Two-State Reactivity, Electromerism, Tautomerism, and “Surprise” Isomers in the Formation of Compound II of the Enzyme Horseradish Peroxidase from the Principal Species, Compound I

The rate constants for the reactions of horseradish peroxidase compound I (k1) and compound II (k2) with three 4-substituted arylboronic acids, which enhance chemiluminescence in the horseradish peroxidase catalyzed oxidation of luminol by hydrogen peroxide, were determined at pH 8.6, total ionic strength 0.11 M, using stopped-flow kinetic measurements. For comparison, the rate constants of the reactions of 4-iodophenol with compounds I and II were also determined under the same experimental conditions. The three arylboronic acid derivatives and their rate constants are: 4-biphenylboronic acid, k1 = (1.21 ± 0.08) × 106 M−1 s−1, k2 = (4.6 ± 0.2) × 105 M−1 s−1; 4-bromophenylboronic acid, k1 = (5.5 ± 0.2) × 104 M−1 s−1, k2 = (3.6 ± 0.2) × 104 M−1 s−1; and 4-iodophenylboronic acid, k1 = (1.1 ± 0.2) × 105 M−1 s−1, k2 = (1.3 ± 0.1) × 104 M−1 s−1. 4-Biphenylboronic acid, which shows comparable luminescent enhancement to 4-iodophenol, has the highest reactivity in the reduction of both compounds I and II among the three arylboronic acid derivatives tested.

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Scavenging of oxygen radicals by heme peroxidases.

Acta Biochimica Polonica ◽

10.18388/abp.1996_4463 ◽

1996 ◽

Vol 43 (4) ◽

pp. 673-678 ◽

Cited By ~ 2

Author(s):

L Gebicka ◽

J L Gebicki

Keyword(s):

Horseradish Peroxidase ◽

Hydroxyl Radicals ◽

Superoxide Radical ◽

Cation Radical ◽

Compound I ◽

Superoxide Radical Anion ◽

Form Compound ◽

Activity Loss ◽

Heme Peroxidases ◽

Compound Ii

The reactions of two heme peroxidases, horseradish peroxidase and lactoperoxidase and their compounds II (oxoferryl heme intermediates, Fe(IV) = O or ferric protein radical Fe(III)R.) and compounds III (resonance hybrids [Fe(III)-O2-. Fe(II)-O2] with superoxide radical anion generated enzymatically or radiolytically, and with hydroxyl radicals generated radiolytically, were investigated. It is suggested that only the protein radical form of compound II of lactoperoxidase reacts with superoxide, whereas compound II of horseradish peroxidase, which exists only in oxoferryl form, is unreactive towards superoxide. Compound III of the investigated peroxidases does not react with superoxide. The lactoperoxidase activity loss induced by hydroxyl radicals is closely related to the loss of the ability to form compound I (oxoferryl porphyrin pi-cation radical, Fe(IV) = O(Por+.) or oxoferryl protein radical Fe(IV) = O(R.)). On the other hand, the modification of horseradish peroxidase induced by hydroxyl radicals has been reported to cause also restrictions in substrate binding (Gebicka, L. & Gebicki, J.L., 1996, Biochimie 78, 62-65). Nevertheless, it has been found that only a small fraction of hydroxyl radicals generated homogeneously does inactivate the enzymes.

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Studies on Horseradish Peroxidase. XII. A Kinetic Study of the Oxidation of Sulfite and Nitrite by Compounds I and II

Canadian Journal of Chemistry ◽

10.1139/v73-089 ◽

1973 ◽

Vol 51 (4) ◽

pp. 588-596 ◽

Cited By ~ 67

Author(s):

R. Roman ◽

H. B. Dunford

Keyword(s):

Horseradish Peroxidase ◽

Ground State ◽

Ph Dependence ◽

Intermediate Formation ◽

Ion Concentration ◽

Compound I ◽

Hydrogen Ion Concentration ◽

Ph Range ◽

Compound Ii ◽

Kinetics Of

The kinetics of the oxidation of sulfite and nitrite by horseradish peroxidase compounds I and II have been studied as a function of pH at 25° and ionic strength 0.11. The pH dependence of the rate of the reaction between compound I and sulfite over the pH range 2–7 is interpreted in terms of two ground state enzyme dissociations with pka values of 5.1 and 3.3, and that for the compound II reaction with sulfite in terms of a single ground state enzyme dissociation with a pKa value of 3.9. Whereas the reaction between compound I and sulfite produces the native enzyme without the intermediate formation of compound II, the reaction of compound I with nitrite yields compound II. The second-order rate constants for the reactions of compounds I and II with nitrite increase linearly with increasing hydrogen ion concentration over the pH range 6–8.

