Urea Cycle

1. 1

   NH₄⁺ + HCO₃⁻ + 2 ATP → carbamoyl phosphate + 2 ADP + Pᵢ

   Carbamoyl phosphate synthetase I (CPS I) 6.3.4.16 ST-0141 Irreversible rate-limiting ATP Pi Bicarbonate/CO2
   In NH4+ HCO3- ATP

   Out Carbamoyl phosphate ADP Pi
   › Notes

   In the mitochondrial matrix, carbamoyl phosphate synthetase I (CPS I) uses MgATP and requires N-acetylglutamate (NAG) as an obligatory allosteric activator. This is the rate-limiting, committed, and essentially irreversible step of the urea cycle; it consumes two high-energy phosphate bonds.

2. 2

   Carbamoyl phosphate + ornithine → citrulline + Pᵢ

   Ornithine transcarbamylase (OTC) 2.1.3.3 ST-0142 Irreversible ATP Pi
   In Carbamoyl phosphate Ornithine

   Out Citrulline Pi
   › Notes

   Ornithine transcarbamylase (OTC) catalyzes this mitochondrial reaction without direct ATP use. Citrulline is exported to the cytosol in exchange for ornithine; the reaction is effectively irreversible under physiological conditions because carbamoyl phosphate is high energy.

3. 3

   Citrulline + L-aspartate + ATP → argininosuccinate + AMP + PPᵢ

   Cytosolic argininosuccinate synthetase (ASS1) 6.3.4.5 ST-0143 Irreversible ATP PPi
   In Citrulline Aspartate ATP

   Out Argininosuccinate AMP PPi
   › Notes

   Cytosolic argininosuccinate synthetase (ASS1) activates citrulline and adds aspartate. MgATP is cleaved to AMP + PPᵢ, making this an essentially irreversible step that consumes two additional high-energy phosphate equivalents.

4. 4

   Argininosuccinate → L-arginine + fumarate

   Argininosuccinate lyase (ASL) 4.3.2.1 ST-0144 Irreversible/directional
   In Argininosuccinate

   Out Arginine Fumarate
   › Notes

   Cytosolic argininosuccinate lyase (ASL) cleaves argininosuccinate without a required redox cofactor. Fumarate enters the aspartate–argininosuccinate shunt, where it can be converted through malate and oxaloacetate to aspartate, linking the cycle to the tricarboxylic acid cycle.

5. 5

   L-arginine + H₂O → L-ornithine + urea

   Arginase I (ARG1) 3.5.3.1 ST-0145 Irreversible H2O
   In Arginine H2O

   Out Ornithine Urea
   › Notes

   Cytosolic arginase I (ARG1), a manganese-containing enzyme, hydrolyzes arginine to urea and ornithine. This essentially irreversible final step regenerates ornithine for mitochondrial import and releases urea into blood for renal excretion.

Showing all 5 steps.

Overview

The urea cycle converts toxic ammonia and the amino nitrogen of aspartate into urea, the principal nitrogenous waste product in humans. It is essential during amino-acid catabolism, especially after a protein-rich meal, prolonged fasting, trauma, or uncontrolled diabetes. One nitrogen of urea enters as free mitochondrial ammonia and the other enters as cytosolic aspartate; its carbonyl carbon is derived from bicarbonate.

Cellular location

The cycle is confined to the liver. Steps 1–2 occur in the mitochondrial matrix of hepatocytes, whereas steps 3–5 occur in the cytosol; ornithine and citrulline traverse the inner mitochondrial membrane through the ornithine–citrulline antiporter (ORNT1). Periportal hepatocytes are specialized for high-capacity ureagenesis, while perivenous hepatocytes scavenge residual ammonia through glutamine synthetase.

Net energetics

The net reaction is approximately: NH₄⁺ + HCO₃⁻ + aspartate + 3 ATP + 2 H₂O → urea + fumarate + 2 ADP + AMP + 4 Pᵢ. The cycle consumes 3 ATP molecules but 4 high-energy phosphate bonds per urea (two ATP → two ADP in CPS I and one ATP → AMP in ASS1). Fumarate oxidation through malate to oxaloacetate can generate one cytosolic NADH, partly offsetting the energetic cost when its reducing equivalent is oxidized.

Clinical significance

The urea cycle prevents accumulation of ammonia generated by amino-acid, nucleotide, and amine metabolism. Inherited defects in NAGS, CPS1, OTC, ASS1 (citrullinemia type I), ASL (argininosuccinic aciduria), or ARG1 cause hyperammonemia, often with vomiting, lethargy, cerebral edema, and encephalopathy. OTC deficiency is X-linked and commonly produces elevated urinary orotic acid because excess mitochondrial carbamoyl phosphate enters cytosolic pyrimidine synthesis; this distinguishes it from CPS1 deficiency.