Ferriheme

Low-spin sulfite reductase were isolated from D. vulgaris (160), De-sulfuromonas acetoxidans (161), and Methanosarcina barker) (DSM 800) (162). The D. vulgaris protein has a molecular mass of 27 kDa and contains a single [4Fe-4S] cluster and one siroheme. The EPR spectrum shows a rhombic signal with g values at 2.44, 2.36, and 1.77, characteristic of a ferriheme low-spin system. This is a unique... 

Figure 18.4 Structures of heme/Cu oxidases at different levels of detail, (a) Position of the redox-active cofactors relative to the membrane of CcO (left, only two obligatory subunits are shown) and quinol oxidase (right), (b) Electron transfer paths in mammalian CcO. Note that the imidazoles that ligate six-coordinate heme a and the five-coordinate heme are linked by a single amino acid, which can serve as a wire for electron transfer from ferroheme a to ferriheme as. (c) The O2 reduction site of mammalian CcO the numbering of the residues corresponds to that in the crystal structure of bovine heart CcO. The subscript 3 in heme as and heme 03 signifies the heme that binds O2. The structures were generated using coordinates deposited in the Protein Data Bank, lari [Ostermeier et al., 1997] Ifft [Abramson et al., 2000] (a) and locc [Tsukihara et al., 1996] (b, c).

A.R. Raitsimring and F.A. Walker, Porphyrin and ligand protons as internal labels for determination of ligand orientations in ESEEMS of low-spin d5 complexes in glassy media ESEEM studies of the orientation of the g tensor with respect to the planes of axial ligands and porphyrin nitrogens of low-spin ferriheme systems, J. Am. Chem. Soc., 1998, 120, 991. 

Helmholtz equation puys chem The relationship stating that the emf (electromotive force) of a reversible electrolytic cell equals the work equivalent of the chemical reaction when charge passes through the cell plus the product of the temperature and the derivative of the emf with respect to temperature. helm,holts i.kwa-zhon hematin org chem C34ff3305N4Fe The hydroxide of ferriheme derived from oxidized heme. he mad an ... 

The Fe(III)-NO complex of NPl is EPR silent (Fig. 3) because it contains an odd-electron (ferriheme) center bound to the odd-electron diatomic NO 24), which creates a FeNO center. The NMR spectrum of NPl Fe(III)-NO is that of a diamagnetic protein 85). However, whether the electron configuration is best described as Fe(II)-NO+ or antiferro-magnetically coupled low-spin Fe(III)-NO- is not completely clear, even though the infrared data 49) discussed earlier (Fig. 7) are consistent with the former electron configuration. Thus, as a prelude to planned detailed studies of the Mossbauer spectra of the nitrophorins and their NO complexes, we have reported the Mossbauer spectrum of the six-coordinate complex of OEPFe(III)-NO 86). 

Fig. 4. Reductive nitrosylation of ferrihemes. Proposed formation of FenNO + as a precursor for nucleophilic attack of OH forming the bound nitrous acid intermediate. The release of nitrite forms a labile Fe11 species which binds NO forming the inert FenNO complex.

The same can be said on the fast NO-release from the ferriheme compounds (22). It has been proposed that the heme FenNO + moieties are highly reactive toward NO release, provided that they are not trapped by a nucleophile or reduced by one electron to the very stable Fen-NO species (14). We are now considering a comparison with the behavior of NP toward dissociation. 

In this section, the complex formations and the coordination structures of ferri- and ferro-protoporphyrineIX(ferriheme and ferrohem, respectively) 22 with... 

The n values of the PLL-ferriheme and -ferroheme complexes in Table 4 are equal to 2, and the six-coordinate heme structure is formed preferentially this may be due to an inherent factor in which PLL differs from the other polymer ligands. 

Table 4. Axial coordination number (n) and formation constant (tf) for polymer-ferriheme and -ferroheme complexes 3 4 ... 

Ligand Solvent n Ferriheme Ferroheme K (1/molforn Ferriheme = 1 l2/mol2 forn = 2) Ferroheme... 

