Compounds 3- dioxygenase system

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 ... 

Usually, the polarity of compounds increases by dihydroxylation to such an extent that re-entry into the active site of dioxygenases is not possible and only single microbial biooxygenation is observed. Ho vever, vhen diols are masked in the form of lipophilic acetonides, a second biooxygenation reaction is possible the diastereoselectivity of both cis-diol systems vas reported to be trans (Scheme 9.30) [240]. 

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. 

Further details of the pathways for the degradation of PAFIs are described in Chapter 6, Section 6.2.1 and in a review (Neilson and Allard 1998). It seems that most of the degradative enzymes are inducible, and this is consistent with the fact that most strains have been isolated after specific enrichment with the xenobiotic. The case of the partially constitutive synthesis of catechol 1,2-dioxygenase in the yeast Trichosporon cutaneum (Shoda and Udaka 1980) has been noted (Section 4.5.2). In the case of biotransformation, however, there are sporadic examples of the constitutive synthesis of enzymes. For example, the system carrying out the O-methylation of halogenated phenolic compounds was apparently constitutive (Neilson et al. 1988) this observation is consistent with the isolation of the strains by enrichment with Q compounds structurally unrelated to the halogenated substrates. The O-methylation reaction may function primarily as a detoxification system, so that in this case constitutive synthesis of the enzyme would clearly be advantageous to the survival of the cells. 

The effect of modifying ligands, which usually occupy the axial position in a complex, was as a rule studied using model compounds that mimic active sites of enzymes, namely, mono- and dioxygenases. At present, examples of catalytic systems are known that substantially increase the rates of catalyzed reactions and, sometimes, the yields of products upon the addition of modifiers. As a rule, the mechanisms of action of additives were not established, although possible schemes were put forward to explain the action of modifying ligands [1]. 

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