Manganese Superoxide Dismutase
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Superoxide is formed in a variety of processes through reaction of mo lecular oxygen (0 2) with reductants such as organic free radicals (la) or reduced metal ions (lb):
0 2 + R# 0 2 + R (In) 0 2 + M“+ 0 2 * + M(/l+1)+ (lb)
The respiratory chain of aerobic organisms is particularly susceptible to these reactions, since reduced electron transfer cofactors (quinones, flavins, iron-sulfur clusters, and cytochromes) lie exposed to dioxygen in these redox systems. In mitochondria, the cytochrome bex complex has been identified as the primary source of super oxide formation , a leak that allows approximately 5% of electrons in the respiratory chain to be diverted to one-electron reduction of 0 2 .