Ferryl States

As briefly summarized in Section 1.2.2.1, extensive evidence has been reported indicating that horseradish peroxidase Compound I is an oxo-ferryl [Fe =0] porphyrin n-cation radical and that Compound II is an oxo-ferryl porphyrin. Groves and co-workers have reported an inorganic model complex for Compound I [53, 54] and Balch and co-workers have described a Compound II model [55, 56]. These models each appear to have the expected compositions for the respective enzyme states that they are designed to mimic. 

The first application of EXAFS spectroscopy to the ferryl states of heme systems was reported by Penner-Hahn et al. in 1983 [143]. This work included a comparative study of the Groves and Balch model complexes, and of horseradish peroxidase Compounds I and II. The EXAFS spectra and corresponding Fourier transforms of the four high-valent systems (two proteins and two models), taken from a subsequent, more complete, analysis of the data [107], are displayed in Figs. 19 and 20. Table 2 contains a summary which shows the Fe-O(oxo) and Fe-Np bond lengths for a variety of oxidized heme proteins and their models. 

From the energies of the X-ray absorption edge, it was concluded that horseradish peroxidase Compounds I and II, and the respective models, were all Fe species. Furthermore, there was essentially no difference in the EXAFS data of the two protein states and their respeetive model eomplexes (compare Figs. 19 and 20). Curve-fitting analyses of the data for all four species suggested the presence of one set of oxygen (or nitrogen) atoms at a distance of 1.6 A [143]. This was consistent with the presence of a short Fe=0 bond, as expected for an oxo-ferryl moiety. A second set of atoms at 2.0 A corresponded to the pyrrole 

EXAFS data analysis procedures are described in the listed references. Abbreviations L, solvent ligand (methanol in the case listed TMP, meso-tetramesitylporphyrinatof—1) radical NMelm, N-methylimidazole TTP, meso-tetratolylporphyrin THF, tetrahydrofuran TpPP, meso-tetrakis(a,G[,(z,(x-o-pivalamidophenyl)porphyrin  

The range of EXAFS data report results horn the use of two different analysis methods (see Ref. 107)l The value reported for Fe-Np actually represents an average value for Fe-Np and Fe-N  

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Cytochrome oxidases. Mitochondrial cytochrome c oxidase uses the energy involved in the oxidation of cytochrome c and reduction of water to generate a proton electrochemical gradient across the inner mitochondrial membrane [57], As stated above, a ferryl state is an essential intermediate in this process. Similar intermediates are to be expected in all similar proton-translocating cytochrome oxidases that contain a binuclear haem-copper centre,... 

Witt and Chan, 1987 Rao et al., 1988) described as ferryl states. The Fe(IV) state has been designated as an iron-oxo complex (Fe02+) the Fe(V) state is a similar complex, the extra electron being lost from the porphyrin jr-system (P +-Fe02+) (Felton et al., 1976 Rakshit et al., 1976). 

The two major classes of heme oxidized above the Fe(III) state, namely, the one-electron oxidized ferryl state and the two-electron oxidized state containing Fe(IV)=0 and the porphyrin ring cation radical, FedVKport), have MCD spectra that provide two useful pieces of information (76). First, the form of the spectrum can indicate whether or not the porphyrin ring system is oxidized, and whether the... 

Figure 8 is a possible redox cycle occurring in an amperometric sensor for hydrogen peroxide involving enzyme-wiring of a typical enzyme (Horseradish peroxidase, HRP) with polyaniline. HRP immobilized on the electrode surface can be oxidized by H2O2 to compound I that contains an oxyferryl centre with the iron in the ferryl state (Fclv = O), and a porphyrin 7r cation radical, followed by further direct (mediatorless) electroreduction of compound I at the electrode surface to the initial HRP state [106], The electrode is considered as an electron donor. 

A state (Figure 39), the 0-0 bond is broken in a concerted reaction. To break the 0-0 bond, four electrons and at least one proton is required. Two of the electrons are donated by the heme iron (forming the ferryl state, Fe ) and one from Cub (which is oxidized to form Cug ). The source of the additional electron depends on whether heme a is oxidized (1), the long route around the circle in Figure 39, or reduced (2), the path through Pr, when O2 binds to the reduced catalytic site. 

Chance et al. have also studied the EXAFS properties of the high-valent oxo-ferryl states of horseradish peroxidase, cytochrome c peroxidase, and myoglobin [144-146]. A two-atom-type constrained amplitude ratio fit and a three-atom-type consistency test were used for the analysis of the EXAFS data [144,146]. These analytical methods differ from that used by Hodgson and co-workers... 

Davydov R, Osborne RL, Shanmugam M, Du J, Dawson JH, Hoffman BM (2010) Probing the oxyferrous and catalytically active ferryl states of Am-phitrite omata dehaloperoxidase by cryoreduction and EPR/ENDOR spectroscopy. Detection of compound I. J Am Chem Soc 132 14995-15004... 

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