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)

Urea is much less toxic than ammonia, therefore organisms which can't 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.

Reactions of cycle:

• NH₃ (ammonia) + CO₂ + 2 ATP → carbamoyl phosphate + 2 ADP + P_i (catalyzed by carbamoyl phosphate synthetase)
• carbomyl phosphate + ornithine → citrulline + P_i
• citrulline + aspartate + ATP → argininosuccinate + AMP + PP_i (catalyzed by argininosuccinate synthetase)
• AMP + ATP → 2 ADP
• PP_i + H₂O → 2 P_i
• argininosuccinate → arginine + fumarate (catalyzed by argininosuccinase)
• arginine + H₂O → ornithine + urea (catalyzed by arginase)
• ornithine + NH₃ + CO₂ → citrulline + H₂O (catalyzed by ornithine transcarbamoylase)

Summary reaction:

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