Figure 2.

Post-translational modifications to the amino acids tyrosine and arginine. (a) Nitration of tyrosine by peroxynitrite results in the formation of 3-nitrotyrosine. This reaction can occur to both peptidyl and free tyrosine residues. (b) Citrullination is the deimination of arginine to citrulline by peptidylarginine deiminase. The substrate for this enzyme is peptidylarginine, the reaction does not occur on free arginine. Production of free citrulline, however, can occur as a by-product of nitric oxide synthase (NOS) or the urea cycle. Both of these modifications are not just passive, they can alter protein function, for example chemokines.