Phenol o-hydroxylase

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... 

By the mid to late seventies the activities of approximately fifty enzymes purportedly had been demonstrated in soils. In recent years the list has been extended to include amidase, phosphodiesterase 15>39>54 adenosine triphosphatase, phenol o-hydroxylase, hydrogenase and... 

Phenol o-hydroxylase is an NADPH-dependent flavin mono-oxygenase which catalyses the oxidation of phenols to o-diphenols. Wainwright described a soil enzyme assay based on the colorimetric measurement of substrate (phenol) disappearance. Soils were incubated for Ih with phenol (O.OIM) in a citrate -phosphate buffer pH 4.0 (optimum) at 37°C (temperature optimum 40-45°C). Under these conditions soil activities were linear with time and the rates of reaction were proportional to the amounts of soil. Soil activities were stimulated by Cu (purified phenol o-hydroxylase has a Cu requirement). 

WAIN 7RIGHT M. 1979. Assay of phenol o-hydroxylase activity in soil. Soil Biology and Biochemistry, 1, 549-551. 

The flavoprotein phenol-2-hydroxylase (EC 1.14.13.7) and the o-phenol-splitting catechol-1,2-oxygenase (EC 1.13.1.1) are prepared from... 

Because LCEC had its initial impact in neurochemical analysis, it is not, surprising that many of the early enzyme-linked electrochemical methods are of neurologically important enzymes. Many of the enzymes involved in catecholamine metabolism have been determined by electrochemical means. Phenylalanine hydroxylase activity has been determined by el trochemicaUy monitoring the conversion of tetrahydro-biopterin to dihydrobiopterin Another monooxygenase, tyrosine hydroxylase, has been determined by detecting the DOPA produced by the enzymatic reaction Formation of DOPA has also been monitored electrochemically to determine the activity of L-aromatic amino acid decarboxylase Other enzymes involved in catecholamine metabolism which have been determined electrochemically include dopamine-p-hydroxylase phenylethanolamine-N-methyltransferase and catechol-O-methyltransferase . Electrochemical detection of DOPA has also been used to determine the activity of y-glutamyltranspeptidase The cytochrome P-450 enzyme system has been studied by observing the conversion of benzene to phenol and subsequently to hydroquinone and catechol... 

Tyrosinase is a monooxygenase which catalyzes the incorporation of one oxygen atom from dioxygen into phenols and further oxidizes the catechols formed to o-quinones (oxidase action). A comparison of spectral (EPR, electronic absorption, CD, and resonance Raman) properties of oxy-tyrosinase and its derivatives with those of oxy-Hc establishes a close similarity of the active site structures in these proteins (26-29). Thus, it seems likely that there is a close relationship between the binding of dioxygen and the ability to "activate" it for reaction and incoiporation into organic substrates. Other important copper monooxygenases which are however of lesser relevance to the model studies discussed below include dopamine p-hydroxylase (16,30) and a recently described copper-dependent phenylalanine hydroxylase (31). 

PHENOL HYDROXYLASE CATECHOL 1,2-DIOXYGENASE CATECHOL 2,3-DIOXYGENASE CATECHOL O-METHYLTRANSFERASE CATECHOL OXIDASE... 

Figure 1.35 Schematic diagram of the phenolic biosynthetic pathway accompanied by the key enzymes involved. Enzyme abbreviations PAL, phenylalanine ammonia-lyase BA2H, benzoic acid 2-hydroxylase C4H, cinnamate 4-hydroxylase COMT-1, caffeic/5-hydroxyferulic acid O-methy I transferase 4CL, p-co um a ra te C o A ligase F5H, ferulate 5-hydroxylase GT, galloyltransferase ACoAC, acetylCoA carboxylase.

Tyrosinase is a copper-containing oxidase (Coche-Guerente et al, 2001 Forzani et al, 2000), which possesses the two different activities illustrated in Figure 57.12. In the first step, referred to as the hydroxylase or cresolase activity, molecular oxygen is used to hydroxylate phenol to form catechol. In the second step, known as the catecholase activity, the enzyme oxidizes catechol to o-quinone, which is simultaneously oxidized by oxygen to its original form, with the production of water. The o-quinone is electro-chemically active and can be reduced back to catechol, as illustrated above in Eq. (57.17). 

Octopamine, a < -4 ml nomet by l)-4-hydroxy benz-enemethauol -taminomethyt1-p-hydroxybenzyt alcohol 1-(p-hydroxyphenyl)-2-aminoethanol norsympatol nor-synephrine p- hyd roxypheny let hanolam i ne WV 569. Cs -H N02 mo] wt 153.18. C 62.72%, H 7.24%, N 9.14%, O 20.89%. A biogenic amine that is the phenol analog of noradrenaline (norepinephrine, q.v,). It is a neurosecretory product found in several vertebrates and invertebrates. Formed by f)-hydroxylation of tyramine by the enzyme dopamine 0 -hydroxylase Pisano et a ., Btochlm. Biophys. 

As shown in Scheme 13.63, tabersonine, in the presence of tabersonine 16-hydroxylase (EC 1.14.13.73), is converted to 16-hydroxytabersonine. The reaction requires both NADPH and oxygen (O2), and NADP and water (H2O) are produced. Tabersonine 16-hydroxylase is a heme-thiolate protein (a P45o).The phenolic hydroxyl group on the aromatic ring undergoes methylation (tabersonine 16-O-methyltransferase, EC 2.1.1.94), while 5-adenosylmethionine is converted to... 

Tyrosinase is an enzyme complex (phenolase, polyphenol oxidase are other names which have been used for this enzyme), which catalyses of the ortho hydroxylation of monohydric phenols. The enzyme, which should not be confused with L-tyrosine hydroxylase mentioned above, contains Cu (I) and catalyses two distinct reactions—the hydroxylation of monohydric phenols to o-diphenols (cresolase activity) and the oxidation of o-diphenols to o-quinones (catecholase or catechol oxidase activity) . Most enzymes of this type, which are widely distributed in both the plant and animal kingdoms, exhibit both cataljrtic functions. Thus typically, the conversion of L-tyrosine (5) to L-dopa (15) and dopaquinone (36) which occurs in melanin biosynthesis is catalysed by an enzyme of the tyrosinase category. The two activities appear, in the majority of cases, to be functions of the same enzyme. However, certain o-diphenol oxidases such as those from tea , sweet potato and tobacco have been reported to show no capacity to catalyse the hydroxylation reaction but this is most probably due to destruction of the cresolase activity during purification. 

The phenolase complex consists of two activities, phenol -hydroxylase ( cresolase, equation 53) and o-diphenol dehydrogenase ( catecholase, equation 54) (3,111,203,244,352,358,462,579,585,... 

The sensitizer acts as a catalyst to the over-all reaction, as it is regenerated by reaction (16). Whether the photochemical oxidation proceeds with or without sensitizer, the triplet state will lower the energy of activation for subsequent reaction with oxygen. Kemula and Grabowska (1960) have described a method of studying the reactivity of aromatic molecules in their lowest triplet state. They irradiated the system benzene-oxygen within the forbidden band (A 2900-3600 A) and obtained phenol and o-benzoquinone. This is the simplest model hydroxylase. 

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