Superoxide
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Superoxide is a reactive anion and free radical of oxygen, formally O2−. It has an unpaired electron, is not particularly stable, and spontaneously decomposes into peroxide and oxygen over time. It is biologically quite toxic and is used by the immune system to kill invading microorganisms.
Superoxide is produced most easily by burning rubidium or caesium in oxygen. The pKa of its conjugate acid, HO2, is 4.88 so that at neutral pH 7 the vast majority of superoxide in the anionic form O2−.
In phagocytes, superoxide is produced in large quantities by the enzyme NADPH oxidase for use in oxygen-dependent killing mechanisms of invading pathogens. Superoxide is also deleteriously produced as a byproduct of mitochondrial respiration.
The biological toxicity of superoxide is due to its capacity to inactivate iron-sulfur cluster containing enzymes (which are critical in a wide variety of metabolic pathways), thereby liberating free iron in the cell, which can undergo fenton-chemistry and generate the highly reactive hydroxyl radical. In its HO2 form, superoxide can also initiate lipid peroxidation of polyunsaturated fatty acids. As such, superoxide is a main cause of oxidative stress.
Since superoxide is toxic, nearly all organisms living in the presence of oxygen contain isoforms of the superoxide scavenging enzyme, superoxide dismutase, or SOD. SOD is an extremely efficient enzyme; it catalyzes the neutralization of superoxide nearly as fast as the two can diffuse together spontaneously in solution. Genetic inactivation ("knockout") of SOD produces deleterious phenotypes in organisms ranging from bacteria to mice. The latter species dies around 21 days after birth if the mitochondrial variant of SOD (Mn-SOD) is inactivated, and suffers from multiple pathologies, including reduced lifespan and liver cancer, when the cytoplasmic (Cu,Zn-SOD) variant is inactivated.
Potassium superoxide is used as an oxygen source in chemical oxygen generators, such as those used on the space shuttle and on submarines.
See also
- ozonide, O3−
- peroxide, O22−
- oxide, O2−
- dioxygenyl, O2+ar:أكسيد فائق