Peroxidases represent enzymes that break down hydrogen peroxide or organic peroxides in any organism. Peroxides are potent cytotoxins produced by numerous biochemical oxidation reactions. Thus, peroxidases are among the most important antioxidants.
What are peroxidases?
Peroxidases are enzymes that, in most cases, break down toxic hydrogen peroxide. However, organic peroxides are also reduced by these enzymes. In these reactions, the corresponding peroxide receives two electrons and two protons each in partial steps. This means that a transfer of hydrogen atoms takes place. In the process, the bond between the two oxygen atoms of the peroxide is broken. Peroxidases include catalase, cytochrome c peroxidase, thyreoperoxidase and glutathione peroxidase. The hydrogen atoms come from so-called hydrogen donors. Hydrogen peroxide or organic peroxides represent the substrates. Catalase is considered a special case. In catalase as a peroxidase, a second hydrogen peroxide molecule acts as a hydrogen donor. It thus transfers hydrogen atoms. In the process, two molecules of hydrogen peroxide are converted into two molecules of water and one molecule of oxygen. Cytochrome c peroxidase, for example, oxidizes the ferroform of cytochrome C with the help of hydrogen peroxide. Thyroperoxidase reduces an iodide ion in the presence of hydrogen peroxide, which immediately reacts with the amino acid tyrosine to form thyroid hormones. An important peroxidase is glutathione peroxidase. This catalyzes the oxidation of glutathione by hydrogen peroxide with the formation of water. Thus, it is one of the most important antioxidants in the organism.
Function, effect, and tasks
The most important task of peroxidases is to break down toxic hydrogen peroxide and organic peroxides in the organism. Peroxides are very aggressive substances that can oxidize the body’s biomolecules very easily. In many oxidative metabolic processes they are often formed as metabolites. When peroxides are broken down, hydrogen peroxide is usually released, which attacks other endogenous substances. In addition, free radicals are always formed as intermediate products during the conversion of peroxides. In order to limit the effect of the free radicals, the peroxides must therefore also be broken down. The enzyme catalase is responsible for the direct degradation of hydrogen peroxide into water and oxygen. Glutathione peroxidase oxidizes glutathione sulfide and simultaneously reduces hydrogen peroxide to water. Glutathione is a tripeptide of glutamic acid, cysteine and glycine. It is the most important active substance in the liver as an antioxidant for detoxifying the body. In the process, it is itself oxidized. When it is used up to the point of exhaustion, acute liver failure can occur because the detoxification reactions that are always necessary no longer take place. As a result, the enzyme glutathione peroxidase is of utmost importance as a component of cellular defense against oxidative stress. A deficiency of this enzyme can lead to arteriosclerosis, tumors and neurodegenerative diseases. Thyroperoxidase, in turn, incorporates iodine into thyroid hormones by oxidizing iodide. Again, hydrogen atoms are transferred from tyrosine to hydrogen peroxide during the oxidation of the iodide ion. In the process, the thyroid hormones and water are formed.
Formation, occurrence, properties, and optimal values
Peroxidases are mainly found in the peroxisomes of the cell. Peroxisomes represent cell organelles closed by membranes in which the aggressive peroxides are degraded. These reactions must take place separated from the rest of the cell because they can be dangerous for the rest of the cell. The peroxisomes contain peroxidases, which break down hydrogen peroxide and other peroxides there undisturbed by the cytoplasm of the cell. The number and size of the peroxisomes and also their equipment with proteins depend on the cell type. The more detoxification reactions have to take place, the greater the number and volume of vesicles. There are about 60 oxidases and monooxygenases in the peroxisomes, which catalyze the degradation of fatty acids and other substances with the inclusion of oxygen. In the process, peroxides are also formed as particularly reactive intermediates, which must therefore be degraded with the help of peroxidases.
Diseases and disorders
Hydrogen peroxide as an intermediate product mainly in the oxidative degradation of fats has great effects on the organism.It has an oxidizing effect itself and can damage the body. Therefore, the peroxidases play a very important role, especially with regard to oxidative stress. The glutathione peroxidases in particular are components of the cellular defense system as antioxidants. The glutathione peroxidases are built up from glutathione. They depend on selenium as a cofactor. Thus, glutathione represents a tripeptide of glutamic acid, cysteine and glycine. Selenium binds to cysteine in each of the enzymes, with selenocysteine representing the reactive center of the enzyme. When there is a malfunction or deficiency of glutathione peroxidase, serious health disorders result. This deficiency is often caused by a selenium deficiency. In this case, the function of the enzyme is impaired. Especially in some areas of China, where the soil is very poor in selenium, the so-called Keshan syndrome (named after Keshan County) occurs. The disease manifests itself in cardiac symptoms such as cardiac arrhythmias, heart failure or even cardiogenic shock. Glutathione peroxidases are produced from glutathione. This is particularly present in the liver and is constantly produced in the body. However, in the presence of larger concentrations of peroxides, glutathione is very quickly consumed by oxidation. This can already happen quickly with physical exercise, emotional stress or infections. An unhealthy lifestyle with poor diet, environmental toxins, alcohol, cigarette smoke and more can also lead to a deficiency of glutathione. This deficiency manifests itself in weakening of the immune system and overloading of the liver. If the health complaints have not yet become chronic, a change in lifestyle quickly leads to recovery, because glutathione is quickly produced again.
