Catechol dioxygenases active site

The intradiol-cleaving enzymes represented by catechol 1,2-dioxygenase and protocatechuate 3,4-dioxygenase (PCD) catalyze the reactions shown in Figure 4. Both enzymes have iron(III) active sites and are spectroscopically well characterized. The enzymes exhibit a distinct bmgundy red color (Xmax 460 nm, 450 3500 cm ) that is... 

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... 

Figure 13 Structures of the active sites of (a) 2,3-dihydroxybiphenyl 1,2-dioxygenase complexed with catechol (PDB IKND) (b) deacetoxycephalosporin synthase with bound a-ketoglutarate (PDB IRXG) (c) isopenicillin N synthase with coordinated substrate and NO (PDB IBLZ) (d) naphthalene dioxygenase with bound dioxygen in the presence of substrate (not shown) (PDB 107M).

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]. 

Two intermediates have been detected in the reaction of catechol 1,2-dioxygenase with pyrogallol and dioxygen [88]. The active site in this enzyme has been studied by NMR spectroscopy a high-spin ferric center gives rise to paramagnetically shifted resonances [89]. 

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