, would be expected to maintain normal activity after an overnight fast, using glucose derived from glycogen or gluconeogenesis, rather than orally ingested glucose.