Enzymes phenol-oxidizing

Enzymic phenolic oxidation of reticuline has given a 4% yield of the corresponding tetrahydrobenzylisoquinoline hydroxylated at the C-a site. An interesting case of selective oxidation of a tetrahydrobenzylisoquinoline involves ferricyanide oxidation of (19) to a mixture of diastereoisomeric dienones (20). Acid-catalysed rearrangement then gave the pentaoxygenated derivative (21). 

Pentosan solutions gel when treated with hydrogen peroxide/peroxidase. This is due to the presence of low levels of ferulic acid (ca. 0.2%). An enzymic phenol oxidation occurs (cf. Fig. 15.18), which causes polymerization. This results in build-up of a network which, along with the low content of branched arabinofuranose, is responsible for the lack of solubility of most pentosans. 

Oxidative Reactions. The majority of pesticides, or pesticide products, are susceptible to some form of attack by oxidative enzymes. For more persistent pesticides, oxidation is frequently the primary mode of metaboHsm, although there are important exceptions, eg, DDT. For less persistent pesticides, oxidation may play a relatively minor role, or be the first reaction ia a metaboHc pathway. Oxidation generally results ia degradation of the parent molecule. However, attack by certain oxidative enzymes (phenol oxidases) can result ia the condensation or polymerization of the parent molecules this phenomenon is referred to as oxidative coupling (16). Examples of some important oxidative reactions are ether cleavage, alkyl-hydroxylation, aryl-hydroxylation, AJ-dealkylation, and sulfoxidation. 

An oxidative activation of the lignin also can be achieved in a biochemical way by adding enzymes (phenol oxidase laccase) to the spent sulfite liquor, whereby... 

Reported redox potentials of laccases are lower than those of non-phenolic compounds, and therefore these enzymes cannot oxidize such substances [7]. However, it has been shown that in the presence of small molecules capable to act as electron transfer mediators, laccases are also able to oxidize non-phenolic structures [68, 69]. As part of their metabolism, WRF can produce several metabolites that play this role of laccase mediators. They include compounds such as /V-hvdi oxvacetan i I ide (NHA), /V-(4-cyanophenyl)acetohydroxamic acid (NCPA), 3-hydroxyanthranilate, syringaldehyde, 2,2 -azino-bis(3-ethylben-zothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), violuric acid, 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpipperidin-iV-oxide radical and acetovanillone, and by expanding the range of compounds that can be oxidized, their presence enhances the degradation of pollutants [3]. 

The monophenolase activity of PPO is generally defined as the first step in the melaniza-tion pathway and consists of the o-hydroxylation of the monophenol to odiphenol. This activity distinguishes PPO from other phenol-oxidizing enzymes, such as laccase and peroxidase, and is characterized by the following facts ... 

Zilly A, de Souza CGM, Barbosa-Tessmann IP, Peralta RM (2002) Decolourisation of industrial dyes by a Brazilian strain of Pleurotus pulmonarius producing laccase as the sole phenol-oxidizing enzyme. Folia Microbiol 47 315-319... 

Enzymatic browning. Phenol-oxidizing enzymes (such as tyrosinase and peroxidase) oxidize tyrosine residues into reactive quinone derivatives, which will condense into colored polymers (melanins). Melanins are rich in carboxyl groups and therefore have high affinity for divalent metal ions such as calcium. 

An alternative mechanism for the oxidation of phenolic compounds is enzyme-catalyzed oxidation. Several classes of enzymes can catalyze this reaction. According to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB), these enzymes are part of the E C. 1 class of oxidoreductases (see the Internet web site http //www.chem.qmul.ac.uk/iubmb/enzyme/ECl). The three main classes of enzymes that catalyze the oxidation of phenolic compounds are the oxidoreductases that use oxygen as electron acceptor (E.C. 1.10.3), the peroxidases (E.C. 1.11.1), and monophenol monooxygenase (E.C. 

The phenol-oxidizing enzyme tyrosinase has two types of activity (/) phenol o-hydroxylase (cresolase) activity, whereby a monophenol is converted into an o-diphenol via the incorporation of oxygen, and (2) cathecholase activity, whereby the diphenol is oxidized. The two reactions are illustrated in Figure 2-6, in the conversion of tyrosine (2.40) to L-DOPA (3,4-dihydroxyphenylalanine (2.41), dopaquinone (2.42), and indole-5,6-quinone carboxylate (2.43), which is further converted to the brown pigment... 

Trusov (1915,1916), as indicated in Section 1.4.1, introduced the concept that polyphenols and quinones contributed to the synthesis of HS. Subsequently, as the result of enzymatic oxidation (with oxidase enzymes from microorganisms), the phenols oxidize to quinones and these, through further condensation, are converted into dark-colored humic-type substances. 

Aitken MD, Massey IJ, Chen T, Heck PE. Characterization of reaction products from the enzyme catalyzed oxidation of phenolic pollutants. Water Res 1994 28 1879-1889. 

The efficiency of the operation is conditioned by three main factors (1) active enzyme concentration, (2) mass transfer rate, and (3) operational parameters, such as plugging. Mass transfer rate depends in large part upon the linear velocity of the fluid through the bed. Hence, in order to maximize the efficiency, high L/D ratios are required, which may also reduce back mixing. However, the length of the bed is limited by the pressure that immobilized particles may withstand [116]. Flow rate, L/D ratio, pH, and temperature are some of the operational parameters that should be optimized for an efficient operation. In Shukla et al., a fixed bed reactor is used for phenol oxidation by HRP [87]. At least three cycles were needed for 80% phenol removal under the optimal L/D ratio, HRT, temperature, and substrate concentration. 

The oxidation of phenols and other organic substrates by MnP Is dependent on Mnll (23,30). Apparently the enzyme first oxidizes Mnll to Mnlll, and Mnlll subsequently oxidizes the organic substrates (30,31,47). As shown In Figure 3, addition of one equivalent of Mnll rapidly reduces MnP compound I to compound II (41). A second equivalent of Mnll reduces MnP compound II to the native ferric enzyme. Similarly, MnP compound I Is reducible by phenolic substrates, albeit at a slower rate. However, phenolic substrates are not able to reduce MnP compound II efficiently (41). Thus the enzyme Is unable to complete Its catalytic cycle efficiently In the absence of Mnll. This would seem to explain the absolute Mnll requirement for catalytic activity. In the conversion of MnP compound I to compound II, the porphyrin tt-cation Is reduced back to a normal porphyrin. This suggests that the porphyrin radical Is exposed In a peripheral site as recently suggested for HRP (48) and that this site may be available to organic substrates and to Mnll. In contrast, the FeIV 0 center of MnP compound II may be partially burled and only available to Mnll Ions. 

Zenk, M.H., Gerardy, R. and Stadler, R. (1995) Phenol oxidative coupling of ben-zylisoquinoline alkaloids is catalyzed by regio- and stereo-selective cytochrome P450 linked plant enzymes salutaridine and berbamrmine.. Chem. Soc. Chem. Com-mun., pp. 1725-27. 

Many past studies on the enzymes of lignin decomposition focused on phenol-oxidizing enzymes, such as laccase and peroxidase, produced by white-rot fungi. It is unlikely, however, that this type of activity is important in structural decomposition (33), although the enzymes may have some other role in lignin decomposition (28). 

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