Reduction of the Ferrous α-Verdoheme−Cytochrome b₅ Complex

Abstract

The ferrous alpha-verdoheme-cytochrome b(5) complex, [Fe(II)(verdoheme)](+), has been prepared and characterized spectroscopically. Anaerobic addition of excess sodium dithionite to [Fe(II)(verdoheme)](+) at pH 10 produces a one-electron-reduced species with spectroscopic characteristics that suggest a ferrous hexacoordinated verdoheme pineutral radical best formulated as a [Fe(II)(verdoheme*)] --> [Fe(I)(verdoheme)] resonance hybrid. At lower pH values (7.0 and 8.0) the one-electron-reduced species is shown to disproportionate to produce the resting state [Fe(II)(verdoheme)](+) complex and the two-electron-reduced [Fe(II)(verdoheme:)](-) anion. The latter might also be formulated as a resonance hybrid [Fe(I)(verdoheme*)](-) --> [Fe(II)(verdoheme:)](-). The disproportionation reaction becomes very slow as the pH is raised above 9.0. Exposure of the one-electron- or two-electron-reduced verdoheme complexes of cytochrome b(5) to O(2) results in rapid and quantitative reoxidation to the resting state [Fe(II)(verdoheme)](+) complex.

Reduction of oxaporphyrin ring of CO-bound α-verdoheme complexed with heme oxygenase-1 by NADPH-cytochrome P450 reductase

“…5B). The g values obtained agreed with those reported for π-neutral radical species of bispyridine-bound ferrous verdoheme dimethyl ester in dimethylformamide/pyridine (6:4) solution at 163 K (g ⊥ = 2.002 and g // =1.937) [22] and those of ferrous verdoheme complexed with cytochrome b 5 at 90 K (g = 2.02, 2.00, and 1.925) [35]. Thus we concluded that the paramagnetic species generated in the rHO-1 system is a π-neutral radical of CO-bound ferrous verdoheme, [Fe II (verdoheme•)], complexed with rHO-1.…”

“…3B). A similar species showing absorption peaks at 454 and 497 nm has been reported as a two-electron reduced form, [Fe II (verdoheme:)] − , in the reduction of verdoheme reconstituted with apocytochrome b 5 at pH 7.0 and 8.0 [35]. Thus we judged that the one-electron reduced π-neutral radical, [Fe II (verdoheme•)], of CO-bound ferrous verdoheme in complex with rHO-1 was generated first, followed by formation of the two-electron reduced species, [Fe II (verdoheme:)] − .…”

“…3A). This absorption spectrum was similar to that of the π-neutral radical of ferrous verdoheme dimethyl ester prepared in dimethylformamide/ pyridine (6:4) solutions [22] and also resembled that of the π-neutral radical of ferrous α-verdoheme reconstituted with apocytochrome b 5 at pH 10.0 [35]. When the applied potentials were lowered further, the absorption peak at 760 nm decreased gradually with a concomitant increase of absorption at 441 and 494 nm (Fig.…”

“…As a model for the reduction of verdoheme in complex with HO, the chemical reduction of α-verdoheme-apocytochrome b 5 complex was previously investigated [35]. reaction as well as a variety of other hemoproteins such as the cytochrome P450s and cytochrome b 5 .…”

Electrochemical reduction of ferrous α-verdoheme in complex with heme oxygenase-1

“…This redox reaction was only partially reversible (data not shown), probably due to a reaction leading to nonproductive decomposition of verdoheme, that does not lead to biliverdin [8]. While the immediate product of the reaction has yet to be characterized, a two-electron-reduced anion form of ferrous verdoheme has been indicated as a possible candidate [32]. Indeed, applying potentials below −0.60 V vs NHE caused further spectral changes, and this over-reduction was completely irreversible (data not shown).…”