Urea cycle:

Urea is synthesized in the liver through the urea cycle.  For excretion in the urine it is then secreted into the bloodstream and taken up through the kidneys.  The urea cycle was the first cyclic metabolic pathway to be discovered through Kurt Henseleit and Hans Krebs in year of 1932; 5 years before Krebs discovered the citric acid cycle. The whole reaction of the pathway is:

NH₄⁺+HCO₃^-+H₂O+3ATP+aspartate →

urea+2ADP+AMP+2P_i+PP_i+fumarate

One of the nitrogen atoms of urea comes from ammonia; the other is transferred from   the amino   acid aspartate,   although   the carbon   atom   comes   from   CO₂. Ornithine, an amino acid which is not in the standard group of 20 amino acids and is not found in proteins, is the carrier of these carbon and nitrogen atoms.  Five enzymatic reactions are included in the urea cycle that is shown in the figure, the first two of that take place in mitochondria and the other three in the cytosol:

1.  Carbamoyl  phosphate  synthetase,  that is technically  not a member of the urea cycle, catalyzes  activation  of ammonia  and the condensation (from the oxidative deamination  of glutamate through glutamate dehydrogenase;) and CO₂  (in the structure of bicarbonate, HCO^-) to built carbamoyl phosphate. Hydrolysis of two ATP molecules makes this reaction basically irreversible.

2. The second reaction also occurs in the mitochondria and includes the transfer of the carbamoyl group from carbamoyl phosphate to ornithine through ornithine transcarbamoylase. This reaction built another nonstandard amino acid citrulline that then has to be transported out of the mitochondrion into the cytosol where the remaining reactions of the cycle take place.

3.  The  citrulline  is  then  condensed  with  aspartate and  the  source  of  the  second nitrogen atom in urea, through the enzyme argininosuccinate synthetase  to form argininosuccinate. This reaction is driven through the hydrolysis of ATP to PP_i and AMP, with subsequent hydrolysis of the pyrophosphate.  Therefore both of the high-energy bonds in ATP are ultimately cleaved.

4. Argininosuccinase then erased the carbon skeleton of aspartate from argininosuccinate in the form of fumarate and leaving the nitrogen atom on the other product arginine. As the urea cycle also generates arginine this amino acid is classified as nonessential in ureotelic organisms. The Arginine is the immediate precursor of urea.

5. The  urea  is then  formed  from  arginine  through the  action  of arginase  with  the regeneration of ornithine. The ornithine is then sending back into the mitochondrion ready to be combined with another molecule of carbamoyl phosphate.