Sandbox Mati

Nickel Superoxide Dismutase (NiSOD) is the newest member in a class of enzymes that protects organisms from oxidative stress caused by superoxide, a harmful free radical byproduct of aerobic metabolism. NiSOD reacts with two molecules of superoxide, to form O₂ and H₂O₂ with rates occurring at or near the diffusion limit. During catalysis, the redox-active nickel center cycles between an oxidized and reduced state. This reaction is termed the pin pong mechanism and is shown below.

:M^{(n + 1)} + O₂^•- → Mⁿ⁺ + O₂ <br />

Mⁿ⁺ + O₂^•- + 2H⁺ → M^{(n + 1)} + H₂O₂ <br />

————————————————— <br />

:2O₂^•- + 2H⁺ → O₂ + H₂O₂ <br />

NiSOD is distinct among SOD's for three reasons: <br />

(1) NiSOD shares no sequence homology with the other known SOD's ('''Fig. 1'''). <br />

(2) The ligands employed in the redox-active metal center are distinct. Cu/Zn, Fe, and MnSOD employ only aspartic acids, waters, and histidines. In NiSOD, the nickel center is coordinated by the side chains of cysteine 2 and cysteine 6, as well as the N-terminal amine, the amide group of cysteine 2 and an axial histidine ligand. <br />

(3) Copper, iron, and manganese are redox active in aqueous media at biological pH. Nickel does not, and requires the coordination of the two cysteine ligands to tune its redox potential.

[[Image:SODs.png|600 px|thumb|'''Fig. 1:''' Active site structures of the four known SOD's.]]