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Peroxidases are enzymes that promote the oxidation of compounds using natural peroxides, reducing them to water. They are found in plants and animals, including humans, and have an active site where the reaction takes place. They play a role in minimizing damage from stressors or insect pests in plants and removing potentially harmful oxidizing agents in humans. They are also used in diagnostic tools in pathology and histochemistry. The investigation into the nature of these enzymes was led by experiments on the decomposition of hydrogen peroxide by Louis Jacques Thénard in the mid-19th century.
A peroxidase is one of several enzymes that act as catalysts to enable a variety of biological processes to take place. In particular, they promote the oxidation of various compounds using natural peroxides, especially hydrogen peroxide (H2O2), which are reduced to form water. Peroxides are created as byproducts of various biochemical reactions within organisms, but they can cause damage as they are oxidizing agents. Peroxidases break these compounds down into harmless substances by adding hydrogen, obtained from another molecule – known as a donor molecule – in a reduction-oxidation (redox) reaction in which the peroxide is reduced to form water and the other molecule is oxidized. There are a large number of these enzymes and they are found in plants and animals, including humans.
structure and properties
Like all enzymes, peroxidases are very large, complex molecules with complicated shapes involving multiple folds. They come in a variety of types, some of which can use a wide variety of donor molecules and reduce a wide range of peroxides, and some that are much more specific. Enzymes have an “active site,” which is the part of the molecule where the reaction takes place. This can be in an easily accessible part of the molecule, or it can be hidden in a fold, where it can only be reached by a molecule of the exact shape. Horseradish peroxidase (HRP) is an example of an enzyme that can utilize a wide variety of donor molecules and peroxides.
Role in biological systems
A number of peroxidases are found in plants, where they can help minimize damage from stressors or insect pests. When plants are subjected to stress – such as drought or high temperatures – or attack by pests, this tends to cause the release of reactive oxygen species (ROS). These are forms of oxygen, or compounds of it, including hydrogen peroxide, in which oxygen is highly reactive and can damage or kill cells. Peroxidases are thought to remove ROS, helping to prevent damage.
In humans and other mammals, a group of these enzymes called glutathiones, which contain the building block selenium, are found both inside and outside cells. Some of these catalyze reactions involving H2O2, while others use peroxidic compounds of lipids (fats and oils). Their main role appears to be to remove these potentially harmful oxidizing agents. Peroxidases in saliva also enable redox reactions between H2O2 and chemicals called thiocyanates, producing compounds that can kill potentially harmful microorganisms. Thyroid peroxidase releases iodine from nutrients to form essential thyroid hormones.
An unusual use of these enzymes occurs in a group of insects known as bombardier beetles. They have a chamber containing a mixture of hydrogen peroxide and chemicals called hydroquinones. When threatened, they mix them with peroxidases, which catalyze a redox reaction in which a lot of heat is released, with the resulting liquid ejected explosively at 212°F (100°C). It is a very effective way to deter predators.
it is used
The study of plant-based peroxidases, such as HRP derived from horseradish root, has delved into the fields of molecular biology and immunohistochemistry, also known as histochemistry. In the former case, HRP is used to detect peroxidase antibodies which may indicate an autoimmune condition causing thyroid problems. It is also used to measure serum or urine glucose levels. As a diagnostic tool in pathology, HRP has the ability to target and bind to certain biomarkers found in cancer cells and produce a staining reaction when introduced into biopsy specimens. In addition to being readily available and inexpensive to obtain, HRP is considered particularly useful for such tests as it is highly stable and open to reacting with a variety of donor molecules.
History
The investigation into the nature of these enzymes was led by experiments on the decomposition of hydrogen peroxide by the French baron and chemist Louis Jacques Thénard in the mid-19th century. Having been the first to produce the compound in the laboratory, he later discovered that many plant and animal materials could convert it into water and oxygen, and that the same sample of material could do so many times. This was curious, as if it was a simple chemical reaction between the peroxide and something in the organic material, it should stop once the active agent is used up. This led to the discovery of catalysts – substances that enable a chemical reaction without actually participating in it – and, in particular, peroxidases.
