Catechol dioxygenases mechanism

Figure 15 Proposed substrate activation mechanism for the intradiol cleaving catechol dioxygenases.

Lin, G., Reid, G., and Bugg, T.D.FI. Extradiol oxidative cleavage of catechols by ferrous and ferric complexes of 1,4,7-triazacyclononane insight into the mechanism of the extradiol catechol dioxygenases, J. Am. Chem. Soc. 123 (2001), 5030-5039. 

Figure 6 Proposed substrate activation mechanism for intradiol-cleaving catechol dioxygenases. Substrate activation is proposed to occur via an iron(II)-semiqumone prior to O2 binding...

Figure 9 Proposed mechanism for the extradiol-cleaving catechol dioxygenases...

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... 

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. 

Proposed mechanism for catechol dioxygenases (modified from Reference 45). The major form A is shown here with the catechol bound in a monodentate fashion. It may sometimes be bound in a bidentate fashion as well (see text). 

DTBSQ occurs also without an added catalyst. This raises the question of how far this mechanism may be involved in catechol dioxygenase model systems. The semiquinone radical is supposed to be an intermediate of 3,5-DTBC oxidation to the quinone. Once there is a path for its formation the reactions of Scheme 11 may also start to occur and contribute to the overall diol cleavage process. Thus it seems that diagnostic tests are required for reactions of Scheme 11, perhaps via... 

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