Ornithine-urea cycle

The synthesis of urea (H₂N.CO.NH₂) from one molecule of CO₂ and two of NH, occurs in the liver of ureotelic vertebrates by a metabolic pathway known as the ornithine-urea cycle, discovered by Kreb and Hensleit in 1932. This is a cyclic pathway that involves five enzyme-catalised reactions.

                                                                           Figure:  Ornithine-urea cycle

You can see in Figure that this cycle has five steps :

1. Ammonia, CO₂ and ATP are converted into carbamoylphosphate by the enzyme, carbamoylphosphate synthetase.
2. Ornithine carbamoyl-transferase then makes citrulline from carbamoylphosphate and ornithine.
3. Citrulline is then used in the presence of ATP and aspartic acid for the synthesis of argininosuccinic acid by the enzyme argininosuccinate synthetase.
4. Argininosuccinic acid then gives rise to arginine and fumararic acid in the presence of argininosuccinate lyase. In the formation of afginine from citrulline, the amino group of as paratic acid is used with the release of the carbon skeleton of the aspertic acid in the form of fumararic acid.
5. Finally, the cycle is completed by splitting arginine into urea and ornithine in the presence of the enzyme arginase.