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Mechanism of inhibition of horseradish peroxidase-catalysed iodide oxidation by EDTA

Biochemical Journal ◽

10.1042/bj2980281 ◽

1994 ◽

Vol 298 (2) ◽

pp. 281-288 ◽

Cited By ~ 11

Author(s):

D K Bhattacharyya ◽

S Adak ◽

U Bandyopadhyay ◽

R K Banerjee

Keyword(s):

Horseradish Peroxidase ◽

Kinetic Studies ◽

Spectral Shift ◽

Spectral Studies ◽

Isosbestic Point ◽

Dependent Manner ◽

Compound I ◽

Iodide Oxidation ◽

Hyperfine Splitting Constant ◽

Compound Ii

EDTA inhibits horseradish peroxidase (HRP)-catalysed iodide oxidation in a concentration and pH-dependent manner. It is more effective at pH 6 than at lower pH values. A plot of log Kiapp. values as a function of pH yields a sigmoidal curve from which a pKa value of 5.4 can be calculated for an ionizable group on the catalytically active HRP for EDTA inhibition. Among the structural analogues of EDTA, tetramethylethylenediamine (TEMED) is 80% as effective as EDTA, whereas the EDTA-Zn2+ chelate and EGTA are ineffective. Kinetic studies indicate that EDTA competitively inhibits iodide oxidation. Spectral studies show that EDTA can quickly reduce compound I to compound II, but reduction of preformed compound II to the native enzyme is relatively slow, as demonstrated by the time-dependent spectral shift from 417 nm to 402 nm through an isosbestic point at 408 nm. Under steady-state conditions, in a reaction mixture containing HRP, EDTA and H2O2, the enzyme remains in the compound-II form, with absorption maxima at 417, 527 and 556 nm. Direct evidence for one-electron oxidation of EDTA by HRP intermediates is provided by the appearance of an e.s.r. signal of a 5,5-dimethyl-1-pyrroline N-oxide (spin trap)-EDTA radical adduct [aN (hyperfine splitting constant) = 1.5 mT] in e.s.r. studies. The signal intensity, however, decreases in the presence of iodide. The KD of the HRP-EDTA complex obtained from optical difference spectroscopy increases with an increase in iodide concentration, and the double-reciprocal plot for EDTA binding indicates that EDTA and iodide compete for the same binding site for oxidation. We suggest that EDTA inhibits iodide oxidation by acting as an electron donor.

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Studies on Horseradish Peroxidase. XI. On the Nature of Compounds I and II as Determined from the Kinetics of the Oxidation of Ferrocyanide

Canadian Journal of Chemistry ◽

10.1139/v73-088 ◽

1973 ◽

Vol 51 (4) ◽

pp. 582-587 ◽

Cited By ~ 213

Author(s):

M. L. Cotton ◽

H. B. Dunford

Keyword(s):

Horseradish Peroxidase ◽

Rate Constant ◽

Active Sites ◽

Ph Dependence ◽

Order Rate Constant ◽

Second Order ◽

Compound I ◽

Order Rate ◽

Compound Ii ◽

Kinetics Of

In order to investigate the nature of compounds I and II of horseradish peroxidase, the kinetics were studied of ferrocyanide oxidation catalyzed by these compounds which were prepared from three different oxidizing agents. The pH dependence of the apparent second-order rate constant for ferrocyanide oxidation by compound I, prepared from ethyl hydroperoxide and m-chloroperbenzoic acid, was interpreted in terms of an ionization on the enzyme with a pKa = 5.3, identical to that reported previously for hydrogen peroxide. The second-order rate constant for the compound II-ferrocyanide reaction also showed the same pH dependence for the three oxidizing substrates. However, with more accurate results, the compound II-ferrocyanide reaction was reinterpreted in terms of a single ionization with pKa = 8.5. The same dependence of ferrocyanide oxidation on pH suggests structurally identical active sites for compounds I and II prepared from the three different oxidizing substrates.

