PEROXIDASES: STRUCTURE, FUNCTIONS AND TYPES - BIOLOGY - 2025

The peroxidases are mostly hemoproteins with enzymatic activity catalyzing the oxidation of a variety of organic and inorganic substrates using for this hydrogen peroxide or other unrelated substances.

In its broadest sense, the term "peroxidase" includes enzymes such as NAD- and NADP-peroxidases, fatty acid-peroxidases, cytochrome-peroxidases, glutathione-peroxidases, and many other non-specific enzymes.

Diagram of a heme-dependent peroxidase protein (Source: Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute via Wikimedia Commons)

However, it is more commonly used to refer to nonspecific enzymes from different sources that have oxidoreductase activity and that employ hydrogen peroxide and other substrates to catalyze their oxidation-reduction reactions.

Heme peroxidases are extremely common in nature. They are found in animals, higher plants, yeasts, fungi, and bacteria.

In mammals, these are produced by white blood cells, the uterus, spleen and liver, salivary glands, stomach walls, lungs, thyroid glands, and other tissues.

In plants, the plant species richest in peroxidases are horseradish and fig tree. Peroxidase purified from horseradish has been extensively studied and used for various purposes in experimental biology and biochemistry.

In eukaryotic cells, these important enzymes are usually found within specialized organelles known as "peroxisomes," which are surrounded by a single membrane and are involved in numerous cellular metabolic processes.

Structure

Despite the little homology that exists between the different classes of peroxidases, it has been determined that their secondary structure and the way it is organized is fairly conserved among the different species.

There are some exceptions, but most peroxidases are glycoproteins and carbohydrates are believed to contribute to their high temperature stability.

These proteins have molecular weights ranging from 35 to 150 kDa, which is equivalent to approximately 250 and 730 amino acid residues.

With the exception of myeloperoxidase, all molecules of this type contain a heme group in their structure which, in the resting state, has an iron atom in the Fe + 3 oxidation state. Plants possess a prosthetic group known as ferroporphyrin XI.

Peroxidases have two structural domains that "wrap around" the heme group and each of these domains is the product of the expression of a gene that underwent a duplication event. These structures are composed of more than 10 alpha helices linked by polypeptide loops and turns.

The proper folding of the molecule seems to depend on the presence of conserved glycine and proline residues, as well as an aspartic acid residue and an arginine residue that form a salt bridge between them that connects both structural domains.

Features

The main function of peroxidase enzymes is the removal of hydrogen peroxide from the cellular environment, which can be produced by different mechanisms and which could represent serious threats to intracellular stability.

However, in this process of removal of this reactive oxygen species (in which oxygen has an intermediate oxidation state), peroxidases use the oxidizing capacity of this substance to fulfill other important functions for metabolism.

In plants, these proteins are an important part of the lignification processes and defense mechanisms in tissues infected with pathogens or that have suffered physical damage.

In the scientific context, new applications have emerged for peroxidases and these include the treatment of wastewater containing phenolic compounds, the synthesis of aromatic compounds and the removal of peroxide from food or waste materials.

In analytical and diagnostic terms, horseradish peroxidase is perhaps the most commonly used enzyme for the preparation of conjugated antibodies that are used for immunological absorption tests such as ELISA (Enzyme-linked immunosorbent assay) and also for the determination of various types of compounds.

Mechanism of action

The catalytic process of peroxidases occurs through sequential steps that begin with the interaction between the active site of the enzyme and hydrogen peroxide, which oxidizes the iron atom in the heme group and generates an unstable intermediate compound known as compound I (CoI).

The oxidized protein (CoI) then has a heme group with an iron atom that went from oxidation state III to state IV and for this process hydrogen peroxide was reduced to water.

Compound I is capable of oxidizing an electron donating substrate, forming a substrate radical and becoming a new chemical species known as Compound II (CoII), which is subsequently reduced by a second substrate molecule, regenerating iron in state III and producing another radical.

Types

-According to the body

Peroxidases are grouped into three classes depending on the organism where they are found:

- Class I: intracellular prokaryotic peroxidases.

- Class II: extracellular fungal peroxidases.

- Class III: secreted vegetable peroxidases.

Contrary to class I proteins, those of classes II and III have disulfide bridges built between cysteine ​​residues in their structures, which gives them considerably greater rigidity.

Class II and III proteins also differ from class I in that they generally have glycosylations on their surface.

-According to the active site

Mechanistically speaking, peroxidases can also be categorized according to the nature of the atoms found in their catalytic center. In this way, hemoperoxidases (the most common), vanadium-haloperoxidases and others have been described.

Hemoperoxidases

As already mentioned, these peroxidases have a prosthetic group in their catalytic center known as the heme group. The iron atom in this place is coordinated by four bonds with the nitrogen atoms.

Vanadium-Haloperoxidases

Instead of a heme group, vanadium-haloperoxidases possess vanadate as a prosthetic group. These enzymes have been isolated from marine organisms and some terrestrial fungi.

The vanadium in this group is coordinated by three non-protein oxygens, a nitrogen from a histidine residue and a nitrogen from an azide bond.

Other peroxidases

Many bacterial haloperoxidases that have prosthetic groups other than heme or vanadium are categorized in this group. Glutathione peroxidases are also in this group, which contain a seleno-cysteine ​​prosthetic group and some enzymes capable of oxidizing lignin.

References

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