Two type of enzymes found both in animals and plants fall into the hydroperoxidase category: peroxidases and catalase.
Hydroperoxidases play an important role in protecting the body against the harmful effects of reactive oxygen species (ROS). ROS are highly reactive oxygen-containing molecules such as peroxides which are formed during normal metabolism, but can be damaging if they accumulate. They are believed to contribute to the causation of diseases such as cancer and atherosclerosis, as well as the aging process in general (see Chapters 21, 44, 54).
Peroxidases Reduce Peroxides Using Various Electron Acceptors
Peroxidases are found in milk and in leukocytes, platelets, and other tissues involved in eicosanoid metabolism (see Chapter 23). Their prosthetic group is protoheme. In the reaction catalyzed by peroxidase, hydrogen peroxide is reduced at the expense of several substances that will act as electron acceptors, such as ascorbate (vitamin C), quinones, and cytochrome c. The reaction catalyzed by peroxidase is complex, but the overall reaction is as follows:
In erythrocytes and other tissues, the enzyme glutathione peroxidase, containing selenium as a prosthetic group, catalyzes the destruction of H2O2 and lipid hydroperoxides through the conversion of reduced glutathione to its oxidized form, protecting membrane lipids and hemoglobin against oxidation by peroxides (see Chapter 21).
Catalase Uses Hydrogen Peroxide as Electron Donor & Electron Acceptor
Catalase is a hemoprotein containing four heme groups. It can act as a peroxidase, catalyzing reactions of the type shown above, but it is also able to catalyze the breakdown of H2O2 formed by the action of oxygenases to water and oxygen:
This reaction uses one molecule of H2O2 as a substrate electron donor and another molecule of H2O2 as an oxidant or electron acceptor. It is one of the fastest enzyme reactions known, destroying millions of potentially damaging H2O2 molecules per second. Under most conditions in vivo, the peroxidase activity of catalase seems to be favored. Catalase is found in blood, bone marrow, mucous membranes, kidney, and liver. Peroxisomes are found in many tissues, including liver. They are rich in oxidases and in catalase. Thus, the enzymes that produce H2O2 are grouped with the enzyme that breaks it down. However, mitochondrial and microsomal electron transport systems as well as xanthine oxidase must be considered as additional sources of H2O2.