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Horseradish peroxidase. XXIX. Reactions in water and deuterium oxide: cyanide binding, compound I formation, and reactions of compounds I and II with ferrocyanide

Canadian Journal of Chemistry ◽

10.1139/v78-468 ◽

1978 ◽

Vol 56 (22) ◽

pp. 2844-2852 ◽

Cited By ~ 52

Author(s):

H. Brian Dunford ◽

W. Donald Hewson ◽

Håkan Steiner

Keyword(s):

Hydrogen Peroxide ◽

Horseradish Peroxidase ◽

Kinetic Isotope Effect ◽

Deuterium Oxide ◽

Isotope Effects ◽

Kinetic Isotope Effects ◽

Compound I ◽

Kinetic Isotope ◽

Cyanide Binding ◽

Compound Ii

The kinetics of the reactions of hydrogen peroxide and cyanide with native horseradish peroxidase, as well as reactions of compounds I and II with ferrocyanide have been studied in ordinary water and in deuterium oxide at 25 °C and ionic strength 0.11 using a stopped-flow apparatus. Rate constants for all reactions were measured over a wide range of acidity in both solvents from which equilibrium and kinetic isotope effects were evaluated. Protonation of an ionizable group on the enzyme with a pKa value of 4.15 ± 0.05 in water inhibits the reactions with both hydrogen peroxide and cyanide. A significant kinetic isotope effect, kH/kD = 1.6 ± 0.1, was measured for compound I formation whereas no significant kinetic isotope effect was found for cyanide binding. On the basis of these findings, a partial mechanism for compound I formation is proposed in which the group of pKa 4.15 plays a crucial role. The pH dependencies of the ferrocyanide reaction in the pH interval 4.5–10.8 confirmed the role of an acid group with a pKa of 5.2 for compound I and for compound II a pKa of 8.6 and another with a value lower than that encompassed by the pH range of the study. Equilibrium isotope effects were found but no kinetic isotope effects for either the reaction of compound I or of compound II This suggests that there are no rate-limiting proton transfers in the reactions between ferrocyanide and compounds I and II of horseradish peroxidase. The only reducing substrates which exhibit positive kH/kD values possess a labile proton.

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Titration Study of Guaiarcol Oxidation by Horseradish Peroxidase

Canadian Journal of Biochemistry ◽

10.1139/o75-090 ◽

1975 ◽

Vol 53 (6) ◽

pp. 649-657 ◽

Cited By ~ 35

Author(s):

Marius Santimone

Keyword(s):

Hydrogen Peroxide ◽

Electron Transfer ◽

Low Temperature ◽

Horseradish Peroxidase ◽

Catalytic Amount ◽

Reaction Scheme ◽

General Mechanism ◽

Compound I ◽

Compound Ii ◽

Titration Study

Titration of guaiacol by hydrogen peroxide in the presence of a catalytic amount of horseradish peroxidase shows that the reduction of hydrogen peroxide proceeds by the abstraction of two electrons from a guaiacol molecule. In the same way, it can be demonstrated that 0.5 mol of guaiacol can reduce, at low temperature, 1 mol of peroxidase compound I to compound II. Moreover, the reaction between equal amounts of compound I and guaiacol at low temperature produces the native enzyme. A reaction scheme is proposed which postulates that two electrons are transferred from guaiacol to compound I giving ferriperoxidase and oxidized guaiacol with the intermediary formation of compound II. The direct two-electron transfer from guaiacol to compound I without a dismutation of product free radicals must be considered as an exception to the general mechanism involving a single-electron transfer.

