Peroxo diferric intermediate

Oxygen uptake studies show that the conversion of 39 to 40 requires 0.6 (1) mol of 02, indicating that four ferrous atoms are oxidized per consumed 02 (63). This stoichiometry is similar to that reported for the autoxidation of ferrous porphyrins. The proposed mechanism, shown in Scheme 5, suggests that the autoxidation is initiated by the interaction of 02 with monomeric units of 39 that are either coordinatively unsaturated or contain a labile ligand such as solvent. The reaction of a second monomeric unit with the transient superoxo ferric complex gives rise to a (fi-peroxo)diferric species similar to that reported for [Fe HB(3,5-iPr2pz)3 (OBz)(CH3CN)] (67). This species is then somehow reduced by two electrons from 39 to yield two molecules of 40. No intermediates in the autoxidation of 39 have yet been detected even at low temperatures (—80 °C), unlike the porphyrin systems in which intermediate species such as [(Por)Fe2+02], [(Por)Fe3+-00-Fe3+(Por)], and [(Por)-Fe4+ = 0] have been spectroscopically identified (68). 

The first spectroscopically characterized intermediate in the reaction of the diferrous state (MMOred) with dioxygen is a peroxo species (Intermediate P). This intermediate subsequently converts to a high-valent bis(/x-oxo)diiron(IV) component (Intermediate Q). Intermediate Q reacts with methane releasing methanol and generating a water molecule. 

Although this proposal must be tested by Raman data, we may now postulate that in all these ferritins, the transient oxidation intermediate is a /r-peroxo-diferric charge-transfer complex similar to that formed in model complexes, in MMOH and in RNR R2 [50-56]. It remains to be determined whether the peroxo is bridging as in Scheme 15-3 or terminal as in Scheme 15-1. In FrHF the diferric-peroxo absorbance may be masked by that of the Fe-tyrosinate. In wild-type EcFtna, absorbance... 

The peroxo diferric unit a conmion intermediate in diiron enzymes... 

Oxidation of iron by EcFtnA produces a blue-colored intermediate with an absorption maximum at 650 nm and a shoulder at 370mn. This intermediate in human H-chain ferritin corresponds to a diferric peroxo species and is likely to be the same for EcFtnA, as mutation of Y24 has indicated that it caimot be an iron-tyrosinate. ... 

Note that reaction (9) indicates a pairwise reaction within subunit dimers rather than individual ferroxidase centers reacting separately. Although reaction (7) indicates the formation of hydrogen peroxide, no diferric peroxo intermediate has been observed for BFR. This may be because such a species is not formed, or, more likely, because such an intermediate is too short lived or has too low an extinction coefficient to be detected. ... 

Rates of Fe binding/ oxidation by recombinant H and L ferritins differ over a 1000-fold. The L type of ferritin protein forms polynuclear complexes, as soon as the iron is oxidized, that are indistinguishable from the mineral (B. H. Huynh and E. C. Theil, unpublished results), whereas the H type ferritin proteins form a series of ferric intermediates that include a diferric-peroxo as the first product. The di-ferric peroxo species is similar to complexes that form in methane monoxygenase and ribonucleotide reductase (see Chapter 16). Thus, the Fe - - O2 inorganic chemistry... 

Studies on the H-type animal ferritins to elucidate the mechanism of Fe oxidation show that the ferroxidase center catalytically converts two Fe and one dioxygen to a diferric oxo/hydroxo mineral precursor and hydrogen peroxide via a diferric peroxo intermediate.Overall these reactions can be summarized in Equations (1) and (2), where represents the apoprotein of net charge z, [Fe2-Pf " a diferrous ferroxidase complex, and [Fe20(0H)2-P] a hydrolyzed /u-oxo-bridged iron(III) complex at the same site. Thus in the ferroxidation reaction hydrogen peroxide is produced from the two-electron reduction of dioxygen at the diiron site (Equation... 

A report has highlighted the delicate interplay between Fe "Fe distances in such systems and the chemistry that results with the point made that this distance has been found to be particularly short at 2.53 A in the peroxo intermediate of ferroxidase activity as deduced from X-ray absorption studies.Although other techniques have been applied and compared to support this result, caution in placing too much reliance on metrical data from EXAFS should be exercised. Nonetheless, the hypothesis that this short distance and the strain that is introduced into the system might account for the fact that in ferroxidase activity the peroxo intermediate is a substrate intermediate rather than a catalytic cofactor as is the case in the related diferric enzymes such as methane monooxygenase. 

Lippard et al detected new intermediates, compounds L and Q in the catalytic cycle of MMOH from M. capsulatus (Bath) [65]. These are similar to the compounds P and Q, respectively, for MMOH from M. trichosporium OB3b. The intermediates were trapped by applying the same method as Lipscomb et al [61, 62]. After addition of dioxygen gas to the diferrous MMOH, the compound L was detected from the sample of 155 ms interval, and the compound Q was detected from the sample of 3 s interval. Resonance Raman spectrum of the compound L shows that the compound L is a diiron peroxo complex [66]. The detailed analysis of the Mossbauer spectrum of the compound L shows that the compound L has a synmietrical structure and suggests that the peroxo ligand of compound L coordinates in the I-T t or the I-T ti binding mode. 

An additional reaction intermediate, labeled intermediate P, has been observed and kinetically as well as speetroseopieally eharaeterized in two variants of E. coli R2 D84E R2 and W48F/D84E R2. Intermediate P is best described as a symmetrically bridged p,-l,2-diferric peroxo complex similar to peroxo intermediates trapped in MMO ( 2.1) and A desaturase ( 4) [99,105,308-313]. 

Jameson GNL, Weili J, Krebs C, Pereira AS, Tavares P, Liu XF, Theil EC, Huynh BH. 2002. Stoichiometric production of hydrogen peroxide and parallel formation of ferric multimers through decay of the diferric-peroxo complex, the first detectable intermediate in ferritin mineralization. Biochemistry 41 13435-13443. 

With regard to the diiron site, its most interesting features involve the mechanism of tyrosyl radical generation. In its fully oxidized form the site is diferric and EPR mute. The reaction starts with the diferrous site which is proposed to be carboxylate bridged. In this form oxygen is activated and forms, via a possible peroxo intermediate, after electron transfer a species denoted cluster X . In this form the diiron site is formally Fe(IV)Fe(III) and EPR active. From cluster X the tyrosyl radical at a close-by residue is formed. These mechanisms, which had been well studied in the past for the E. coli RNR, have recently been characterized also for RNR from mouse by stopped-flow absorption and rapid-freeze EPR spectroscopy. A combined EPR and crystallographic study in an E. coli mutant... 

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