Kinetics of the reaction of compound II of horseradish peroxidase with hydrogen peroxide to form compound III

Amperometric biosensor based on horseradish peroxidase for biogenic amine determinations in biological samples

An amperometric biosensor for total biogenic amine determinations, using a carbon paste electrode modified with horseradish peroxidase (HRP) enzyme is described. The HRP immobilization on graphite was made using bovine serum albumin, carbodiimide and glutaraldehyde. The biosensor response was optimized using serotonin and it presented the best performance in 0.1 mol l(-1) phosphate buffer (pH=7.0) containing 10 micromol l(-1) of hydrogen peroxide. Under optimized operational conditions at -50 mV versus SCE, a linear response range from 40 to 470 ng ml(-1) was obtained. The detection limit was 17 ng ml(-1) and the response time was 0.5s. The proposed sensor presented a stable response during 4h under continuous monitoring. The difference of the response between six sensor preparations was <2%. The sensor was applied in the determination of total biogenic amines (neurotransmitters) in rat blood samples with success, obtaining a recovery average of 102%.

Hydrogen peroxide sensor based on horseradish peroxidase immobilized on a silver nanoparticles/cysteamine/gold electrode

A third-generation hydrogen peroxide biosensor was prepared by immobilizing horseradish peroxidase (HRP) on a gold electrode modified with silver nanoparticles. A freshly-cleaned gold electrode was first immersed in a cysteamine-ethanol solution, and then silver nanoparticles were immobilized on the cysteamine monolayer, and finally HRP was adsorbed onto the surfaces of the silver nanoparticles. This self-assemble process was examined via atomic force microscopy (AFM). The immobilized horseradish peroxidase exhibited an excellent electrocatalytic response toward the reduction of hydrogen peroxide. The linear range of the biosensor was 3.3 microM to 9.4 mM, and the detection limit was estimated to be 0.78 microM. Moreover, the biosensor exhibited a fast response, high sensitivity, good reproducibility, and long-term stability.

Kinetics of Interconversion of Ferrous Enzymes, Compound II and Compound III, of Wild-type Synechocystis Catalase-peroxidase and Y249F

With the exception of catalase-peroxidases, heme peroxidases show no significant ability to oxidize hydrogen peroxide and are trapped and inactivated in the compound III form by H2O2 in the absence of one-electron donors. Interestingly, some KatG variants, which lost the catalatic activity, form compound III easily. Here, we compared the kinetics of interconversion of ferrous enzymes, compound II and compound III of wild-type Synechocystis KatG, the variant Y249F, and horseradish peroxidase (HRP). It is shown that dioxygen binding to ferrous KatG and Y249F is reversible and monophasic with apparent bimolecular rate constants of (1.2 +/- 0.3) x 10(5) M(-1) s(-1) and (1.6 +/- 0.2) x 10(5) M(-1) s(-1) (pH 7, 25 degrees C), similar to HRP. The dissociation constants (KD) of the ferrous-dioxygen were calculated to be 84 microm (wild-type KatG) and 129 microm (Y249F), higher than that in HRP (1.9 microm). Ferrous Y249F and HRP can also heterolytically cleave hydrogen peroxide, forming water and an oxoferryl-type compound II at similar rates ((2.4 +/- 0.3) x 10(5) M(-1) s(-1) and (1.1 +/- 0.2) x 10(5) M(-1) s(-1) (pH 7, 25 degrees C)). Significant differences were observed in the H2O2-mediated conversion of compound II to compound III as well as in the spectral features of compound II. When compared with HRP and other heme peroxidases, in Y249F, this reaction is significantly faster ((1.2 +/- 0.2) x 10(4) M(-1) s(-1))). Ferrous wild-type KatG was also rapidly converted by hydrogen peroxide in a two-phasic reaction via compound II to compound III (approximately 2.0 x 10(5) M(-1) s(-1)), the latter being also efficiently transformed to ferric KatG. These findings are discussed with respect to a proposed mechanism for the catalatic activity.

