Figure 5.

Model of electronic bifurcation of the Q_o site accommodating the SQ_o–FeS coupled system. (a) Bound QH₂ is flanked by oxidized heme b_L and oxidized FeS. (b) FeS withdraws an electron from QH₂, which leads to the formation of the SQ_o–FeS triplet state. (c) The SQ_o–FeS distance increases (by movement of the FeS head domain and/or SQ_o), breaking spin exchange interaction, exposing separate spectra of SQ_o and reduced FeS. (d) Heme b_L is reduced by SQ_o generating Q. (e) In the noninhibited enzyme, heme b_L rapidly transfers an electron across the membrane to heme b_H directly or through heme b_L in the other monomer^44,45 (not shown). The enzyme goes through further states to reach the initial state a. Antimycin prevents oxidation of heme b_H, interrupting the transition from state d to state e. Black and red denote the oxidized and reduced cofactor, respectively, while the dot with an arrow indicates the paramagnetic state of the center. Orbitals engaged in spin exchange are shown as gray ovals. Blue, black, magenta, and green spectra are EPR spectra of heme b_L, SQ_o, FeS, and the SQ_o–FeS triplet state, respectively. Green arrows show transitions between the enzyme states. The blue box denotes the state that was detected as a major fraction of SQ. The scheme does not consider the still unknown proton transfers that may influence transitions between the states.