Horseradish peroxidase (HRP) is a heme-dependent peroxidase isolated from the root tissue of horseradish (Armoracia rusticana) and belongs to the oxidoreductase family. HRP uses hydrogen peroxide (H₂O₂) or organic peroxides as oxidants to catalyze the oxidation of a wide variety of aromatic amines, phenols, and synthetic chromogenic/chemiluminescent substrates. It is one of the most widely used reporter enzymes in enzyme-linked immunoassays, immunoblotting, immunohistochemistry, biosensing, and biocatalysis.HRP combines high catalytic efficiency, broad substrate scope, flexible choice of chromogenic/chemiluminescent systems, and ease of conjugation to antibodies, nucleic acids, and nanomaterials, which underpins its central role in research and diagnostic applications. At the same time, its strong dependence on H₂O₂ concentration and limited stability under certain environmental conditions have driven ongoing development of recombinant HRP, immobilized HRP, and engineered HRP variants.
I. Overview of Horseradish Peroxidase
1.1 Basic definition and origin
(1) Enzymatic definition
HRP is a heme-containing peroxidase and belongs to EC 1.11.1.7-type oxidoreductases. Using H₂O₂ or organic peroxides as electron acceptors, HRP transfers electrons from a broad range of hydrogen-donor substrates to peroxides, resulting in a net two-electron oxidation of the donor. Because its catalytic cycle proceeds through high-valent iron–oxo intermediates, HRP exhibits strong oxidizing power toward many aromatic and heterocyclic substrates.
(2) Primary sources and recombinant production
Native HRP is primarily extracted from roots of the cruciferous plant horseradish. After crude extraction, high-purity enzyme preparations can be obtained via multi-step chromatography such as salting out, ion-exchange chromatography, and gel filtration.With increasing demand for batch-to-batch consistency and structural homogeneity, recombinant expression systems based on yeast and plant cells have been developed to produce sequence-defined, structurally controlled HRP variants. These systems support production of research-grade and diagnostic-grade enzymes and provide a basis for enzyme engineering.
1.2 HRP as a central tool in analysis and biotechnology
(1) Core reporter enzyme in classical immunoassay systems
HRP offers high specific activity, compatibility with many chromogenic and chemiluminescent substrates, and good stability in commercial formulations. It is therefore the most commonly used reporter enzyme in ELISA, Western blotting, immunohistochemistry (IHC/ICC), and nucleic acid hybridization assays.By covalently coupling HRP to antibodies or nucleic acid probes, specific recognition events (antigen–antibody or nucleic acid hybridization) are converted into optical or electrochemical signals. HRP has thus become one of the most mature enzymatic detection modules in modern bioanalysis.
(2) Key tool molecule in biosensing and biocatalysis
In biosensing, HRP is often coupled with oxidases that generate H₂O₂ (such as glucose oxidase) to construct highly sensitive colorimetric or electrochemical sensors for analyte concentration.In biocatalysis and environmental engineering, HRP can catalyze oxidative degradation of phenols, amines, and related pollutants, or promote oxidative polymerization of monomers for green synthesis and wastewater treatment. In these contexts, HRP provides both catalytic function and signal-transduction capability, serving as a key enzyme at the interface of chemistry, materials science, and bioanalysis.
II. Molecular Structure and Catalytic Mechanism
2.1 Molecular structure and isoenzyme features
(1) Polypeptide chain and prosthetic group
HRP is a monomeric glycoprotein, with a polypeptide chain of approximately 33.9 kDa and an overall molecular weight of about 44 kDa including glycan and other post-translational modifications. It contains a heme cofactor (commonly heme b, i.e., iron–protoporphyrin IX), which is responsible for catalysis. The heme iron is embedded in a hydrophobic pocket and coordinated by a proximal histidine residue, while distal residues (e.g., histidine, arginine) participate in H₂O₂ activation and proton transfer.The overall fold is dominated by multiple α-helices that pack around the heme to form a compact globular structure, providing a rigid framework for catalysis and stability.
