Ornithine

From Wikipedia, the free encyclopedia

L-Ornithine
IUPAC name	2,5-Diaminopentanoic acid
Identifiers
CAS number	[70-26-8]
PubChem	389
MeSH	Ornithine
SMILES	C(CC(C(=O)O)N)CN
Properties
Molecular formula	C5H12N2O2
Molar mass	132.16 g/mol
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Ornithine is an amino acid which plays a role in the urea cycle.

Contents 1 Role in urea cycle 2 Lactamization 3 Other reactions 4 References

[edit] Role in urea cycle

L-Ornithine is one of the products of the action of the enzyme arginase on L-arginine, creating urea. Therefore, ornithine is a central part of the urea cycle, which allows for the disposal of excess nitrogen. Ornithine is recycled and in a manner is a catalyst. First, ammonia is converted into carbamoyl phosphate (phosphate-CONH₂), which creates one half of urea. Ornithine is converted into a urea derivative at the δ (terminal) nitrogen by carbamoyl phosphate. Another nitrogen is added from aspartate, producing the denitrogenated fumarate, and the resulting arginine (a guanidinium compound) is hydrolysed back to ornithine, producing urea. The nitrogens of urea come from the ammonia and aspartate, and the nitrogen in ornithine remains intact.

Ornithine is not an amino acid coded for by DNA, and, in that sense, is not involved in protein synthesis. However, in mammalian non-hepatic tissues, the main use of the urea cycle is in arginine biosynthesis, so as an intermediate in metabolic processes, ornithine is quite important.

[edit] Lactamization

It is believed to not be a part of genetic code because polypeptides containing unprotected ornithines undergo spontaneous lactamization. This proved to be a problem when ornithine was artificially incorporated in 21st amino acid systems.^{[citation needed]}

[edit] Other reactions

Ornithine, via the action of ornithine decarboxylase (E.C. 4.1.1.17), is the starting point for the synthesis of polyamines such as putrescine.

In bacteria, such as E. coli, ornithine can be synthesized from L-glutamate.^[1]

[edit] References

1. ^ "Ornithine Biosynthesis". http://www.chem.qmul.ac.uk/iubmb/enzyme/reaction/AminoAcid/Orn.html. Retrieved on 2007-08-17. 

v • d • e Amino acid metabolism metabolic intermediates K→acetyl-CoA lysine→ Saccharopine · Allysine · α-Aminoadipic acid · α-Aminoadipate · Glutaryl-CoA · Glutaconyl-CoA · Crotonyl-CoA · β-Hydroxybutyryl-CoA leucine→ α-Ketoisocaproic acid · Isovaleryl-CoA · 3-Methylcrotonyl-CoA · 3-Methylglutaconyl-CoA · HMG-CoA tryptophan→alanine→ N'-Formylkynurenine · Kynurenine · Anthranilic acid · 3-Hydroxykynurenine · 3-Hydroxyanthranilic acid · 2-Amino-3-carboxymuconic semialdehyde · 2-Aminomuconic semialdehyde · 2-Aminomuconic acid · Glutaryl-CoA G G→pyruvate→citrate glycine→serine→ 3-Phosphoglyceric acid glycine→creatine: Glycocyamine · Phosphocreatine · Creatinine G→glutamate→ α-ketoglutarate histidine→ Urocanic acid · Imidazol-4-one-5-propionic acid · Formiminoglutamic acid · Glutamate-1-semialdehyde proline→ 1-Pyrroline-5-carboxylic acid arginine→ Ornithine · Putrescine · Agmatine other cysteine+glutamate→glutathione: γ-Glutamylcysteine G→propionyl-CoA→ succinyl-CoA valine→ α-Ketoisovaleric acid · Isobutyryl-CoA · Methacrylyl-CoA · 3-Hydroxyisobutyryl-CoA · 3-Hydroxyisobutyric acid · 2-Methyl-3-oxopropanoic acid isoleucine→ 2,3-Dihydroxy-3-methylpentanoic acid · 2-Methylbutyryl-CoA · Tiglyl-CoA · 2-Methylacetoacetyl-CoA methionine→ generation of homocysteine: S-Adenosyl methionine · S-Adenosyl-L-homocysteine · Homocysteine conversion to cysteine: Cystathionine · alpha-Ketobutyric acid+Cysteine threonine→ α-Ketobutyric acid propionyl-CoA→ Methylmalonyl-CoA G→fumarate phenylalanine→tyrosine→ 4-Hydroxyphenylpyruvic acid · Homogentisic acid · 4-Maleylacetoacetate G→oxaloacetate see urea cycle Other Cysteine metabolism Cysteine sulfinic acid see also enzymes, disorders