Question

The stage of ornithine cycle at which arginase enzyme used is
A. Ornithine$\to$citrulline
B. Arginine$\to$ornithine
C. Citrulline$\to$Argino succinic acid
D. Ornithine$\to$Urea

Answer 1

Hint:In the ornithine cycle arginine is fused to form urea and ornithine by the enzyme arginase. This is the last step of the urea cycle.

Complete answer:
The urea cycle (also known as the ornithine cycle) is a biochemical reaction cycle that creates urea from ammonia. This process is present in ureotelic organisms. The urea cycle transforms extremely poisonous ammonia into urea for excretion. This mechanism was the first biochemical mechanism to be identified five years prior to the discovery of the TCA cycle. This period was described in more detail later by Ratner and Cohen. The urea cycle occurs mainly in the liver and, to a lesser degree, in the kidneys.
Ammonia is converted to carbamoyl phosphate before the urea cycle starts. The reaction is catalysed by carbamoyl phosphate synthetase I and needs the use of two ATP molecules. Then the carbamoyl phosphate enters the urea cycle. The steps of the urea cycle are listed below:-
1. Carbamoyl phosphate has been converted to citrulline. With ornithine transcarbamoylase catalysis, the carbamoyl phosphate group is donated to ornithine and the phosphate group is released.
2. A condensation reaction takes place between the aspartate amino group and the citrulline carbonyl group to form argininosuccinate. This reaction is ATP based and catalysed by argininosuccinate synthetase.
3. Argininosuccinate goes through cleavage by argininosuccinase to produce arginine and fumarate.
4. Arginine is fused to form urea and ornithine by arginase. Ornithine is then transferred back to the mitochondria to resume the urea cycle again.

Thus, the correct answer is option “D”. i.e., Ornithine$\to$Urea.

Note:The urea or ornithine cycle transforms extra ammonia to urea in the mitochondria of the liver cells. The urea produces and enters the bloodstream, filtered through the kidneys, and gradually excreted in the urine.