The superoxide ion,  is generated in biological systems by reduction of molecular oxygen. It has an unpaired electron, so it behaves as a free radical. It is a powerful oxidizing agent. These properties render the superoxide ion very toxic and are deployed to advantage by phagocytes to kill invading microorganisms. Otherwise, the superoxide ion must be destroyed before it does unwanted damage in a cell. The superoxide dismutase enzymes perform this function very efficiently.
The formal oxidation state of the oxygen atoms is −. In solutions at neutral pH, the superoxide ion disproportionates to molecular oxygen and hydrogen peroxide.
:2  + 2 H → O + HO
In biology this type of reaction is called a dismutation reaction. It involves both oxidation and reduction of superoxide ions. The superoxide dismutase (SOD) group of enzymes increase the rate of reaction to near the diffusion-limited rate. The key to the action of these enzymes is a metal ion with variable oxidation state that can act either as an oxidizing agent or as a reducing agent.
:Oxidation: M +  → M + O
:Reduction: M +  + 2 H → M + HO.
In human SOD, the active metal is copper, as Cu(II) or Cu(I), coordinated tetrahedrally by four histidine residues. This enzyme also contains zinc ions for stabilization and is activated by copper chaperone for superoxide dismutase (CCS). Other isozymes may contain iron, manganese or nickel. The activity of Ni-SOD  involves nickel(III), an unusual oxidation state for this element. The active site nickel geometry cycles from square planar Ni(II), with thiolate (Cys and Cys) and backbone nitrogen (His and Cys) ligands, to square pyramidal Ni(III) with an added axial His side chain ligand.