(2) Glycosylation, disulfide bonds, and structural stability
HRP contains multiple N-glycosylation sites. Glycan chains coat the protein surface, enhancing solubility, resistance to proteolysis, and thermal stability. Multiple disulfide bonds are also present, and correct disulfide pairing is critical for proper folding and structural integrity—one reason HRP folds poorly in the reducing environment of the cytosol.The combination of glycosylation pattern and disulfide network gives different HRP preparations distinct isoelectric points, hydrophilicity, and thermostability.
(3) Isoenzyme forms and physicochemical differences
HRP extracted from horseradish roots is in fact a mixture of isoenzymes. These differ in amino acid sequence, glycosylation level, and isoelectric point, leading to variations in charge, stability, and substrate preference.The commonly used HRP-C isoenzyme is abundant in native preparations and structurally well characterized, and is often regarded as the “representative” HRP molecule in structural and mechanistic studies. Specific isoenzymes can be enriched by chromatographic separation based on pI or affinity, to meet application needs that require particular pH ranges, stability, or substrate profiles.
2.2 Peroxidase catalytic cycle
(1) Resting state and formation of Compound I
In the resting state, HRP heme iron is in the ferric Fe³⁺ state. Fe³⁺–HRP first binds a molecule of H₂O₂ and undergoes a redox reaction to form the high-valent iron–oxo intermediate Compound I. This species can be described as Fe⁴⁺=O, together with a porphyrin or nearby amino acid radical cation, giving the intermediate strong oxidizing power.This step involves cleavage of H₂O₂ and release of one molecule of water, corresponding to a two-electron transfer.
(2) Compound II and enzyme regeneration
Compound I then oxidizes the first hydrogen-donor substrate via a one-electron step, generating a substrate radical (or partially oxidized product) and reducing HRP to Compound II (often described as Fe⁴⁺=O without a porphyrin radical).Compound II reacts with a second substrate molecule, performing another one-electron oxidation and reducing Fe⁴⁺=O back to resting Fe³⁺–HRP. The second oxidized substrate is released, and the enzyme returns to its resting state, completing a full catalytic cycle. Overall, one molecule of H₂O₂ is reduced to two molecules of water and two substrate molecules are oxidized.
(3) Substrate diversity and downstream radical reactions
HRP accepts a wide range of hydrogen-donor substrates, including phenols, aromatic amines, heterocyclic compounds, and commonly used chromogenic/chemiluminescent substrates (e.g., TMB, ABTS, OPD, luminol).The substrate radicals generated by HRP can undergo dimerization, polymerization, or further rearrangements, producing different colored, fluorescent, or light-emitting products. The chemistry of the substrate determines signal type (colorimetric, chemiluminescent, fluorescent), signal stability, and dynamic range; therefore, specialized substrate systems are typically chosen for specific applications.
III. Enzymatic Properties and Influencing Factors
3.1 pH and temperature characteristics
The optimal pH of HRP is strongly dependent on the specific hydrogen-donor substrate. Most chromogenic and chemiluminescent systems are operated in mildly acidic to neutral pH. HRP itself generally maintains good structural stability near neutral pH, whereas some substrates show better color development in slightly acidic conditions.In practice, the working pH is usually determined by the substrate system rather than HRP alone. For temperature, HRP displays high activity and acceptable stability between room temperature and 37 °C. Prolonged incubation above ~50 °C promotes structural loosening and irreversible inactivation.
3.2 Stability, inactivation mechanisms, and inhibitors
(1) H₂O₂-induced “suicide inactivation”
H₂O₂ is both an essential substrate and a major cause of HRP inactivation. When H₂O₂ concentration is too high or hydrogen-donor substrates are depleted, Compound I/II is not efficiently reduced by substrate and may instead react with solvent or with the protein itself. This can induce heme degradation or long-lived abnormal iron states, leading to “suicide inactivation.”Experimental design therefore must balance H₂O₂ dosage with enzyme stability and avoid conditions where significant inactivation occurs before the signal reaches a usable plateau.
