Arene dioxygenase

Mutants of Pseudomonas putida were found to exhibit an arene dioxygenase activity, which has been exploited in whole-cell reactions for the regio- and enantioselective preparation of cw-dihydrodiols starting from benzene, substituted benzenes, and polycyclic or heteroaromatic compounds [48], The products are invaluable precursors for natural product synthesis, as exemplified in Scheme 8 [49],... 

Arene dioxygenases, or Rieske dioxygenases (RDO), are enzymes that contain both a mononuclear, nonheme iron center as well as a Rieske-type, Fe2S2 cluster. They have a wide-ranging... 

Figure 29 Proposed mechanism for the cis-dihydroxylation reaction of arene dioxygenases.

Among bacteria that are capable of metabolizing arenes, certain strains express arene dioxygenase enzymes capable of effecting the dearomatizing dihydroxylation of an arene 1 to give a cis-cyclohexa-3,5-diene-l,2-diol 2 (Scheme 32.1). Ordinarily, such diene diols are merely fleeting... 

More than 400 arene cis-dihydrodiols derived from a wide variety of aromatic substrates have now been reported. For a more in-depth coverage, the reader is directed to several excellent reviews that have appeared recently [23]. Bicychc and heterocychc arenes are viable substrates for the transformation, although when 5-membered heteroarenes (furan, thiophene) undergo this transformation, the products are not always stable. Arene dioxygenases are membrane-bound proteins, and as such, this biotransformation is not carried out using isolated enzymes. Rather, whole-cell fermentation approaches are employed. While these are not as operationally simple as the use of isolated enzymes, they can nevertheless be carried out without recourse to any particularly unusual... 

SCHEME 32.2 Regio- and stereoselectivity of arene dihydroxylation by arene dioxygenases, (a) For TDO (toluene dioxygenase) and BPDO (biphenyl dioxygenase), (b) For BZDO (benzoate dioxygenase). 

Lipoxygenase Catechol dioxygenases a-Ketoglutarate-dependent dioxygenases (Clavaminate synthase) Arene dioxygenases... 

Arene dioxygenases catalyze the first step in the metabolism of unactivated aromatic compounds, yielding cw-dihydrodiol of aromatics. As shown in eq. (22), the reaction requires nicotinamide adenine nucleotide (NADH) and molecular oxygen. Substrate specificity of enzymes is high, and products shown in Fig. 27 are produced by benzene-[351, 352], toluene- [353-356], naphthalene- [357-361], biphenyl- [362-364], benzoate-[365-370], and phthalate-dioxygenases [371-378]. In the cases of benzoate [379], 4-sulphobenzoate [380], and 6>-nitrotoluene [381], catechols are formed via unstable dihydroxylated intermediates as shown in eq. (23)... 

Figure 27. Dihydroxylated products formed by arene dioxygenases.

Mechanism for oxygen activation and dihydroxylation by these enzymes is far from established. A mechanism in Scheme 23 [387] which is proposed for putidamonooxin is suggestive for arene dioxygenases. 

Scheme 23. Proposed reaction cycle of putidamonooxin for hydroxylaton of 4-methoxybenzoate [387] which is referred to arene dioxygenases.

Putidamonooxin contains a mononuclear iron center and a Rieske [2Fe-2S] cluster and oxidatively demethylates 4-methoxylbenzoate to 4-hydroxybenzoate [387]. Possibly diol (R-(0H)2) may be formed in the step 4 in case of arene dioxygenases, but no information has been obtained about the oxygen transfer process, e.g. whether the oxygen transfer occurs simultaneously or stepwise. 

Arene.. dioxygenases catalyze reactions other than dihydroxylation of aromatics. For example, toluene dioxygenase catalyzes monooxygenation of indene and indan to 1-indenol and 1-indanol, respectively [382], oxidation of polyhalogenated compounds such as trichloroethylene [eq. (24)] [363, 388-390], and stereoselective sulfoxidation of sulfides [391, 392]. 

Several different important milestones in applications to synthesis of arene dioxygenases have been reported by the groups of Banwell [161], Hudlicky and others [154], Key contributions have been the synthesis of PGE, (+) and (-)-pinitol, pancratistatin, lycoricidine, narddasine, (+)-codeine, and oseltamivir (Scheme 9.29) [154]. 

