Urea Cycle
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Direct answer
The urea cycle (Krebs-Henseleit cycle) is the only pathway that detoxifies ammonia, converting it to urea — the principal nitrogenous waste in urine. It operates exclusively in the liver, beginning in the mitochondrial matrix and completing in the cytosol. Of the two nitrogen atoms in urea, one enters as free ammonia through carbamoyl phosphate synthetase I (CPS-I), the rate-limiting enzyme, and the other enters as aspartate.
What you must remember
- Five enzymes in order: carbamoyl phosphate synthetase I and ornithine transcarbamylase (mitochondrial), then argininosuccinate synthetase, argininosuccinate lyase and arginase (cytosolic); arginase releases urea and regenerates ornithine.
- CPS-I is rate-limiting and absolutely requires N-acetylglutamate as allosteric activator; NAG is synthesised from acetyl-CoA and glutamate after a protein-rich meal.
- Energetics: three ATP are consumed per urea formed — two at CPS-I and one converted to AMP at argininosuccinate synthetase, meaning four high-energy phosphate bonds.
- Link with the TCA cycle (the "Krebs bicycle"): fumarate released by argininosuccinate lyase enters the citric acid cycle, and aspartate is regenerated from oxaloacetate by transamination.
- Inborn errors cause hyperammonaemia with vomiting, lethargy, seizures and intellectual disability; ornithine transcarbamylase deficiency is the commonest and the only X-linked one.
- Urinary orotic acid separates them — high in OTC deficiency (carbamoyl phosphate diverted to pyrimidine synthesis) and low in CPS-I deficiency.
- Management: protein restriction, nitrogen-scavenging agents (sodium benzoate, phenylbutyrate), arginine or citrulline supplementation, and haemodialysis in acute hyperammonaemic crisis.
Common confusion
CPS-I and CPS-II are constantly confused. CPS-I is mitochondrial, uses ammonia, participates in the urea cycle and is activated by N-acetylglutamate; CPS-II is cytosolic, uses glutamine, builds pyrimidines and is inhibited by UTP. A second trap is OTC deficiency versus hereditary orotic aciduria (UMP synthase defect): both raise urinary orotic acid, but only the latter produces a megaloblastic anaemia unresponsive to B12 and folate.
Exam-focused takeaway
Theory answers should list the reactions with sites, the two nitrogen donors, energetics (three ATP, four high-energy bonds) and regulation by N-acetylglutamate, closing with the hyperammonaemia disorders. In the viva, be ready for why the cycle is partly mitochondrial and how the urea cycle connects with the TCA cycle. MCQs test the NAG requirement, the X-linked inheritance of OTC deficiency, the orotic acid differentiator and citrullinaemia-related enzyme deficiencies.
Frequently asked questions
Where does the urea cycle occur?
Only in the liver — the first two steps (CPS-I, ornithine transcarbamylase) are mitochondrial and the last three are cytosolic.
What activates carbamoyl phosphate synthetase I?
N-acetylglutamate, formed from acetyl-CoA and glutamate, is the obligatory allosteric activator; without it the rate-limiting step stops.
How many ATP does one urea molecule cost?
Three ATP, equal to four high-energy phosphate bonds, per turn of the cycle.
Why does ornithine transcarbamylase deficiency cause orotic aciduria?
Blocked conversion of carbamoyl phosphate to citrulline diverts it to cytosolic pyrimidine synthesis, so orotic acid accumulates and spills into urine.
How are urea cycle disorders treated?
With a low-protein diet, sodium benzoate and phenylbutyrate to scavenge nitrogen, arginine or citrulline supplementation, and dialysis for acute crises.