(2) Inhibitors and environmental factors
Azide (e.g., NaN₃) and cyanide strongly inhibit HRP by coordinating with the heme iron. Buffers or antibody storage solutions containing azide will significantly diminish HRP signal. Strong reducing agents, certain metal ions, and high concentrations of surfactants may also modify the heme environment or overall conformation, reducing activity.Strong acids and bases, high levels of organic solvent, and prolonged light exposure further accelerate HRP inactivation.
3.3 Kinetic parameters and substrate dependence
HRP exhibits substrate-dependent Km and kcat values. For H₂O₂, Km is typically in the micromolar to low millimolar range, while kinetic parameters for different chromogenic substrates vary widely.These kinetic properties directly influence sensitivity, linear range, and reaction time of detection systems. Low Km and high kcat favor detection of low analyte concentrations but can also increase the risk of substrate or product inhibition at high substrate levels.In practical use, working concentrations of substrate and H₂O₂ are often determined by preliminary tests so that the signal is quasi-linear within the intended reading time.
IV. Preparation, Purification, and Recombinant Expression
4.1 Extraction and purification of native HRP
HRP is extracted from horseradish roots by low-temperature homogenization in buffer, followed by solid–liquid separation, salting out, and multi-step chromatographic purification. Early workflows typically combine ammonium sulfate precipitation with ion-exchange chromatography to enrich HRP, and use gel filtration to remove small molecules and aggregates.When separation of individual isoenzymes is required, additional chromatographic steps based on pI or specific interactions can be used. In native preparations, N-glycan positions and glycan composition show some heterogeneity; glycan differences between batches can cause modest changes in pI, stability, and enzymatic properties.
4.2 Recombinant HRP and engineered expression systems
To enhance batch consistency and investigate structure–function relationships, various recombinant HRP expression systems have been developed.Prokaryotic systems can yield high expression levels but typically produce inclusion bodies lacking correct glycosylation and disulfide pairing. These require in vitro refolding and heme reconstitution to recover partial activity.Eukaryotic systems (such as yeast and plant cells) are better suited for expressing correctly glycosylated and folded HRP, with properties closer to native isoenzymes. Recombinant systems also enable site-directed mutagenesis and directed evolution to generate HRP variants with improved thermostability, tolerance to organic solvents or extreme pH, or altered substrate profiles.
V. Typical Applications of Horseradish Peroxidase
5.1 Immunoassays and protein detection
(1) Enzyme label module in ELISA
In ELISA, HRP is covalently conjugated to detection antibodies or antigens, forming “recognition unit + catalytic unit” hybrid molecules. Upon addition of chromogenic substrates such as TMB or ABTS, HRP generates colored products with characteristic absorbance peaks, enabling quantitative determination of analyte concentration by absorbance measurement.Because HRP has high specific activity and diverse substrate options, detection schemes with different sensitivities and linear ranges can be designed, covering analyte concentrations from pg-level to μg-level.
(2) Western blot and chemiluminescent detection
In protein immunoblotting, HRP is usually attached to secondary antibodies that bind primary antibody–antigen complexes on membranes. Chemiluminescent substrates (e.g., luminol-based systems) are then added; HRP catalyzes formation of reactive intermediates that emit light upon return to the ground state.Combined with CCD imaging, this approach provides high sensitivity and a broad linear range and is a mainstream method for routine protein expression analysis.
(3) Immunohistochemistry and in situ colorimetric localization
In tissue sections or cell samples, HRP-labeled antibodies bind target antigens. Addition of insoluble chromogenic substrates such as DAB produces brown precipitates at antigen locations. Light microscopy can then be used for direct visualization of target protein localization and relative expression levels. This is widely used in pathology and basic research for in situ detection.
5.2 Nucleic acid detection and signal amplification
HRP can be conjugated to nucleic acid probes, strand-displacement products, or aptamers for use in nucleic acid hybridization, rolling-circle amplification, and multiplex detection. HRP-catalyzed chromogenic or chemiluminescent reactions greatly amplify signals resulting from specific nucleic acid recognition, enabling highly sensitive detection of target sequences.Combining HRP with nanoparticles and multienzyme labeling strategies further lowers detection limits, supporting pathogen detection, mutation analysis, and microarray readouts.
