Intradiol oxygenation

Intradiol oxygenation produces mainly the carboxymethylfuranone derivative 15 via the corresponding muconic acid anhydride DBMUA. The extradiol path affords mainly formylfuranone 5 via the cyclic... 

Despite a typical substrate in the enzymatic system, protocatechuic acid was found to be oxygenated by model complexes not in organic solvents but in water. The highly selective and catalytic intradiol oxygenation of 4-fm-butylcatechol indicated that various types of catechols other than 3,5-di-fert-butylcatechol can be used as substrates in the model systems and that oxygenations by nonheme iron systems in water are attractive. The reactivity and selectivity are dependent on the substituent on catechols and pH of the solution. 

In the mechanism of the intradiol oxygenation, the first step is the activation of catechol by coordination to a Fe(III) center. The chelate coordination of catechols in the enzymatic systems (protocatechuate 3,4-dioxygenase, 3,4-PCD) has been clarified by the X-ray analysis. in addition, the analysis indicated... 

Recently, experimental supports for the oxygen activation process (B) were reported to explain the extradiol oxygenation of the Fe" -(DTBC) complex. The process is favored by the complex having facial tridentate ligands rather than tetradentate ligands in the former 02 and substrate can occupy the opposite face and form an intermediate that leads to the extradiol products (Fig. 9). The A process was proposed for the intradiol oxygenations for the complexes with meridional... 

The products are grouped to the following three types. Intradiol oxygenation ... 

Compared with extensive studies on the intradiol oxygenation by model complexes, the extradiol oxygenation has been little studied. The extradiol oxygenation is performed not only by Fe -enzymes but also Fe -enzymes in the case of 3-substituted catechols [119]. Thus, the explanation for the extradiol oxygenation by an Fe complex in Scheme 8 is reasonable. However, the remarkable promotion of extradiol oxygenation by using Fe " " in place of Fe " has indicated that such oxygenation is catalyzed by Fe rather than Fe " ", as in enzymatic systems. The reaction has been explained by Scheme 11 [31]. The Fe " " species may be formed from the Fe species by the side-reaction of oxidation of catechol to quinone. 

Because ferric iron does not react with triplet oxygen, intradiol dioxygenases must use a mechanism besides oxygen activation as the first step. 

Fig. 7. Model for oxygen activation by intradiol dioxygenases. PCA, the substrate pro-tochatechuic acid.

The enzyme catalyzes the intradiol cleavage of catechol with the insertion of 2 atoms of molecular oxygen to form e/s.c/s-muconic acid. The enzyme also catalyzes the oxidation of various catechol derivatives, including 4-methylcatechol, 4-chloro-catechol, 4-formylcatechol (protocatechualdehyde), 4,5-dichlorocatechol, 3,5-di-chlorocatechol, 3-methylcatechol, 3-methoxycatechol, and 3-hydroxycatechol (pyro-gallol). All of these substrates give products having an absorption maximum at around 260 nm characteristic of t7s-c/s-muconic acid derivatives. However, when 3-methylcatechol is used as a substrate, the product formed shows an absorption maximum at 390 nm besides that at 260 nm. These two absorption maxima are found to be attributable to two different products, 2-hydroxy-6-oxo-2,4-heptadienoic acid (7), and 5-carboxy-2-methyl-2,4-pentadienoicacid(2-methylmuconic acid), (2) [Eq. (19)]96. ... 

Figure 18-22 Some possible intermediates in the action of extradiol (left) and intradiol (right) aromatic dioxygenases. Although the steps depict the flow of pairs of electrons during the formation and reaction of peroxide intermediates, the mechanisms probably involve free radicals whose formation is initiated by 02. The asterisks show how two atoms of labeled oxygen can be incorporated into final products.

Dioxygenases catalyze the incorporation of both atoms of molecular oxygen into the substrate. Microbial pyrocatechol dioxygenases contain non-heme iron as the sole cofactor and catalyze the oxidative cleavage of pyrocatechol to as,cis-muconic acid (intradiol equation 19) or to a-hydroxymuconic s-semialdehyde (extradiol equation 20).63"65 On... 

The visible spectra of the intradiol dioxygenases (Fig. 2) are characterized by a broad absorption band centered near 460 nm with molar extinction coefficients of 3000-4000 M-1 cm-1 16>34). The color disappears upon reduction of the ferric ion with dithionite and is regenerated upon exposure of the solution to oxygen. Resonance Raman studies on these enzymes15 35-38) have been reported by several laboratories (Table 4). These spectra are characterized by a set of four peaks at ca. 1605, 1505, 1270, and 1175 cm-1, which are assigned to ring vibrations of Fe(III) coordinated tyrosinate ligands. Similar spectra are obtained for the transferrins as well as for model iron-phenolate complexes (Table 4). A new class of iron proteins seems to... 

Dioxygenase enzymes are known that contain heme iron, nonheme iron, copper, or manganese.The substrates whose oxygenations are catalyzed by these enzymes are very diverse, as are the metal-binding sites so probably several, possibly unrelated, mechanisms operate in these different systems. For many of these enzymes, there is not yet much detailed mechanistic information. However, some of the intradiol catechol dioxygenases isolated from bacterial sources have been studied in great detail, and both structural and mechanistic information is available. These are the systems that will be described here. 

Protocatechuate 3,4-dioxygenase. Crystalline preparations of the enzyme were obtained as described previously (2). The enzyme catalyzes the intradiol cleavage of protocatechuic acid with the insertion of two atoms of molecular oxygen to form /8-carboxy cis,cis-muconic acid (Reaction 3). 

Intradiol cleavage products Extradiol cleavage products Oxygenation products... 

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