Which pair of urea cycle reactions occurs inside the mitochondrion?

• A CPS-I and argininosuccinate synthetase
• B Ornithine transcarbamylase and argininosuccinate lyase
• C Carbamoyl phosphate synthetase-I and ornithine transcarbamylase ✓
• D Argininosuccinate synthetase and arginase

Explanation

The first two steps are mitochondrial: carbamoyl phosphate synthetase-I generates carbamoyl phosphate from ammonia and CO2 using two ATP, and ornithine transcarbamylase transfers the carbamoyl group to ornithine forming citrulline. Citrulline then exits the mitochondrion, and the remaining three reactions (argininosuccinate synthetase, argininosuccinate lyase, arginase) are cytosolic. This compartmentalization explains why OTC deficiency produces mitochondrial carbamoyl phosphate overflow into pyrimidine synthesis with urinary orotic acid.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.