Horseradish peroxidase, ferryl

Although ferryl intermediates of horseradish peroxidase and microperoxidase-8 have been produced in reactions with photogenerated [Ru(bpy)3]3+ [5], analogous experiments with P450s were unsuccessful, presumably due to the inefficiency of electron transfer from the buried heme active site through the protein backbone [6]. Photoactive molecular wires (sometimes referred to as metal-diimine wires, sensitizer-tethered substrates, or electron tunneling wires) were developed to circumvent this problem by providing a direct ET pathway between [Ru(bpy)3]3+ and the heme. These molecular wires, which combine the excellent photophysical properties of metal-diimine complexes... 

Several of the proteins with ferryl intermediates have been crystalised at sufficient resolution to allow the elucidation of their 3-dimensional structure. These include cytochrome c peroxidase [95], horseradish peroxidase [96], catalase [97], myeloperoxidase [98], ribonucleotide reductase [99], cytochrome P-450 [100] and myoglobin [101]. Of these only cytochrome c peroxidase has proved stable enough to crystallise with the iron in the ferryl form [26]. High-resolution structures exist for small FeIV model compounds, both in the presence [102] and absence [7,8] of an Fe=0 bond. These compounds can have sulphur, nitrogen and chloride ligation to the iron and the iron can be five [7,8] or six [8] coordinate. 

MCD spectra perhaps provide the best fingerprint for the existence of an FeIV=0 structure. Fig. 8 shows that there is a great similarity between the spectra of horseradish peroxidase compound II, horseradish peroxidase compound X, cytochrome c peroxidase compound I, Pseudomonas aeruginosa peroxidase compound I and ferryl myoglobin at acid pH. Similar features are seen in the spectra of catalase [170] and myoglobin [171] compound II. 

The reasons for the difference between the spectra of ferryl myoglobin at acid and alkaline pH are not clear. However, it has been suggested that deprotonation of the proximal histidine ligand at alkaline pH may be responsible [175,176], Furthermore a third form of ferryl iron was detected in varying amounts in preparations of horseradish peroxidase compound II, ferryl myoglobin and cytochrome c peroxidase compound I [162], To account for these spectra it was proposed that the iron-histidine bond was broken, leaving a five-coordinate ferryl haem. 

Mossbauer spectra has been extensively used to probe the structure of the iron nucleus in biological FeIV=0 compounds. These include horseradish peroxidase compoundl[134,180,181], horseradish peroxidase compound II [182,183], horseradish peroxidase compound X [181], Japanese-radish peroxidase compounds I and II [184], chloroperoxidase compound I [185], cytochrome c peroxidase compound I [186] and ferryl myoglobin [183]. Examples of Mossbauer spectra attributed to non-porphyrin-bound FeIV are only available from synthetic model compounds. These include compounds with [130] and without [4-8] an FeIV=0 bond. 

Unlike the case of optical or MCD spectroscopy, the presence of a nearby free radical (porphyrin or amino acid) has only a small effect on the Mossbauer spectra of ferryl iron (by contrast the nature of the axial ligand appears to have a greater effect on the Mossbauer spectra). Thus in the absence of a magnetic field there is little difference between the Mossbauer spectra of horseradish peroxidase compounds I and II [134,181,183]. Due to... 

Myoglobin has the same prosthetic group as some peroxidases, such as horseradish peroxidase (HRP) or cytochrome c peroxidase, and reacts with H2O2 to produce a ferryl species, PFe(IV)=0, observed in the native peroxidase (see Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases). However, the catalytic activity of myoglobin toward substrate oxidation is very low, because... 

Fig 17. MCD spectra recorded at 5 T and at 50 or 100 K of Fe(IV)(por) in various protein environments HRPCII, horseradish peroxidase compound II HRPCX, horseradish peroxidase compound X YCCP, yeast cytochrome c peroxidase compound ES PsCCP, compound I of the diheme cytochrome c peroxidase Mb pH 3.5, the ferryl form of myoglobin formed at pH 3.5 Mb pD 9.0, the same compound found at pD 9.0. 

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. 

For horseradish peroxidase (HRP), it was long thought that only a small fraction of the free-radical content was ESR detectable conventional ESR spectra show a narrow component equivalent to about 0.01 spins/heme. However, recent work has demonstrated that much of the free-radical spectrum is extremely broad due to interaction of the radical with ferryl ion, probably through an anisotropic exchange interaction [109]. When these broad components are taken into account, the number of free... 

Filatov, M., N. Harris, and S. Shaik (1999). A theoretical study of electronic factors affecting hydroxylation by model ferryl complexes of cytochrome P-450 and horseradish peroxidase. J. Chem. Soc. Perkin Trans. 2, 399-410. 

Ferryl complexes have been implicated in the reaction mechanisms of peroxidases and cytochromes P450. 38,1596 Pqj. horseradish peroxidase, two intermediates are spectroscopically detectable. Compound I, formed upon addition of peroxide to the resting Fe form of the enzyme, is a green species that is formally two oxidation levels higher than the resting state, and is widely believed to consist of an (Fe =0) + unit complexed by a porphyrin jt-cation radical. The [(P" ) Fe =0]+ complexes are discussed in Section 9. Compound II, which is red, and is obtained upon one-electron reduction of Compound I, also possesses a (Fe =0) + unit, in this case complexed by a normal porphyrin dianion, PFe =0. The fifth ligand, provided by the protein in the various enzymes, is a cysteine thiolate for the cytochromes nitric oxide synthases... 

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... 

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... 

The functional importance of the alkaline transition in Compound II is intriguing. Hayashi and Yamazaki [153] have shown that the ferryl/ferric redox potential is dramatically decreased at high pH. Temer and co-workers [154] recently observed that the resonance Raman oxidation state marker frequency (V4 [155, 156]) of alkaline pH ferric horseradish peroxidase was quite similar to that of Fe hemes. They proposed that oxidation of alkaline ferric horseradish peroxidase (Fe" -OH) to the Fe =0 state is promoted by the distal histidine acting as a base catalyst [154]. Likewise, the higher ferryl/ferric redox potential at lower pH [153] is consistent with the idea that hydrogen bonding to the 0x0... 

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