Intradiol aromatic dioxygenases

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.

Dioxygenase. Although it is not involved in the transformation of PAHs, it may be noted for the sake of completeness that a catechol intradiol dioxygenase is involved in the fission of 1,2,4-trihydroxybenzene that is formed from a number of aromatic substrates (Rieble et al. 1994). 

Dioxygenases which act on aromatic ortho-diphenols may either cleave the aromatic ring by an intradiol pathway or by an extradiol route, as shown in equation (77),... 

The intradiol cleaving catechol dioxygenases are bacterial iron-containing enzymes that serve as a component of nature s mechanism for degrading aromatic compounds in the environment. These enzymes, represented by catechol 1,2-dioxygenase (CTD) and protocatechuate(3,4-dihydroxybenzoate) 3,4-dioxygenase (PCD), catalyze the reaction... 

With one exception, in all the substrates the ene-diol structure is part of an aromatic nucleus. Oxidative cleavage of the ene-diol yields a dicarboxylic acid which retains the geometry of the remaining double bonds. There are eight of these so-called intradiol dioxygenases (/, 2, 83). 

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