MCAT Biochemistry Question 180: Answer and Explanation

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Question: 180

15. Which process is expected to begin earliest in a prolonged fast?

  • A. Ketone bodies are used by the brain.
  • B. Glycogen storage is halted.
  • C. Proteins are broken down.
  • D. Enzyme phosphorylation and dephosphorylation.

Correct Answer: D

Explanation:

A prolonged fast is characterized by an increase in glucagon, which accomplishes its cellular activity by phosphorylating and dephosphorylating metabolic enzymes. Glycogen storage, choice (B), is then halted, but this requires enzyme regulation by glucagon to occur. Later in the postabsorptive state, protein breakdown, choice (C), begins. Eventually, in starvation, ketone bodies, choice (A), are used by the brain for its main energy source.

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