Peroxidases catalyzed reactions

Figure 24 Peroxidase reaction catalyzed by 13G10-Fe(ToCPP) and 7G12-Fe(MPIX) porphyrin-antibody complexes and X-ray structure of the 7G12-MPIX complex.

H2O2 was 2.9 x 10-6 M. A requirement for O2 could not be detected, suggesting that 02 is not involved in the luminescence reaction. The luciferase was found to be a peroxidase that catalyzes peroxidation of the luciferin, and it can be substituted with horseradish peroxidase. 

Peroxidases are found in milk and in leukocytes, platelets, and other tissues involved in eicosanoid metabolism (Chapter 23). The prosthetic group is protoheme. In the reaction catalyzed by peroxidase, hydrogen peroxide is reduced at the expense of several substances that will act as electron acceptors, such as ascorbate, quinones, and cytochrome c. The reaction catalyzed by peroxidase is complex, but the overall reaction is as follows ... 

It participates in the decomposition of potentially toxic hydrogen peroxide in the reaction catalyzed by glutathione peroxidase (Ghapter 20). 

Isoflavones have been implicated in goiter induction. Soybean extracts inhibit reactions catalyzed by thyroid peroxidase (TPO), essential to the synthesis of thyroid hormones (Divi et al., 1997). Genistein and daidzein (at about 1-10 p,M of IC50) may act as alternative substrates for tyrosine iodination (Divi et al., 1997). Furthermore, genistein and daidzein have also been shown to cause the irreversible inactivation of TPO in the presence of hydrogen peroxide. Genistein also inhibits thyroxine synthesis in the presence of iodinated... 

The enzymatic reactions of peroxidases and oxygenases involve a two-electron oxidation of iron(III) and the formation of highly reactive [Fe O] " species with a formal oxidation state of +V. Direct (spectroscopic) evidence of the formation of a genuine iron(V) compound is elusive because of the short life times of the reactive intermediates [173, 174]. These species have been safely inferred from enzymatic considerations as the active oxidants for several oxidation reactions catalyzed by nonheme iron centers with innocent, that is, redox-inactive, ligands [175]. This conclusion is different from those known for heme peroxidases and oxygenases... 

Mechanism of the Interaction of Peroxidases with Hydroperoxides Role of Superoxide and Hydroxyl Radicals in Reactions Catalyzed by Peroxidases... 

The mechanism of free radical formation in the reactions catalyzed by MPO and other heme peroxidase may be presented as follows [180] ... 

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

It should be mentioned that the MP0Fe(II)02 intermediate is very similar to the analogous hydroxylating species of cytochrome P450. Thus, the participation of hydroxyl radicals in the reactions catalyzed by peroxidases remains questionable. 

Furthermore, it was suggested that nitrite is a physiological substrate for mammalian peroxidases because it may compete with other physiological substrates (Cl-, Br, and SCN ) in the reactions catalyzed by MPO, EPO, and LPO. 

The iodinated tyrosine residues monoiodotyrosine (MIT) and diiodoty-rosine (DIT) combine (couple) to form iodothyronines in reactions catalyzed by thyroid peroxidase. Thus, two molecules of DIT combine to form T4, and MIT and DIT join to form T3. 

At the same time as the Belousov-Zhabotinsky reaction provided a chemical prototype for oscillatory behavior, the first experimental studies on the reaction catalyzed by peroxidase [24] and on the glycolytic system in yeast (to be discussed in Section 111) demonstrated the occurrence of biochemical oscillations in vitro. These advances opened the way to the study of the molecular bases of oscillations in biological systems. 

Peroxidase Source Catalyzed Reaction Biological Functions... 

The peroxidase-catalyzed transformations are classified as (1) the peroxidase reaction (Table 2, entries 1 -4), (2) the peroxygenase reaction (Table 2, entries 4-6) [25], and (3) the oxidase reaction (Table 2,entries 7-9). 

The majority of reactions catalyzed by horseradish peroxidase can be summarized by Eq. (1), in which AH2 represents a reducing substrate. 

