Other Peroxidases

UV-Vis spectroscopic studies showed that PANI enzymatically synthesized was able to be de-doped and re-doped by treatment with aqueous ammonia solution and CSA solution, respectively, which allowed using these nanofibers in ammonia sensing devices. 

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Nitric oxide and nitrite react with other peroxidase enzymes such as horseradish peroxidase (HRP) (138a,139), lactoperoxidase (138a) and eosinophil peroxidase (140) similarly. The rate constants for reaction of NO with compounds I and II in HRP were found to be 7.0 x 105M 1s 1 and 1.3 x 106M 1s 1, respectively (139). Catalytic consumption of NO as measured by an NO sensitive electrode in the presence of HRP compounds I and II is shown in Fig. 5 where accelerated consumption of NO is achieved even in deoxygenated solutions (140). 

The detailed mechanism of P aeruginosa CCP has been studied by a combination of stopped-flow spectroscopy (64, 65, 84, 85) and paramagnetic spectroscopies (51, 74). These data have been combined by Foote and colleagues (62) to yield a quantitative scheme that describes the activation process and reaction cycle. A version of this scheme, which involves four spectroscopically distinct intermediates, is shown in Fig. 10. In this scheme the resting oxidized enzyme (structure in Section III,B) reacts with 1 equiv of an electron donor (Cu(I) azurin) to yield the active mixed-valence (half-reduced) state. The active MV form reacts productively with substrate, hydrogen peroxide, to yield compound I. Compound I reacts sequentially with two further equivalents of Cu(I) azurin to complete the reduction of peroxide (compound II) before returning the enzyme to the MV state. A further state, compound 0, that has not been shown experimentally but would precede compound I formation is proposed in order to facilitate comparison with other peroxidases. 

Lignin peroxidase, secreted by the white-rot fungus Phanerochaete chrysosporium in response to nutrient deprivation, catalyzes the H202-dependent oxidation of non-phenolic aromatic substrates. The present report summarizes the kinetic and structural characteristics of lignin peroxidase isozymes. Our results indicate that the active site of lignin peroxidase is more electron deficient than other peroxidases. As a result, the redox potential of the heme active site is higher, the heme active site is more reactive and the oxycomplex is more stable than that of other peroxidases. Also discussed is the heme-linked ionization of lignin peroxidase. 

Compound I is a two-electron oxidized enzyme intermediate containing a oxyferryl iron and a porphyrin cation radical while compound II is an one-electron oxidized intermediate (13), With lignin pa oxidase, as with other peroxidases, the substrate oxidation products are fir radicals which undergo nonenzymatic disproportionation reactions to give rise to the final products. 

Effect of pH on Lignin Peroxidase Catalysis. The oxidation of organic substrates by lignin peroxidase (Vmax) has a pH optimum equal to or possibly below 2. Detailed studies have been performed on the pH dependency of many of the individual reactions involved in catalysis. The effect of pH on the reaction rates between the isolated ferric enzyme, compounds I or II and their respective substrates has been studied. Rapid kinetic data indicate that compound I formation from ferric enzyme and H2O2 is not pH dependent from pH 2.5-7.5 (75,16). Similar results are obtained with Mn-dependent peroxidase (14). This is in contrast to other peroxidases where the pKa values for the reaction of ferric enzyme with H2O2 are usudly in the range of 3 to 6 (72). 

However, the distal histidine apparently has no effect on lignin and Mn-dependent peroxidase compound I formation. Although all active site amino acid residues that are proposed to participate in compound I formation of peroxidase (37) are conserved in lignin and Mn-dependent pa oxidases, the lack of pH dependence may be a result of some inherent structural and conformational differences between lignin and Mn-dependent peroxidases and other peroxidases. 

Figure 2. Comparison of P. chrysosporium MnP-1 and other peroxidases at regions near the proximal and distal histidines. The peroxidase sequences used were manganese peroxidase (MnP) (20), cytochrome c peroxidase (CCP) QS), horseradish peroxidase (HRP)...

For foreign compounds, the majority of oxidation reactions are catalyzed by monooxygenase enzymes, which are part of the mixed function oxidase (MFO) system and are found in the SER (and also known as microsomal enzymes). Other enzymes involved in the oxidation of xenobiotics are found in other organelles such as the mitochondria and the cytosol. Thus, amine oxidases located in the mitochondria, xanthine oxidase, alcohol dehydrogenase in the cytosol, the prostaglandin synthetase system, and various other peroxidases may all be involved in the oxidation of foreign compounds. 

