Editing Ornithine (section)

==Role in urea cycle==
<small>L</small>-Ornithine is one of the products of the action of the enzyme [[arginase]] on <small>L</small>-[[arginine]], creating [[urea]]. Therefore, ornithine is a central component of the [[urea cycle]], which enables the disposal of excess [[nitrogen]]. Ornithine itself is recycled and, in a sense, acts as a catalyst.

First, ammonia is converted into [[carbamoyl phosphate]] ({{chem|H|2|NC(O)OPO|3|2−}}) by [[carbamoyl phosphate synthetase]]. [[Ornithine transcarbamylase]] then catalyzes the reaction between carbamoyl phosphate and ornithine to form [[citrulline]] and phosphate (P<sub>i</sub>). Another [[amino group]] is contributed by [[aspartate]], leading to the formation of arginine and the byproduct [[fumarate]]. The resulting arginine, a [[guanidinium]] compound, is subsequently hydrolyzed by arginase to regenerate ornithine and release urea.

The two nitrogen atoms in urea are derived from ammonia and aspartate, while the nitrogen atoms in ornithine remain unchanged.

[[Image:OTC reaction.png|thumb|left|458px|'''Reaction mechanism:'''. The [[Side chain|side-chain]] amino group of ornithine (Orn) nucleophilically attacks the carbonyl carbon of carbamoyl phosphate (CP), ''left'', forming a tetrahedral transition state, ''middle''. Charge rearrangement then releases citrulline (Cit) and phosphate (P<sub>i</sub>), ''right''.<ref name="pmid10747936">{{cite journal |vauthors=Langley DB, Templeton MD, Fields BA, Mitchell RE, Collyer CA |title=Mechanism of inactivation of ornithine transcarbamoylase by Ndelta-(N'-Sulfodiaminophosphinyl)-L-ornithine, a true transition state analogue? Crystal structure and implications for catalytic mechanism |journal=The Journal of Biological Chemistry |volume=275 |issue=26 |pages=20012–9 |date=June 2000 |pmid=10747936 |doi=10.1074/jbc.M000585200 |doi-access=free }}</ref>]] {{clear|left}}

Ornithine is not an amino acid directly coded for by [[DNA]]; that is, it is not a [[proteinogenic amino acid]]. However, in mammalian non-hepatic tissues, the primary role of the urea cycle is often the biosynthesis of arginine. As an intermediate in metabolic pathways, ornithine is thus quite important.<ref>{{cite journal |vauthors=Weber AL, Miller SL |title=Reasons for the occurrence of the twenty coded protein amino acids |journal=Journal of Molecular Evolution |volume=17 |issue=5 |pages=273–284 |year=1981 |pmid=7277510 |doi=10.1007/BF01795749 |bibcode=1981JMolE..17..273W |s2cid=27957755 |url=http://physwww.mcmaster.ca/~higgsp/3D03/WeberReasons.pdf }}</ref> {{clear|left}}