A High-Valent Fe Intermediate of Tryptophan 2,3-Dioxygenase

Although a similar Fe(IV)=0 intermediate of IDO has not yet been directly observed in the catalytic cycle of TDO, it has been trapped and characterized in the enzyme reactivation study. The ferric form of TDO can be reactivated by H202in the presence of Trp through a complex mechanism [40]. When Fe(lll)-TDO reacts with H2O2 in the absence of T rp, the Fe ion becomes EPR-silentand a protein-based radical is observed (Fig. 15.6). The free radical decays overtime, while the EPR signal of the high-spin ferric heme is restored. The ferric ion is shown by Mossbauer spectroscopy to 

The formation and decay of the compound l-type ferryl intermediate in the reaction of 150 jjiM tryptophan 2.3-dioxygenase with 900 xM hydrogen peroxide monitored by EPR spectroscopy at 10 K. (a) Seven representative EPR spectra (traces A to G) are shown in a 2D plot for the reaction of 0,12,30,60,90,240, and 600 s in the parallel samples, (b) The high spin EPR signal of TDO (A), H2O2 treated TDO at 30 s (B). 

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