Electrochemical and electrocatalytic properties of myoglobin and hemoglobin incorporated in carboxymethyl cellulose films

Protein-CMC films were made by casting a solution of myoglobin (Mb) or hemoglobin (Hb) and carboxymethyl cellulose (CMC) on pyrolytic graphite electrodes. In pH 7.0 buffers, Mb and Hb incorporated in CMC films gave a pair of well-defined and quasi-reversible cyclic voltammetric peaks at about -0.34 V vs. SCE, respectively, characteristic of heme Fe(III)/Fe(II) redox couples of the proteins. The electrochemical parameters such as apparent standard heterogeneous electron transfer rate constants (k(s)) and formal potentials (E degrees ') were estimated by square wave voltammetry with nonlinear regression analysis. In aqueous solution, stable CMC films absorbed large amounts of water and formed hydrogel. Scanning electron microscopy of the films showed that interaction between Mb or Hb and CMC would make the morphology of dry protein-CMC films different from the CMC films alone. Positions of Soret absorbance band suggest that Mb and Hb in CMC films retain their secondary structure similar to the native states in the medium pH range. Trichloroacetic acid, nitrite, oxygen, and hydrogen peroxide were catalytically reduced at protein-CMC film electrodes.

Anomalous oxidation of MDL 73,404 by horseradish peroxidase

3,4-Dihydro-6-hydroxy-N,N,N-2,5,7,8-heptamethyl-2H-1-benzopyran-2-ethanaminium-4-methylbenzene sulfonate (MDL 73,404) is a cardioselective water-soluble quaternary ammonium analogue of Vitamin E which is synthesized to augment the antioxidant defence in situations of free radical injury such as myocardial infarction/reperfusion. Its oxidation by any peroxidative enzyme has not been studied kinetically. This paper describes its enzymatic oxidation by horseradish peroxidase (HRP). The activity was followed spectrophotometrically at 255nm, and the experimental results were simulated using the program "KINETIC 3.1" for Windows 3.x. The MDL 73,404 was oxidized by horseradish peroxidase in the presence of H2O2 to its corresponding MDL 73,404 quinone. During this oxidation, the horseradish peroxidase showed an unexpectedly slow kinetic response with time, which contrast with the linear product accumulation curve measured with 2,2'-azino-bis-(3-estilbenzotiazol-6-sulfonic acid) (ABTS). This response was dependent on the respective concentrations of enzyme, MDL 73,404 and H2O2. However, when the enzyme was incubated with H2O2, the slow kinetic response disappeared and a lag period was observed. Furthermore, when p-coumaric acid (PCA) was added, the activity increased and the slow kinetic response became a straight line. In order to explain this anomalous behaviour, a kinetic model has been proposed and its differential equations simulated. From the correlation between experimental and simulated results it is concluded that MDL 73,404 can act as a slow response substrate for peroxidase, probably due to the presence of a quaternary ammonium side chain that confers on it a slow capacity to convert compound III into ferriperoxidase.

Stabilization of activity of oxidoreductases by their immobilization onto special functionalized glass and novel aminocellulose film using different coupling reagents

Glucose oxidase (GOD), horseradish peroxidase (HRP), and lactate oxidase (LOD) were covalently immobilized on special NH(2)-functionalized glass and on a novel NH(2)-cellulose film via 13 different coupling reagents. The properties of these immobilized enzymes, such as activity, storage stability, and thermostability, are strongly dependent on the coupling reagent. For example, GOD immobilized by cyanuric chloride on the NH(2)-cellulose film loses approximately half of its immobilized activity after 30 days of storage at 4 degrees C or after treatment at 65 degrees C for 30 min. In contrast, GOD immobilized by L-ascorbic acid onto the same NH(2)-cellulose film retains 90% of its initial activity after 1 year of storage at 4 degrees C and 92% after heat treatment at 65 degrees C for 30 min. Unlike GOD, in the case of LOD only immobilization on special NH(2)-functionalized glass, e.g., via cyanuric chloride, led to a stabilization of the enzyme activity in comparison to the native enzyme. The operational stability of immobilized HRP was up to 40 times higher than that of the native enzyme if coupling to the new NH(2)-cellulose film led to an amide or sulfonamide bond. Regarding the kinetics of the immobilized enzymes, the coupling reagent plays a minor role for the enzyme substrate affinity, which is characterized by the apparent Michaelis constant (K(M,app)). The NH(2)-functionalized support material as well as the immobilized density of the protein and/or immobilized activity has a strong influence on the K(M,app) value. In all cases, K(M,app) decreases with increasing immobilized enzyme protein density and particularly drastically for GOD.

