Superoxide dismutase

• Cu⁺²-SOD + O₂^- → Cu⁺¹-SOD + O₂
• Cu⁺¹-SOD + O₂^- + 2H⁺ → Cu⁺²-SOD + H₂O₂.

There are at least three forms of superoxide dismutase in nature. Human erythrocytes contain an SOD enzyme with divalent copper and divalent zinc (bovine erythrocyte: PDB 1SXA (http://www.rcsb.org/pdb/cgi/explore.cgi?pid=26081034364676&page=0&pdbId=1SXA), EC 1.15.1.1). Chicken liver mitochondria and E. coli contain a form with trivalent manganese (human mitochondrion: PDB 1ABM (http://www.rcsb.org/pdb/cgi/explore.cgi?pid=17181034364361&page=0&pdbId=1ABM), EC 1.15.1.1). E. coli also contains a form of the enzyme with trivalent iron ( PDB 1ISA (http://www.rcsb.org/pdb/cgi/explore.cgi?pid=28071034364826&page=0&pdbId=1ISA), EC 1.15.1.1). The Cu-Zn enzyme is a dimer of molecular weight 32,500. The two subunits are joined by a disulfide bond.

Although the enzyme isn't especially fast relative to the spontaneous dismutation of superoxide, the ability of the enzyme to provide some protection to organisms is shown by the existence of a motor neuron disease in individuals who have point mutations in SOD and by the finding that the absence of SOD may lead to a form of anemia.

A short but substantive overview of SOD and its literature can be found here (http://www.worthington-biochem.com/manual/S/SOD).