The urea cycle spans two cellular compartments. Which set of reactions occurs within the mitochondrial matrix?
- A Condensation of citrulline with aspartate to form argininosuccinate
- B Synthesis of carbamoyl phosphate and formation of citrulline ✓
- C Cleavage of argininosuccinate to arginine and fumarate
- D Hydrolysis of arginine to ornithine and urea
Correct answer: B. Synthesis of carbamoyl phosphate and formation of citrulline
Explanation
Carbamoyl phosphate synthetase-I and ornithine transcarbamylase are mitochondrial matrix enzymes; their products must cross the inner membrane via the ornithine-citrulline antiporter. The remaining three steps, argininosuccinate synthetase, argininosuccinate lyase, and arginase, are cytosolic. Option D kills itself because arginase hydrolysis is the classic cytosolic final step that regenerates ornithine for re-entry into the mitochondrion.
Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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