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A peroxidase is an enzyme, usually containing heme, that catalyzes a reaction of the form:

ROOR' + electron donor (2 e-) + 2H+ → ROH + R'OH

Many of these compounds are optimally designed to process hydrogen peroxide.

The nature of the electron donor is very dependent on the specific nature of the enzyme itself. For a compound such as horseradish peroxidase, the number of organic compounds it can use as electron donors is substantial. Horseradish peroxidase has a broad and accessible active site and many compounds can reach the site of the reaction. For a compound such as cytochrome c peroxidase, the compounds that donate electrons are very specific, because there is a very closed active site.

Peroxidases are sometimes used as histological markers. Cytochrome c peroxidase is used as a soluble, easily purified model for cytochrome c oxidase.

Glutathione peroxidase is a peroxidase found in humans containing selenocysteine.

see also: peroxide, catalase, hemoprotein