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The Urea Cycle is a cycle of biochemical reactions
occurring in many animal organisms that produces urea from ammonia.
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 + CO2 + 2 ATP →
carbamoyl phosphate + 2 ADP + Pi (catalyzed by carbamoyl phosphate synthetase)
- carbomyl phosphate + ornithine → citrulline + Pi
- citrulline + aspartate + ATP → argininosuccinate + AMP + PPi (catalyzed by argininosuccinate
synthetase)
- AMP + ATP → 2 ADP
- PPi + H2O → 2 Pi
- argininosuccinate
→ arginine + fumarate (catalyzed by
argininosuccinase)
- arginine + H2O → ornithine + urea (catalyzed
by arginase)
- ornithine + NH3 + CO2 → citrulline + H2O (catalyzed
by ornithine transcarbamoylase)
Summary reaction:
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