What Is Peroxidase and What Does It Do?

Peroxidases are enzymes found across virtually all living organisms whose core job is to use hydrogen peroxide to drive chemical reactions. They show up in settings as different as a white blood cell killing bacteria and a fungus digesting a rotting log. What makes them unusual as a family is the sheer range of tasks biology has assigned them: some peroxidases protect your cells from oxidative damage, others deliberately create toxic molecules to fight infection, still others build structural materials in plant cell walls, and one is essential for making thyroid hormones. Understanding peroxidases means understanding a toolkit that life has reinvented multiple times for wildly different purposes.

The Basic Reaction

At its simplest, a peroxidase takes hydrogen peroxide and a second molecule (called a substrate), and uses the peroxide’s oxidizing power to chemically transform that substrate. Most peroxidases contain a heme group at their active site, the same iron-containing ring structure found in hemoglobin. When hydrogen peroxide reaches the heme iron, it reacts to form a highly reactive intermediate known as Compound I. This intermediate then pulls electrons from nearby molecules in one or two successive steps, returning the enzyme to its resting state and leaving the substrate chemically altered.

Whether Compound I strips electrons one at a time or two at once depends partly on the architecture of the enzyme’s active site. Research using ultrafast crystallography on a bacterial peroxidase showed that the presence or absence of water molecules in the pocket around the heme iron shifts the enzyme between these two modes: a “wet” pocket favors sequential one-electron transfers, while a “dry” pocket produces what looks like a two-electron process, because the intermediate between the steps is so short-lived it barely accumulates.¹ This fine-tuning of the same basic chemistry is part of what allows peroxidases to serve such different biological roles.

Not One Family but Several

Despite sharing the name “peroxidase,” these enzymes did not all descend from a single ancestor. At least four major superfamilies of heme peroxidases evolved independently, each with its own protein fold and active-site design.² Three of these superfamilies trace back to ancient bacteria, and over evolutionary time, each branched into specialized forms in plants, animals, and fungi. The peroxidase-catalase superfamily alone contains hundreds of distinct sequences across organisms, including hybrid types that blur the boundaries between established classes.³

Beyond the heme peroxidases, there are also thiol peroxidases, which use a sulfur-containing amino acid (cysteine or selenocysteine) instead of an iron-heme group to break down peroxides. These include glutathione peroxidases and peroxiredoxins, both of which play major antioxidant roles in cells.⁴ The practical upshot is that “peroxidase” is less a single enzyme and more a job description filled by many unrelated proteins, all converging on the same basic trick of harnessing peroxide chemistry.

Killing Microbes in Your Immune System

One of the most dramatic roles for peroxidases is in innate immunity. Neutrophils, the most abundant white blood cells, carry large amounts of myeloperoxidase (MPO). When a neutrophil engulfs a bacterium, it seals the invader inside a compartment called a phagosome, then floods that compartment with hydrogen peroxide. MPO uses the peroxide along with chloride ions to generate hypochlorous acid, the same active ingredient found in household bleach.⁵ Real-time imaging of individual neutrophil phagosomes has confirmed that hypochlorous acid is produced inside these compartments during engulfment of bacteria like Staphylococcus aureus.⁶

This system is extremely effective at killing pathogens, but it can also damage your own tissues when neutrophils become overactive. Hypochlorous acid production by MPO outside of phagosomes is implicated in the progression of several inflammatory diseases.⁷ The enzyme has been linked to cardiovascular disease specifically: MPO-driven reactions appear to contribute to atherosclerosis at multiple stages, from the initial damage to artery walls through plaque formation and eventual rupture.⁸ MPO levels in blood are sometimes measured as a biomarker for heart attack risk, which makes it an example of a peroxidase where too much activity becomes the problem rather than the solution.

Peroxidases in Saliva, Tears, and Milk

Your body’s mucosal surfaces use a gentler peroxidase-based defense system. Lactoperoxidase, found in saliva, tears, and breast milk, belongs to the same broad family as myeloperoxidase but produces a milder oxidant.⁹ Instead of making bleach, lactoperoxidase uses hydrogen peroxide and thiocyanate (a naturally occurring ion in secretions) to produce hypothiocyanite, a compound that inhibits bacterial growth by oxidizing critical sulfur-containing molecules on bacterial surfaces.¹⁰

Research on E. coli showed that the killing power of the lactoperoxidase system correlated directly with how much it oxidized bacterial sulfhydryl groups, proteins with exposed sulfur atoms that are essential for bacterial metabolism.¹¹ The beauty of this system is that hypothiocyanite is strong enough to slow bacterial growth but mild enough not to damage the delicate tissues of your mouth, eyes, or an infant’s gut. Some toothpaste and mouthwash formulations now include lactoperoxidase for this reason, aiming to boost the mouth’s natural antimicrobial chemistry rather than introducing a foreign disinfectant.

