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urea cycle
Biochemistry:
- pathway confined to liver.
- in terrestial vertebrates, excess ammonia formed from the hepatic degradation of amino acids, is converted into urea, then excreted
- one of the nitrogen atoms of urea is transferred from aspartate, the other is derived from ammonia
- the carbon atom is derived from carbon dioxide
- aspartate is consumed in the urea cycle & fumarate is generated
* figure
- urea cycle 1
- enzyme: carbamoyl phosphate synthase NH3
- substrate: CO2 + NH3 + 2 ATP
- product: carbamoyl phosphate + 2 ADP + H2PO4-
- urea cycle 2
- enzyme: ornithine carbamoyltransferase
- substrate: ornithine + carbamoyl_phosphate
- product: citrulline + H2PO4-
- urea cycle 3
- enzyme: argininosuccinate synthetase
- substrate: citrulline + aspartate + ATP
- product: argininosuccinate + AMP + H2P2O7-2
- urea cycle 4
- enzyme: argininosuccinate lyase
- substrate: argininosuccinate
- product arginine + fumarate
- urea cycle 5
- enzyme: arginase-1
- substrate: arginine + H2O
- product: ornithine + urea
Pathology:
1) carbamoyl phosphate synthase 1 deficiency
2) ornithine transcarbamylase deficiency
3) argininosuccinate synthetase deficiency (citrullinemia)
4) argininosuccinate lyase deficiency (argininosuccinic aciduria)
5) N-acetyl glutamate synthetase deficiency
6) arginase deficiency
Laboratory:
1) arterial blood gas
2) plasma citrulline
3) urine orotic acid (random) [2]
Related
urea (carbamide)
urea cycle disorder
General
molecular pathway
Database Correlations
Kegg map/map00220
References
- http://www.genome.ad.jp/kegg/pathway/map/map00220.html
- UpToDate 2005
http://www.utdol.com