Causes and consequences of cysteine S-glutathionylation

Abstract

Post-translational S-glutathionylation occurs through the reversible addition of a proximal donor of glutathione to thiolate anions of cysteines in target proteins, where the modification alters molecular mass, charge, and structure/function and/or prevents degradation from sulfhydryl overoxidation or proteolysis. Catalysis of both the forward (glutathione S-transferase P) and reverse (glutaredoxin) reactions creates a functional cycle that can also regulate certain protein functional clusters, including those involved in redox-dependent cell signaling events. For translational application, S-glutathionylated serum proteins may be useful as biomarkers in individuals (who may also have polymorphic expression of glutathione S-transferase P) exposed to agents that cause oxidative or nitrosative stress.

Keywords: Cysteine-mediated Cross-linking; Glutathione; Glutathione S-Transferase; Glutathionylation; Kinase Signaling; Nitric Oxide; Nitrosylation; Peroxidases; Peroxiredoxin; Serpin.

FIGURE 1.

Chemical basis for the biological S-glutathionylation of regular Cys (A) and Sec (B). A, surface protein cysteines at physiological pH are not reactive because of high pK_a values. In a basic microenvironment, the thiol deprotonates to a thiolate anion, which attacks the sulfhydryl of intracellular GSSG, with the subsequent formation of disulfide (pathway 1). Under oxidative (pathway 2) or nitrosative (pathway 3) stress, a surface protein (Pr) sulfhydryl becomes activated through cysteine sulfenate or nitro-S-cysteine formation and then undergoes S-glutathionylation. Deglutathionylation of glutathionylated cysteines is predominately catalyzed by Grx. Site-specifically, thioredoxin (TRx) can also catalyze deglutathionylation. Sulfiredoxin (Srx) catalyzes the reduction of a protein cysteine sulfinic acid into sulfenate for 2-Cys peroxiredoxins (Prx; pathway 4). Generally, overoxidation of protein cysteines is terminal (pathway 5). Similar to protein thiols, the cysteine of GSH can be activated through pathways 6–8, resulting in protein S-glutathionylation. The relative abundance of intracellular GSH influences the probability and specificity of particular reactions (see text). B, for proteins containing Sec (GSH peroxidases, etc.), its low pK_a results in the formation of a selenate anion at physiological pH, permanently activated (pathway 1). S-Glutathionylation of protein Sec residues is affected by ROS/RNS (pathways 2 and 3), but the mechanism of the latter is unclear. S-Glutathionylation of Sec could be further accelerated by activation of GSH (pathways 4–6), but the mechanism of such is not yet defined.