Oxyferryl porphyrin

The EPR and ENDOR spectroscopy was used for studies of catalytic intermediates in native and mutant cytochrome P450cam in cryogenic temperatures (6 and 77K) (Davydov et al., 2001). The ternary complex of camphor, dioxygen, and ferrous-enzyme was irradiated with y-rays to inject the second electron. This process showed that the primary product upon reduction of the complex is the end -on intermediate. This species converts even at cryogenic temperatures to the hydroperoxo-ferriheme form and after brief annealing at a temperature around 200 K, causes camphor to convert to the product. In spite of conclusions derived from x-ray analysis (Schlichtich et al., 2000) no spectroscopic evidence for the buildup of a high-valance oxyferryl/porphyrin rc-cation radical intermediate during the entire catalytic circle has been obtained. 

Col, which has been described as an oxyferryl porphyrin % cation radical [27], accepts one electron and one proton from a reducing substrate (RH) to yield the corresponding free radical (R ) and the oxyferryl heme intermediate known as compound II (Coll) ... 

Once activated, MV-CCP reacts with 1 equiv of H2O2 in a bimolecu-lar reaction, presumably to form compound 0. In YCCP and HRP this species is referred to as compound ES or compound I, respectively, and contains oxyferryl heme and either a porphyrin n -cation radical (HRP) or an amino acid radical (YCCP). However, the presence of an extra reducing equivalent on the second heme in CCP suggests that such an oxidizing radical species close to the active site heme will be very shortlived and readily form compound I (Fig. 10), which is formally Fe(HI) Fe(IV)=0. The bimolecular rate constant for compound I formation is reported to be very close to the diffusion limit (84). 

Compound I is a two-electron oxidized enzyme intermediate containing a oxyferryl iron and a porphyrin cation radical while compound II is an one-electron oxidized intermediate (13), With lignin pa oxidase, as with other peroxidases, the substrate oxidation products are fir radicals which undergo nonenzymatic disproportionation reactions to give rise to the final products. 

In Section 3.6, we have described the method to produce unstable species in inert gas matrices by laser-photolysis and simultaneously determine their structures by RR spectroscopy. The example given was oxyferryl(IV) porphyrins produced by laser photolysis of the corresponding oxyiron(III) porphyrins ... 

HRP exhibits the typical peroxidase fold and active site stractme as shown in Figme 6(b). As with CcP, the native resting form of HRP-C contains a five-coordinate, high-spin Fe + heme. Compound I of HRP contains two oxidizing equivalents, one as oxyferryl (Fe" +-0) and the other as porphyrin radical. " A transient Trp r-cation radical has been detected in the Phe221Trp mutant of HRP-C compound... 

Electron-transfer reactions between cytochrome c and cytochrome c peroxidase have been studied extensively because of the well-characterized structures and biophysical properties of the reactants [146-150]. It is well known that the resting ferric form of cytochrome c peroxidase is oxidized by hydrogen peroxide to compound I, which contains an oxyferryl heme moiety in which the iron atom has a formal oxidation state of 4-1- [146-150]. The other is a porphyrin n radical cation or organic radical (R +) localized on an amino acid residue of Trp-191 [151-154] this is formed by transfer of the oxidized equivalent to the amino acid side chain [150]. The site of electron transfer in the reduction of compound I has been controversial and two forms of compound II have been identified, (P)Fe =0 containing the oxyferryl heme Fe(IV) [155-158] and [(P)Fe ] + containing Fe(III) and the porphyrin % radical cation which oxidizes the amino acid side-chain to produce an organic radical [(P)Fe +, R" ] [159 165] as shown in Scheme 10. 

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. 

The catalytic mechanism of heme CATs is a two-step reaction. In the first step, interaction between ferriCAT (Fe +) and hydrogen peroxide leads to the heterolytic cleavage of the 0-0 bond in the substrate molecule and oxygen is bound to the 6th valency of the porphyrin iron. In this reaction Compound I (in which formal oxidation state of Fe is +5), a spectroscopically distinct and enzymatically active form of CAT, is formed. Compound I is an oxyferryl species in which one oxidation equivalent is removed from the iron and one from the porphyrin ring to generate a porphyrin rr-cation radical [206] ... 

In compound I the two oxidizing equivalents are separated the iron is present in an oxyferryl (Fe = O) form [107,108] while the other equivalent is a free radical which, depending on the specific peroxidase, can be either a porphyrin radical cation or a radical localized on an amino acid in the polypeptide. 

Figure 2. Direct bioreduction of hydrogen peroxide using a peroxidase modified enzyme at a potential below than 0.6 vs SCE, Compound-I is the oxyferryl Jt-cation radical heme intermediate 1 and compound-II is the oxyferryl intermediate 2, where both of them are the oxidised forms of the native ferriperoxidase. P is the cation radical, which is localised on porphyrin ring or polypeptide chain.

The mechanism for (most) heme peroxidases is believed to be as shown in Scheme 3. The first step involves 0-0 bond cleavage and the resultant formation of an oxyferryl unit and an attendant radical. In most cases, the radical is located on the porphyrin ring leading to Compound I. The electronic structure of Compound I is subtle and has itself been the subject of a num-... 

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