Aerobic life requires the continual production, utilization and removal of metabolites capable of mediating reduction–oxidation (redox) reactions on biomolecules. Reactive oxygen species (ROS), including superoxide, hydrogen peroxide and hydroxyl radicals, were the first class of redox-active metabolites to be extensively characterized. Since then, additional families of redox metabolites have been recognized, including reactive nitrogen species (RNS) and reactive sulfur species (RSS). Because these molecules are highly reactive toward DNA, RNA and proteins, the prevailing view was that tight homeostatic control of cellular redox levels was required to preserve signaling while preventing oxidative damage associated with disorders ranging from cancer to aging. Increasingly, however, these metabolites are understood not simply as harmful byproducts of metabolism but also as important signaling mediators. Despite their chemical diversity, many redox metabolites converge mechanistically on the modification of cysteine residues in proteins and peptides, providing a common chemical basis for redox regulation. Given the close interplay between chemistry and biology in these processes, Nature Chemical Biology has long served as a forum for publishing discoveries in redox biology. In this Focus issue we present a collection of articles highlighting emerging areas of redox research and the chemical tools that are enabling new insights into the roles of these metabolites in biology.

Hydrogen peroxide (H2O2) is now appreciated as a major second messenger that functions largely through the reversible oxidation of cysteine residues in proteins. However, the difficulty of tracking and detecting H2O2 in real time has long hindered efforts to decipher its biological roles. The development of genetically encoded and small-molecule fluorescent sensors represented a major advance in visualizing H2O2 dynamics in living cells, although early probes were limited by pH sensitivity, oxygen interference and restricted dynamic range. Two Articles in this issue, one from Lee et al. and one from Potekhina et al., highlighted in a News and Views by Fransen and Lismont, describe improved biosensors (oROS-HT635 and HyPerFLEX) that enable more accurate and dynamic tracking and imaging of H2O2 in real time. These probes function effectively even in highly oxidizing or hypoxic environments in which earlier sensors were less reliable.

ROS also have central roles in several forms of regulated cell death. Their involvement is particularly prominent in ferroptosis, an iron-dependent mode of cell death driven by ROS-mediated lipid peroxidation of cellular membranes. This process is normally counteracted by glutathione peroxidase 4 (GPX4). A recently described non-canonical ferroptosis pathway operates independently of GPX4 and instead relies on ROS and p53 signaling to suppress tumor growth. Xia et al. show that this pathway involves shuttling of the peroxidase GPX1 to the endoplasmic reticulum (ER), mediated by OSBPL8 (an ER lipid-binding protein); at the ER GPX1 suppresses both ROS accumulation and lipid peroxidation1. Whereas ROS promote lipid peroxidation during ferroptosis, the precursor species superoxide was recently shown by Chen et al. to inhibit lipid peroxidation and preserve nuclear envelope integrity during aging. Inhibition of lipid peroxidation delayed aging in Caenorhabditis elegans and in senescent mammalian cells2. Cancer cells are also characterized by elevated ROS levels yet maintain rapid proliferation despite extreme oxidative stress. This metabolic vulnerability has motivated therapeutic strategies designed to exploit ROS chemistry. In one recent example, Ming et al. implemented ROS-activated bioorthogonal tetrazine chemistry to enable H2O2-mediated uncaging and release of prodrugs3.

The cellular concentration of H2O2 is tightly controlled by peroxide-consuming enzymes such as the peroxiredoxins (Prxs). Prxs are highly reactive toward H2O2, participate in redox relay signaling and can undergo hyperoxidation that temporarily inactivates their catalytic activity. Although the catalytic activity of Prxs has been extensively characterized, their supramolecular organization has remained less clear. It was generally assumed that Prx isoforms assemble as homooligomeric complexes in equilibrium between homodimers and homodecamers associated with catalytic and chaperone functions. However, recent work from Zimmermann et al., highlighted in a News and Views by Mazon, Selles and Rahuel-Clermont, demonstrates that Prxs can also form functional chimeric heterodimers and heterooligomeric assemblies across a wide range of organisms, from humans to the protozoan parasite Leishmania.

RSS represent another important class of redox metabolites whose biological roles have become increasingly apparent. RSS can exist across a wide spectrum of oxidation states, ranging from hydrogen sulfide to sulfate. A Review by Nagy et al. provides an updated overview of the chemical biology of RSS, the current detection methods and their roles in physiological and pathological processes. Among these species, hydropersulfides have attracted particular interest. Recent work has shown that hydropersulfides can suppress superoxide-mediated ferroptotic cell death by lowering intracellular free-radical oxidation chemistry4.

ROS and RSS frequently react with cysteine residues to generate oxidative post-translational modifications that modulate protein structure and activity. Advances in chemoproteomics, coupled with chemical probe design and bioinformatic analysis, have provided a global view of oxidant-mediated covalent modification of reactive cysteine residues across the proteome. These approaches have revealed how oxidative modifications influence signaling pathways and cellular stress responses5,6. In a Perspective by Carroll and Yang, the authors propose a conceptual and experimental framework for decoding the cysteine redoxome, integrating chemical probe design with chemoproteomic measurements of site-specific oxidation, occupancy and redox flux to distinguish reactive, redox-sensitive and regulatory cysteine residues.

We hope this Focus issue highlights emerging themes in redox biology and the chemical innovations that are driving the field forward. Although the trajectory of redox research is promising, many questions remain unanswered. In our accompanying Feature article, we asked a diverse group of redox biologists to share their perspectives on the most exciting frontiers in the field and the developments needed to advance it further. Their insights provide a roadmap that we hope will inspire the community to address these challenges and continue expanding our understanding of redox chemistry in biology.