Urea cycle
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Urea cycle
The Urea Cycle is a cycle of biochemical reactions occurring in many animal organisms that produces urea from ammonia. This cycle, also known as ornithine cycle, was the first metabolic cycle discovered (Krebs and Hensenleit, 1932) ...
The Urea Cycle
Urea is the chief nitrogenous waste of mammals.
Most of our nitrogenous waste comes from the breakdown of amino acids.
In the urea cycle, the enzyme arginase cleaves (hydrolyzes) the guanidinium group to yield urea and the L-amino acid ornithine. Ornithine is lysine with one fewer methylene groups in the side chain. L-ornithine is not normally found in proteins.
Carbamoyl phosphate synthetase An enzyme that begins the urea cycle by catalyzing the synthesis of carbamoyl phosphate from bicarbonate, ammonium ion, and ATP. The enzyme also catalyzes the initial reaction in pyrimidine biosynthesis.
Most of these defects cause significant disability that cannot be treated satisfactorily by such other approaches as modifying the diet or supplying the enzyme. Some examples include adrenoleukodystrophy, galactosemia, and the urea cycle disorders.
 See also: Urea, Trans, Organ, Metabolism, Cells
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