Peroxidase oxidation mechanisms

It has been pointed out earlier that peroxidases oxidize hydrogen peroxide by two-electron transfer mechanism to form Compound I. Thus for MPO, we have ... 

Kreps, E.M., "fyurin, V.A., Gorbunov, N.V., Maksimovich, A.A., Polyakov, V.N., Plyusnin, V.V. and Kagan, V.E. (1986). Activation of peroxidase oxidation of lipids during migrational stress in gorbuscha possible mechanism of adaptation (In Russian). Doklady Akademii Nauk SSSR 286,1009-1012. 

Another important aspect of peroxidase reactions is the relation between the substrate one-electron redox potential and the redox potential of compound I and compound II, since this restricts the number of possible redox partners (see Chap. 4 for a detailed description). Table 6.1 reports the redox potentials of some selected peroxidases as it can be seen, the values span an interval ranging from 1.35 V for reduction of myeloperoxidase (MPO) compound I to 1.0 V for reduction of HRP compound I [13-15]. But the selection of the preferred enzyme for a given radical reaction must consider not only the complementarities in the redox potentials but also the mechanism preferred by the enzyme, since some peroxidases, such as CPO and MPO, and also LPO in some cases, react through a two-electron oxidation mechanism. 

Guengerich FP, Yun CH, Macdonald TL. Evidence for a 1-electron oxidation mechanism in N-dealkylation of N,N-dialkylanilines by cytochrome P450 2B1. Kinetic hydrogen isotope effects, linear free energy relationships, comparisons with horseradish peroxidase, and studies with oxygen surrogates. J Biol Chem 1996 271 27321-9. 

As one may expect from the peroxidase reaction mechanism described in Eqs. (2-4), the reactivity of the enzyme intermediates towards a particular substrate may be estimated a priori on the basis of the thermodynamic driving force of these two electron-transfer reactions, which is directly related with the difference between the oxidation/reduction potentials of both the enzyme active intermediates (i.e.. Col and Coll) and the substrate radicals. Thus, the thermodynamic driving force for the reaction of Col (or Coll) with the reducing substrates is the difference between the mid-point potentials of the CoI/CoII (or CoII/Felll) and the substrate radical/substrate (R, Hr/RH) redox couples ... 

CuZnSOD has been demonstrated to act as a peroxidase, oxidizing various substrates, among them nitrite (to NO2) [57] and relatively bulky molecules such as 5,5-dimethyl- 1-pyrroline N-oxide (DMPO to DMPO-OH), tyrosine (to dityrosine) or 2,2 -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS to a cation radical). However, the enzyme becomes inactivated in the presence of H2O2. Apparently, hydrogen peroxide reduces Cu- at the active site of the enzyme. Subsequent reaction of H2O2 with Cu generates a potent oxidant (Cu+-0 or Cu + CH) that can attack the adjacent histidine residue and thus inactivate the enzyme, or oxidize an alternative substrate [3]. Most authors assume that the mechanism of the peroxidative action of CuZnSOD, significantly accelerated in the presence of bicarbonate, consists in oxidation of bicarbonate to a carbonate radical anion able to oxidize other substrates [58,59] ... 

There are two possible routes by which electron transfer could result in the oxidation of the methyl substituent on heme O. The first is via outer-sphere electron transfer, as depicted in Figure 5. In this mechanism, the cofactor heme B binds and activates O2 to form compound I, and then heme O is oxidized via a peroxidase-type mechanism. In the second, related mechanism, HAS oxidizes heme O via autoxidation. In this case, heme O binds and activates O2 to form compound I, while heme B is presumably involved in shuttling electrons from a putative ferredoxin to the active site. Heme O would then be oxidized by internal electron transfer, similar to the mechanism of heme cross-linking elucidated by Ortiz de Montellano and coworkers (22). While the labeling experiments of HAS strongly suggest that heme O is oxidized via electron transfer, they do not allow us to distinguish between these two possible scenarios, and additional experiments are required. 

Besides the ordinary HoO -consuming oxidation, peroxidase also catalyzes Oo-consuming oxidation (the peroxidase-oxidase reaction). In recent years considerable attention has been directed to elucidating the peroxidase-oxidase mechanism. Controversy was centered about the participation of ferrous enzyme in O2 activation. Is peroxidase reduced to the ferrous state during the reaction 3,13,19, 21, 23) Does peroxidase compound III, which appears in the reaction, correspond to oxygenated ferroperoxidase (3, 5, 15) If so, is O2 in Compound III activated (15) As discussed here, these problems seem to be almost solved, and it is very likely that the peroxidase-oxidase reaction is a good model for analyzing the mechanism of other oxidases. 

The electrochemical and enzymic (peroxidase) oxidations outlined in this section clearly support the view that electrochemical techniques can provide useful information regarding the pathways and mechanisms of enzymic redox reactions. Work currently underway in the author s laboratory on the electrochemical and enzymic oxidation of guanine, adenine, hypoxanthine, xanthine, and various nucleoside and nucleotide species indicates that there is a considerable parallelism between the electrochemical and enzyme-catalyzed processes. 

Peroxidase-catalyzed polymerization behavior of coniferyl alcohol has been compared with that by laccase (285). Peroxidase oxidized the substrate faster than laccase in the presence of hydrogen peroxide. As to the laccase-catalyzed polymerization, the oxidation rate and reaction mechanism depended on the enzyme origin. 

Varner, 1988). Extensin is secreted as single soluble monomeric hydroxyproline forms that are slowly insolubilized in the cell wall, probably via the oxidative formation of isodityrosine cross-links (Cooper and Varner, 1983 Fry, 1986a). Wall-catalyzed cross-linking of soluble extensin is inhibited by ascorbate, indicating that this reaction is dependent on an oxidative mechanism (Cooper and Varner, 1984). Thus, it is probable that a cell wall-bound peroxidase-ascorbate oxidase system controls the redox state at the wall, and hence the extensibility of cell wall, by controlling the cross-linking of wall glycoproteins. 

The data reviewed above clearly show that ascorbate and urate may contribute substantially to the anti-oxidative balance in bodi pre- and post-pasteurized milk. Moreover, die results clearly show that peroxidases and xanthine oxidase are prominent enzymes in relation to oxidation of ascorbate and urate, and that die oxidative mechanisms responsible for oxidation of ascorbate and urate are very conqilex. 

Figure 7. Proposed mechanism for peroxidase oxidation of EGCG and EGC.

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... 

Martmez-Parra, J. and Munoz, R., An approach to the characterization of betanine oxidation catalyzed by horseradish peroxidase, J. Agric. Food Chem., 45, 2984, 1997. Martmez-Parra, J. and Munoz, R., Characterization of betacyanin oxidation catalyzed by a peroxidase from Beta vulgaris L. roots, J. Agric. Food Chem., 49, 4064, 2001. Ashie, l.N.A. Simpson, B.K., and Smith, J.P., Mechanisms for controlling enzymatic reactions in foods, Crit. Rev. Food Sci. Nutr., 36, 1, 1996. 

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