The Catalytic Cycle of Horseradish Peroxidase

In some cases, the formation of compoimd I was accompanied by the oxidation of an amino acid residue instead of the porphyrin structure [53]. 

The investigation of the reaction of compoimd II with o-diphenols also revealed a two-step mechanism, in which the first step corresponds to the formation of an enzyme-substrate complex, and the second to electron transfer from the substrate to the iron atom. The size and the hydrophobicity of the substrate control their access to the hydrophobic binding site of HRP, but the electron density in the hydroxyl group of C-4 was foimd to be the most important feature for the electron-transfer step [68]. 

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Figure 4.3. The catalytic cycle of horseradish peroxidase with ferulate as reducing substrate. The rate constants Ki, K2, and K3 represent the rate of compound I formation, rate of compound I reduction, and rate of compound II reduction, respectively.

Fig. 5. Catalytic cycle of horseradish peroxidase. The overall charge on the resting state 10 and Compound I (II) is plus one (the dot and the positive charge oa 11 indicate the radical state and electron deficiency of the n electron system of the porphyrin ring), while Compound II (12) is neutral. Adapted from Ref. [20]...

The peroxidase activity of PGHS is comparable to that of better known peroxidases such as horseradish peroxidase (HRP). The catalytic cycle of HRP is shown in Figure 5 [9], Its first step is the formation of an intermediate very often found in hemoproteins by transfer of an oxygen atom from various oxygen atom donors to the Fe(III) heme (Eq. 6). It is a high-valent iron-oxo species, at least formally a Fe(V)=0 complex. In fact, the detailed electronic structure of this intermediate depends on the environment of the heme provided by the protein. In HRP, this intermediate (called compound I) is a (porphyrin radical-cation)-Fe(IV)=0 complex, as shown by many spectroscopic techniques [9],... 

The catalytic cycle of peroxidases (Fig. 5) begins with the oxidation of the high-spin, pentacoordinate ferric native enzyme 10) by hydrogen peroxide to form a semi-stable intermediate called Compound I (//). Compound I is a high-valent oxo-iron complex that is two oxidation equivalents above ferric horseradish peroxidase. Although formally an Fe heme. Compound 1 is generally thought to be an Fe porphyrin 71-cation radical [51, 52]. 

We investigated the kinetics of the catalytic cycle of single horseradish peroxidase enzymes when hydrogen peroxide (H2O2) as an electron donor is processed for oxidizing (dihydro)Rhodamine 123, thereby generating Rhodamine 123 (Rhl23) (30 and references therein). 

One of the most used systems involves use of horseradish peroxidase, a 3-diketone (mosl commonly 2,4-pentandione), and hydrogen peroxide." " " Since these enzymes contain iron(II), initiation may involve decomposition of hydrogen peroxide by a redox reaction with formation of hydroxy radicals. However, the proposed initiation mechanism- involves a catalytic cycle with enzyme activation by hydrogen peroxide and oxidation of the [3-diketone to give a species which initiates polymerization. Some influence of the enzyme on tacticity and molecular... 

Extensive studies have established that the catalytic cycle for the reduction of hydroperoxides by horseradish peroxidase is the one depicted in Figure 6 (38). The resting enzyme interacts with the peroxide to form an enzyme-substrate complex that decomposes to alcohol and an iron-oxo complex that is two oxidizing equivalents above the resting state of the enzyme. For catalytic turnover to occur the iron-oxo complex must be reduced. The two electrons are furnished by reducing substrates either by electron transfer from substrate to enzyme or by oxygen transfer from enzyme to substrate. Substrate oxidation by the iron-oxo complex supports continuous hydroperoxide reduction. When either reducing substrate or hydroperoxide is exhausted, the catalytic cycle stops. 

Rodriguez-LopezJ. N. Lowe D. J. Hemandez-RuizJ. Hiner A. N. P. Garcia-Canovas F. Thomeley R. N. F. Mechanism of reaction of hydrogen peroxide with horseradish peroxidase identification of intermediates in the catalytic cycle. J. Am. Chem. Soc. 2001, 123, 11838-11847. 

Peroxidases are known to catalyze the one electron oxidation of a wide range of structurally diverse organic and inorganic compounds, Fig. (1). The catalytic cycle, first described by Chance [26] for horseradish peroxidase, may be described as follows. H2O2 oxidizes the ferric form of the enzyme (Felll), in a two electronic oxidation, to yield the enzyme intermediate compound I (Col) ... 

Another example on fluorescence lifetimes of beinoproteins peroxidases (donor H2O2, oxidoreductase E.C. 1.11.1.7) are heme enzymes that catalyze oxidative reactions that use hydrogen perox dase as an electron acceptor. The seed coat soybean peroxidase (SBP) belongs to class III of the plant peroxidase super family, which includes horseradish (HRP), barely (BPl) and peanut (PNP) peroxidases. All the en mes of this class contain a protoheme located within a pocket that plays an important role in the catalytic cycle. Soybean peroxidase (SBP) is a glycoprotein of molecular mass equal to 37 kDa. It is a monomer composed of 326 amino acids with a single tryptophan at position 117 (Figure 7.15). 

The MnP catalytic cycle resembles that of the classical heme peroxidase horseradish peroxidase [141], and also LiP [142], A two-electron transfer from hydrogen peroxide to native MnP results in MnP Compound I, which can then perform a one... 

Electrode processes in experiments with redox indicators involved one or a few electrons and were, therefore, inherently low-yield reactions. Recently catalytic processes have been used to collect as many electrons as possible ([327, 454, 455] E. Palecek, M. Fojta, and L. Havran, unpublished). Thorp [327] used a soluble mediator that moved close to G residues present only in target DNA (but absent in the probe) and shuttled electrons to the polymer-modified ITO electrode. The reduced form of the mediator [Ru(bipy)3] + was oxidized by holding the electrode at a sufficiently positive potential. The oxidized form of the mediator removed electrons from G residues, generating reduced [Ru(bipy)3] + and completing a catalytic cycle. About 100 electrons per hybridized G could be collected under favorable conditions. Horseradish peroxidase coupled to target DNA was applied to detect the hybridization by electrocatalytic reduction of hydrogen peroxide... 

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