The Urea cycle

This cycle is active in the liver. The liver cleans the blood. It removes with this cycle ammonia (NH4 +) from the body in the form of Urea. Urea gets excreted through the kidneys in the urine. The ammonia originates from the break down of amino acids. Amino acids are broken down when to much amino acids are present to be used directly. In western countries this is mostly the case, because of the great amount of meat (protein) which is eaten.

The Urea cycle in reactions:

One of both nitrogen atoms in urea originates from aspartate (acid rest group of the amino acid aspartic acid). The other nitrogen atom comes from NH₄⁺ and the carbon atom of CO₂.
The amino acid ornithine (which is not a building block of proteins) is the transporter of this carbon and nitrogen atoms.
Urea is formed from arginine. Arginine becomes hydrolysed to form urea and ornithine through the enzyme arginase.
The remaining reactions of the urea cycle lead to the regeneration of arginine from ornithine. Firstly a carbamoyl group of carbamoyl phosphate is transferred onto ornithine, by which citrulline is formed. This reaction is catalysed by the enzyme ornithine transcarbamoylase.
Subsequently, argininosuccinate synthetase catalyses the condensation of citrulline and aspartate. The energy source necessary for the formation of argininosuccinate is provided by splitting ATP in AMP and pyrophosphate and the subsequent hydrolysis of the formed pyrophosphate.
Finally the enzyme argininosuccinase splits up argininosuccinate in arginine and fumarate. The amine group of aspartate is transferred to form arginine by the last two reactions. The remaining carbon skeleton of aspartate is left over in the form of fumarate.

Carbamoyl phosphate is formed in the mitochondria of the cell from CO₂, H₂O, NH₄⁺ and ATP.

Reaction:

CO₂ + NH₄⁺ + 2 ATP + H₂O         + 2 ADP + P_i + 3 H⁺

                           Carbamoyl phosphate

The Urea cycle and the citric acid cycle are coupled through the intermediate fumarate.

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Literature

[1]. Stryer, Lubert;- Biochemistry - fourth edition; New York: W. H. Freeman and Company
      (1995). ISBN 0-7167-2009-4

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