Figure 2. Formation of protein persulfides by H2S.

(A) Modification of cysteine residues by H₂S. H₂S cannot directly modify thiol groups (i.e. cysteine residues). The thiol group must first be must first be oxidised into a disulfide (disulfide bond formation), sulfenic acid (S-Sulfenylation), glutathiolated cysteine (S-Glutathiolation), or a S-Nitroso Cysteine (S-Nitrosation). From these oxidised thiol groups H₂S can react to form persilfides, thiols, and a variety of by-products dependent on the type of oxidised thiol it is reacting with. The sulfur atom from the H₂S molecule is highlighted in orange to show where in the product it incorporates. (B) Persulfidation is a reversible post-translational modification and can be readily removed by the action of glutathione and thioredoxin. ROS, Reactive oxygen species; GSH, Glutathione; NO, Nitric oxide; NOH, Nitroxyl; SNOH, Thionitrous acid; GSSH, Glutathione persulfide; Trx-S⁻, Thioredoxin.