Question

By which process ammonia is converted into urea in liver?

Answer 1

Hint: It is the final step of the urea cycle which is catalyzed by an enzyme called Arginase. The urea arises, which goes into the circulation, is absorbed by the liver, and eventually excreted in the urine.

Complete answer:
To solve this question, first we should know the urea cycle. In humans and mammals, approximately 80 percent of the excreted nitrogen is in the form of urea, which is formed by a sequence of reactions that occur in the liver cell cytosol and mitochondrial matrix. Such reactions are usually known as the urea cycle or Krebs-Henseleit cycle.
The cycle of urea is a sequence of five reactions catalyzed by many main enzymes. The first two steps in the process occur in the mitochondrial matrix and the most of the steps occur in the cytosol.

1. The ammonia formed in the mitochondria is transformed to carbamoyl phosphate by an enzyme known carbamoyl phosphate synthetase I in the first step of the Krebs-Henseleit cycle.
2. The second step includes the transformation from carbamoyl phosphate to ornithine to produce citrulline of a carbamoyl group. This step is facilitated by the Transcarbamoylase (OTC) enzyme ornithine.
3. The third step is facilitated by an enzyme called synthetase argininosuccinate, which utilizes citrulline and ATP to produce an intermediate citrullyl-AMP, which produce argininosuccinate from aspartate.
4. The four steps include the argininosuccinate cleavage to form fumarate and arginine. The enzyme which catalyzes the reaction is argininosuccinate lyase.
5. Arginine is hydrolyzed in the fifth and last step of the urea cycle to form urea and ornithine. Arginase catalyzes the reaction.
   The urea or ornithine cycle in the mitochondria of the liver cells transforms excess ammonia into urea.

Note: The primary aim of the urea cycle is to remove toxic ammonia from the body. Per day nearly 10 to 20 g of ammonia is eliminated from a healthy adult’s body.