Oxo-ferryl

In fact, spectroscopic measurements indicate that form P contains an oxo-ferryl ion with the second oxygen of the original 02 converted to an OH ion and probably coordinated with CuB.136a 136c 142/142a c P may also contain an organic radical, perhaps formed from tyrosine 244 as indicated in Fig. 18-11. 

Peroxidases (EC 1.11.1.7). Peroxidases are hemoproteins, produced mainly by microorganisms and plants, which catalyze oxidation of the recalcitrant nonphenolic lignin units in the presence of hydrogen peroxide (Duran and Esposito, 2000). This is possible because of the formation of a high redox potential oxo-ferryl intermediate during the reaction of the heme cofactor with H202 (Martinez et al., 2005). Dubey et al. (1998) studied the polymerization of catechol by plant peroxidases and found that the resultant polymers consisted of phenylene and oxyphenylene units (Figure 2.14). 

It is important to bear in mind that ferrylmyoglobin has two forms the radical species as described above, and the non-radical iron-oxo ferryl form in the iron(IV) state. 

Figure 16. Proposed L-arginine-assisted NOS oxygen activation. First, substrate L-arginine (only guanidinium shown) donates a proton to the per-oxo-iron, facilitating 0-0 band cleavage and conversion to a proposed oxo-ferryl 7r-cation radical species. The radical species then rapidly hydrox-ylates the neutral guanidinium to A "-hydroxy-L-arginine. (Adapted from Ref. [100].)...

Liu, M.H. and Y.O. Su (1998). Selective electrocatalysis of alkene oxidations in aqueous media. Electrochemical and spectral characterization of oxo-ferryl porphyrin, oxo-ferryl porphyrin radical cation and their reaction products with alkenes at room temperature. J. Electroanal. Chem. 452, 113-125. 

Perhaps the most important is the enzyme s ability to control the delivery of protons to the developing negative charge of the iron-dioxygen complex as electrons are introduced into the center. The protonation/deprotonation events at the bound superoxo, peroxo, and hydroperoxo anions were theoretically analyzed and claimed to account for the definitive reaction sequence in the formation of the active oxo-ferryl porphyrin... 

As non-toxic chiral Fe complexes have recently been used as catalysts [118-120], increased knowledge of their structure-reactivity relationships becomes pertinent. X-band CW-EPR spectra of [Fe °Cl(l)], reported by Bryliakov et al. [121], were found to be typical of high-spin S = 5/2 Fe complexes with EID K, 0.15. Using this complex, the conversion and selectivity of the asymmetric sulphide oxidation reaction was investigated in a variety of solvents. In previous studies [122], the active site was proposed to be the [Fe =0(l)] species. However an alternative active species was proposed [121]. Oxo-ferryl 7i-cation radicals are expected to have typical S = 3/2 spectra with resonances at geff 4... 

Another endogenous HNO source relies on the oxidation of hydroxylamine (HA), or other alcohol amine, such as hydroxyurea or A -hydroxy arginine. In vivo, such a process is postulated to depend on the activity of several heme proteins, which are able to stabilize oxo ferryl species (compound I and compound II), such as peroxidases. Recently, Donzelli et al. evaluated HNO production by this mechanism (22), with a newly developed selective assay in which the reaction products, GS(0)NH, in the presence of reduced glutathione (GSH) are quantified by HPLC. Their results showed that metmyoglobin, horse radish peroxidase, and myeloperoxidase were efficient HNO producers using hydroxylamine as substrate. However, there are several remaining unresolved questions concerning the proposed mechanism (which is outlined below, Eq. (2)). 

There have been a number of recent review articles on the application of EXAFS spectroscopy to the study of metalloproteins [1-8]. The theory of EXAFS spectroscopy and the historical development of the field have also been extensively discussed [1-5,7-19]. Here we will briefly cover the practical aspects of data analysis for biological heme (iron porphyrin) systems eind the appropriate model complexes. We will then focus on the EXAFS of two types of biological heme system (a) thiolate-ligated heme enzymes and (b) oxo-ferryl [oxo-iron (IV), Fe =0] states of heme enzymes. All of the enzymes discussed herein have in common the iron protoporphyrin IX ( heme ) as the prosthetic group, Fig. 1. 

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. 

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... 

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