Question:medium
The rate limiting step in the urea cycle is catalyzed by which enzyme?
The rate limiting step in the urea cycle is catalyzed by which enzyme?
Show Hint
First, allosteric, mitochondrial step activated by N-acetylglutamate.
Updated On: Jun 24, 2026
- Arginase
- Argininosuccinase
- Carbamoyl-phosphate synthase
- Ornithine transcarbamylase
Show Solution
The Correct Option is C
Solution and Explanation
Step 1: Think about which reaction sets the pace of nitrogen disposal. In any pathway the rate limiting enzyme usually sits at the first irreversible, regulated, energy-investing step.
Step 2: For urea synthesis that step is the very first one: condensing $NH_3$ and $CO_2$ into carbamoyl phosphate at the cost of two ATP, done by carbamoyl phosphate synthase I inside the mitochondrion. Its allosteric on-switch is N-acetylglutamate, confirming it as the regulated, rate controlling enzyme.
Step 3: Run down the remaining choices, all of which act downstream. Ornithine transcarbamylase builds citrulline, argininosuccinase liberates arginine and fumarate, and arginase performs the final hydrolysis yielding urea plus ornithine. None of these governs overall cycle speed.
Step 4: Note the trap: a second carbamoyl phosphate synthase (CPS-II) exists in the cytosol but it serves pyrimidine synthesis and uses glutamine, so it is irrelevant to the urea cycle.
\[\boxed{\text{Carbamoyl-phosphate synthase (CPS-I)}}\]
Step 2: For urea synthesis that step is the very first one: condensing $NH_3$ and $CO_2$ into carbamoyl phosphate at the cost of two ATP, done by carbamoyl phosphate synthase I inside the mitochondrion. Its allosteric on-switch is N-acetylglutamate, confirming it as the regulated, rate controlling enzyme.
Step 3: Run down the remaining choices, all of which act downstream. Ornithine transcarbamylase builds citrulline, argininosuccinase liberates arginine and fumarate, and arginase performs the final hydrolysis yielding urea plus ornithine. None of these governs overall cycle speed.
Step 4: Note the trap: a second carbamoyl phosphate synthase (CPS-II) exists in the cytosol but it serves pyrimidine synthesis and uses glutamine, so it is irrelevant to the urea cycle.
\[\boxed{\text{Carbamoyl-phosphate synthase (CPS-I)}}\]
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