文摘
We report here that photoinitiated electron flow involving a metal-substituted (M = Mg, Zn) myoglobin (Mb) and its physiological partner protein, cytochrome b5 (cyt b5) can be 鈥渟ymmetrized鈥? the [Mb:cyt b5] complex stabilized by three D/E 鈫?K mutations on Mb (D44K/D60K/E85K, denoted MMb) exhibits both oxidative and reductive ET quenching of both the singlet and triplet photoexcited MMb states, the direction of flow being determined by the oxidation state of the cyt b5 partner. The first-excited singlet state of MMb (1MMb) undergoes ns-time scale reductive ET quenching by Fe2+cyt b5 as well as ns-time scale oxidative ET quenching by Fe3+cyt b5, both processes involving an ensemble of structures that do not interconvert on this time scale. Despite a large disparity in driving force favoring photooxidation of 1MMb relative to photoreduction (未(鈭捨?i>G0) 鈮?0.4 eV, M = Mg; 鈮?0.2 eV, M = Zn), for each M the average rate constants for the two reactions are the same within error, 1kf > 108 s鈥?. This surprising observation is explained by considering the driving-force dependence of the Franck鈥揅ondon factor in the Marcus equation. The triplet state of the myoglobin (3MMb) created by intersystem crossing from 1MMb likewise undergoes reductive ET quenching by Fe2+cyt b5 as well as oxidative ET quenching by Fe3+cyt b5. As with singlet ET, the rate constants for oxidative ET quenching and reductive ET quenching on the triplet time scale are the same within error, 3kf 鈮?105 s鈥?, but here the equivalence is attributable to gating by intracomplex conversion among a conformational ensemble.