Horseradish peroxidase, reaction cycle

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... 

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. 

Horseradish peroxidase (and urease) played an important role in the development of the modern concept of the nature of an enzyme and the role of metal ions (Sumner and Somers, 1943 Willstatter, 1965). The species now known as compound II (HRP-II) formed as a result of the reaction of HRP with H202, was discovered in 1937 (Keilen and Mann, 1937). Later compound I (HRP-I), formed prior to HRP-II was identified (Theorell, 1941). The spectra of HRP-I and HRP-II in the 400 nm (Soret band) region have been determined (Chance, 1949 a, b) and measurements have also been extended to the visible region (Chance, 1952). Formation of HRP-I is first order in H202 and HRP (Chance, 1943) and the -OOH group is essential for the oxidation of HRP by peroxide. The enzymatic cycle can be summarised by the following equations (George, 1952),... 

On reaction with a stoichiometric amount of hydroperoxide, catalase and horseradish peroxidase are converted to a green colored intermediate. Compound I (5). The chemical nature of Compound I has been extensively debated since its discovery by Theorell 59). Recently, Dolphin et al. 60) have demonstrated that upon one-equivalent oxidation several metalloporphyrins are converted to stable porphyrin jr-cation radicals, the absorption spectra of which possess the spectral characteristics of Compound I, namely, a decreased Soret w-n transition and an appearance of the 620-670-nm absorption bands. Since Moss et al. 61) proposed the presence of Fe(IV) in Compound I of horseradish peroxidase from Mossbauer spectroscopic measurements, it is attractive to describe Compound I as Fe(IV)-P, where P is a porphyrin w-cation radical. Then, Compound I and Compound ES become isoelectronic. Both contain Fe(IV) and a radical the former as a porphyrin radical (P ) and the latter as a protein radical (R ). Then the reaction cycles of horseradish and cytochrome c peroxidases may be compared as shown in Fig. 4. 

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... 

The use of biocatalysis for synthetic chemistry is significantly important for reducing the environmental footprint of chentical processes. The possibility of setting up a cascade of enzyme-catalyzed reactions in the same pot is very attractive. In nature, many biochemical transformations are achieved through a combination of several different proteins [65]. For example, the enzymes in mitochondria were settled on the surface of a compartment, in its interior, in its membrane or in any combination of these for the citric acid-catalyzed cycle. Van Dongen et al. mimicked this method to design one porous polymersome to anchor enzymes at three different locations in their lumen (glucose oxidase, GO ), in their bilayer membrane Candida antarctica lipase B, CalB), and on their surface (horseradish peroxidase, HRP). As shown in Scheme 8.22, a mixture of block... 

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). 

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