hydrogen peroxide. Hydrogen peroxide is immediately acted upon by catalase, generating water. Hydrogen peroxide can also be removed by the action of peroxidases. A peroxidase uses the oxidation of a substrate molecule (usually having a phenol structure, C-OH, which becomes a quinone, C=O, after the reaction) to react with hydrogen peroxide, converting it to water. Hydrogen peroxide can also be acted upon by ascorbate peroxidase, which uses ascorbate as the hydrogen donor for the reaction, resulting in water formation. The oxidized ascorbate is regenerated by the action of a series of enzymes (Fig. 3.5)