evolutionary conserved Q-cycle

  4 H⁺ per e^- pair CoQH₂ thru Complex III

1.    CoQH₂ binds to Q_o site outer site
2a.  releases 2H⁺ into perimito space
2b.  1e^- passed to Cyto-C via an FeS
3.    1e^- passes to cytoB_L-->B_H
           & reduces CoQ at matrix Q_i site
4.    oxidized CoQH₂ dissociates at Q_o site
5.    2nd CoQH₂ binds to open Q_o
6a.  repeat of step 2a.
6b.  repeat of step 2b.
7.    repeat of step3.
8.    2H⁺ of matrix reduce CoQ --> CoQH₂
9.    CoQH₂ dissociates from Q_i site
10.  open Q_i site binds new CoQ

                           repeat