Which metabolite donates the second nitrogen atom of the urea molecule?
- A Aspartate, condensed with citrulline ✓
- B Free ammonia entering carbamoyl phosphate
- C Glutamine via ornithine transcarbamylase
- D Alanine via the glucose-alanine cycle
Correct answer: A. Aspartate, condensed with citrulline
Explanation
One nitrogen enters as ammonia captured by carbamoyl phosphate at CPS-I. The second nitrogen is contributed by aspartate, which condenses with citrulline in the argininosuccinate synthetase step, consuming ATP. Argininosuccinate lyase then releases fumarate, linking the cycle to the TCA cycle through cytosolic fumarase and malate dehydrogenase. Alanine and glutamine are nitrogen shuttles delivering substrate to the cycle, not direct donors.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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