Figure 1.

Schematic presentation of the generation of ROS in mitochondria. ROS are generated from the transfer of electrons (e^-) to molecular oxygen to form superoxide (O₂•^-) at the mitochondrial electron transport chain complex I and III. Once generated, superoxide is decomposed enzymatically by superoxide dismutase 1 (SOD1) in the intermembrane space and by SOD2 (MnSOD) in the matrix to form hydrogen peroxide, which is further catabolized to water by the action of enzymes such as catalase (CAT), glutathione peroxidases (GPx), and thioredoxin reductase (TPx) to avoid possible buildup of oxidative stress. However, under mitochondrial stress, superoxide may react with nitric oxide to form the potent oxidant and nitrating agent peroxynitrite (ONOO^-). Hydrogen peroxide can also form the highly reactive hydroxyl radical (OH•) in the presence of Fe²⁺ cations. These highly reactive radicals may cause damage to proteins, lipids, and nucleic acids. CoQ, coenzyme Q; Cyt C, cytochrome C.