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ROS as signalling molecules: mechanisms that generate specificity in ROS homeostasis

Key Points

  • Reactive oxygen species (ROS) homeostatic pathways in microbes operate instant and feedback-regulated adjustments of intracellular ROS concentration. They use redox sensors that 'measure' ROS concentration and proportionally set the expression of ROS scavengers. Because of the high sensitivity of these pathways, sensors can be equated to receptors and ROS signals can be equated to agonists.

  • ROS chemistry dictates reactivity towards selective atomic targets in proteins. The hydroxyl radical HO·is indiscriminate; the superoxide anion O2 is active towards iron–sulphur ([Fe–S]) clusters and hydrogen peroxide (H2O2) targets reactive Cys residues. The subcellular colocalization of ROS and their targets contributes to mammalian ROS signalling specificity.

  • Among prokaryotic [Fe–S]-cluster-based sensors, SoxR is O2-specific; both FNR (which senses oxygen) and IscR (which senses [Fe–S]-cluster biosynthesis status) also respond to H2O2 and O2.

  • Among prokaryotic peroxide sensors, OxyR and OhrR use a reactive Cys residue that oxidizes to a disulphide bond, and PerR uses a non-haem iron centre, the two coordinating His residues of which oxidize to 2-oxo-His.

  • The Hsp33 chaperone and RsrA anti-sigma factor use a Cys–zinc redox centre to respond to a combination of H2O2 and heat (Hsp33) and to diamide (RsrA). Their sensitivity is much lower than that of the ROS receptors.

  • Thiol-based peroxiredoxins and GPX-like peroxidases carry H2O2 receptors and have redox transducing functions. Inactivation by overoxidation and reactivation by sulphiredoxin regulate peroxiredoxin antioxidant functions, H2O2 receptor functions and redox transduction functions.

  • Yeast H2O2 homeostatic pathways use redox relays that comprise a thiol peroxidase as the H2O2 receptor that oxidizes a transcription factor: Yap1 in Saccharomyces cerevisiae and Pap1 in Schizosaccharomyces pombe.

  • Instead of ROS homeostatic pathways, mammals use global differentiation programmes, such as those regulated by the tumour-suppressor p53, peroxisome proliferator-activated receptor-γ (PPARγ) coactivator-1α, the oncogene c-Myc and class O forkhead box transcription factors (FOXOs). These pathways provide either long-lasting oxidant-protective responses or cell death as clearance mechanisms for oxidatively damaged cells.

  • The KEAP1–NRF2 pathway constitutes the closest fit to a ROS receptor in mammals, and regulates oxidant and xenobiotic stress-protective responses. Sensing involves a Cys–zinc redox centre.

  • The lack of instant ROS homeostatic control in higher eukaryotes might be the inescapable consequence of using ROS as diffusible signals that modulate multiple intracellular signalling pathways.


Reactive oxygen species (ROS) have been shown to be toxic but also function as signalling molecules. This biological paradox underlies mechanisms that are important for the integrity and fitness of living organisms and their ageing. The pathways that regulate ROS homeostasis are crucial for mitigating the toxicity of ROS and provide strong evidence about specificity in ROS signalling. By taking advantage of the chemistry of ROS, highly specific mechanisms have evolved that form the basis of oxidant scavenging and ROS signalling systems.

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Figure 1: Microbial ROS receptors.
Figure 2: Model of the Saccharomyces cerevisiae Orp1–Yap1 redox relay.
Figure 3: A PRX operates in the two parallel Schizosaccharomyces pombe H2O2 response pathways.
Figure 4: Schematic of the speculative two-site interaction model of KEAP1–NRF2.


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We acknowledge members of the Toledano laboratory for discussions, C. Mann for critical reading of the manuscript, and grants from the Agence Nationale pour la Recherche (ANR), Association de Recherche pour le Cancer (ARC) and the CEA ToxNuc programme to M.B.T. We regret that we were unable to cite much interesting work in this area because of space limitations.

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Correspondence to Michel B. Toledano.

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Supplementary information

Supplementary information S1 (table)

Kinetic parameters of selected peroxiredoxins and catalases (PDF 181 kb)

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Iron–sulphur clusters

Iron–sulphur clusters are metal centres that consist of sulphide (S2−) and iron. The most common structures are the diamond [2Fe–2S] and the cubane [4Fe–4S] clusters. Iron–sulphur clusters are usually coordinated by Cys or His residues at the iron atoms.

Reactive nitrogen species

These are derived from the reaction of nitric oxide (NO) with oxygen or superoxide and include nitrogen trioxide (N2O3), peroxinitrite (ONOO) and nitrogen dioxide (NO2).


Peroxide-reducing enzymes that function by heterolytic cleavage of the O–O bond. Peroxidases fall into different classes depending on the nature of their catalytic site or according to the mechanisms that regenerate the active form.


The equilibrium constant of proton (H+) exchange reactions between acids and bases according to the Brönsted theory. It reflects the strength of an acid to donate its proton as pKa decreases.


A modification to the –SNO form of the thiol moiety of a Cys residue, caused by reaction with peroxinitrite (ONOO) or nitrogen trioxide (N2O3).


A chemical modification that involves the transfer of a carbon chain to any other atom.

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D'Autréaux, B., Toledano, M. ROS as signalling molecules: mechanisms that generate specificity in ROS homeostasis. Nat Rev Mol Cell Biol 8, 813–824 (2007).

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