Five of the twenty proteinogenic amino acids can undergo enzyme-mediated phosphorylation by ATP on a side chain nitrogen, creating N–PO₃²⁻ linkages. Most surprising from a chemical perspective is N-phosphorylation of arginine guanidinium side chains that create phosphagens, reversible donors of phosphoryl groups to ADP for ATP formation in tissues, such as muscle, with high energy demands. Aminokinases, generating the phosophoramidate linkages, are relatively rare in the pantheon of metabolic kinases that normally use oxygen nucleophiles in phosphoryl transfer events.