Kinetic studies have shown that the product formed in the reaction of the fully oxidized enzyme with hydrogen peroxide is catalytically inactive. Reaction of the half-reduced enzyme with hydrogen peroxide leads to an enzymatically active compound, in which the Fe" heme is oxidized to Fe, and the FeIU heme is oxidized to the FeIV ferryl species. No stoichiometric formation of a radical species is observed, unlike the case for other peroxidases. The peroxide-oxidized enzyme will then oxidize two molecules of reduced cytochrome c. Mechanistic details are still unclear, particularly with regard to the interaction between the two heme groups, a phenomenon revealed by ESR studies.1373... 

After the first discovery of the asymmetric sulfoxidation by Kobayashi et al. [226], it could be shown that a large number of aryl alkyl sulfides are oxygenated with enantiomeric excesses higher than 98% [227-229]. Other peroxidases also catalyze this reaction. Interestingly, the plant peroxidase HRP [230] yields the (S)-sulfoxide, whereas mammalian myeloperoxidase [223] and lactoperoxidase [231] catalyze the formation of the R-enantiomers. The stereospecific sulfoxidation of aryl alkyl sulfides by purified toluene dioxygenase (TDO) from P. putida was also studied in this context [232] and showed that sulfoxidation yielded the (S)-sulfoxides in 60-70% yield, whereas CPO under the same conditions yielded 98% (R)-sulfoxides (Scheme 2.15). CPO is thus again an exception from the rule in that it produces R-enantiomeric sulfoxides, besides its bacterial origin [227]. The reason for this behavior lies in the... 

Most other peroxidases are Fe-heme-containing systems, which function as two-electron redox catalysts (Scheme 8). Dihydrogen peroxide oxidizes the Fe-heme moiety by two electrons, forming Compound 1 (a heme + FeIV=0 species) [97], Compound 1 oxidizes the halide ion, forming the active halogenating species. This mechanism cannot be operative in V-BrPO because the vanadium is already in its highest accessible oxidation state. Moreover, native V-BrPO does not oxidize bromide without an acceptable peroxide source. However, it should... 

These observations suggest that vitamin E may play a regulatory role in PG biosynthesis by controlling the formation of key intermediates such as hydroperoxides and cyclic endoperoxides. In these experiments, peroxidase activity associated with the PGH synthase could not be measured because of the contamination of other peroxidases like cyt P-450 hydroperoxidase in crude microsomal preparations. Our attempts to measure the differences in catalytic rates employing indomethacin were not successful. Nevertheless, it is not too unreasonable to assume that probably both activities of PGH synthase are equally affected by vitamin E deficiency, since both require heme as cofactor. 

Other peroxidases have a similar mechanism, but peroxidases vary significantly in terms of their rate constants and their susceptibility to side reactions that may cause temporary or permanent inactivation. 

Other peroxidase enzymes have been found to halogenate various substrates. Thus lac-toperoxidase catalyzes the bromination of tyrosine, thyroglobulin and bovine serum albumin380, and myeloperoxidase brominates nucleic acids and related compounds381. 

Block with three percent hydrogen peroxide or other peroxidase blocking reagent. Using a new bottle of hydrogen peroxide, perform a three percent H202 peroxidase block, followed by DAB and an appropriate counterstain. 

Several simple halogenated pyrroles are produced by marine organisms. Even though the oceans are richer in chloride than bromide, most marine halogenated compounds, of any type, contain bromine rather than chlorine. This is likely a consequence of the greater susceptibility of bromide to enzymatic oxidation by bromoperoxidase or other peroxidases. 

Compared with plant and other peroxidases, white-rot fungal peroxidases are characterized by their high redox potential, related to the architecture of the heme environment (see Chap. 4). This is required to perform their role in nature, namely the oxidative biodegradation of the recalcitrant lignin polymer present in the cell wall of all vascular plants [30-32], By contrast, one of the roles of plant peroxidases... 

Notwithstanding the phenol dimers are builder compounds with respect to the starting substrate, thus making their access to the peroxidase active site more difficult, they have a lower redox potential and compete with monomeric phenols in the reaction with compound I and compound II. Furthermore, the phenoxy radical can oxidize a phenol dimer to its radical form. This results in the formation of oligomeric and then polymeric compounds at longer reaction times [17]. Thus, if the products of interest are the diphenolic compounds, the reaction must be carried out in mildest conditions and only the products formed in the initial phase have to be collected. But in some cases [18-20], the o.o -biphenyl is the principal product of phenol oxidation, such as with p-cresol. tyrosine, etc. All peroxidases can be employed for these reactions, but HRP is usually preferred with respect to other peroxidases due to its higher availability and to its broad specificity. Whole cell... 

Other peroxidases, albeit non-heme, are able to catalyze similar oxidations. CPO from Pseudomonas pyrrocinia has been successfully employed for the preparation of the antibiotic pyrrolnitrine [60]. The amino group of the precursor molecule is directly transformed into a nitro group by the CPO active species (Fig. 6.4b). 

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