If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Amino Acids Biosynthesis → Other Amino Acid Biosynthesis → L-citrulline Biosynthesis|
L-citrulline is a non-standard amino acid that is not normally incorporated into proteins during protein synthesis. The name citrulline was coined in 1930 from Citrullus, the Latin name of the watermelon, from which it was first isolated. Free citrulline is formed mainly by catabolism of amino acids in the small intestine (see L-citrulline biosynthesis), as an intermediate in the conversion of ammonia to urea in the urea cycle, and as a by-product during the production of nitric oxide (see L-citrulline-nitric oxide cycle). In addition, citrulline is also formed by modification of arginine residues in proteins (see protein citrullination).
About This Pathway
Free citrulline metabolism involves three key enzymes: nitric oxide synthase (NOS, EC 18.104.22.168), which catalyzes the production of nitric oxide from arginine, generating citrulline as a by-product, ornithine carbamoyltransferase (OCT, EC 22.214.171.124), which produces citrulline by condensing carbamoyl phosphate and L-ornithine, and argininosuccinate synthase (ASS, EC 126.96.36.199), which converts citrulline into argininosuccinate.
The tissue distribution of these enzymes results in three orthogonal metabolic routes for citrulline in mammals, all of which are depicted in this pathway:
1. In the liver, citrulline is locally synthesized by the combined action of arginase 1, ammonia-dependent carbamoyl-phosphate synthase and ornithine carbamoyltransferase, and metabolized by argininosuccinate synthase and argininosuccinate lyase back to arginine. The main purpose of this cycle is the production of urea, and it is described in greater detail in the pathway urea cycle.
2. In most of the tissues producing NO, citrulline is generated from arginine in a single step by NOS, and then recycled back into arginine via argininosuccinate synthase and argininosuccinate lyase. The main purpose of this cycle is the production of NO, and it is described in greater detail in the pathway L-citrulline-nitric oxide cycle.
3. In the gut citrulline is synthesized from glutamine and other amino acids (such as proline) by the combined action of many enzymes, including glutaminase, kidney isoform, δ-1-pyrroline-5-carboxylate synthase, mitochondrial proline dehydrogenase 1, ornithine aminotransferase and ornithine carbamoyltransferase. Citrulline is then released into the blood, and converted back into arginine in the kidneys by argininosuccinate synthase and argininosuccinate lyase. The main purpose of this pathway is the transport of arginine in the blood, avoiding captation by the liver, and it is described in the pathway L-citrulline biosynthesis.
Citrulline has long been administered in the treatment of inherited urea cycle disorders, and recent studies suggest that it may be used to control the production of NO.
In plants, citrulline has additional roles. It has been shown that in drought-tolerant wild watermelon leaves citrulline functions both as a compatible solute and ahydroxyl radical scavenger [Akashi01, Yokota02].
Unification Links: AraCyc:PWY-5004
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