urea cycle
Definition
Target type: biologicalprocess
The sequence of reactions by which arginine is synthesized from ornithine, then cleaved to yield urea and regenerate ornithine. The overall reaction equation is NH3 + CO2 + aspartate + 3 ATP + 2 H2O = urea + fumarate + 2 ADP + 2 phosphate + AMP + diphosphate. [GOC:pde, GOC:vw, ISBN:0198506732]
The urea cycle, also known as the ornithine cycle, is a metabolic pathway that converts ammonia (NH3), a toxic byproduct of protein metabolism, into urea, a less toxic compound that can be excreted in urine. This process primarily occurs in the liver and is crucial for maintaining nitrogen homeostasis in the body.
The urea cycle consists of five key enzymatic reactions:
1. **Carbamoyl phosphate synthetase I (CPS I):** This enzyme catalyzes the first committed step of the cycle, combining ammonia, carbon dioxide, and ATP to form carbamoyl phosphate. This reaction occurs in the mitochondria.
2. **Ornithine transcarbamylase (OTC):** Carbamoyl phosphate is then transferred to ornithine, an amino acid, by OTC, forming citrulline. This reaction also takes place within the mitochondria.
3. **Citrulline transport:** Citrulline is then transported out of the mitochondria into the cytosol.
4. **Argininosuccinate synthetase (ASS):** In the cytosol, citrulline reacts with aspartate, another amino acid, to form argininosuccinate. This reaction requires ATP and is catalyzed by ASS.
5. **Argininosuccinate lyase (ASL):** Argininosuccinate is then cleaved by ASL, releasing fumarate and arginine.
6. **Arginase:** Finally, arginine is hydrolyzed by arginase, releasing urea and regenerating ornithine, which can enter the cycle again.
The urea cycle is tightly regulated by several factors, including the availability of substrates like ammonia and aspartate, the activity of the enzymes involved, and the levels of certain hormones.
The urea cycle plays a crucial role in removing excess nitrogen from the body, preventing ammonia toxicity. Ammonia accumulation can lead to serious neurological problems, including coma and death. Disorders affecting the urea cycle can result in hyperammonemia, a buildup of ammonia in the blood, which can lead to various health issues.
Overall, the urea cycle is an essential metabolic pathway that removes nitrogen waste from the body, protecting against ammonia toxicity and maintaining nitrogen homeostasis.'
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Proteins (4)
| Protein | Definition | Taxonomy |
|---|---|---|
| Arginase-1 | An arginase-1 that is encoded in the genome of cow. [OMA:Q2KJ64, PRO:DNx] | Bos taurus (cattle) |
| Arginase-2, mitochondrial | An arginase-2, mitochondrial that is encoded in the genome of human. [PRO:DNx, UniProtKB:P78540] | Homo sapiens (human) |
| Arginase-1 | An arginase-1 that is encoded in the genome of human. [PRO:WCB, UniProtKB:P05089] | Homo sapiens (human) |
| Ornithine transcarbamylase, mitochondrial | An ornithine transcarbamylase, mitochondrial that is encoded in the genome of human. [PRO:DNx, UniProtKB:P00480] | Homo sapiens (human) |
Compounds (17)
| Compound | Definition | Classes | Roles |
|---|---|---|---|
| catechol | catechols | allelochemical; genotoxin; plant metabolite | |
| phosphoric acid | phosphoric acid : A phosphorus oxoacid that consists of one oxo and three hydroxy groups joined covalently to a central phosphorus atom. phosphoric acid: concise etchant is 37% H3PO4 | phosphoric acids | algal metabolite; fertilizer; human metabolite; NMR chemical shift reference compound; solvent |
| diphosphoric acid | diphosphoric acid : An acyclic phosphorus acid anhydride obtained by condensation of two molecules of phosphoric acid. | acyclic phosphorus acid anhydride; phosphorus oxoacid | Escherichia coli metabolite |
| hydroxyhydroquinone | benzene-1,2,4-triol : A benzenetriol carrying hydroxy groups at positions 1, 2 and 4. | benzenetriol | mouse metabolite |
| methylene diphosphonate | medronic acid : A 1,1-bis(phosphonic acid) consisting of methane substituted by two phosphonic acid groups. | 1,1-bis(phosphonic acid) | bone density conservation agent; chelator |
| 2-aminoimidazole | 2-aminoimidazole: from catabolism of arginine | ||
| 3,4-dihydroxybenzylamine | 3,4-dihydroxybenzylamine: RN given refers to parent cpd; structure | catechols | |
| 3,4-dihydroxybenzyl alcohol | 3,4-dihydroxybenzyl alcohol: structure given in first source | ||
| n(omega)-hydroxyarginine | N(5)-[(Z)-amino(hydroxyimino)methyl]-L-ornithine : An N(5)-[amino(hydroxyimino)methyl]-L-ornithine in which the double bond has Z-configuration. N(omega)-hydroxyarginine: can cause vasorelaxation of bovine intrapulmonary artery; structure given in first source | amino acid zwitterion; N(5)-[(E)-amino(hydroxyimino)methyl]ornithine; N(5)-[(hydroxyamino)(imino)methyl]ornithine; N(5)-[(Z)-amino(hydroxyimino)methyl]ornithine; N(5)-[amino(hydroxyimino)methyl]-L-ornithine; N(5)-[amino(hydroxyimino)methyl]ornithine; N(omega)-hydroxy-L-arginine | |
| n(delta)-(phosphonoacetyl)-l-ornithine | |||
| a 3253 | 4-aminocatechol: RN given refers to parent cpd | catechols | |
| methyl 3,4-dihydroxybenzoate | catechols; methyl ester | antioxidant; neuroprotective agent; plant metabolite | |
| Nomega-hydroxy-nor-l-arginine | L-alpha-amino acid | ||
| 3,3',4,5'-tetrahydroxystilbene | 3,3',4,5'-tetrahydroxystilbene: demethyl derivative of isorhapontigenin; structure in first source; a Syk kinase inhibitor; found in heartwood of FABACEAE; inhibitor of photosynthesis in spinach chloroplasts; may be inhibitor of plant growth; RN given refers to (E)-isomer piceatannol : A stilbenol that is trans-stilbene in which one of the phenyl groups is substituted by hydroxy groups at positions 3 and 4, while the other phenyl group is substituted by hydroxy groups at positions 3 and 5. | catechols; polyphenol; resorcinols; stilbenol | antineoplastic agent; apoptosis inducer; geroprotector; hypoglycemic agent; plant metabolite; protein kinase inhibitor; tyrosine kinase inhibitor |
| caffeic acid | trans-caffeic acid : The trans-isomer of caffeic acid. | caffeic acid | geroprotector; mouse metabolite |
| (S)-2-amino-6-boronohexanoic acid | (S)-2-amino-6-boronohexanoic acid : L-Norleucine substituted at C-6 with a borono group. | non-proteinogenic L-alpha-amino acid; organoboron compound | |
| 2-amino-6-boronohexanoic acid |