The Catechol Dioxygenases

The nonheme iron-containing dioxygenases other than the catechol dioxygenases mentioned above are listed in Table 6. For the details of these enzymes, the readers are referred to the original references of individual enzymes or the review articles14,81. ... 

The emphasis on the study of hemoproteins and the iron-sulfur proteins often distracts attention from other iron proteins where the iron is bound directly by the protein. A number of these proteins involve dimeric iron centres in which there is a bridging oxo group. These are found in hemerythrin (Section 62.1.12.3.7), the ribonucleotide reductases, uteroferrin and purple acid phosphatase. Another feature is the existence of a number of proteins in which the iron is bound by tyrosine ligands, such as the catechol dioxygenases (Section 62.1.12.10.1), uteroferrin and purple acid phosphatase, while a tyrosine radical is involved in ribonucleotide reductase. The catecholate siderophores also involve phenolic ligands (Section 62.1.11). Other relevant examples are transferrin and ferritin (Section 62.1.11). These iron proteins also often involve carboxylate and phosphate ligands. These proteins will be discussed in this section except for those relevant to other sections, as noted above. 

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 asymmetric bonding motif of the catecholate to the ferric ion in these complexes duplicates the binding motif in the enzyme-substrate structures that have been characterized. While the original belief that this asymmetry in Fe—O bond lengths was critical to the reactivity of the systems has since proven incorrect,the complexes remain as functional models for the catechol dioxygenases due to both their structural and reactivity features. All complexes have been characterized as high-spin ferric catecholate complexes by UV-visible, EPR and NMR spectroscopy, and an examination of C—O bond length in these complexes shows all of the complexes to bind in the catecholate form with no semiquinonate character. ... 

In contrast to the high substrate specificity of 4,5-PCD for PCA, 2,3-PCD has the largest number of alternate substrates in the catechol dioxygenases [41]. As shown in Table 6, Wolgel et al. found that the substrate range of 2,3-PCD is larger than those of... 

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