Regulation of phosphatase by the redox state. Cysteine molecules have sulfur atoms (S) that are protonated and not reactive in most proteins. However, on some molecules, such as phosphatases, S can form thiolates (S-) at normal pH and these can be reversibly oxidized. The top of the figure shows the balance between phosphatase activity (which dephosphorylates molecules) and kinase activity (which phosphorylates and activates molecules). Phosphatase activity is regulated by the redox state as shown in the cycle below the bracket. Oxidation to sulfenic acid (-S-OH) is reversible. This can occur by glutathiolation (GSH) or by the formation of disulfides. However, excessive oxidation leads to sulfinic acid, which cannot easily be converted back to reduced forms of sulfur.