Peroxidases are a family of enzymes found in animals, plants, and microorganisms. Their role is to catalyze an oxidation-reduction reaction, where the enzyme transfers oxygen from a peroxide molecule to another substance. This process alters the chemical structure of the receiving substance, and these enzymes are involved in many biological activities.
The Peroxidase Chemical Reaction
A peroxidase enzyme breaks down a peroxide, often hydrogen peroxide (H₂O₂), by using it to oxidize another molecule called a substrate. The enzyme contains a heme group with an iron atom at its center, which is the active site where the reaction occurs. The process begins when hydrogen peroxide binds to this iron atom.
Once bound, the enzyme breaks the oxygen-oxygen bond within the hydrogen peroxide molecule, transferring one oxygen atom to the substrate. This transfer changes the substrate’s chemical properties and behavior. The reaction effectively neutralizes the potentially damaging peroxide while modifying the substrate molecule for a specific task.
After the initial reaction with hydrogen peroxide, the enzyme enters a highly reactive state. It returns to its original resting state through two one-electron reduction steps. During these steps, the oxidized substrate molecules donate electrons back to the enzyme, completing the catalytic cycle and preparing it for a new reaction.
Biological Roles of Peroxidase
In plants, peroxidases are involved in growth and defense. For instance, horseradish peroxidase, a well-studied example, participates in lignification, the process that strengthens and stiffens plant cell walls. This structural reinforcement provides stability and a physical barrier against invading microbes.
Plants also use peroxidases for defense against pathogens. When a plant is wounded or attacked, it increases peroxidase production at the site of the injury. These enzymes generate reactive molecules toxic to the invading organisms, which helps contain infections and protect the plant from further damage.
In humans, different peroxidases have specialized and distinct functions. Thyroid Peroxidase (TPO), located in the thyroid gland, is necessary for producing thyroid hormones. TPO helps incorporate iodine into a protein called thyroglobulin, a step in synthesizing thyroxine (T4) and triiodothyronine (T3), which are hormones that regulate metabolism.
Myeloperoxidase (MPO) is a human peroxidase found in high concentrations within neutrophils, a type of white blood cell. When neutrophils engulf pathogens, they release MPO, which uses hydrogen peroxide to generate hypochlorous acid. This substance, the active ingredient in household bleach, destroys the invaders.
Peroxidase and Human Health
Disruptions in peroxidase function can lead to medical conditions. Thyroid Peroxidase (TPO) is involved in autoimmune thyroid diseases like Hashimoto’s thyroiditis, where the immune system produces antibodies against TPO. This attack damages the thyroid gland’s ability to produce hormones, leading to hypothyroidism, a state characterized by a slowed metabolism.
In Graves’ disease, antibodies stimulate the TPO enzyme, leading to an overproduction of thyroid hormones and causing hyperthyroidism. The presence of anti-TPO antibodies in the blood is a diagnostic marker used to confirm these autoimmune thyroid disorders. This demonstrates how an immune response to a single enzyme can have profound and opposite effects on the body’s metabolic rate.
Elevated levels of Myeloperoxidase (MPO) in the bloodstream are an indicator of inflammation and cardiovascular risk. When inflammation occurs in arteries, immune cells release MPO, which contributes to tissue damage and the modification of LDL cholesterol, promoting plaque formation. Measuring blood MPO levels helps doctors assess a person’s risk for events like heart attacks and strokes, making it a biomarker in cardiovascular medicine.
Practical Applications of Peroxidase
Peroxidase enzymes have practical and industrial applications, especially in medical diagnostics using a technique called the enzyme-linked immunosorbent assay (ELISA). In an ELISA test, an enzyme like horseradish peroxidase is attached to an antibody. When this antibody binds to its target, such as a virus or protein, the peroxidase is carried along with it.
To detect this binding, a substrate is added that the peroxidase converts into a colored product. The color’s intensity, which can be measured, corresponds to the amount of the target substance in the sample. This sensitive method is used to diagnose many conditions, from infectious diseases like HIV to hormonal imbalances, by detecting the presence of specific antigens or antibodies.
The catalytic activity of peroxidases is also used in environmental and industrial applications. In wastewater treatment, these enzymes break down toxic pollutants like phenols from industrial processes. In the pulp and paper industry, peroxidases offer an environmentally friendlier alternative to chlorine-based chemicals for bleaching wood pulp, reducing harmful dioxin release.