Fig. 4. Schematic representation of the structure of polymer-heme complexes the ferriheme complex with poly(4-vinylpyridine) (a), poly(L-lysine) (b), (c), or poly(7-benzyl-L-glutamate) with a pendant imidazole (d)...

Table 4 was unity, which indicated the five-coordinate structure as in the PVMI-heme complex. PBLGIm forms an a-helix, and the helix content and intrinsic viscosity were unchanged in the PBLGTm ferriheme complex. The formation constant of the ferriheme complex with PBLGTm was not so different from that of the imidazole complex (Table 4). The strong coordination was thought to be due to an additional hydrogen bond between a propionic residue of ferriheme and a carbonyl residue in the side chain of PBLGTm, as shown in Fig. 4(d)w ...

We recently studied the ESR spectra of the mixed heme complex 24, where the nitrogen monoxide radical is inserted into the sixth coordinate site of heme-iron as a probe28 in order to examine the electronic structure of the heme complex29. Figure 6(a) shows the ESR spectrum of the NO radical which coordinates to the imidazole-ferriheme complex. The hyperfine stmcture (hfs) due to the N of the... 

One of the major difficulties encountered in attempts to prepare 1 1 dioxygen-iron complexes which can desorb molecular oxygen, particularly oxygenated complexes of ferroheme, is the strong driving force toward the irreversible formation of the stable p-oxo ferriheme dimer, as represented in Eq. (11). The oxygenated... 

Peroxidases and catalases contain high-spin Fe(III) and resemble metmyoglobin in properties. The enzymes are reducible to the Fe(II) state in which form they are able to combine (irreversibly) with 02. We see that the same active center found in myoglobin and hemoglobin is present but its chemistry has been modified by the proteins. The affinity for 02 has been altered drastically and a new group of catalytic activities for ferriheme-containing proteins has emerged. 

The decay curves of the triplet excited state of zinc porphyrin for TZnCCP and the [TZnCCP/cyt c] complex with reduced horse cytochrome c heme are exponential with the same decay rate. Upon addition of horse cyt c with the oxidized heme to the solution of ZnCCP, the TZnCCP decay remains exponential but the decay rate increases until a 1 1 ratio is reached and then remains constant. The form of the dependence between the rate and the concentration of cyt c indicates that ZnCCP and the cyt c form a strong 1 1 complex. These results indicate electron tunneling at the distance of 25 A to be the reason for the enhancement of TZnCCP decay in the presence of cyt c. The rate of electron transfer from TZnCCP to the low-spin ferriheme within the [ZnCCP/horse cyt c] complex was found to be 17 3 s 1 at 293 K [70]. 

The rate of electron tunneling from TZnCCP to the ferriheme of the yeast cytochrome c was found to be roughly 10 times bigger than that observed in the case of the homologous horse cytochrome c. This difference demonstrates the fine degree of species specificity involved in biological electron transfer and must reflect subtle structural differences between horse and yeast cytochromes [70],... 

NOTE Ferroprotoporphyrin is the term used for reduced heme (mol. wt. 616 Windholz et al., 1976). Hematin (mol. wt. 633) is one of several terms used for the oxidized or ferric heme. Other terms include ferriporphyrin hydroxide or ferriheme hydroxide, due to the binding of a hydroxyl to the ferric heme iron. Hornsey (1956) used the term acid hematin (mol. wt. 652) to describe the heme oxidation product in acidified acetone solutions. The more common term hemin is used in this unit. It is also known as chlorohemin, due to binding of a chloride ion to the ferric heme iron. 

Analogously, in bis-histidine ferriheme proteins, cytochromes b (see Fig. 5.20B) [50,51], cytochromes C3 [52] and cytochromes C7 [53], the chemical shifts for each methyl proton at 298 K appear to be dependent on the

structural angles according to the heuristic equation [48] ... 

Fig. 5.20. Hyperfine shifts of methyl protons in (A) Met80Ala cytochrome c-CN-, and (B) cytochrome b5. The former is a histidine-cyanide ferriheme protein, since the axial ligand methionine is substituted with alanine, the latter is a bis-histidine ferriheme protein (labeling as in Fig. 5.7B).

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