Figure 4.

Oxidative thiol modifications in redox-sensitive cysteine residues. Active thiol groups are easily oxidized to sulfenic acids (RSOH), the initial oxidation product. These transient sulfenic acids can also result from the hydrolysis of S-nitrosothiols (RSNO), which are the oxidative products of thiol groups in response to RNS. Sulfenic acids often condense with nearby thiols to form intermolecular or intramolecular disulfide bonds (RS-SR' or RS-SR), or with GSH resulting in S-glutathionylation (RS-SG). These oxoforms are reversible and can be restored to free thiols through the action of cellular reductants. Alternatively, sulfenic acids can be further oxidized to sulfinic acids (RSO₂H) and, under more severe oxidizing conditions, to sulfonic acids (RSO₃H), both of which are irreversible modifications. Cys, cysteine; ROS, reactive oxygen species; RNS, reactive nitrogen species; GSH, reduced glutathione; Reduc, Reductants