Ferryl species, formation

Fig. 14. Manifold of reactive species produced from the reaction of a heme group with oxygen and two reducing equivalents. The rate of conversion of A to B limits the lifetime (and therefore reactivity) of the Fe peroxo anion. The rate of formation of the ferryl species C via the Fe -OOH complex B competes with the intramolecular hydroxylation reaction to give hydroxyheme. Reactions of the Fe -hydroperoxy complex B with exogenous electrophilic substrates must compete with conversion of the intermediate to both C and meso-hydroxyheme. The Fe -OOH complex B can also be formed directly with H2O,.

Kinetic studies have shown that the product formed in the reaction of the fully oxidized enzyme with hydrogen peroxide is catalytically inactive. Reaction of the half-reduced enzyme with hydrogen peroxide leads to an enzymatically active compound, in which the Fe" heme is oxidized to Fe, and the FeIU heme is oxidized to the FeIV ferryl species. No stoichiometric formation of a radical species is observed, unlike the case for other peroxidases. The peroxide-oxidized enzyme will then oxidize two molecules of reduced cytochrome c. Mechanistic details are still unclear, particularly with regard to the interaction between the two heme groups, a phenomenon revealed by ESR studies.1373... 

Ferryl species are well-documented and play a major role in P-450-type systems. In general, Fe(II)-containing enzymes try to avoid the formation of -OH in their reaction with H202. A similar situation seems to prevail in the case of Fe2+ complexed by DTPA (Rahhhal and Richter 1988), and one has to be keep in mind when discussing Fenton and Fenton-type reactions that complexation and possibly also the pH may shift the Fenton reaction from OH to Fe(IV) as the reactive intermediate. 

Upon the addition of H202 to a solution of the reconstituted Mb, the characteristic spectrum assigned as the ferryl species (FeIV=0) was observed with a formation rate constant of 1.6 x 103 s-1 at 20°C. A comparison of the reactivities between ferryl Mb and the reconstituted Mb is summarized in Fig. 19. Interestingly, the relative rate constants toward neutral substrate oxidation by the reconstituted Mb are remarkably higher than those observed... 

Harris DL, Loew GH. Investigation of the proton-assisted pathway to formation of the catalytically active, ferryl species of P450s by molecular dynamics studies of P450eryF. J Am Chem Soc 1996 118 6377-6387. 

Shen X, Tian J, Li J, Li X, Chen Y. Formation of the excited ferryl species following Fenton reaction. Free Radic Biol Med 1992 13 585-592. 

In view of the formation of a highly reactive Compound I ferryl species, and the fact that the porphyrin radical cation of this intermediate is reduced in enzymes such as CcP by a protein residue, it is not surprising that permanent covalent modifications are autocatalytically introduced into some protein frameworks. Two examples of autocatalytic protein modification, those of LiP and the catalase-peroxidases, are summarized here to illustrate the maturation of peroxidase protein structures that can have important functional consequences. 

It is possible to generate ferryl species by peroxide treatment of ferrous iron ions [18,249], The two-electron oxidation of ferrous (as opposed to ferric) iron does not require the formation of a cation radical, although subsequent reactions may generate hydroxyl radicals. These reactions therefore provide an alternative mechanism to the Fenton reaction for free radical damage associated with low-molecular-weight iron species. In the absence of a protective protein environment, however, such low-molecular-weight ferryl species are unstable and difficult to detect and therefore their existence is controversial [see the review by Koppenol in this volume (Chapter 1)]. 

A mechanism to account for the formation of the ferryl species, was postulated. According to this mechanism (equations 5 and 6), one of the iron atoms in the ii-oxo dimer 31 is oxidized by two electrons. 

Photoinduced electron-transfer in the opposite direction was demonstrated upon irradiation of the Ru(bpy)3 +-Mb system in the presence of Co +(NH3)5Cl as a sacrificial electron acceptor (Figure 44B) [244]. The photochemical reaction results in the formation of ferryl species (i.e., Fe(IV)-heme), with the intermediate formation of the porphyrin cation radical (as demonstrated using laser flash photolysis [237]). The electron-transfer cascade includes the primary oxidative quenching of the excited chromophore, Ru(bpy)3"+, by Co +(NH3)5Cl to yield Ru(bpy)3 + [E° = +1.01 V vs. SCE). The resulting oxidant efficiently takes an electron from the porphyrin ring (fcet = 8.5 x 10 s ) and the porphyrin cation radical produced further oxidizes the central iron atom, converting it from the Fe(III) state to the Fe(IV) state (/cet = 4.0 x 10 s at pH 7.5). 

Several mechanisms can be envisaged for heme alkylation that are consistent with the experimental data, none of which involves a concerted transfer of the oxygen to the rr-bond. Subsequent to possible formation of a charge transfer complex between the ferryl species and the olefin -ir-bond, addition of the oxygen to the Tr-bond could give a... 

Product analysis alone provides reasonable evidence for hydroxyl radical reactivity in vivo. However, the exact identity of the hydroxylating species remains elusive. It is worth noting that production of either a ferryl species or hydroxyl radical via Fenton chemistry requires iron, whereas formation of, and hydroxylation by, ONOO is metal independent. 

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