The chemical compound citrulline is an amino acid whose structure is:

H₂N-CONH-CH₂-CH₂-CH₂-CHNH₂-COOH

L-citrulline is made from L-ornithine and carbamoyl phosphate in one of the central reactions in the urea cycle. It is also produced from L-arginine as a by-product of the reaction catalyzed by the enzyme NO synthase. L-citrulline, while being an amino acid, is not involved in protein synthesis and is not one of the amino acids coded for by DNA.