Ammonia to urea, in the liver: the first two steps in the mitochondrial matrix, the rest in the cytosol
Route: Carbamoyl phosphate + Ornithine → Citrulline + Aspartate → Argininosuccinate → Arginine + Fumarate → Urea + Ornithine
Amino acid breakdown releases ammonium, which is toxic. The liver turns it into urea, which the body excretes in urine. One nitrogen comes from ammonium, the other from aspartate. Ornithine is both the first and the last compound: it picks up carbamoyl phosphate and the cycle turns again. Making one urea costs four high-energy phosphate bonds (two ATP in the first step, one ATP cut to AMP and pyrophosphate in the third). The fumarate joins the Krebs cycle, which links the two cycles.
Practice this pathway All pathways
| Step | Enzyme | EC | In | Out |
|---|---|---|---|---|
| 1 | Carbamoyl phosphate synthetase I | 6.3.4.16 | NH₄⁺, HCO₃⁻, ATP, ATP | ADP, ADP, Pᵢ |
| 2 | Ornithine transcarbamylase | 2.1.3.3 | Pᵢ | |
| 3 | Argininosuccinate synthetase | 6.3.4.5 | ATP | AMP, PPᵢ |
| 4 | Argininosuccinate lyase | 4.3.2.1 | ||
| 5 | Arginase | 3.5.3.1 | H₂O |
Enzyme names and EC numbers are those used on the interactive board. Checked against sources: see the claim table in the repository (tools/pathway_data/checks).