FIGURE 12-27 Generation and inactivation of toxic reactive oxygen species. Electrons from the electron-transport chains of mitochondria and chloroplasts, as well as some generated through other enzymatic reactions, reduce molecular oxygen (O₂), forming the highly reactive radical anion superoxide (O₂^•−). Superoxide is rapidly converted by superoxide dismutase (SOD) to hydrogen peroxide (H₂O₂), which in turn can be converted by metal ions such as Fe²⁺ to hydroxyl radicals (OH^•) or inactivated to H₂O by enzymes such as catalase. Because of their high chemical reactivity, O₂^•−, H₂O₂, OH^•, and similar molecules are called reactive oxygen species (ROS). They cause oxidative and free-radical damage to many biomolecules, including lipids, proteins, and DNA. This damage leads to cellular oxidative stress that can cause disease and, if sufficiently severe, can kill cells. In addition, ROS can function as intra- and intercellular signaling molecules.