Ferriheme protein

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).

The ferriheme protein metmyoglobin (metMb) at the physiological pH 7.4 was reported to bind the NO molecule reversibly yielding the nitrosyl adduct [metMb(NO)] the kinetics of the association and dissociation processes were investigated and a limiting dissociation mechanism was proposed (58,68). 2-His-l-Glu nonheme iron center engineered into myoglobin was reported capable to bind Fe(II) and reduce NO to N2O (69). 

In view of the finding that some ferriheme proteins in insects (37) are able to carry nitric oxide, a NO carrier role for the metmyogiobin in vivo can be considered. As was shown, NO binds reversibly to the six-coordinate, high-spin iron(III) center of metmyogiobin (38) possessing a water and histidine molecule as distal and proximal ligand, respectively. Activation parameters determined for NO interaction with metMb (Table 4) clearly... 

Indolamine dioxygenase (- Tryptamine oxygen 2,3-oxidoreductase (decyclizing). This enzyme which is either a ferroheme protein which uses oxygen as dioxygen substrate or a ferriheme protein using superoxide ion 129, 130) is non-specific in its choice of substrates (2,127). 

Astashkin AV, Raitsimring AM, Walker FA. 1999. Two- and four-pulse ESEEM studies of the heme binding center of a low-spin ferriheme protein the importance of a multi-frequency approach. Chem Phys Lett 306( l-2) 9-l 7. 

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).

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). 

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. 

In studying ET between proteins, complications may arise because the systems exhibit more than one stable conformational state (66). In hemoglobin (Hb), for example, the U2P2 tetramer exists in two distinct states, T (deoxyhemoglobin) and R (oxyhemoglobin). Electron transfer between subunits in hemoglobin hybrids, [ai(Fe), P2(Zn)] and [ai(Zn), p2(Fe)], has been studied by Hoffman and co-workers (47, 66, 121, 151). The association of the Fe and Zn subunits has been extensively characterized (121) photogenerated Zn-protoporphyrin transfers an electron to a ferriheme acceptor, as outlined in Scheme V. 

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