Table 3.

Some important amino acid modifications by reactive oxygen/nitrogen species.

Modification	Reaction description	Most commonly affected amino acids
Thiol modification (Barford, 2004)	It results in formation of sulfenic acids, intra- and intermolecular disulfides, cyclic sulfenamides, glutathionylation, sulfenyl-amide linkages, and S-nitrosation. Some of the reactions are reversible	Cysteine
Methionine oxidation (Stadtman et al., 2003)	Similar to cysteine, methionine has sulfur in its structure. Its oxidation by ROS results in formation of methionine sulfoxide. The reaction is reversible by methionine sulfoxide reductases. Further oxidation to methionine-S-sulfone may not be reversible	Methionine
Nitrosylation (Alvarez and Radi, 2003)	Addition of nitrosyl group to the protein. S-nitrosation refers to the reaction with cysteine and methionine	Cysteine, methionine, tyrosine, tryptophan, phenylalanine, histidine
Carbonylation (Wong et al., 2010)	Introducing the carbonyl group to the amino acid. May be reversible by a decarbonylation process. Carbonyl groups may form cross linkage with lysine residue of another protein. Detection of carbonylated proteins is an important method for detection of the ROS effect	Proline, arginine, lysine, threonine
Reactive aldehyde formation (Hazen et al., 1998a,b)	ROS (particularly HOCl) can virtually affect all amino acids to form reactive aldehydes. Generally irreversible	Most amino acids