Reactive oxygen species (ROS) are now recognized as essential physiological regulators of intracellular signaling pathways, rather than solely harmful byproducts. ROS modulate specific target proteins through covalent modification of reactive cysteine residues, leading to reversible changes in enzymatic activity. ROS play roles in diverse physiological processes, including growth factor signaling and inflammatory responses, and dysregulated ROS signaling is linked to various diseases. Otto Warburg's early observations on oxygen consumption in fertilized eggs revealed that ROS are purposefully produced for physiological functions, such as shell formation in sea urchin eggs. Recent studies show that ROS are used in signaling pathways, with NADPH oxidases and mitochondria as major sources. ROS can be generated by various enzymes, including NADPH oxidases, xanthine oxidase, and cytochrome P450 enzymes. ROS regulate signaling through the oxidation of specific cysteine residues, which can alter enzymatic activity. For example, ROS can modulate tyrosine phosphatases, affecting signaling pathways. ROS can also influence other signaling molecules, such as p21ras and 14-3-3 isoforms, and even the ATM protein kinase, which is involved in DNA repair and cell survival. Antioxidants, including superoxide dismutase, catalase, and peroxiredoxins, help maintain redox homeostasis. These antioxidants are not merely scavengers but also participate in redox signaling. For instance, thioredoxin (Trx) can regulate signaling pathways by interacting with proteins like ASK1 and HDACs. Similarly, peroxiredoxins can modulate signaling by binding to and regulating other molecules. ROS are also involved in the regulation of stem cell function and aging. For example, high levels of ROS can impair stem cell self-renewal, while antioxidant treatments can rescue this defect. ROS are also linked to the regulation of mitochondrial biogenesis and the maintenance of redox balance. In disease, ROS play a complex role. For example, increased ROS levels are associated with insulin resistance and type 2 diabetes, but ROS can also be necessary for insulin secretion. ROS are also involved in the pathogenesis of inflammatory diseases, such as the NLRP3 inflammasome. Overall, ROS are essential for various physiological processes and their dysregulation can lead to disease. Understanding the mechanisms of ROS signaling is crucial for developing therapeutic strategies for diseases involving oxidative stress.Reactive oxygen species (ROS) are now recognized as essential physiological regulators of intracellular signaling pathways, rather than solely harmful byproducts. ROS modulate specific target proteins through covalent modification of reactive cysteine residues, leading to reversible changes in enzymatic activity. ROS play roles in diverse physiological processes, including growth factor signaling and inflammatory responses, and dysregulated ROS signaling is linked to various diseases. Otto Warburg's early observations on oxygen consumption in fertilized eggs revealed that ROS are purposefully produced for physiological functions, such as shell formation in sea urchin eggs. Recent studies show that ROS are used in signaling pathways, with NADPH oxidases and mitochondria as major sources. ROS can be generated by various enzymes, including NADPH oxidases, xanthine oxidase, and cytochrome P450 enzymes. ROS regulate signaling through the oxidation of specific cysteine residues, which can alter enzymatic activity. For example, ROS can modulate tyrosine phosphatases, affecting signaling pathways. ROS can also influence other signaling molecules, such as p21ras and 14-3-3 isoforms, and even the ATM protein kinase, which is involved in DNA repair and cell survival. Antioxidants, including superoxide dismutase, catalase, and peroxiredoxins, help maintain redox homeostasis. These antioxidants are not merely scavengers but also participate in redox signaling. For instance, thioredoxin (Trx) can regulate signaling pathways by interacting with proteins like ASK1 and HDACs. Similarly, peroxiredoxins can modulate signaling by binding to and regulating other molecules. ROS are also involved in the regulation of stem cell function and aging. For example, high levels of ROS can impair stem cell self-renewal, while antioxidant treatments can rescue this defect. ROS are also linked to the regulation of mitochondrial biogenesis and the maintenance of redox balance. In disease, ROS play a complex role. For example, increased ROS levels are associated with insulin resistance and type 2 diabetes, but ROS can also be necessary for insulin secretion. ROS are also involved in the pathogenesis of inflammatory diseases, such as the NLRP3 inflammasome. Overall, ROS are essential for various physiological processes and their dysregulation can lead to disease. Understanding the mechanisms of ROS signaling is crucial for developing therapeutic strategies for diseases involving oxidative stress.