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Oxidation of 4-tert-Butylcatechol and Dopamine by Hydrogen Peroxide Catalysed by Horseradisch Peroxidase

Biological Chemistry ◽

10.1515/bc.1999.085 ◽

1999 ◽

Vol 380 (6) ◽

Cited By ~ 8

Author(s):

M. García-Moreno ◽

M. Moreno-Conesa ◽

J.N. Rodríguez-López ◽

F. García-Cánovas ◽

R. Varón

Keyword(s):

Hydrogen Peroxide ◽

Electron Transfer ◽

Horseradish Peroxidase ◽

Resting State ◽

Intermediate Compound ◽

Transfer Reactions ◽

Single Electron Transfer ◽

Compound I ◽

Compound Ii ◽

Enzyme Intermediate

AbstractThe catalytic cycle of horseradish peroxidase (HRP; donor:hydrogen peroxide oxidoreductase; EC 1.11.1.7) is initiated by a rapid oxidation of it by hydrogen peroxide to give an enzyme intermediate, compound I, which reverts to the resting state via two successive single electron transfer reactions from reducing substrate molecules, the first yielding a second enzyme intermediate, compound II. To investigate the mechanism of action of horseradish peroxidase on catechol substrates we have studied the oxidation of both 4-

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Horseradish peroxidase. XLII. Oxidations of L-tyrosine and 3,5-diiodo-L-tyrosine by compound II

Canadian Journal of Biochemistry ◽

10.1139/o80-170 ◽

1980 ◽

Vol 58 (11) ◽

pp. 1270-1276 ◽

Cited By ~ 16

Author(s):

Isobel M. Ralston ◽

H. Brian Dunford

Keyword(s):

Horseradish Peroxidase ◽

Reaction Rate ◽

Acid Group ◽

Amino Groups ◽

Compound I ◽

Ph Profile ◽

Rate Versus ◽

Phenolic Group ◽

Ph Range ◽

Compound Ii

The oxidations of both L-tyrosine and 3,5-diiodo-L-tyrosine by compound II of horseradish peroxidase were studied over the pH range of approximately 3 to 10 at 25 °C and at a constant ionic strength of 0.11. The rate versus pH profile for the tyrosine – compound II reaction illustrates the influences of at least two acid group ionizations. An enzyme dissociation (pKa ~ 6.2) has a small effect on the reaction rate; whereas, a second pKa of 9.2, which may be attributed to either the enzyme or substrate, has a greater influence on the rate. The oxidation of tyrosine by compound II is fastest at pH 7.6. In the case of the diiodotyrosine – compound II reaction, three acid dissociations are necessary to describe the plot of log (kapp) versus pH. These include two enzyme pKa values of 3.6 and 8.6, and one substrate pKa of 6.6. The rate optimum for the reaction occurs at pH 5.2 and deprotonation of the phenolic group of diiodotyrosine results in a dramatic decrease in kapp. Diiodotyrosine is required in only a 0.5 M equivalent for the conversion of horseradish peroxidase compound I to compound II. The diiodotyrosine pKa values were estimated as 6.4 and 9.4 for the phenolic and amino groups, respectively.

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Horseradish peroxidase. XXXIV. Oxidation of compound II to I by periodate and inorganic anion radicals

Canadian Journal of Biochemistry ◽

10.1139/o79-137 ◽

1979 ◽

Vol 57 (8) ◽

pp. 1080-1083 ◽

Cited By ~ 4

Author(s):

A. Nadezhdin ◽

H. B. Dunford

Keyword(s):

Horseradish Peroxidase ◽

Ultraviolet Light ◽

Room Temperature ◽

Sodium Periodate ◽

Compound I ◽

Inorganic Anion ◽

Temperature Irradiation ◽

The One ◽

Compound Ii ◽

Anion Radicals

The one-electron oxidation of horseradish peroxidase compound II to compound I by sodium periodate was observed. The bimolecular rate constant for the NaIO4–compound II interaction is equal to 9.5 ± 1 × 10−3 M−1 s−1 at room temperature. Irradiation, using ultraviolet light, of the solution containing compound II and persulfate in the presence of bicarbonate, chloride, or bromide, leads to the fast accumulation of compound I due to the oxidative action of [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] anion radicals, which are products of the photolysis.

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