5.3 Biosensing and electrochemical detection
In electrochemical biosensors, HRP is immobilized at electrode surfaces or on nanomaterials. By catalyzing redox reactions of H₂O₂ or other substrates, HRP converts analyte concentration into measurable current or potential changes.A common strategy is to couple HRP to oxidases that generate H₂O₂, such as glucose oxidase–HRP systems for glucose determination. By carefully designing electrode materials (e.g., carbon nanotubes, graphene, gold nanoparticles) and immobilization methods, electron transfer efficiency and sensor sensitivity can be significantly improved.
5.4 Biocatalysis and environmental engineering
HRP can catalyze oxidation and coupling/polymerization of various phenols, aromatic amines, and dyes under mild conditions, making it suitable for green synthesis and pollutant degradation.In industrial wastewater treatment, HRP can oxidize refractory phenols and azo dyes into higher-molecular-weight products that are more easily removed (e.g., as precipitates). In materials synthesis, HRP can catalyze formation of functional polymers or surface coatings with controlled structure. Compared with conventional chemical oxidants, HRP offers higher selectivity, milder reaction conditions, and relatively benign byproducts.
VI. Safety and Practical Considerations
6.1 Biosafety and personal protection
HRP is a non-pathogenic protein enzyme and can be handled under standard biochemical laboratory conditions. Powdered reagents should be handled to minimize inhalable dust generation, with mask and eye protection recommended. If liquid HRP solutions contact skin or eyes, they should be rinsed immediately with plenty of water.Long-term handling should follow standard laboratory biosafety practices.
6.2 Safety management of H₂O₂ and substrates
HRP reaction systems typically contain H₂O₂ and various chromogenic or chemiluminescent substrates. Concentrated H₂O₂ is irritating and potentially corrosive to skin and mucosa, and some synthetic substrates and their oxidized products may have toxic or sensitizing properties.Preparation and waste handling should comply with safety data sheet (SDS) instructions, using appropriate protective equipment and segregated collection and disposal procedures, avoiding direct release into the environment.
6.3 Data reliability and control of experimental conditions
To ensure reliability of HRP-based assays, attention should be paid to storage conditions, freeze–thaw cycles, and timing of working solution preparation. Expired substrates and H₂O₂ solutions should be avoided.Experiments should include blank, negative, and positive controls, and standard curves should be designed so that measured values fall within the linear range. For highly sensitive assays, background sources must be carefully evaluated, including nonspecific binding, endogenous peroxidase activities, and potential redox-active interferents in the system.
VII. Related Products (Aladdin)
Catalog No. | Product Name | Grade and Purity / Specification |
Horseradish Peroxidase (HRP) | EnzymoPure™, ≥250 U/mg, Rz ≥ 3 | |
Horseradish Peroxidase (HRP) | EnzymoPure™,≥150 U/mg powder, Rz ≥ 1.5 | |
Horseradish Peroxidase (HRP) | EnzymoPure™,>100 U/mg (pyrogallol), Rz > 1 | |
Horseradish Peroxidase (HRP) | EnzymoPure™,>150 U/mg, Rz > 2 | |
Horseradish Peroxidase (HRP) | EnzymoPure™,>200 U/mg, Rz 2–4 | |
Horseradish Peroxidase (HRP) | Bioactive, Recombinant, ActiBioPure™, High Performance, EnzymoPure™, ≥250U/mg enzyme powder, Rz ≥3; expressed in Nicotiana benthamiana | |
Horseradish Peroxidase (HRP) | Bioactive, Recombinant, ActiBioPure™, High Performance, EnzymoPure™, ≥150U/mg enzyme powder, Rz ≥2; expressed in Nicotiana benthamiana | |
