Ferryl species

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,.

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

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. 

Although there is still debate as to whether hydroxyl radicals or ferryl species are the key oxidants in Fenton systems, most literature reports on the mechanisms of degradation of organic compounds invoke the hydroxyl radical. Based on the reports discussed above, it seems likely that hydroxyl radical is a major oxidant during Fenton degradations. Although ferryl ions or other highly oxidized forms of iron may occur, either to a limited extent or more abundantly under specific conditions, this section will deal with documented reaction pathways and kinetics for hydroxyl radical or species assumed to be hydroxyl radical. The reader should keep in mind that ferryl pathways may need to be considered under certain conditions. 

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. 

As already mentioned, in some enzymes radicals generated on surface tryptophan and tyrosine radicals by electron transfer to the ferryl species are involved in abstraction of electrons from substrates [33-36, 40]. Mutation of Trpl71 on the surface of P. chrysosporium LiP to a phenylalanine or serine completely suppresses the veratryl alcohol oxidizing activity of the enzyme [40]. A similar depression in the oxidation of veratryl alcohol occurs on mutation of Trpl64 in the versatile peroxidase from P. eryngii [34, 35]. 

The peroxidases produce a reactive intermediate, Compound I, whose reactions are controlled by the protein environment. In conventional peroxidases with a histidine iron ligand, access to the Compound I ferryl species is restricted by the protein, favoring the transfer of single electrons from the substrate to an exposed heme edge. If the protein has a suitably placed oxidizable residue such as a tyrosine or... 

The role of medium polarization in the vicinity of the model compound I-like the ferryl species, [Por (HS)-Fe (IV) O], and the effect of hydrogen bonding of the fifth... 

But such an oxidation would again require a strong oxidizing power (E° close to 1V). Most ferric complexes would not fulfill this requirement. Hypothetically, Fe(IV) ferryl species could play a more significant role in peroxyl radical production from hydroperoxides. 

There are now good theoretical descriptions of the electronic structures contributing to the optical absorption bands in spectra of porphyrin radicals and ferryl species [160,167] most charge-transfer bands in the latter are due to a transition from a porphyrin p orbital to an Fe-0 tt orbital [167], However, in the absence of a prior knowledge of the structure around the Felv site (and/or spectra of a variety of synthetic model compounds) it is not straightforward to assign an optical spectrum to a ferryl species. Thus the intermediate assumed to be the ferryl species in the binuclear haem c /Cub centre of cytochrome c oxidase [168] has a spectrum at 580 nm essentially identical [169] to that of low-spin ferric haem a3 compounds (e.g. cyanide). 

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