Oxidant sensing by reversible disulfide bond formation

Abstract

Maintenance of the cellular redox balance is crucial for cell survival. An increase in reactive oxygen, nitrogen, or chlorine species can lead to oxidative stress conditions, potentially damaging DNA, lipids, and proteins. Proteins are very sensitive to oxidative modifications, particularly methionine and cysteine residues. The reversibility of some of these oxidative protein modifications makes them ideally suited to take on regulatory roles in protein function. This is especially true for disulfide bond formation, which has the potential to mediate extensive yet fully reversible structural and functional changes, rapidly adjusting the protein's activity to the prevailing oxidant levels.

Keywords: Antioxidants; Oxidative Stress; Reactive Oxygen Species (ROS); Redox Signaling; Stress Response.

FIGURE 1.

Reversible and irreversible cysteine modifications. Oxidation of cysteine thiol (RSH/RS⁻) by ROS, RNS, or RCS leads to the formation of highly reactive sulfenic acid (RSOH), which can react either with another thiol to form a disulfide bond (RSSR) or with GSH to become S-glutathionylated (RSSG). These oxidative modifications are reversible, and reduction is catalyzed by the Trx and/or Grx system. Further oxidation of sulfenic acid to sulfinic acid (RSO₂H) and sulfonic acid (RSO₃H) is thought to be generally irreversible in vivo.

FIGURE 2.

Redox-mediated activation of oxidative stress defense proteins. A, HOCl-mediated activation of Hsp33 chaperone activity involves formation of two intramolecular disulfide bonds, zinc release (red spheres), and unfolding of the C-terminal redox switch domain. Once activated, Hsp33 functions as an ATP-independent chaperone holdase, able to bind numerous unfolding client proteins to prevent their aggregation. When oxidative stress conditions subside, the disulfide bonds of Hsp33 are re-reduced by the Trx system, triggering the transfer of client proteins to the DnaK/DnaJ/GrpE system for refolding. B, H₂O₂-mediated activation of the transcriptional activator OxyR involves formation of an intramolecular disulfide bond between two cysteines, which are separated by 17 Å in the reduced form. These extensive conformational changes allow binding of RNA polymerase and transcription of target genes. The oxidative modifications are reversed by the Grx system, whose expression is under OxyR control.

FIGURE 3.

Versatile life of peroxiredoxins. Peroxiredoxins (red) catalyze the decomposition of H₂O₂, transiently forming a sulfenic acid in this process. Disulfide bond formation and subsequent reduction by Trx regenerate the active site cysteine thiol in peroxiredoxin (arrow A). In selected eukaryotic peroxiredoxins, high levels of H₂O₂ lead to the overoxidation of sulfenic acid to sulfinic acid, causing inactivation of the peroxidase function and concomitant activation as a chaperone holdase (arrow B). ATP-dependent sulfiredoxins convert the sulfinic acid back to sulfenic acid, which is subsequently reduced by Trx. Peroxiredoxins also undergo thiol-disulfide exchange reactions, leading to the oxidative activation of selected proteins. After oxidation of their active site cysteine to sulfenic acid, an intermolecular disulfide bond forms between peroxiredoxin (e.g. Orp1) and the client protein (e.g. Yap1p) (green). This disulfide bond is subsequently resolved, leading to the formation of an intramolecular disulfide bond in the client protein and the release of reduced peroxiredoxin (arrow C).