Spin Ferric Forms of Heme Proteins

II ferric heme centers are also characterized by a electronic ground 

Two of the pioneers of EPR analyses of low-spin ferric heme proteins are undoubtedly J. Peisach and W.E. Blumberg. Their so-called truth tables allow for a prediction of the axial ligands of die iron heme center from the principal g values [18]. Although the truth tables are based on crude assumptions and do not include type 1 or type 111 centers, they can nevertheless give in many cases a valuable first clue as to the type of axial ligands. As an example, based on X-band cw-EPR lysine was predicted to be the sixfli ligand of native cytochrome/ [19,20]. However, in order to obtain more detailed and conclusive information on the heme pocket, the X-band cw-EPR studies need to be supplemented wifli more advanced EPR experiments. 

All foregoing studies were performed at X-band microwave frequencies. B. Hoffman and coworkers showed, however, in a number of studies on chloroperoxi-dase and P450cam that a combination of cw-, Davies- and Mims-ENDOR experiments at Q-band (35 GHz) can largely facilitate the analysis of low-spin ferric heme proteins [31,32]. Indeed, due to the magnetic-field dependence of the nuclear Zeeman interaction, the spectral contributions stemming from N, N, H, H, and 

It is important to note that, despite the above remarks, the recording of the ESE-detected EPR spectra is still favored over cw-EPR spectroscopy to determine the lowest principal g value in low-spin ferric systems with high g anisotropy (supplementary material of [44]). Furfliennore, for type I systems, 2D nutation spec- 

Finally, the recent major evolutions in quantum chemistry (especially the development of DFT [density functional theory]) have made it feasible to compute the EPR parameters of low-spin heme systems, as was demonstrated for simple ferric porphyrin systems [46,47], However, state-of-the-art quantum chemistry is still a long way from an identical match between experimental and computed EPR parameters, and the further development of these quantum-chemical methods form undoubtedly one of the biggest challenges for the next years. 

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High-Spin Ferric Forms of Heme Proteins... 

Although a similar Fe(IV)=0 intermediate of IDO has not yet been directly observed in the catalytic cycle of TDO, it has been trapped and characterized in the enzyme reactivation study. The ferric form of TDO can be reactivated by H202in the presence of Trp through a complex mechanism [40]. When Fe(lll)-TDO reacts with H2O2 in the absence of T rp, the Fe ion becomes EPR-silentand a protein-based radical is observed (Fig. 15.6). The free radical decays overtime, while the EPR signal of the high-spin ferric heme is restored. The ferric ion is shown by Mossbauer spectroscopy to... 

Low-spin ferric iron is conunon in heme and in some iron-sulfiir proteins, but nitrile hydratase is presently the lone example among mononuclear, non-heme, non-iron-sulfur iron proteins [31]. The ferric form of the enzyme is obtained by photolysis of the as-isolated nitric oxide d form. The g-values of the photodissoci-ated ferric protein are 2.27, 2.13, and 1.97, and photointermediates of differing g-values are observed. The relatively narrow linewidths allowed the Fe hyperfine to be measured at (40 MHz), establishing that the g- and iron A-tensors are 45° out of alignment. 

Trandafir F, ter Heerdt P, Fittipaldi M, Vinck E, Dewilde S, Moens L, van Doorslaer S. 2007. Stud5dng high-spin ferric heme proteins using pulsed EPR spectroscopy analysis of the ferric form of the E7Q mutant of human neuroglohin. Appl Magn Reson 31 553-572... 

MCD spectroscopy in range 300 to 2000 nm at both ambient and liquid helium (4.2 K) temperatures can yield information about the spin, oxidation, and coordination states of each heme in a multiheme protein such as CCP (75). This technique, in combination with low-temperature X-band EPR spectroscopy, was used to great effect in characterizing the properties of the fully oxidized and MV forms of the P. aeruginosa CCP in solution. At 4.2 K, both hemes in the oxidized enzyme are low-spin ferric, with diagnostic features in the near infrared-MCD (NIR-MCD) spectrum consistent with one heme with His/Met axial coordination and the other with bis-histidine axial coordination this is entirely consistent with the crystal structure. In contrast, at room temperature only the low-potential (bis-histidine coordinated) heme in the C-terminal domain remains completely low-spin, whereas the high-potential (His/Met coordinated) heme exists as mixture of high- and low-spin forms 58). 

Electronic absorption maxima for LIP, MnP, horseradish peroxidase (HRP) and various llganded forms of these enzymes are shown in Table I. The spectra of native LIP and MnP are characteristic of high-spin ferric heme proteins, with Soret and visible maxima ( 407, 502 and... 

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