Arginine

Arginine is a protein-building amino acid that humans can synthesize but may need from food during growth, illness, or other stress. It also supplies nitrogen and carbon for nitric oxide, urea, and other compounds.

Nitric oxide synthase: An enzyme family that converts arginine and oxygen into nitric oxide and citrulline. This enzyme consumes arginine to make nitric oxide, a signaling molecule.
Amino acid: An organic molecule with amino and carboxyl groups that serves as a protein building block or metabolic intermediate. Arginine belongs to this molecular class and is incorporated into proteins.
Conditional essential amino acid: An amino acid the body usually synthesizes but may not produce in sufficient amounts under particular conditions. Arginine requirements can exceed endogenous production during growth or severe stress.
Lysine: An essential, positively charged amino acid with a terminal amino group in its side chain. Like arginine, lysine is basic, but it is essential and lacks arginine’s guanidinium group.
Urea cycle: A series of reactions that converts toxic ammonia into urea for excretion. Arginine is formed and then cleaved in this pathway to release urea.
Guanidinium group: A positively charged functional group with a carbon bonded to three nitrogen atoms. Arginine’s side chain ends in this group, which remains positively charged at physiological pH.
Dietary supplement: A product intended to add nutrients or other substances to the diet. Arginine is sold as a supplement for proposed vascular and exercise effects.
Histidine: A protein-building amino acid whose imidazole side chain can gain or lose a proton near physiological pH. Histidine is another basic amino acid, but its side-chain chemistry differs from arginine’s.
Arginase: An enzyme that hydrolyzes arginine into ornithine and urea. It diverts arginine from nitric oxide production into urea-cycle metabolism.
Peptide bond: A covalent amide bond linking amino acids in peptides and proteins. Arginine joins protein chains through the same peptide bonds as other amino acids.

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