Peroxo-ferric

SCHEME 4.3 Cytochrome P450 and peroxidase pathways to hydroperoxo-ferric intermediate or Compound 0 (5). Ferric cytochrome P450 (1) is reduced to the ferrous state (2), which can hind dioxygen to form oxy-ferrous complex (3). Reduction of this complex results in the formation of peroxo-ferric complex (4), which is protonated to give hydroperoxo-ferric complex (5). The same hydroperoxo-ferric complex is formed in peroxidases and catalases via reaction with hydrogen peroxide. 

Somewhat surprisingly, all heme proteins showed similar pattern in EPR spectra of (hydro)peroxo-ferric complexes.106 This fact has been attributed to the location of the... 

TABLE 4.1 EPR Parameters of (Hydro)Peroxo-Ferric Complexes in Heme Proteins... 

X-ray crystal stmcture of hydroperoxo-ferric complex in CPO was determined as a result of cryogenic photoreduction of oxy-ferrous precursor during data collection in the synchrotron beam.133 Three data sets with low, medium, and high dose have been prepared from the data collected on multiple crystals with the goal to resolve the structures with well-defined redox state. Because of the fast reduction, the low-dose data set already had sufficient contribution of Compound 0, and the stmcture of Compound III could not be determined. The medium-dose data set was resolved to 1.75 A resolution and revealed a well-defined 0-0 ligand with 100% occupancy. Assignment of this stmcture to hydroperoxo- ferric versus peroxo-ferric was done based on the QM/MM calculation. 

Oxygen is found to be coordinated in the bent end-on mode (with a Fe-O-0 angle of 136°). This structure provides a starting point for the formation of the end-on bound peroxo-ferric complex, which is the first of three unstable and highly reactive intermediates in the P450 catalytic cycle. 

Reduction of Oxy-Ferrous P450 and Formation of Peroxo-Ferric Complexes Properties, Stability, and Spectroscopy... 

Recently, this approach was further expanded to understand the detailed electronic structure and stability of peroxo-ferric intermediates in heme -41, 49, 50, 53,157,159,179-181 jjQjjjjeme systems -... 

Table 5.1. Electron Paramagnetic Resonance (EPR) Parameters of Mononuclear Peroxo-Ferric Complexes from Select S5mthetic Models and Enzymes...

Figure 6.40. Conversion of enolized analogue of the mtural aromatase substrate to the corresponding aromatized compound is catalyzed by a model peroxo ferric porphyrin complex.

These first detailed characterizations of the peroxo- and hydroperoxo-ferric intermediates in CYPIOIAI [185, 187] provided several important results and enabled further experimental studies with other heme proteins. Clear EPR signatures for the unprotonated peroxo-ferric (gi<2.27) and protonated hydroperoxo-ferric (gi>2.27) intermediates in cytochromes P450... 

Table 3.2 EPR parameters of (hydro)peroxo-ferric complexes in heme proteins...

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