Electrochemistry and electrocatalysis with heme proteins in chitosan biopolymer films

Protein-chitosan (CS) films were made by casting a solution of proteins and CS on pyrolytic graphite electrodes. Myoglobin (Mb), hemoglobin (Hb), and horseradish peroxidase (HRP) incorporated in CS films gave a pair of stable, well-defined, and quasi-reversible cyclic voltammetric peaks at about -0.33V vs saturated calomel electrode in pH 7 buffers, respectively, while catalase (Ct) in CS films showed a peak pair at about -0.46V which was not stable. All these peaks are located at the potentials characteristic of heme Fe(III)/Fe(II) redox couples of the proteins. The electrochemical parameters such as formal potentials (E degrees (')) and apparent heterogeneous electron-transfer rate constants (k(s)) were estimated by square-wave voltammetry with nonlinear regression analysis. Chitosan films contained considerable water and formed hydrogel in aqueous solution. Positions of the Soret absorbance band suggest that Mb and Hb in CS films keep their secondary structure similar to the native states in the medium pH range, while HRP and Ct retain their native conformation at least in the dry CS films. Scanning electron microscopy of the films demonstrated that interaction between the proteins and CS would make the morphology of dry protein-CS films very different from the CS films alone. Oxygen, trichloroacetic acid, nitrite, and hydrogen peroxide were catalytically reduced by all four proteins in CS films.

Renewable reagentless hydrogen peroxide sensor based on direct electron transfer of horseradish peroxidase immobilized on colloidal gold-modified electrode

A novel renewable reagentless hydrogen peroxide (H(2)O(2)) sensor based on the direct electron transfer of horseradish peroxidase (HRP) is proposed. The direct electrochemistry of HRP immobilized on a colloidal gold-modified carbon paste electrode (Au-CPE) was investigated using electrochemical methods. The immobilized HRP displayed a pair of redox peaks in 0.1M phosphate buffer (PB), pH 7.0, with a formal potential of -0.346 V. The response showed a surface-controlled electrode process with an electron transfer rate constant of 6.04+/-0.18s(-1) determined in the scan rate range from 120 to 500 mV/s. The biosensor displayed an excellent electrocatalytic response to the reduction of H(2)O(2) without the aid of an electron mediator. The sensor surface could be renewed quickly and reproducibly by a simple polish step. The calibration range of H(2)O(2) was 0-0.3mM with linear relation from 0.48 to 50 microM and a detection limit of 0.21 microM at 3 sigma. The response showed Michaelis-Menten behavior at higher H(2)O(2) concentrations. The K(app)(M) value of HRP at HRP-Au-CPE was determined to be 3.69+/-0.71 mM.

Extremely high stability of African oil palm tree peroxidase

A detailed kinetic study on thermal inactivation of African oil palm tree peroxidase (AOPTP) at different pHs has been carried out. The enzyme does not undergo inactivation over a broad range from pH 2 to 12 at ambient temperature. Complete inactivation of AOPTP is observed only at 70 degrees C and extremal pHs like <3.0 and >12.0, whereas under neutral conditions, its activity shows no changes. The study of AOPTP inactivation kinetics in the presence of dithiothreitol (DTT) and ethylenediaminetetraacetic acid (EDTA) showed that calcium ions, disulfide bonds and the interaction between apo-AOPTP and heme are important structural elements responsible for the enzyme stability. The guanidium hydrochloride (GdHCl)-induced inactivation of AOPTP indicated that the hydrogen-bonding network plays also a significant role in stabilizing the active structure of the enzyme. AOPTP is stable toward hydrogen peroxide treatment, especially under neutral conditions. The comparison of AOPTP stability to that of other peroxidases shows that AOPTP is the most stable peroxidase reported so far.

