Ferrous center

The kinetics and the mechanism of superoxide reduction by SORs have been studied by several researchers. It was suggested that SORs react with superoxide via an inner-sphere mechanism, binding superoxide at ferrous center to form a ferric hydroperoxo intermediate [46,48 50]. The rate constant for this reaction is equal to 108 109 1 mol-1 s-1 [46,49], This... 

The bpza ligand in 4a coordinates with the anti lone pair of the carboxylate donor to the ferrous center. Thus, the as5unmetric carboxylate IR absorption (Vas = 1653cm ) appears at even lower wavenumbers compared to that of 4b. 

Moura I, Tavares P, Moura JJ, et al. 1990. Purification and characterization of desul-foferrodoxin. A novel protein from Desulfovibrio desulfuricans (ATCC 27774) and from Desulfovibrio vulgaris (strain Hildenborough) that contains a distorted rubredoxin center and a mononuclear ferrous center. J Biol Chem 265 21596-602. 

Photosystem 1 Reaction Center 4 bacteriochlorophylls, 2 bacteriopheophytins and a non-heme ferrous center [Fe(N-His) (0-Glu)] 131... 

Solid-state magnetic susceptibility data for 36 is consistent with little or no exchange coupling between the two ferrous centers (58, 59). The Mossbauer spectrum (zero field, 4.2 K) of 36 shows a broad asymmetric doublet consistent with two distinct sites (8i = 1.26 mm s 1, AEq = 2.56 mm s-1 82 = 1.25 mm s-1, A Eq = 3.30 mm s-1). EPR spectrum (X-band, 7 K) of the diferrous system contains a broad feature at g 16, indicative of an integer S = 4 spin state. 

NO reacts with both ferric and ferrous centers in hemoproteins to form the respective iron(II) and iron(III) nitrosyl adducts, whose structural features are similar to those observed for iron (II) and iron(III) porphyrin nitrosyls. These analogies are also reflected in similar chemical reactivity observed for nitrosylated ferri- and ferroproteins and their respective porphyrin models. For example, NO-adducts of Fe(III) undergo reductive nitrosylation in the presence of an excess of NO, and a similar process is commonly observed for synthetic Fe(III) porphyrins. The first step of this reaction involves nucleophilic attack of OH on the nitrosyl ligand coordinated to the iron center, as presented in reaction (13) (33,60) ... 

Another important contribution of pulse radiolysis is in the evaluation of redox processes in native SODs and development of SOD mimics. SOD is an endogenous antioxidant enzyme which catalyzes the conversion of Oj radicals to H2O2. Different types of SODs are present in cells such as Mn-SOD in mitochondria and Cu, Zn-SOD in the cytosol and in extracellular surfaces. Reactions of O " radicals with the active site of native SODs from bacterial and animal sources have been examined. In one recent study involving superoxide reductase (SOR) from Desulfoarculus baarsii, the precise step responsible for the catalytic action was examined. Its active site contains an unusual mononuclear ferrous center. Since protonation processes are essential for the catalytic action, the pH dependence of the redox properties of the active site, both in the absence and in the presence of O radicals, was studied using pulse radiolysis. The results confirmed that the reaction of SOR with O2" radicals involves two reaction intermediates, an iron(III)-peroxo species and an iron(III)-hydroperoxo species. The protonation takes place in the second step, and therefore responsible for its catalytic activity. 

Nonheme ferrous centers in some metalloproteins react reversibly with NO forming nitrosyl complexes with S = 312 characterized by the g values of about 4.0 and 2.0 [51]. The EPR spectrum of the nitrosylated NorR (abacterial NO-responsive transcription factor, the enhancer binding protein) is typical of a d high-spin Fe NO", where the S = 5/2 iron is antiferromagnetically coupled to the NO (Fig. 5, [52]). This is confirmed by the X-ray, resonance Raman, MCD, Mossbauer spectroscopies, and DFT calculations. Similar structures were proposed for the classical complexes, [Fe(NO)(l-isopropyl-4,7-(4-ferf-butyl-2mercaptobenzyl)-l,4, 7-triazacyclononane)], [53], Fe(EDTA)NO [54—56], the brown-ring compound, Fe(H20)5N0 [57], and for the Fe(N/V ,N -trimethyl-l,4,7-triazacyclononane) (N3)2N0 [54]. Interestingly, for the latter a spin equilibrium between the valence tautomers 5=1/2 and 3/2 in the solid state was observed. 

When NO binds to the reduced PCA-3,4-PCD complex (PCA-E ), PC A binds to the ferrous center only by the 4-carbon-OH group of PCA as shown in Fig. 13. Corresponding to the two possible structures, 5- or 6-coordinate forms, two types of the peroxo species in Fig. 14 are proposed for the oxygenation process, though it is difficult to select one from these two types. 

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