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).

Related Topics:
Biochemical - Urea - Ammonia - Krebs

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Urea is much less toxic than ammonia; therefore, organisms that cannot easily and quickly remove ammonia usually have to convert it to some other substance, like urea or uric acid. 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.

Related Topics:
Urea - Uric acid - Genetic disorder - Inborn errors of metabolism - Liver failure - Hepatic encephalopathy

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Reactions of cycle:

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:(a) carbamyl phosphate + ornithine (1) → citrulline (2) + Pi (catalyzed by ornithine transcarbamoylase)

Related Topics:
Carbamyl phosphate - Ornithine - Citrulline - Ornithine transcarbamoylase

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:(b) citrulline (2) + aspartate + ATP → argininosuccinate (3) + AMP + PPi (catalyzed by argininosuccinate synthase)

Related Topics:
Citrulline - Aspartate - Argininosuccinate - AMP - PP_i - Argininosuccinate synthase

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:(c) argininosuccinate (3) → arginine (4) + fumarate (catalyzed by argininosuccinate lyase)

Related Topics:
Argininosuccinate - Arginine - Fumarate - Argininosuccinate lyase

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:(d) arginine (4) + H2O → ornithine (1) + urea (catalyzed by arginase)

Related Topics:
Arginine - Ornithine - Urea - Arginase

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Summary reaction:

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• 2 NH₃ + CO₂ + 4 ATP + aspartate → urea + fumarate + 4 ADP + 4 P_i
 
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