Ferric Forms of Heme Proteins

High-Spin Ferric Forms of Heme Proteins... 

Fig. 5. Electronic spectra of a mixture of ferric cytochrome c and enhanced green fluorescent protein EGFP before (—) and after (—) 490 nm photolysis indicating the formation of the ferrous form of heme c (6) with maxima at 413, 520, and 550 nm. Adapted from Ref (37).

The similarities between CCP (Im") and CAT tend to reinforce the belief that the anionic character of the proximal ligand in these two proteins promotes peroxidase activity or at least stabilizes the active form of these proteins. A recent study ( ) of a model ferric heme system in which internally hydrogen-bonded imidazoles were shown to increase peroxidase activity also supports this hypothesis. 

Both catalase and ascorbate peroxidase are heme-containing proteins with peroxidase activities. The mechanism by which SA inhibits the activity of both these enzymes appears to be similar [112]. The native ferric form of catalase (Fe III) undergoes a two electron... 

Scheme 1.5. Pathways for oxygen-atom transfer from the active ferryl species 7 of heme-thiolate enzymes such as cytochrome P450 to form the product alcohol coordinated to the ferric, resting form of the protein (8).

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... 

How does nature prevent the release of hydrogen peroxide during the cytochrome oxidase-mediated four-electron reduction of dioxygen It would appear that cytochrome oxidase behaves in the same manner as other heme proteins which utilize hydrogen peroxide, such as catalase and peroxidase (vide infra), in that once a ferric peroxide complex is formed the oxygen-oxygen bond is broken with the release of water and the formation of an oxo iron(IV) complex which is subsequently reduced to the ferrous aquo state (12). Indeed, this same sequence of events accounts for the means by which oxygen is activated by cytochromes P-450. 

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