Horseradish Peroxidase (HRP) | Bioactive, Recombinant, ActiBioPure™, High Performance, EnzymoPure™, ≥150U/mg enzyme powder, Rz ≥2.0 | |
Horseradish Peroxidase (HRP) | Bioactive, ActiBioPure™, Native, High Performance, EnzymoPure™, ≥300U/mg enzyme powder, Rz≥3; from Horseradish | |
Avidin, HRP conjugate |
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COX IV Mouse mAb (HRP) | ExactAb™, Validated, High Performance, 0.5 mg/mL | |
Cofilin Mouse mAb (HRP) | ExactAb™, Validated, 0.5 mg/mL | |
GAPDH Mouse mAb (HRP) | ExactAb™, Validated, 0.5 mg/mL | |
Goat Anti-Chicken IgY H&L (HRP) | ExactAb™, High Performance, Validated, 1 mg/mL | |
Goat Anti-Chicken IgY H&L (HRP) | ExactAb™, High Performance, Validated, Azide Free, 1.0 mg/mL | |
Goat Anti-Human IgG (HRP) | ExactAb™, High Performance, Validated, Azide Free, 1.0 mg/mL | |
Goat Anti-Human IgG H&L (HRP) | ExactAb™, High Performance, Validated, 1 mg/mL | |
Goat Anti-Mouse IgG H&L (HRP) | ExactAb™, High Performance, Validated, Azide Free, 1.0 mg/mL | |
Goat Anti-Mouse IgG H&L (HRP) | ExactAb™, Azide Free, Validated, High Performance, Pre-adsorbed, 1.0 mg/mL | |
Goat Anti-Mouse IgG H&L (HRP) | ExactAb™, High Performance, Validated, 1 mg/mL | |
Goat Anti-Rabbit IgG H&L (HRP) | ExactAb™, High Performance, Validated, Azide Free | |
Goat Anti-Rabbit IgG H&L (HRP) | ExactAb™, Azide Free, Validated, High Performance, Pre-adsorbed, 1.0 mg/mL | |
HA tag Mouse mAb (HRP) | ExactAb™, Validated, High Performance, 0.5 mg/mL | |
HRP Antibody Labeling Kits |
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MBP Tag Mouse mAb (HRP) | ExactAb™, Validated, Azide Free, High Performance, 0.5 mg/mL | |
Mouse Anti-Human IgG H&L (HRP) | Carrier Free, ExactAb™, Azide Free, Validated, See COA | |
Profilin 1 Mouse mAb (HRP) | ExactAb™, Azide Free, Validated, High Performance, 0.5 mg/mL | |
Rabbit Anti-Goat IgG H&L (HRP) | ExactAb™, High Performance, Validated, 1.0 mg/mL | |
Rabbit Anti-Goat IgG H&L (HRP) | ExactAb™, Validated, Azide Free, 1.0 mg/mL | |
Rabbit Anti-Mouse IgG (HRP) | ExactAb™, High Performance, Validated, Azide Free, 1.0 mg/mL | |
Rabbit Anti-Mouse IgG H&L (HRP) | ExactAb™, High Performance, Validated, 1.0 mg/mL | |
Sumo tag Mouse mAb (HRP) | ExactAb™, Validated, High Performance, 0.5 mg/mL | |
Vinculin Mouse mAb (HRP) | ExactAb™, Validated, 0.5 mg/mL | |
beta Actin Mouse mAb (HRP) | ExactAb™, High Performance, Validated, Azide Free, 0.5 mg/mL | |
beta Tubulin Mouse mAb (HRP) | ExactAb™, Validated, Azide Free, High Performance, 0.5 mg/mL | |
Avidin, HRP conjugate |
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With advances in structural biology, enzyme engineering, and nanotechnology, the application scope of HRP continues to expand. High-resolution structural analysis and computational design enable development of HRP variants with improved catalytic efficiency, stability, and tailored substrate profiles for ultra-sensitive detection or challenging process conditions.In parallel, integration of HRP with microfluidic chips, paper-based analytical devices, and wearable electrochemical sensors is driving progress in point-of-need testing and individualized monitoring. In biocatalysis and environmental remediation, scalable production of recombinant HRP and maturation of immobilization technologies are enhancing its feasibility for green synthesis and pollutant treatment.Overall, as a structurally well-characterized, mechanistically understood, and highly engineerable peroxidase, horseradish peroxidase is expected to maintain long-term technical value in bioanalysis, diagnostic technologies, and sustainable catalysis.