Hemoproteins peroxidase reactions

McCarthy and White have noted the ability of a series of seven hemoproteins, P-450lm2, P-450lm4, P-420lm2, HRP, CPO, catalase, and metmyoglobin, as well as hemin to catalyze a set of five oxidative reactions (57, 55). These reactions are typical of the peroxidase reaction [oxidation of pyrogallol to purpuro-gallin, Eq. (8)] and three characteristic P-450 reactions (aliphatic hydroxylation, aromatic hydroxylation, and olefin epoxidation). 

The enzyme horseradish peroxidase is a hemoprotein and the region of the Soret band exhibits large differences between the position and extinction coefficients of the uncombined and combined forms. Both forms were first studied by spectrophotometry, but the E—S complexes were 0 labile that they could not be examined extensively by any other spectroscopic method. Using rapid-scanning spectrophotometry and rapid mixing, Chance was able to distinguish the spectra of compound I and II and determine the various rate constants of the multistep reaction with rather poor precision. 

Peroxidases (EC 1.11.1.7). Peroxidases are hemoproteins, produced mainly by microorganisms and plants, which catalyze oxidation of the recalcitrant nonphenolic lignin units in the presence of hydrogen peroxide (Duran and Esposito, 2000). This is possible because of the formation of a high redox potential oxo-ferryl intermediate during the reaction of the heme cofactor with H202 (Martinez et al., 2005). Dubey et al. (1998) studied the polymerization of catechol by plant peroxidases and found that the resultant polymers consisted of phenylene and oxyphenylene units (Figure 2.14). 

The reaction sequence at the heme active site starts with the binding of unactivated triplet dioxygen forming the so-called oxy-heme complexes. The iron center in 02-activating heme enz5maes is then thought to be converted into a peroxo anion species. It can be protonated to form a ferric hydroperoxo intermediate usually termed compormd 0 (183), which is a crucial reactive species in catalase and peroxidase enz5nne catalysis (Fig. 21). These hydroperoxo intermediates of hemoproteins are important... 

Figure 6 The most common paradigm for hemoprotein-catalyzed substrate oxidation involves heterolytic scission of the 0-0 bond of an iron-bound peroxo species to give an Fe(IV) = O ferryl intermediate and either a porphyrin radical or a protein radical. The peroxo intermediate is generated in the cytochromes P450 by in situ NAD(P)H-dependent reduction of O2, and in the peroxidases by reaction with H2O2.

Electron-transfer reactions of amines are of significant importance in biochemical systems. Enzymes known to catalyze the oxidative dealkylation of amines include monoamine oxidase [16, 17], cytochrome-P450 [18, 184-186], horseradish peroxidase [187], hemoproteins [188, 189], and chloroperoxidase [187, 188]. N-dealkylation of amines by peroxidases are generally accepted to occur via one-electron transfer, whereas the role of electron transfer in reactions catalyzed by enzymes such as monoamine oxidase [16, 17] and cytochrome P-450 [18, 184, 185] is currently a topic of debate. 

In the editorial preface to the first volume of Advances in Catalysis the decision was made known not to publish reviews of specialized topics in biocatalysis but from time to time to bring reports in which the relationship and parallelism between this special field and normal catalysis are discussed. This is the first of these reports. Its purpose is to examine the reactions of four hemoproteins, hemoglobin, myoglobin, peroxidase, and catalase, which all contain the same coordination compound of iron—ferrous or ferric protoporphyrin attached to different protein molecules, with oxygen, hydrogen peroxide, and in a few cases additional reducing substances. Some of these reactions are specific ... 

In studying the reaction of the heme group in hemoproteins it is always necessary to ensure that no alteration in protein structure is affecting the results. Complete denaturation is easily recognized by precipitation and flocculation and the liberation of heme of hematin, but minor changes can also occur. In the case of peroxidase two forms were isolated by Theorell (25) and named peroxidase I and II. The first of these, now called paraperoxidase (Theorell, 26), has been found to have undergone some alteration possibly the removal of the carbohydrate portion of the protein molecule or possibly reversible denaturation to a slight extent (Keilin and Hartree, 48). The paraperoxidase shows enzymatic activity comparable to that of the intact enzyme. 

It has been shown above that the catalytic action of catalase and peroxidase is intimately connected with the ability of these hemoproteins to form complexes with hydrogen peroxide (or alkyl peroxides). By choosing the experimental conditions the existence of three different complexes can be demonstrated spectroscopically. The chemical nature of these complexes is as yet unknown, and mechanisms have been represented as bimolecular reactions between substrate and complex. 

It is the reaction of the ferric forms of these hemoproteins with hydrogen peroxide that provides the sharpest contrast to ionic iron. Complexes are formed in both systems. Although ferric ion only gives an ion pair complex Fe02H2+, whereas peroxidase can give three com-... 

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