Guarding Cells Against Oxidative Damage

While some peroxidases weaponize hydrogen peroxide, others exist to get rid of it. Hydrogen peroxide is a normal byproduct of cellular metabolism, but if it accumulates, it can react with iron to form hydroxyl radicals, which are among the most destructive molecules in biology. Glutathione peroxidases (GPxs) are a family of enzymes that neutralize hydrogen peroxide and other peroxides by converting them to harmless water and alcohols. In mammals, at least four of the eight GPx family members contain selenium in their active site in the form of selenocysteine, which is why selenium is an essential dietary mineral.¹²

Different GPx family members protect different cellular compartments. GPx1 works mainly in the cytoplasm, GPx3 is secreted into plasma, and GPx4 is specialized for reducing lipid hydroperoxides embedded in cell membranes, something the other GPxs cannot do effectively.¹³ GPx4 is so important for membrane integrity that its complete loss is lethal in mice.

Peroxiredoxins (Prxs) form another major branch of the antioxidant peroxidase network. These cysteine-based enzymes react extremely rapidly with hydrogen peroxide and also reduce organic hydroperoxides and peroxynitrite.¹⁴ But Prxs do more than just scavenge peroxides. Because they react with hydrogen peroxide so fast, with rate constants ranging from a million to a hundred million per molar per second, they effectively control how much peroxide is available to act as a signaling molecule inside cells.¹⁵ In one model of how this works, a peroxiredoxin gets oxidized by hydrogen peroxide and then passes that oxidation along to a target protein like a transcription factor or a phosphatase, effectively relaying the peroxide signal without letting the peroxide itself roam free. This dual role as both antioxidant and signal relay is one of the more elegant arrangements in cell biology.

Making Thyroid Hormones

Thyroid peroxidase (TPO) is the enzyme responsible for synthesizing thyroid hormones, which regulate metabolism, growth, and development.¹⁶ TPO sits on the surface of thyroid cells facing into the colloid, the protein-rich gel inside thyroid follicles. There, it uses hydrogen peroxide to attach iodine atoms to tyrosine residues on a large protein called thyroglobulin, and then couples those iodinated residues together to form the hormones T3 and T4.

This is a case where the peroxidase is not defending against anything or breaking anything down. It is running a biosynthetic assembly line. The enzyme’s dependence on hydrogen peroxide makes the thyroid gland one of the most peroxide-intensive tissues in the body, which is why the thyroid also maintains high levels of antioxidant enzymes as a counterbalance. Autoimmune thyroid diseases like Hashimoto’s thyroiditis often involve antibodies directed against TPO, and measuring anti-TPO antibodies in blood is one of the standard diagnostic tests for autoimmune thyroid dysfunction.

Building and Defending Plant Cell Walls

Plants have their own rich collection of peroxidases, particularly the Class III family, which are secreted into the cell wall space. These enzymes serve two overlapping purposes: they help construct the cell wall by cross-linking lignin, the tough polymer that gives wood its rigidity, and they participate in defense against pathogens by generating reactive oxygen species and reinforcing the wall as a physical barrier.

A clear example comes from citrus plants. A peroxidase called CsPrx25 in sweet orange was shown to both maintain the balance of reactive oxygen species and enhance lignin deposition in cell walls, strengthening them against invasion by the bacterium that causes citrus canker.¹⁷ In Arabidopsis, a model plant widely used in research, a peroxisomal ascorbate peroxidase protects against oxidative stress that originates specifically in peroxisomes, the organelles where certain metabolic reactions generate hydrogen peroxide as a byproduct.¹⁸ Interestingly, this protection did not extend to oxidative stress originating in chloroplasts, showing that even antioxidant peroxidases in plants are compartment-specific rather than general-purpose.

How Fungi Digest Wood

White-rot fungi are among the only organisms on Earth that can fully break down lignin, and they do it with secreted peroxidases. Lignin peroxidases (LiPs) and manganese peroxidases are the key enzymes in this process. They use hydrogen peroxide to generate radicals powerful enough to crack open the aromatic ring structures that make lignin so chemically stable.

