Fig. 3. Schemes to show potential electron transfer pathways involved in electron equilibration between monomers at the level of heme b_L.

A. The redox centers of the dimer are shown, and the Q_o- and Q_i-sites indicated by stigmatellin and antimycin, respectively. The scheme shows the dimer with the Q_i-site blocked in both monomers by antimycin (blue crosses), and the Q_o-site of one monomer blocked by myxothiazol (red cross). Under these conditions, if electron transfer between hemes b_L could occur, both hemes b_H could be reduced by turnover of the uninhibited Q_o-site (broken red arrows). B. The dimer interface, showing residues referred to in the text. The dotted line shows the 10.21 Å distance between heme b_L 4-vinyl groups (taken from PDB file 2qjy, chains A–F).