Peroxidase catalytic cycle

Fig. 5.2 Generic peroxidase catalytic cycle. The square of four nitrogens around the iron atom is a representation of the prosthetic heme group of the peroxidase...

All the catalytic intermediates of the peroxidase catalytic cycle as well as CIII present characteristic spectroscopic properties, which provide invaluable information on the structure of the porphyrin and its ligands. Here we discuss the evidence regarding the structure of Compound III. 

Fig. 10.4. Peroxidase catalytic cycle. Four pathways are shown for the return of Compound I to the resting state (1) oxidative dehydrogenation, where RH is the substrate (2) oxidative halogenation, where X" is a halogen ion and RH is the substrate (3) peroxide disproportionation, and (4) oxygen transfer, where R is the substrate.

Heme-containing peroxidases thus belong to the rather restricted group of the redox enzymes for which DET has been shown [11,12]. If, however, the electronic commnnication between the cofactor of the peroxidase and the electrode is slow, small redox molecnles can be exploited as mediators to carry the electrons between the enzyme and the electrode. This mediated electron transfer (MET) proceeds in accordance with the peroxidase catalytic cycle (reactions 1-3), where the role of the mediator can be carried by the reducing substrate AH. The differ-... 

Scheme 5 Side reactions in the peroxidase catalytic cycle... 

Arajno, J.C., Prieto, T, Prado, RM., Trindade, RJ., and Nunes, G.L. Peroxidase catalytic cycle of MCM41 entrapped microperoxidase-11 as a mechanism for phenol oxidation, JMiwoScfAa/io Tech 7(10), 3643-52 (2007). 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

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... 

Figure 2-9. Reaction scheme for the complete catalytic cycle in glutathione peroxidase (left). Numbers represent calculated reaction barriers using the active-site model. The detailed potential energy diagram for the first elementary reaction, (E-SeH) + H2O2 - (E-SeOH) + H2O, calculated using both the active-site (dashed line) and ONIOM model (grey line) is shown to the right (Adapted from Prabhakar et al. [28, 65], Reprinted with permission. Copyright 2005, 2006 American Chemical Society.)...

MnP is the most commonly widespread of the class II peroxidases [72, 73], It catalyzes a PLC -dependent oxidation of Mn2+ to Mn3+. The catalytic cycle is initiated by binding of H2O2 or an organic peroxide to the native ferric enzyme and formation of an iron-peroxide complex the Mn3+ ions finally produced after subsequent electron transfers are stabilized via chelation with organic acids like oxalate, malonate, malate, tartrate or lactate [74], The chelates of Mn3+ with carboxylic acids cause one-electron oxidation of various substrates thus, chelates and carboxylic acids can react with each other to form alkyl radicals, which after several reactions result in the production of other radicals. These final radicals are the source of autocataly tic ally produced peroxides and are used by MnP in the absence of H2O2. The versatile oxidative capacity of MnP is apparently due to the chelated Mn3+ ions, which act as diffusible redox-mediator and attacking, non-specifically, phenolic compounds such as biopolymers, milled wood, humic substances and several xenobiotics [72, 75, 76]. 

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. 

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.

WRF are key regulators of the global C-cycle. Some WRF produce all three LME, while others produce only one or two of them [10]. The main LME are oxidor-eductases, that is two types of peroxidases, LiP and MnP, and a phenoloxidase Laccase. Catalytic cycles of peroxidases and laccases are given in Figs. 1 and 2, respectively. LME are produced by WRF during their secondary metabolism. 

Fig. 1 Generic scheme of the catalytic cycle of peroxidases (taken from [24])...

Substitution of zinc(ll) ions into cytochrome c peroxidase (ZnCcP) has been used to exploit photoactivation of electron transfer (eT) reactions since the mid-1990s. The ZnCcP triplet state ( ZnCcP) reduces Fe(III) cytochrome c, and then back electron transfer recombines the charge separation to complete the catalytic cycle (see Figure 7.36). 

Such an involvement of an amino acid side-chain ligand switch within each catalytic cycle was a novel proposal and as such needs to be scrutinized by a variety of experimental procedures as well as analysis in the context of information known for cytochrome cd nitrite reductase from another source (see later discussion). However, it is interesting to note that something similar has been proposed for the protocate-chuate 3,4-dioxygenase enzyme from Pseudomonas putida (15). On the other hand, bacterial cytochrome c peroxidase offers an example where ligand switching seemingly relates only to an activation phenomenon. 

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