The urea cycle, also known as the ornithine cycle, is a cycle of biochemical reactions occurring in many animal organisms that produces urea from ammonia (NH₄⁺). This cycle was the first metabolic cycle discovered (Krebs and Hensenleit, 1932).

Function

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Organisms that cannot easily and quickly remove ammonia usually have to convert it to some other substance, like urea or uric acid, which are much less toxic. Insufficiency of the urea cycle occurs in some genetic disorders (inborn errors of metabolism), and in liver failure. The result in liver failure is accumulation of nitrogenous waste, mainly ammonia, which leads to hepatic encephalopathy.

Reactions

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The urea cycle consists of five reactions - two mitochondrial and three cytosolic. The cycle converts two amino groups, one from NH₄⁺ and one from Asp, and a carbon atom from HCO₃^-, to relatively nontoxic excretion product, urea, at the cost of four "high-energy" phosphate bonds (3 ATP hydrolyzed to 2 ADP and one AMP). Orn is the carier of these carbon and nitrogen atoms.

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