Mechanistic and molecular investigations on stabilization of horseradish peroxidase C

The enzyme horseradish peroxidase (HRP) shows a decreasing activity when the enzyme's substrate hydrogen peroxide is present with the degree of inactivation being dependent on the incubation time and the hydrogen peroxide concentration. Incubation times of some minutes do not inactivate the enzyme independent of the H2O2 concentration. After several hours, only 50% of the activity is found for a medium H2O2 excess, and a >100-fold excess of H2O2 completely inactivates the enzyme. Polymeric additives, in particular Gafquat, lead to higher residual activities, whereas stabilizers, such as aminopyrine, preserve the full activity. Circular dichroism (CD) measurements reveal that the enzyme structure remains more or less unchanged when hydrogen peroxide is added. Only when a 1000-fold excess of hydrogen peroxide is present are structural changes observed. UV spectra highlight that the heme group in the enzyme is affected by hydrogen peroxide in a first step. Without any prolonged incubation, a decrease of the Soret band to approximately 50% is found for low hydrogen peroxide concentrations (HRP/H2O2 from 1:1 to 1:100). Higher H2O2 concentrations lead to the formation of catalytically inactive HRP forms. Preincubation of Gafquat, which is a copolymer from vinylpyrrolidone and derivatized methyl methacrylate, with hydrogen peroxide shifts the influence of hydrogen peroxide to higher concentrations, the shift being dependent on the Gafquat concentration. This effect is not observed for other polymers, such as dextrans, but it is also found for the stabilizer aminopyrine. Extended incubation times (24 h) of HRP together with H2O2, however, lead to an at least partial recovery of the Soret band for lower H2O2 concentrations (H2O2/HRP from 1:1 to 1:100). When hydrogen peroxide is used in a >100 fold excess, the heme group is irreversibly destroyed, and even the characteristic band of cpd III is not found. Here, the presence of Gafquat only reduces the degree of destruction. Computer modeling of the interaction between the polymers and the enzyme shows no specific binding sites for the functional groups of the vinylpyrrolidone-methacrylate copolymer Gafquat or of DEAE-dextran on the enzyme, whereas for the only activating polymer, polyethylenimine clustering of binding sites is observed.

Suicide inactivation of peroxidases and the challenge of engineering more robust enzymes

As the number of industrial applications for proteins continues to expand, the exploitation of protein engineering becomes critical. It is predicted that protein engineering can generate enzymes with new catalytic properties and create desirable, high-value, products at lower production costs. Peroxidases are ubiquitous enzymes that catalyze a variety of oxygen-transfer reactions and are thus potentially useful for industrial and biomedical applications. However, peroxidases are unstable and are readily inactivated by their substrate, hydrogen peroxide. Researchers rely on the powerful tools of molecular biology to improve the stability of these enzymes, either by protecting residues sensitive to oxidation or by devising more efficient intramolecular pathways for free-radical allocation. Here, we discuss the catalytic cycle of peroxidases and the mechanism of the suicide inactivation process to establish a broad knowledge base for future rational protein engineering.

Mediated electrochemistry of horseradish peroxidase. Catalysis and inhibition

A precise determination of the complex mechanism of catalysis and inhibition involved in the reaction of HRP with H(2)O(2) as substrate and an outersphere single electron donor ([Os(bpy)(2)pyCl](+)) as cosubstrate is made possible by a systematic analysis of the cyclic voltammetric responses as a function of the scan rate and of the substrate and cosubstrate concentrations, complemented by spectrophotometric steady-state and stopped-flow experiments. The bell-shaped calibration curve relating the electrochemical response to the concentration of H(2)O(2) is qualitatively and quantitatively explained by taking into account the conversion of the catalytically active forms of the enzyme into the inactive oxyperoxidase in addition to the primary catalytic cycle. These characteristics should be kept in mind in biosensor applications of HRP. The ensuing analysis and data allow one to predict biosensor amperometric responses in all practical cases. From a mechanistic standpoint, conditions may, however, be defined which render inhibition insignificant, thus allowing an electrochemical characterization of the primary catalytic cycle. At very low concentrations of H(2)O(2), its diffusion tends to control the electrochemical response, resulting in proportionality with H(2)O(2) concentration instead of the square root dependence characteristic of the classical catalytic currents. Intriguing hysteresis and trace crossings behaviors are also quantitatively explained in the framework of the same mechanism. As a consequence of the precise dissection of the rather complex reaction mechanism into its various elementary steps, a strategy may be devised for gaining a better understanding of the mechanism and reactivity patterns of each elementary step.

