Catechol dioxygenase reaction

When a benzene ring is cleaved by a dioxygenase reaction, hydroxylation of the benzene ring usually proceeds to form catechol or phenol derivatives. When catechol derivatives are cleaved by the action of individual dioxygenases, three modes of ring fission have been demonstrated by microbial enzymes85 ... 

The dioxygenation of unsaturated a-diols (catechol and benzoin. Table 6-2) by the O2/Fe l(DPAH)2 system parallels that of the catechol dioxygenase enzymes, which are nonheme iron proteins. -l Hence, the reactive intermediate (1, Scheme 6-1) of the Feh(DPAH)2/O2 reaction may be a useful model and mimic for the activated complex of dioxygenase enzymes. ... 

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... 

A large family of these enzymes is now known, and their enzymology and structures have been reviewed. A number of crystal structures have been obtained for enzymes in this family, and in each case the mononuclear iron(II) center is coordinated by a His,His,Glu motif, also observed in the extradiol catechol dioxygenases, and in other nonheme iron-dependent enzymes. Structural studies on clavaminic acid synthase have indicated the structural basis for the separate hydroxylation and oxidative cyclization/ desaturation reactions catalyzed by this enzyme. ... 

Superoxide complexes are also invoked as intermediates in the reactions of bleomycin, catechol dioxygenases, and cytochrome P450. They can be expected to occur in cases where the redox cofactor consists of a single metal ion in the absence of other readily oxidizable cofactors. 

Coordination of the active site metal by water molecules and substrates is obviously much more variable and the X-ray structure may be of little assistance. In order to determine experimentally the orientation of the substrate, a crystal would need to be grown with the substrate bound. Notwithstanding the problems of obtaining X-ray quality crystals in the first place, by its very nature, the enzyme will attempt to convert the substrate to product and capturing a bound state may not be possible. However, many reactions require an additional reactant—say a molecule of O2—and thus the substrate-bound form may remain stable under anaerobic conditions. The structure of catechol dioxygenase with substrate bound has been determined in this way [36]. 

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