Intradiol dioxygenases oxygen activation

Because ferric iron does not react with triplet oxygen, intradiol dioxygenases must use a mechanism besides oxygen activation as the first step. 

Fig. 7. Model for oxygen activation by intradiol dioxygenases. PCA, the substrate pro-tochatechuic acid.

In the mechanism of the intradiol oxygenation, the first step is the activation of catechol by coordination to a Fe(III) center. The chelate coordination of catechols in the enzymatic systems (protocatechuate 3,4-dioxygenase, 3,4-PCD) has been clarified by the X-ray analysis. in addition, the analysis indicated... 

Table 1. Comparison of relative catalytic activities of oxygenation of substituted catechols by intradiol catechol dioxygenases ... 

Importance of the ferrous state for dioxygenation is mostly observed, but intradiol catechol dioxygenases and lipoxygenase require the ferric state for oxygenation. The oxidation state of iron important for oxygenation is sometimes different from that of isolated enzymes and in the resting state. In the enzymes which require the ferric state, activation of substrate by the iron center seems to proceed in preference to that of oxygen, and vice versa in the ferrous enzymes. 

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