Fig. 8.

Three likely mechanisms for S-sulfhydration. At physiological pH, most cysteine thiol groups in proteins are protonated (-SH) and hence display low reactivity towards H2O2. However, in some proteins where the cysteine residue is flanked by basic amino acids, the cysteine-SH group exists as the highly oxidizable thiolate anion (-S-). This introduces an element of specificity in H₂O₂-mediated signaling, suggesting that mainly proteins containing low pKa cysteine residues undergo S-sulfhydration.