The electron transport chain (ETC) and NADPH oxidases (NOX) take up oxygen to generate superoxide. Superoxide is dismutated by MnSOD and Cu-ZnSOD to generate H₂O₂. Oxidation of cysteine residues of peroxiredoxin (PRDX) and thioredoxin (TXN) proteins converts H₂O₂ to water. H₂O₂ is also converted to water by the glutathione peroxidase-glutathione reductase (GPX-GSR). In the presence of Fe²⁺ ions, H₂O₂ is converted to hydroxyl radicals (^•OH) via the Fenton reaction. H₂O₂ and ^•OH radicals oxidize macromolecules, altering cellular signaling and oxidatively damaging DNA. This leads to gene mutations. The ^•OH radicals start lipid oxidation of alkyl (R^•) groups, which are oxidized to alkoxyl (RO^•) and peroxyl (ROO^•) radicals. The dotted blue arrows show the source of ROS while purple numbers show the half-life of these radicals (in seconds). Adapted from Purohit et al., 2019 [1].