Figure 2.

Proposed intracomplex electron shuttle mechanism in cyt bc₁. (Left) Schematic representations of the different steps of electron transfer (A, B, D, F, and H) and movement (C, E, and G) in the uninhibited cyt bc₁. The [2Fe2S] cluster domain, the cyt c complement (c₁ + c_y/c₂) and the Q_o site are shown as a brown ellipsoid, orange rectangle, and tan square, respectively. Black dots represent electrons located on the [2Fe2S] cluster and the cyt c heme complement before flash oxidation (A). Arrows refer to electron transfers between cofactors and movement of the [2Fe2S] cluster domain between its Q_o and c₁ positions. (Insets) Kinetics of cyt c oxidation (downward deflection) and cyt c reduction (upward deflection). The dotted parts of the traces in D, F, and H indicate the predicted unresolved fast phase. On flash activation of the reaction center and oxidation of the cyt c complement (B), the [2Fe2S] cluster domain is proposed to move from its Q_o to c₁ position (C), transfer an electron to cyt c₁ of the cyt c complement (D), and return to the Q_o position (E). Then, QH₂ is oxidized to ubiquinone (Q); one electron is transferred to the [2Fe2S] cluster; and the other is transferred to the low potential cyt b chain (F). The [2Fe2S] cluster domain moves back to the c₁ position (G) where it transfers another electron (H). (Right) Representation of the steps C (movement) and D (electron transfer) shown in Left in inhibited states. In the presence of the Q_o site inhibitor myxothiazol (Top), normal movement of the [2Fe2S] cluster domain will yield partial reduction of the cyt c complement with an unresolved fast phase (blue traces). In the presence of stigmatellin or in a mutant locking the [2Fe2S] cluster domain in Q_o position (Middle), the cyt c complement should remain completely oxidized (red traces), whereas in a mutant slowing the movement (Bottom), this fast phase should be time-resolved (blue traces).