Expression of peroxidases that catalyze crosslinking reactions can he induced by wounding or infection. Well-documented examples include the production of acidic peroxidase isoenzymes in woimded tissue of tobacco (132), potato (133), and tomato (134), and also in wheat plants infected with the powdery mildew fungus, Erysiphe graminis (135). It is known that peroxidase activity can be induced on wounding of horseradish leaves (56), but other mechanisms of defense in horseradish remain to be demonstrated. 

In contrast, the physiological significance of the reaction catalyzed by bacterial cytochrome c peroxidase is not so clear. The reaction is... 

In reactions catalyzed by peroxidases one cannot easily avoid the presence of hydrogen peroxide as this is the natural oxidant employed by the enzyme. However, it is necessary to maintain the steady-state concentration of hydrogen peroxide as... 

Hoft reported about the kinetic resolution of THPO (16b) by acylation catalyzed by different lipases (equation 12) °. Using lipases from Pseudomonas fluorescens, only low ee values were obtained even at high conversions of the hydroperoxide (best result after 96 hours with lipase PS conversion of 83% and ee of 37%). Better results were achieved by the same authors using pancreatin as a catalyst. With this lipase an ee of 96% could be obtained but only at high conversions (85%), so that the enantiomerically enriched (5 )-16b was isolated in poor yields (<20%). Unfortunately, this procedure was limited to secondary hydroperoxides. With tertiary 1-methyl-1-phenylpropyl hydroperoxide (17a) or 1-cyclohexyl-1-phenylethyl hydroperoxide (17b) no reaction was observed. The kinetic resolution of racemic hydroperoxides can also be achieved by chloroperoxidase (CPO) or Coprinus peroxidase (CiP) catalyzed enantioselective sulfoxidation of prochiral sulfides 22 with a racemic mixmre of chiral hydroperoxides. In 1992, Wong and coworkers and later Hoft and coworkers in 1995 ° investigated the CPO-catalyzed sulfoxidation with several chiral racemic hydroperoxides while the CiP-catalyzed kinetic resolution of phenylethyl hydroperoxide 16a was reported by Adam and coworkers (equation 13). The results are summarized in Table 4. 

As already reported in Section II.A.2, the enzymes chloroperoxidase (CPO) and Copri-nus peroxidase (CiP) catalyze the enantioselective oxidation of aryl alkyl sulfides. If a racemic mixture of a chiral secondary hydroperoxide is used as oxidant, kinetic resolution takes place and enantiomerically enriched hydroperoxides and the corresponding alcohols can be obtained together with the enantiomerically enriched sulfoxides. An overview of the results obtained in this reaction published by Wong and coworkers, Hoft and... 

The peroxidase-catalyzed oxidation of ABTS (103) by H2O2 can also be performed by hydroperoxides, yielding a green-colored free radical. This reaction, catalyzed by HRP-C, was applied to the identification of an oxidation product of skatole isolated by HPLC, as a hydroperoxide (40). This assignment was confirmed by MS analysis (Section V.C.3) . 

A typical chemical system is the oxidative decarboxylation of malonic acid catalyzed by cerium ions and bromine, the so-called Zhabotinsky reaction this reaction in a given domain leads to the evolution of sustained oscillations and chemical waves. Furthermore, these states have been observed in a number of enzyme systems. The simplest case is the reaction catalyzed by the enzyme peroxidase. The reaction kinetics display either steady states, bistability, or oscillations. A more complex system is the ubiquitous process of glycolysis catalyzed by a sequence of coordinated enzyme reactions. In a given domain the process readily exhibits continuous oscillations of chemical concentrations and fluxes, which can be recorded by spectroscopic and electrometric techniques. The source of the periodicity is the enzyme phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate by ATP, resulting in the formation of fructose-1,6 biphosphate and ADP. The overall activity of the octameric enzyme is described by an allosteric model with fructose-6-phosphate, ATP, and AMP as controlling ligands. 

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