Figure 1.

Model diagram of the bc₁ dimer. (A) Dimeric model of the bc₁ complex is shown with major redox centers and partial reactions. Midpoint potentials for the redox centers on the complex are from Table S1 and represented at pH 7. The model assumes only one Q_p site is active per turnover (31). Two turnovers at the Q_p site are required per turnover at the Q_n site. The model lumps oxidation of QH₂ at the Q_p site into a two-electron step. The first electron is used to reduce cytochrome c, and the second electron is deposited on the bc₁ complex. The monomer order during QH₂ oxidation at the Q_p site is random. Quinone reduction at the Q_n site is also random. Electrons on the complex distribute themselves among the redox centers according to the Boltzmann distribution. Electron transport between b hemes and the Q_n site is electrogenic. Coulombic interaction energies between intramonomer b_L and b_H hemes and intermonomer b_L and b_L hemes are included. Blue and yellow circles are approximate locations of Q_p- and Q_n-site binding pockets, respectively. The dimer cartoon was generated from the crystal structure by Esser et al. (13) (PBD: 5KLV). The depicted proton uptake and release pathways are only for visual purposes. (B) State representation of the model is shown where E_i is the ith electronic state corresponding to the number of electrons residing on the complex. The state-transition rate constants, k_ij, are given in the Supporting Material. Probabilities <0.4 are shown in gray. (C) For each enzyme state, E_i, the probability of finding an electron on each redox center in the model is shown for the following conditions: Q pool 10% reduced, membrane potential of 0 mV, pH 7 on both sides of the membrane, no cytochrome c present, and under anoxia. Cartoon rendering of the bc₁ complex was done using the software PyMOL (108). To see this figure in color, go online.