Extradiol-Cleaving Dioxygenases

Co-factor studies have been performed for all the enzymes listed, except for dihydroxy benzoate extradiol dioxygenase. Tests for dependence on ferrous ions and reducing agents and of inhibition by ferrous chelators and oxidizing agents indicate the presence of iron in the ferrous state in the active forms of metapyrocatechase 116), protocatechuate 83,117), dihydroxykynurenate (2), homoprotocatechuate 118), carboxy- 

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Table 3 compares some key properties of the enzyme-substrate complexes of the intradiol and extradiol cleaving dioxygenases. A mechanism for extradiol cleavage must account for the differing metal requirements of the two classes of enzymes and their distinct regiospecificities. A key difference is the reactivity of the ES complexes toward NO. Whereas the ES complexes of extradiol enzymes readily react with NO to form ES—NO adducts, those of intradiol enzymes do not react with NO unless the Fe(III) center is reduced prior to exposure to NO [167], Thus the Fe(II) center in the extradiol cleaving enzymes appears... 

Iron-containing extradiol-cleaving dioxygenases are well known (see Iron Proteins with Mononuclear Active Site. ... 

Figure 8 Schematic views of extradiol-cleaving dioxygenases.

In addition to the classical extradiol cleaving dioxygenases, the dioxygenases which cleave o-amino phenols (44) and p-diphenols (45) (Scheme 22) also appear to share the same catalytic properties. 

Figure 13.21 Mononuclear non-haem iron enzymes from each of the five families in structures which are poised for attack by 02. (a) The extradiol-cleaving catechol dioxygenase, 2,3-dihydroxy-biphenyl 1,2-dioxygenase (b) the Rieske dioxygenase, naphthalene 1,2-dioxygenase (c) isopenicillin N-synthase (d) the ot-ketoglutarate dependent enzyme clavaminate synthase and (e) the pterin-dependent phenylalanine hydroxylase. (From Koehntop et al., 2005. With kind permission of Springer Science and Business Media.)...

Scheme 2.4 Catalytic cycle for extradiol ring- characterized by X-ray crystallography [22]. cleaving dioxygenases (R = CH2COOH or N02) During the reaction cycle, various protonated as proposed by Lipscomb and coworkers [22,56]. and unprotonated amino acid residues near the The amino acids are numbered according active site serve as proton donors and acceptors...

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

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... 

Fig. 4.92. Reactions catalyzed by intra- and extradiol ring cleaving dioxygenases.

Lipscomb, J. D. (2008). Mechanism of extradiol aromatic ring-cleaving dioxygenases. Current Opinion in Structural Biology, 18, 644-649. 

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