Figure 3. Proposed mechanisms of protein S-glutathiolation.
There are four mechanisms by which S-glutathiolated proteins (Pr-SSG) can be formed. Mechanism A illustrates thiol-disulfide exchange between protein-SH (Pr-SH) and GSSG. Since the thiol-disulfide reaction requires a significant increase in GSSG levels, the formation of Pr-SSG via this mechanism would be mostly associated with pathological conditions. Mechanisms B-D illustrate the interactions of Pr-SH or GSH with S-nitrosyl (-SNO), sulfenic acid (-SOH), or thiyl radical (-S˙) derivatives. Two electron oxidation of Pr-SH generates Pr-SOH that reacts with GSH to form Pr-SSG. During nitrosative stress, Pr-SSG is formed from increased reactions of Pr-SNO or GSNO with GSH or Pr-SH respectively (mechanism C). Pr-SSG can also be formed through reactions with thiyl radicals of Pr-SH (Pr-S˙) and GSH (GS˙). One electron oxidation of Pr-SH or GSH gives rise to the respective thiyl radicals which react with GSH or Pr-SH, respectively, yielding Pr-SSG (mechanism D). Additionally, glutaredoxin-catalyzed reaction of GS˙ with Pr-SH produces Pr-SSG. S-glutathiolated proteins can be formed through two-steps reaction of the activated thiyl radical forms of the protein thiol (Pr-S˙) and GSH (GS˙). Firstly, one electron oxidation of protein or GSH give rise to the respective thiyl radical; this radical further reacts with the GSH and protein thiol, respectively, to form the S-glutathiolated protein (mechanism D). Intracellularly, glutaredoxin can catalyzes the reaction of GS˙ with proteins to generate S-glutathiolated proteins. In parallel, single-step reaction between the thiyl radicals of protein and GSH also forms the S-glutathiolated protein (mechanism D). Additionally, glutaredoxin-catalyzed reaction of GS˙ with Pr-SH produces Pr-SSG. Finally, a single-step reaction between Pr-S˙ and GS˙ also produced Pr-SSG (mechanism D).