Question

Arginine is converted to urea and ornithine by the action of enzyme

• A. Arginase
• B. Arginosuccinase
• C. Arginosuccinate synthetase
• D. Ornithine transcarbamoylase

Hint:

Arginase is the hydrolase enzyme that completes the urea cycle.
It is highly abundant in the liver of ureotelic animals, but absent or poorly expressed in non-ureotelic species (like birds).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The urea cycle (or Krebs-Henseleit cycle) is a metabolic pathway occurring primarily in the liver to convert toxic ammonia into urea for excretion.

Step 2: Detailed Explanation:
The urea cycle consists of five main enzymatic steps.
The final, hydrolytic cleavage step of this cycle occurs in the cytosol of hepatocytes:
- Arginase (A): This enzyme catalyzes the hydrolytic cleavage of the guanidino group of L-arginine.
This reaction yields one molecule of urea and regenerates one molecule of L-ornithine.
The newly formed urea is transported via the blood to the kidneys for excretion in urine.
The regenerated ornithine is transported back into the mitochondria to initiate another round of the urea cycle by reacting with carbamoyl phosphate.
Let us review the other enzymes:
- Arginosuccinate synthetase (C) condenses citrulline and aspartate to form arginosuccinate.
- Arginosuccinase (B) (arginosuccinate lyase) cleaves arginosuccinate into arginine and fumarate.
- Ornithine transcarbamoylase (D) condenses carbamoyl phosphate and ornithine to form citrulline.

Step 3: Final Answer:
Arginine is converted to urea and ornithine by Arginase.