Ammonia is toxic, especially to the brain. The liver converts it to urea - non-toxic, water-soluble - for excretion by the kidneys. The urea cycle is the metabolic pathway that does this, spanning the mitochondrial matrix and cytoplasm of hepatocytes.
The urea cycle and the Krebs bicycle
Pathway map
1
Scroll sideways to see the whole map.
Mitochondrial matrix Cytosol CPS-I: the regulated step ATP spent
Aspartate inThe second nitrogen atom of urea does not arrive as free ammonia. It comes in on aspartate, made from oxaloacetate in the TCA cycle. So one nitrogen comes from the mitochondrial ammonia pool and one comes from the amino acid pool.
Fumarate outArgininosuccinate lyase releases fumarate, a TCA intermediate. It becomes malate, then oxaloacetate, which can either restart the cycle as aspartate or leave for gluconeogenesis. This shared traffic is why the two cycles are called the Krebs bicycle.
When it failsAny enzyme deficiency backs ammonia up into the blood. Hyperammonaemia is neurotoxic: ammonia pulls α-ketoglutarate out of the TCA cycle to make glutamate, starving neurons of ATP. OTC deficiency is the most common and is X-linked.
Two nitrogens in, one urea out, at a cost of four ATP equivalents. Ammonia is toxic and cannot be stored or exhaled, so the liver spends real energy to package it as a neutral, water-soluble molecule the kidney can excrete. Everything else on this map exists to serve that one conversion.
The urea cycle connects directly to amino acid catabolism (previous section) and to the TCA cycle. Transamination collects amino-nitrogen onto glutamate. Glutamate dehydrogenase releases that nitrogen as free ammonia, which immediately enters the urea cycle for safe disposal. The urea cycle also generates fumarate as a byproduct - which flows directly into the TCA cycle. So the two cycles share a metabolite and are functionally linked. This link is often called the “Krebs bicycle” because two cycles (urea + TCA) share a common intermediate.
The Net Reaction
NH3+CO2+Aspartate+3 ATP→Urea+Fumarate+2 ADP+AMP+PPi+4 Pi
Two nitrogens end up in urea: one from free NH3, one from aspartate. The fumarate connects the urea cycle to the TCA cycle.
Ornithine transcarbamylase (mitochondrial) · citrulline exits to cytoplasm
–
3
Citrulline + aspartate → argininosuccinate
Argininosuccinate synthetase · second N arrives via aspartate
-1 ATP (→AMP)
4
Argininosuccinate → arginine + fumarate
Argininosuccinate lyase · fumarate feeds the TCA cycle
+fumarate
5
Arginine + H₂O → urea + ornithine
Arginase · ornithine recycles into step 2
+urea
Net cost: 4 high-energy phosphate bonds (2 ATP → 2 ADP; 1 ATP → AMP + PPi). Urea carries 2 nitrogens (one from NH₃, one from aspartate) to the kidney for excretion.
Mnemonic
Urea Cycle Defects
Any urea cycle enzyme deficiency causes hyperammonemia. Ammonia accumulates and damages the brain, causing altered mental status, seizures, vomiting, and potentially coma. Serum levels of specific intermediates help diagnose which enzyme is missing.
Ornithine transcarbamylase (OTC) deficiency: most common. X-linked. High ammonia, high orotate, low citrulline (because the block is before citrulline). Elevated orotate is pathognomonic.
CPS-I deficiency: also high ammonia; low citrulline and low orotate.
Treatment includes dietary protein restriction and medications that divert nitrogen via alternative pathways (phenylbutyrate, benzoate).
What is the function of the urea cycle?
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To convert toxic ammonia (from amino acid catabolism) into non-toxic urea for excretion by the kidneys. Each urea molecule contains 2 nitrogens: one from free NH3 (input to CPS-I), one from aspartate (input to argininosuccinate synthetase). The cycle consumes 4 high-energy phosphate bonds per urea.
Which urea cycle enzyme is the most commonly deficient, and what is its diagnostic feature?
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Ornithine transcarbamylase (OTC) deficiency. It is X-linked. The enzyme normally converts carbamoyl phosphate + ornithine → citrulline. Without it, carbamoyl phosphate accumulates in mitochondria and diffuses to cytoplasm, where it is diverted into pyrimidine synthesis, producing excess orotate. High serum ammonia + high orotate + low citrulline is the classic triad.
What happens if the urea cycle is impaired?
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Hyperammonemia - ammonia accumulates in the blood because it cannot be converted to urea. Ammonia is especially toxic to the brain, causing altered mental status, seizures, vomiting, and coma. Treatments include dietary protein restriction and nitrogen-scavenging drugs (phenylbutyrate, benzoate) that use alternative pathways to excrete nitrogen.
