CHAPTER 24: Unit 14. Urea Cycle

Ammonia NH₃ is made from the breakdown of amino acids. It is the end product of amino acid degradation. It is toxic and it is not allowed to stay in the liver and therefore, it is converted to urea inside the liver.

There are several reactions needed to convert ammonia to urea H₂N-CO-NH₂.

NH₃       +      H₂O        <—–>      NH₄⁺    +    OH^–

2 NH₄⁺    +     CO₂       —–>     H₂N-CO-NH₂        +        H₂O    +     2 H⁺

The figure below illustrates the general process of the Urea Cycle:

Reference: https://www.sciencedirect.com/topics/neuroscience/urea-cycle

The videos below illustrate the Urea Cycle:

https://www.news-medical.net/health/The-Urea-Cycle-Step-by-Step.aspx

The figure below illustrates the steps of Urea Cycle

Reference: https://www.slideshare.net/mahalingam12/urea-cycle-its-regulationUrea Cycle Reaction Steps:There are 5 steps: Reference: https://www.ncbi.nlm.nih.gov/books/NBK22450/

1.  The First Step (Reaction): The formation of Carbamoyl Phosphate: (it occurs in the mitochondria)The reaction of ammonium ion NH4+ with HCO3 – to form carbamoyl phosphate. The enzyme used is carbamoyl phosphate synthetase.

2. The Second Step (Reaction): (it occurs in the mitochondria)

The carbamoyl group of carbamoyl phosphate (high transfer potential due to its anhydride bond) conversion into ornithine to form citrulline. The enzyme used is ornithine transcarbamoylase.

3. The Third Step: The Condensation of Citrulline with Aspartate: (it occurs in the cytosol):

The reaction is catalyzed by enzyme argininosuccinate synthetase

4. The Fourth Step: Argininosuccinate Cleavage: (it occurs in the cytosol):

The reaction is catalyzed by enzyme Argininosuccinase.

5. Urea Formation from Arginine Hydrolysis: (it occurs in the cytosol):

The hydrolysis yields urea and ornithine. The reaction is catalyzed by the enzyme arginase. After that, ornithine is transported back into the mitochondria to start a new urea cycle.