ROS-dependent signal transduction

Abstract

Reactive oxygen species (ROS) are no longer viewed as just a toxic by-product of mitochondrial respiration, but are now appreciated for their role in regulating a myriad of cellular signaling pathways. H2O2, a type of ROS, is a signaling molecule that confers target specificity through thiol oxidation. Although redox-dependent signaling has been implicated in numerous cellular processes, the mechanism by which the ROS signal is transmitted to its target protein in the face of highly reactive and abundant antioxidants is not fully understood. In this review of redox-signaling biology, we discuss the possible mechanisms for H2O2-dependent signal transduction.

Figure 1. Endogenous sources of ROS signal

Intracellular ROS is primarily produced by NADPH oxidase enzymes (NOXs), the mitochondria, the endoplasmic reticulum, and the peroxisome. Cytosolic superoxide (O₂⁻) is rapidly converted into hydrogen peroxide (H₂O₂) by superoxide dismutase 1 (SOD1). H₂O₂ can either act as a signaling molecule by oxidizing critical thiols within proteins to regulate numerous biological processes, including metabolic adaptation, differentiation, and proliferation or be detoxified to water (H₂O) by the scavenging enzymes peroxiredoxin (PRX), glutathione peroxidase (GPX), and catalase (CAT). In addition, H₂O₂ can react with metal cations (Fe²⁺ or Cu⁺) to generate the hydroxyl radical (OH•), which causes irreversible oxidative damage to lipids, proteins, and DNA.

Figure 2. H₂O₂-mediated cysteine oxidation of redox-sensitive proteins

Critical cysteine thiol groups of target proteins exist as a thiolate anion (S⁻) and are readily oxidized by hydrogen peroxide (H₂O₂) to yield sulfenic acid (SO⁻), a reversible modification which alters protein activity. When H₂O₂ levels are high, SO⁻ can be hyperoxidized to generate sulfinic (SO₂⁻) and sulfonic (SO₃⁻) acids (orange box). While SO₃⁻ generally represents an irreversible oxidative modification, SO₂⁻ can be converted back to the SO⁻ intermediate by the enzymatic activity of sulfiredoxin (SRX). To protect the target protein from irreversible oxidation, the SO⁻ intermediate commonly forms reversible disulfide (S-S) or sulfenic-amide (S-N) bonds (green boxes). SO⁻ can either form a disulfide bond by reacting with an intra- or intermolecular cysteine or with glutathione (S-SG) or form a sulfenic-amide bond by reacting with the backbone amide nitrogen atom. The enzymatic activity of glutaredoxin (GRX) and thioredoxin (TRX) restore protein function by returning the oxidized protein back to its reduced state.

Figure 3. Possible mechanisms for H₂O₂-dependent signal transduction

(A) The redox relay mechanism uses a scavenging enzyme such as glutathione peroxidase (GPX) or peroxiredoxin (PRX) to transduce the H₂O₂ signal and oxidize the target protein. (B) With the floodgate model, H₂O₂ inactivates the scavenger, perhaps through hyperoxidation to sulfinic (SO₂⁻) acid or through a post-translational modification (PTM), to allow for H₂O₂-mediated oxidation of the target protein. (C) The scavenging enzymes accept H₂O₂ oxidation and transfer the oxidation to an intermediate redox protein such as thioredoxin (TRX), which subsequently oxidizes the target protein. (D) Dissociation of the target protein from the oxidized scavenging enzyme results in target protein activation.