Peroxygenase

In plant tissues, various enzymes convert the hydroperoxides produced by LOX to other products, some of which are important as flavor compounds. These enzymes include hydroperoxide lyase, which catalyzes the formation of aldehydes and oxo acids hydroperoxide-dependent peroxygenase and epoxygenase, which catalyze the formation of epoxy and hydroxy fatty acids, and hydroperoxide isomerase, which catalyzes the formation of epoxyhydroxy fatty acids and trihydroxy fatty acids. LOX produces flavor volatiles similar to those produced during autoxidation, although the relative proportions of the products may vary widely, depending on the specificity of the enzyme and the reaction conditions. 

In addition to a well-known NADPH-dependent hydroxylation mechanism (Reaction (2)), cytochrome P-450 is able to catalyze the oxidation of substrates by peroxygenase mechanism (Reaction (8)) where XOOH presents the peroxy compound acting as the oxygen donor. 

The peroxidase-catalyzed transformations are classified as (1) the peroxidase reaction (Table 2, entries 1 -4), (2) the peroxygenase reaction (Table 2, entries 4-6) [25], and (3) the oxidase reaction (Table 2,entries 7-9). 

As mentioned above, a prosthetic group of Mb, heme b, is the same as that of some heme peroxides such as HRP and CcP, and there are two characteristic histidines, the proximal and distal, in the heme pocket. However, the peroxidase and peroxygenase activities of Mb are much lower than those observed for the native heme peroxidase, which were discussed in Section V. In addition, Mb has... 

The initial turnover number of the thioanisole oxidation by the reconstituted Mb is clearly larger than that observed for the native Mb, and the kcat/Km value is more than seven fold higher than that of the native protein. The epoxidation of styrene is also effectively catalyzed by the reconstituted Mb by 10-fold increase compared to native Mb. These findings indicate that the appropriate modification of the heme propionate side chains forms a substrate-binding domain that enhances the peroxidase and peroxygenase activities of Mb, as shown in Table VI. 

As mentioned above, both the point-mutation on the distal side of the Mb and the modification of the heme-propionate side chains are effective to convert the Mb into peroxidase and peroxygenase. Thus, one can imagine that the combination of an amino acid mutation and a modified-heme reconstitution may more effectively allow us a new catalyst by oxygen storage. Recently, two examples, which demonstrated the hybrid modification of Mb, have been reported. One is the T67R/S92D Mb reconstituted with the modified hemin where a histidine is linked at the terminal of the heme-propionate side chain (108). The peroxidase activities toward p-hydroxyphenylpropionic acid and tyramine oxidations by the reconstituted mutant Mb are increased by 24- and 2.3-fold, respectively, based on the kciJKm value compared to those observed for the native Mb. 

CPO due to its ability to transfer oxygen not only to typical CPO substrates but also to different aromatics, such as naphthalene or benzene [45, 46]. Therefore, the name aromatic peroxygenase has been suggested for the A. aegerita heme-thiolate peroxidase [83, 84]. 

Although CPO was first describe in 1966 [85] it was not until 1995 that its crystal structure was solved [27]. The crystal structure of the novel heme-thiolate aromatic -peroxygenase from A. aegerita has not been yet published, therefore, the CPO molecular structure is described herein as representative for the heme-thiolate peroxidases. 

Pecyna MJ, Ullrich R, Bittner B et al (2009) Molecular characterization of aromatic peroxygenase from Agrocybe aegerita. Appl Microbiol Biotechnol 84 885-897... 

Savenkova MI, Kuo JM, Ortiz de Montellano PR (1998) Improvement of peroxygenase activity by relocation of a catalytic histidine within the active site of horseradish peroxidase. Biochemistry 37 1828-10836... 

Matsunaga I, Sumimoto T, Ayata M, Ogura H (2002) Functional modulation of a peroxygenase cytochrome P450 novel insight into the mechanisms of peroxygenase and peroxidase enzymes. FEBS Lett 528 90-94... 

Ozaki S, Matsui T, Watanabe Y (1996) Conversion of myoglobin into a highly stereospecific peroxygenase by the L29FI/H64L mutation. J Am Chem Soc 118 9784-9785... 

The oxygen transfer reactions catalyzed by peroxidases are very interesting for synthetic applications. The overall reaction for this peroxygenase activity is... 

The P450 shunt pathway and peroxygenase activity of peroxidases share identical overall reaction equations. P450s generally have high Km values for H 202 values of 15 mM... 

The P450 scaffold has evolved to serve as a ubiquitous monooxygenase. Nature manipulated this structure to create an immense library of P450s with finely-tuned activities and specificities. The CPO scaffold has not become a ubiquitous oxidation catalyst, but it supports high peroxygenase activity and may be superior to P450s for chemical transformations of certain substrates, particularly when it has been evolved in the laboratory for optimal performance in these applications. 

One set of site-directed mutagenesis studies improved the peroxygenase activity of Mb [128] in a different random mutagenesis study the peroxidase activity was improved. Clearly these reactions are very different from one another, and neither is myoglobin s natural function. Here is another example where an alternative scaffold supports functions that are characteristic of other conserved structures. Heme enzyme function is flexible and highly evolvable. 

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