Evolutionary analysis suggests that efficient lignin degradation in fungi co-evolved with the appearance of lignin in plants. A study reconstructing ancestral fungal peroxidases found that over evolutionary time, the enzymes became progressively better at attacking lignin, with the rate-limiting catalytic step improving up to 50-fold from the earliest peroxidase ancestor to modern forms.¹⁹ The critical innovation was a solvent-exposed tryptophan residue on the enzyme surface that acts as a long-range electron-transfer site, allowing the enzyme to oxidize the bulky lignin polymer without requiring it to fit into a conventional active-site pocket.

Not all fungal peroxidases follow this same blueprint, though. The lignin peroxidase from Trametes cervina lacks that conserved tryptophan entirely and instead appears to use a tyrosine residue at a different position on the protein surface, suggesting a completely independent solution to the same problem.²⁰ Infrared spectroscopy of lignin before and after treatment with a fungal lignin peroxidase confirmed substantial disruption of the polymer’s chemical bonds, including breakdown of aromatic rings and ether linkages.²¹

The Workhorse of the Diagnostic Lab

Horseradish peroxidase (HRP), originally purified from the roots of the horseradish plant, has become one of the most widely used enzymes in medical and research laboratories. Its popularity comes from a practical combination of traits: it is small, stable, easy to produce, and generates a visible color change when it reacts with certain chemical substrates. This makes it ideal for enzyme-linked immunosorbent assays (ELISAs), the standard format for detecting everything from pregnancy hormones to infectious disease markers in blood samples.²²

In a typical ELISA, HRP is attached to an antibody. When that antibody binds its target, adding hydrogen peroxide and a chromogenic substrate causes HRP to produce a color change proportional to how much target is present. More recent versions use fluorescent substrates instead of colorimetric ones for greater sensitivity. One platform using HRP-triggered fluorescence achieved detection limits in the sub-nanogram-per-milliliter range for cardiac troponin I (a heart attack marker) and the SARS-CoV-2 nucleocapsid protein.²³

An engineered version of a different peroxidase, called APEX2 (derived from soybean ascorbate peroxidase), has opened up an entirely separate application. When fused to a protein of interest inside a living cell, APEX2 can generate short-lived radicals that tag nearby proteins with biotin. This proximity labeling happens in about a minute and gives researchers a snapshot of which proteins are physically close to each other inside specific cellular compartments.²⁴ It has become a go-to method for mapping protein neighborhoods in organelles that are difficult to isolate by traditional biochemical methods.²⁵

Cleaning Up Polluted Water

The same peroxide-driven oxidizing chemistry that works inside cells also works in a wastewater treatment tank. Peroxidases from several plant sources can polymerize phenolic pollutants in water, essentially gluing them together into large, insoluble clumps that drop out of solution and can be filtered away. Soybean peroxidase removed at least 95% of several phenolic compounds from synthetic wastewater, including bisphenol A and chlorinated phenols.²⁶

The cost of purified enzymes has historically limited this approach, so researchers have explored cheaper alternatives. Potato pulp, a waste product of the starch industry, contains large amounts of active peroxidase and achieved over 95% phenol removal at optimized concentrations, making it a candidate for low-cost wastewater treatment.²⁷ Horseradish peroxidase has also been tested on real biorefinery wastewater, where it achieved about 97.5% conversion of phenols at neutral pH.²⁸ The challenge is keeping the enzyme active long enough in a harsh industrial setting, but the principle is sound and the field is actively working on enzyme immobilization strategies to extend working life.

Why Food Processors Care About Peroxidase

If you have ever seen a package of frozen vegetables that says “blanched,” peroxidase is part of the reason. In food processing, peroxidase is used as an indicator enzyme for blanching adequacy. Because peroxidases are among the most heat-resistant enzymes in fruits and vegetables, the logic is straightforward: if the blanching treatment was hot enough and long enough to inactivate peroxidase, it was almost certainly sufficient to inactivate every other enzyme that could degrade color, flavor, or nutritional quality during storage.²⁹ Testing for residual peroxidase activity after blanching is a standard quality-control step across the frozen-vegetable industry.³⁰

This might seem like a minor technical detail, but it affects the quality of a large fraction of processed plant foods. An under-blanched frozen vegetable can develop off-flavors, brown discoloration, and loss of vitamins during months of freezer storage, all driven by residual enzyme activity. Peroxidase testing is the fastest and most reliable way to catch that problem before the product ships.