Iron center diferrous form

Structure of the Iron Center Formation of the Iron Center and Tyrosyl Radical Spectroscopy of the Diferric Iron Center Spectroscopy of the Tyrosyl Radical Redox Properties of the Iron Center Mixed-Valent Form of the Iron Center Diferrous Form of the Iron Center Inhibitors to Iron-Containing Ribonucleotide Reductase Methane Monooxygenase A. Spectroscopy of the MMOH Cluster X-Ray Structure of MMOH... 

The site in the active Fe ribonucleotide reductase contains two Fe(III) ions 3.3 A apart, bridged by one carboxylate from a glutamate residue and a water-derived oxo bridge (57). The function of this iron center appears to be the formation and stabilization of a free radical on a tyrosine about 5 A away. This radical is formed by reaction of the reduced, diferrous center with 02, probably through peroxide and ferryl intermediates. This unusually stable tyrosyl radical is thought to partic-... 

The iron center of protein R2, which is one of the focal points of this review, is an antiferromagnetically coupled pair of high-spin ferric ions in the active state. Figure 3 shows the known redox states of the iron/ tyrosyl radical site in the protein. The crystal structure of the met form, i.e., the diferric form without radical, around the iron site (Figs. 

An interesting observation is that the radical neighboring form and the met form of the iron center of E. coli protein R2 gave slightly different EPR spectra after radiolytic reduction at 77 K, indicating a different ligand geometry in the diferric states in the two cases (100). At present the EPR differences cannot be directly interpreted in terms of structural differences. 

The fully reduced iron center in E. coli R2 is structurally characterized from analogy with a Mn-containing R2 protein (101). There was no evidence of any bridging ligand in the structure, also in agreement with preliminary data on the diferrous form of R2 (Par Nordlund, personal communication). 

Mossbauer, MCD, and EPR spectroscopy have been used to study the diferrous form of the iron center. EPR spectra of the fully reduced... 

Structurally characterized models for the diferrous oxidation state thus far number only three. The first reported by Wieghardt, [(Fe2(0H)(0Ac)2(Me3TACN)2](C104) (70, 87), has a (p.-hydroxo)bis( x-carboxylato)diiron(II) core. Its structure is closely related to that of the diferric form, but the Fe-fi-O and Fe-Fe distances of 1.99 and 3.32 A, respectively, are longer than their ferric counterparts. The Fe(II) ions in this complex are antiferromagnetically coupled with aJ of —13 cm" and the complex is thus EPR silent. The J value of the complex is comparable to that found for deoxyHr, suggesting that deoxyHr is likely to have a hydroxide bridging the iron centers. 

In contrast to MMOH and RNR, Hr has the diferrous form as the resting unreacted state, and there is a single five-coordinate site for end-on binding of O2. After O2 binding, a Fe end-on bound hydroperoxide (OOH) intermediate is formed, with the iron centers becoming diferric. In Hr, the O2 binding step occurs comparatively earlier, and the oxidation states of the first two intermediates are reversed compared to RNR and MMO. The resting states of both are diferric... 

Spectroscopic studies have demonstrated that the MMOHs have the stable diiron centers of diferric Fe(III)Fe(III), mixed-valent Fe(II)Fe(III), and diferrous Fe(II)Fe(II) redox states [33]. Although the electronic absorption spectrum of the MMOH from M. capsulatus (Bath) shows an absorption peak at 280 nm due to the protein, there is no significant optical features that can be attributed to the iron prosthetic group. The Mdssbauer Fe enriched spectrum of the isolated MMOH from M. trichosporium OB3b shows two quadrupole doublets of equal area. Their parameters at 4.2 K are AEq =1.16 mm/s and the isomer shift 5 = 0.51 mm/s for site 1 and AEq = 0.87 mm/s and the isomer shift 6 = 0.50 mm/s for site 2 [34]. The Mdssbauer spectrum of the isolated MMOH from Af. capsulatus (Bath) at 80 K shows a quadrupole doublet having AEq = 1.05 mm/s and the isomer shift 6 of 0.50 mm/s [35]. These spectra show that the isolated MMOH is diferric form. 

Recombinant MIOX from Mus musculus kidney was recently subjected to a detailed spectroscopic and kinetic characterization. The nature of the iron active site in the presence and absence of substrate wyo-inositol was examined using both EPR and Mfissbauer methodologies [387]. The outcome of this study has not only confirmed that MIOX contains a binuclear non-heme iron center, but also that its diiron cluster is stable in its mixed-valent form and the fully oxidized state. Specifically, the low-temperature X-band EPR spectrum of fully reduced, diferrous MIOX reveals a geff 16 resonance that disappears upon sample exposure to O2, consistent with a conversion of the diferrous center to its mixed-valent form. The... 

The latter compound attracts special interest because it forms more rapidly in the absence of substrates (k = 1.2 s 1) than it autodecays (k = 0.05 s 1) and, therefore, can be directly investigated by physicochemical methods. The Mossbauer spectrum of compound Q from M. trichosporium indicates that the diiron center consists of two high-spin antiferromagnetically-coupled iron atoms, each in the Fe(IV) state bridged by oxygen atom. Compound Q reacts very quickly with methane and other substrates with the formation of compound T. The latter releases a product and is transformed to diferric MMOH. 

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