An electrochemical multienzymatic biosensor for determination of cholesterol

This paper describes an electrochemical biosensor for free cholesterol monitoring. The sensor is a multienzymatic electrodic system in which horseradish peroxidase and cholesterol oxidase are simultaneously immobilized within a polymeric film, on the surface of a pyrolitic graphite electrode. From voltammetric and amperometric (flow-injection) data obtained, the efficiency, reproducibility and stability of the system are discussed. Results obtained, of interest for basic and applied biochemistry, represent a first step for construction of a mediator-free biosensor with potentialities for a successful application in the biosensor area.

Catalase-like activity of horseradish peroxidase: relationship to enzyme inactivation by H2O2

H2O2 is the usual oxidizing substrate of horseradish peroxidase C (HRP-C). In the absence in the reaction medium of a one-electron donor substrate, H2O2 is able to act as both oxidizing and reducing substrate. However, under these conditions the enzyme also undergoes a progressive loss of activity. There are several pathways that maintain the activity of the enzyme by recovering the ferric form, one of which is the decomposition of H2O2 to molecular oxygen in a similar way to the action of catalase. This production of oxygen has been kinetically characterized with a Clark-type electrode coupled to an oxygraph. HRP-C exhibits a weak catalase-like activity, the initial reaction rate of which is hyperbolically dependent on the H2O2 concentration, with values for K(2) (affinity of the first intermediate, compound I, for H2O2) and k(3) (apparent rate constant controlling catalase activity) of 4.0 +/- 0.6 mM and 1.78 +/- 0.12 s(-1) respectively. Oxygen production by HRP-C is favoured at pH values greater than approx. 6.5; under similar conditions HRP-C is also much less sensitive to inactivation during incubations with H2O2. We therefore suggest that this pathway is a major protective mechanism of HRP-C against such inactivation.

Direct electrochemistry of horseradish peroxidase immobilized on a colloid/cysteamine-modified gold electrode

Direct electron transfer of immobilized horseradish peroxidase on gold colloid and its application as a biosensor were investigated by using electrochemical methods. The Au colloids were associated with a cysteamine monolayer on the gold electrode surface. A pair of redox peaks attributed to the direct redox reaction of horseradish peroxidase (HRP) were observed at the HRP/Au colloid/cysteamine-modified electrode in 0.1 M phosphate buffer (pH 7.0). The surface coverage of HRP immobilized on Au colloid was about 7.6 x 10(-10) mol/cm(2). The sensor displayed an excellent electrocatalytic response to the reduction of H(2)O(2) without the aid of an electron mediator. The calibration range of H(2)O(2) was 1. 4 microM to 9.2 mM with good linear relation from 1.4 microM to 2.8 mM. A detection limit of 0.58 microM was estimated at a signal-to-noise ratio of 3. The sensor showed good reproducibility for the determination of H(2)O(2). The variation coefficients were 3. 1 and 3.9% (n = 10) at 46 microM and 2.8 mM H(2)O(2), respectively. The response showed a Michaelis-Menten behavior at higher H(2)O(2) concentrations. The K(app)(M) value for the H(2)O(2) sensor was found to be 2.3 mM.

The inactivation and catalytic pathways of horseradish peroxidase with m-chloroperoxybenzoic acid. A spectrophotometric and transient kinetic study

