8 min readPathway sheetVery often a 10-mark answer

Urea cycle

The liver turns toxic ammonia into urea in five steps, starting in the mitochondria and finishing in the cytoplasm, using 3 ATP per urea.

WhereLiver: mitochondria → cytoplasm

In → OutNH₃ + CO₂ + aspartate → urea

Energy3 ATP (4 high-energy bonds)

Control pointCPS I, switched on by N-acetylglutamate

The 5 steps

Steps 1–2 run in the mitochondria; steps 3–5 in the cytoplasm. Urea carries two nitrogens: one from free ammonia (step 1) and one from aspartate (step 3). Starred steps are the key ones.

1★
CO₂ + NH₃ + 2 ATP → carbamoyl phosphate
Carbamoyl phosphate synthetase I (CPS I) · mitochondria · uses 2 ATP · rate-limiting
2
Carbamoyl phosphate + ornithine → citrulline
Ornithine transcarbamoylase · mitochondria; citrulline then moves out to the cytoplasm
3★
Citrulline + aspartate + ATP → argininosuccinate
Argininosuccinate synthetase · cytoplasm · ATP → AMP + PPi (2 high-energy bonds) · second N from aspartate
4
Argininosuccinate → arginine + fumarate
Argininosuccinate lyase · fumarate links the cycle to the TCA cycle
5★
Arginine + H₂O → urea + ornithine
Arginase · urea leaves the liver; ornithine returns to the mitochondria

★ one-way, controls the pace

Energy count

Step 1 (CPS I)2 ATP → 2 ADP

Step 3 (argininosuccinate synthetase)1 ATP → AMP + PPi (2 high-energy bonds)

Total per urea3 ATP, 4 high-energy bonds

Regulation and the TCA link

N-acetylglutamate (NAG) is the required activator of CPS I. It is made from acetyl-CoA and glutamate, and arginine speeds up its formation. After a protein-rich meal, more glutamate and arginine mean more NAG, so the cycle runs faster. Over days, a high-protein diet or starvation increases the amount of the cycle enzymes. Fumarate from step 4 enters the TCA cycle, becomes malate and then oxaloacetate, which can be transaminated back to aspartate. This link between the two cycles is called the Krebs bicycle.

When the cycle fails: hyperammonaemia

Inherited (urea cycle disorders)A defect in any cycle enzyme raises blood ammonia: CPS I deficiency (hyperammonaemia type I), OTC deficiency (type II, the most common, X-linked), citrullinaemia, argininosuccinic aciduria and argininaemia. Babies show vomiting, poor feeding, drowsiness and fits. Managed with a low-protein diet and drugs such as sodium benzoate that help remove nitrogen.

Acquired (liver disease)In cirrhosis or severe hepatitis the liver cannot make enough urea, so ammonia builds up. The brain suffers first: flapping tremor, confusion and finally hepatic coma.

Why ammonia harms the brain: the brain uses up α-ketoglutarate to trap ammonia as glutamate and glutamine, so the TCA cycle slows and brain energy falls.

Nutrition link

More dietary protein means more amino acids broken down, so more urea is made and passed in urine. In liver disease the cycle slows and ammonia builds up, so the dietitian plans protein intake carefully rather than simply cutting it out. In kidney failure urea cannot be excreted, so blood urea rises.

Exam angle

2 marks: “Name the activator of carbamoyl phosphate synthetase I.” N-acetylglutamate (NAG), made from acetyl-CoA and glutamate.

5 marks: “What is hyperammonaemia? Why is ammonia toxic?” Raised blood ammonia from urea cycle enzyme defects or liver disease; ammonia drains α-ketoglutarate in the brain, cutting its energy supply.

10 marks: “Describe the urea cycle with its regulation and energetics.” Draw the cycle across mitochondria and cytoplasm, write the five steps with enzymes, the 3 ATP count, NAG control and the fumarate–TCA link.

Remember it

"Ornithine Comes Around Again": Ornithine → Citrulline → Argininosuccinate → Arginine → back to ornithine, releasing urea. Carbamoyl phosphate brings the first N; aspartate brings the second.