Catechol dioxygenases iron coordination

There are many discussions on the coordination numbers of the iron species in the resting and intermediate states. It is noteworthy that in many cases a six coordination is first proposed, but importance of five coordination is demonstrated. As seen in catechol dioxygenase, substrates coordinate to iron with displacing attached ligands such as histidine with forming five coordinate species. The unsaturated coordination around the iron center may be important for the oxygen activation and oxygenation. 

Figure 2.3 Molecular structure of [Fe(Me3TACN)(DBC)CI], a model complex for a catechol dioxygenase coordinated to its substrate molecule [28]. Hydrogen atoms have been omitted for clarity. The l,4,7-trimethyl-l,4,7-triazacyclononane (Me3TACN) ligand coordinates facially to the iron center. The remaining three coordination sites are occupied by 3,5-di-tert-butylcatecholate (DBC) and a chlorido ligand.

Little is known about the nature of the iron active sites in these enzymes. With the application of modern spectroscopic methods to the catechol enzymes, we are beginning to get a glimpse of the iron coordination during the catalytic process. Much remains to be done before a well-defined picture emerges. Similar studies on the other systems are still to be undertaken. We have limited the scope of this discussion to these three classes of enzymes because most progress has been achieved in these systems. Still to be studied are the extradiol cleaving catechol dioxygenases and the... 

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. ... 

Since in catechol dioxygenases there are two tyrosinates coordinated to iron in the active site [94,95],the above complex may be regarded as a mimic for the enzyme center. Iron(III) is not necessarily reduced by coordinated catecholate, and the high-spin iron(III) can... 

The X-ray absorption spectra of a series of 28 model Fem complexes and those of ovatransferrin, protocatechuate 3,4-dioxygenase and catechol 1,2-dioxygenase have been analyzed, and appear to give information on the coordination number of the protein-bound iron. Thus the transferrin complexes appear to be six- or seven-coordinate, while the dioxygenase complexes could be five-or six-coordinate.816... 

The anaerobic addition of catechol to catechol 2,3-dioxygenase (2,3-CTD) results in the formation of an ES complex that retains the Fe(II) oxidation state of the as-isolated enzyme as indicated by a number of spectroscopic (Mossbauer [162,163], XAS [158], near IR CD and MCD [157]) methods. These methods also indicate that the iron center remains 5-coordinate. Crystallographic studies... 

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