Superoxide is a reactive oxygen species that is produced in large amounts during photosynthesis and aerobic cellular respiration.

[4] The disproportionation mechanism involves a reduction-oxidation cycle where a single electron transfer is catalyzed by the Ni2+/Ni3+ redox couple.

All of the amino acids involved in catalysis and nickel binding are located within the first six residues from the N-terminus of each subunit.

After nickel binds, this motif takes on a highly ordered structure and forms the enzyme's active site.

The equatorial ligands include the thiolates of cysteine-2 and cysteine-6, as well as a deprotonated backbone amide nitrogen and the N-terminal amine.

[4][6] In its oxidized (Ni(III)) state, the coordination geometry of nickel is square pyramidal.

[3][7] If histidine is not a ligand in the reduced enzyme, the nickel(II) cofactor would be square planar.

[4][5][7] However, the His-1 may remain in place throughout the redox cycle, meaning that the nickel cofactor would always have a square-pyramidal geometry.

H+ is most likely carried into the active site by the substrate, meaning that superoxide enters the enzyme in its protonated form (HO2).

[4] Nickel superoxide dismutase is an incredibly efficient enzyme, indicating the redox mechanism is very fast.

This means that large structural rearrangements or dramatic changes to the coordination sphere are unlikely to be involved in the catalytic mechanism.

The only known example of a eukaryote expressing a nickel containing superoxide dismutase is in the cytoplasm of a number of green algae species.

Some of the Actinomycetes species that express nickel containing superoxide dismutatses are Micromonospora rosia, Microtetraspora glauca and Kitasatospora griseola.

In particular, expression of iron superoxide dismutase (Fe-SOD) is repressed by nickel in Streptomyces coelicolor.

[3] When nickel is present Nur binds to the promoter of sodF, stopping the production of iron superoxide dismutase.