The kinetics of the catalytic cycle and irreversible inactivation of horseradish peroxidase C (HRP-C) reacting with m-chloroperoxybenzoic acid (mCPBA) have been studied by conventional and stopped-flow spectrophotometry. mCPBA oxidized HRP-C to compound I with a second order-rate constant k1 = 3.6 x 10(7) M-1 s-1 at pH 7.0, 25 degrees C. Excess mCPBA subsequently acted as a one-electron reducing substrate, converting compound I to compound II and compound II to resting, ferric enzyme. In both of these reactions, spectrally distinct, transient forms of the enzyme were observed (lambdamax = 411 nm, epsilon = 45 mM-1 cm-1 for compound I with mCPBA, and lambdamax = 408 nm, epsilon = 77 mM-1 cm-1 for compound II with mCPBA). The compound I-mCPBA intermediate (shown by near infrared spectroscopy to be identical to P965) decayed either to compound II in a catalytic cycle (k3 = 6.4 x 10(-3) s-1) or, in a competing inactivation reaction, to verdohemoprotein (ki = 3.3 x 10(-3) s-1). Thus, a partition ratio of r = 2 is obtained for the inactivation of ferric HRP-C by mCPBA. The intermediate formed from compound II with mCPBA is not part of the inactivation pathway and only decays via the catalytic cycle to give resting, ferric enzyme (k5 = 1.0 x 10(-3) s-1). The data are compared with those from earlier steady-state kinetic studies and demonstrate the importance of single turnover experiments. The results are discussed in terms of the physiologically relevant reactions of plant peroxidases with hydrogen peroxide.

Inactivation of horseradish peroxidase by phenol and hydrogen peroxide: a kinetic investigation

Inactivation of horseradish peroxidase (HRP) was examined in the presence of hydrogen peroxide alone and in the presence of hydrogen peroxide plus phenol. HRP is inactivated upon exposure to hydrogen peroxide (H2O2) by the combination of two possible pathways, dependent upon hydrogen peroxide concentration. At low H2O2 concentrations (below 1.0 mM in the absence of phenol), inactivation is predominantly reversible, resulting from the formation and accumulation of catalytically inert intermediate compound III. As H2O2 concentrations increase, an irreversible mechanism-based inactivation process becomes predominant. The overall inactivation comprised of both processes exhibits a second-order inactivation rate constant (kapp) of 0.023 +/- 0.005 M-1 s-1 at pH 7.4 and 25 degrees C. In the presence of both hydrogen peroxide fixed at 0.5 mM and phenol, HRP was inactivated in an irreversible, time- and phenol concentration-dependent process, also mechanism-based, with a kapp of 0.019 +/- 0.004 M-1 s-1.

A reagentless amperometric biosensor for alcohol detection in column liquid chromatography based on co-immobilized peroxidase and alcohol oxidase in carbon paste

A reagentless carbon paste electrode chemically modified with covalently bound alcohol oxidase and horse-radish peroxidase was examined as a selective sensor in flow injection and column liquid chromatography. A combination of carbodiimide, glutaraldehyde, and polyethyleneimine was used for immobilizing the enzymes in the paste. The surface of the electrodes was protected by first forming a layer of electropolymerized ortho-phenylenediamine followed by deposition of a cation exchange membrane (Eastman AQ 29D). The electrodes were used for detection of hydrogen peroxide, methanol, ethanol, propanol, isopropanol, and butanol. Preliminary investigations of the use of this sensor for bioprocess control are reported.

Electron spin resonance study of peroxidase activity and kinetics

An electron spin resonance (ESR) assay has been developed for peroxidase activity. The assay measures the formation of the paramagnetic nitroxide Tempol from the oxidation of its hydroxylamine derivative (TOLH) by short-lived radicals produced by peroxidase cycle intermediates, Compounds I and II. Using phenol as a peroxidase electron donor, the ESR approach is suitable for measurements of peroxidase activity ( > or = 0.003 U/ml) and micromolar quantities of H2O2 in sample sizes as small as 2 microliters. In addition, the ESR method can be used to continuously monitor activity in cell suspensions and other media that are susceptible to optical artifacts. The high membrane permeability of TOLH also makes it possible to estimate peroxidase activity in membrane-enclosed compartments, provided that TOLH oxidation rates can be stimulated with exogenous peroxidase reductants, e.g., phenol. Analysis of TOLH oxidation rates under conditions of low electron donor concentrations and high concentrations of H2O2 also shows clear indications of substrate-dependent inhibition and increased catalytic activity. Computer simulations indicate that the results obtained are consistent with the peroxidase reaction scheme proposed by Kohler et al. (1988, Arch. Biochem. Biophys. 264, 438-449) modified to correct for a nitroxide dependent stimulation of peroxidase catalytic activity.