superoxidjon

Superoxidjon, known in English as the superoxide anion, is the reactive oxygen species with the chemical formula O2−. It is formed by the one-electron reduction of molecular oxygen and is a radical anion due to its unpaired electron. In aqueous solution its chemistry is pH-dependent; the hydroperoxyl radical HO2• has a pKa around 4.8, so at physiological pH superoxide exists mainly as O2−. It readily participates in redox reactions and is relatively unstable, tending to undergo dismutation to oxygen and hydrogen peroxide: 2 O2− + 2 H+ -> O2 + H2O2. It can also react with various organic and inorganic substrates, initiating radical chain processes.

Biological context is central to the significance of the superoxide anion. It is produced as a byproduct of mitochondrial respiration and, in immune cells, by the NADPH oxidase complex during the respiratory burst to help kill pathogens. Superoxide can react with nitric oxide to form peroxynitrite (ONOO−), a potent oxidant and nitrating agent. Cellular antioxidant defenses, notably the enzyme superoxide dismutase, catalyze the dismutation of superoxide into hydrogen peroxide and oxygen, helping to limit damage. Imbalances in production and clearance contribute to oxidative stress and have been linked to inflammation, cardiovascular and neurodegenerative diseases, and ischemia-reperfusion injury. Defects in NADPH oxidase cause chronic granulomatous disease, underscoring the enzyme’s role in host defense.

In chemical and environmental contexts, superoxide participates in radical reactions, redox cycling, and catalytic processes, serving as both reducing and oxidizing agent depending on the coupled reactions. Its study informs understanding of redox biology, oxidative stress, and related pathologies.