Superoxide dismutase

Superoxide dismutase (SOD, ) is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide (O2-) radical into ordinary molecular oxygen (O2) and hydrogen peroxide (H2O2). Superoxide is produced as a by-product of oxygen metabolism and, if not regulated, causes many types of cell damage. Hydrogen peroxide is also damaging and is degraded by other enzymes such as catalase. Thus, SOD is an important antioxidant defense in nearly all living cells exposed to oxygen. One exception is Lactobacillus plantarum and related lactobacilli, which use a different mechanism to prevent damage from reactive O2-. SODs catalyze the disproportionation of superoxide: 2H+ + 2O2− → O2 + H2O2 In this way, O2− is converted into two less damaging species. The general form, applicable to all the different metal−coordinated forms of SOD, can be written as follows: M(n+1)+−SOD + O2− → Mn+−SOD + O2 Mn+−SOD + O2− + 2H+ → M(n+1)+−SOD + H2O2 The reactions by which SOD−catalyzed dismutation of superoxide for Cu,Zn SOD can be written as follows: Cu2+−SOD + O2− → Cu+−SOD + O2 (reduction of copper; oxidation of superoxide) Cu+−SOD + O2− + 2H+ → Cu2+−SOD + H2O2 (oxidation of copper; reduction of superoxide) where M = Cu (n=1); Mn (n=2); Fe (n=2); Ni (n=2) only in prokaryotes. In a series of such reactions, the oxidation state and the charge of the metal cation oscillates between n and n+1: +1 and +2 for Cu, or +2 and +3 for the other metals . Irwin Fridovich and Joe McCord at Duke University discovered the enzymatic activity of superoxide dismutase in 1968. SODs were previously known as a group of metalloproteins with unknown function; for example, CuZnSOD was known as erythrocuprein (or hemocuprein, or cytocuprein) or as the veterinary anti-inflammatory drug "Orgotein". Likewise, Brewer (1967) identified a protein that later became known as superoxide dismutase as an indophenol oxidase by protein analysis of starch gels using the phenazine-tetrazolium technique.