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Fundamentals of Bioanalytical Electrochemistry
Many enzymes are oxidoreductases. This means that the enzyme can catalyze oxidation and reduction reactions. Some examples of such enzymes include such well known enzyme classes as cytochromes, dehydrogenases, reductases, dismutases, and others. These usually occur with the help of enzyme cofactors such as NAD/NADH, FAD/FADH, hemes and other transition metals.
The overall reaction catalyzed by the enzyme superoxide dismutase is :
2 O2- + 2H+ <=> H2O2 + O2
This is an example of a disproportionation reaction, in which the superoxide is in an intermediate oxidation state (-1/2) and products are formed in the -1 and the 0 oxidation state. In order to decipher the half reactions, we must write the reduction and oxidation reactions separately:
Reduction Half Reaction:
O2- + 2H+ <=> H2O2
Oxidation Half Reaction:
O2- <=> O2
In the first reaction, superoxide is reduced to peroxide, and it is classified as an oxidizing agent or oxidant. Oxidizing agents are always reduced in a reaction and reducing agents are always oxidized. Thus, in the second reaction, superoxide is the reducing agent. For the reverse overall reaction, H2O2 is oxidized and O2 is reduced. Thus O2 is the oxidant and is the reductant.....