Sequences of proteins containing Rieske-type clusters have been deduced from the complete operons of several dioxygenases these dioxygenases require electrons from NAD(P)H to convert aromatic compounds to cis-arene diols. The water-soluble dioxygenase systems consist of a reductase and a terminal dioxygenase many dioxygenases also contain a [2Fe-2S] ferredoxin (20). The terminal oxygenases contain a Rieske-type cluster and the ferredoxins may contain either a Rieske-type or a 4-cysteine coordinated [2Fe-2S] cluster. 

Rieske-type clusters are found in aromatic-ring hydroxylating dioxygenase systems (20). These enzymes catalyze the conversion of different aromatic compounds into cis-arene diols ... 

Arene hydrocarbon dioxygenases are capable of carrying out a number of reactions other than the introduction of both atoms of oxygen into the substrate. Illustrative examples of monooxygenation carried out by dioxygenases include the following ... 

The first step in any of the degradation pathways is the formation of 2-quinolinone/2-hydroxyquinoline/2-oxo-l, 2-dihydroquinoline and quinoline2-oxidoreductase has been shown to be responsible for this initial metabolic reaction [318], When the oxidation is carried out by a dioxygenase, it resulted in the formation of ds-hydrodiol derivatives and when oxidation is carried out by a monooxygenase monohydroxilated derivatives are formed, via the arene oxide intermediates [323],... 

The 2-oxoacid p-hydroxyphenylpyruvate is decar-boxylated by the action of a dioxygenase (Eq. 18-49). The product homogentisate is acted on by a second dioxygenase, as indicated in Fig. 25-5, with eventual conversion to fumarate and acetoacetate. A rare metabolic defect in formation of homogentisate leads to tyrosinemia and excretion of hawkinsin97 a compound postulated to arise from an epoxide (arene oxide) intermediate (see Eq. 18-47) which is detoxified by a glutathione transferase (Box 11-B). 

Phenanthrene is transformed to trans-9,10- (major), trans-1,2- (minor), and trans-3,4-dihydrodiol (minor) metabolites via monooxygenase-catalyzed formation of arene oxides, followed by epoxide hydrolase-catalyzed hydration in mammalian liver systems.219-221 In bacterial cultures, phenanthrene is converted to cis-3,4- (major) and cis-1,2- dihydrodiols (minor) through the action of dioxygenase enzymes and molecular oxygen.221,222 Recently, Boyd et al.10 have prepared trons-3,4-dihydroxy-1,2,3,4-tetrahydrophenanthrene (359) and cis-3,4-dihydroxy-1,2,3,4-tetrahydrophenanthrene (360) in optically pure forms. These compounds have made possible the determination of the configurations of the trans- and cis-3,4-dihydrodiol metabolites of phenanthrene (361 and 362) as (-)-(3R,4R) and ( + )-(3S,4R), respectively. 

What is remarkable, however, is the stereochemical influence of a 13-hydroxyl group, p-hydroxycarbocations such as 31 are formed not only from arene oxide as precursors but from arene dihydrodiols. As shown for the parent benzene dihydrodiols in Scheme 23, arene dihydrodiols exist as cis-and /ra/rv-isomers. The m-isomers are obtained as products of the action on the aromatic molecule of dioxygenase enzymes and have been prepared on a large scale by fermentation.92 The trans-isomers are normally accessible by straightforward synthesis, for example, from the arene oxide. Both isomers undergo acid-catalyzed dehydration to the parent aromatic molecule, as is also shown in Scheme 23. It is clear that their reactions should involve a common carbocation intermediate,163 164 and in so far as there is little difference in the stabilities of the isomers,165 their difference in reactivities might have been expected to be small. 

For alkene dihydroxylations, heavy metal oxides such as 0s04 and Ru04 can be applied. They are efficient catalysts but their toxitity makes their use less desirable and there is a dear need for non-toxic metal catalysts. Nevertheless, only a few reports have focused on the use of iron catalysts for alkene dihydroxylations. All systems described so far try to model the naturally occurring Rieske dioxygenase, an enzyme responsible for the biodegradation of arenes via cis-dihydroxylation by soil baderia [66]. 

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