Lumen acidification, qE quenching and photosynthetic control

Sousa and colleagues (2013) placed the Q-cycle late in bioenergetic evolution. They argued that prokaryotes which built cytochrome bc type respiratory chains probably also invented the Q-cycle, the membrane-phase, quinone-based analogue of flavin-based electron bifurcation.¹ They noted that flavins are far more universal than quinones among prokaryotes.¹ In their account, methanogens and acetogens first solved the pumping problem with an Na+/H+ antiporter and flavin-based bifurcation, and quinones, the Q cycle and cytochromes b and c arose later.²

Protasoni and Zeviani (2021) described the cycle in mitochondrial complex III. Complex III oxidizes QH2 to Q and passes electrons to cytochrome c.³ Coenzyme Q appears in three forms: ubiquinone, semiquinone and ubiquinol. The Qo site faces the intermembrane space and oxidizes ubiquinol. The Qi site faces the matrix and reduces ubiquinone.⁴ Brzezinski, Moe and Ädelroth (2021) stated that the mechanism requires a bifurcated transfer from QH2 at the QP site, with one electron going to the FeS cluster and one to heme bL.⁵

Brzezinski and co-authors compared supercomplex structures of M. smegmatis and C. glutamicum.⁶ In one M. smegmatis supercomplex the cyt. cc domain of complex III took a closed and an open conformation in the two halves. They hypothesized that its movement could mediate electron transfer from MQH2, but said it was unknown whether the movement is stochastic or linked to other reactions.⁶ In the C. glutamicum supercomplex and another M. smegmatis structure, all elements of the electron-transfer chain appear fixed, which suggests the Q-cycle can be realized without domain movements.⁶ They read this as variability in the structural solution to the Q cycle.⁶

Didaran and co-authors (2026) describe the proton motive force as divided between ΔpH and Δψ. ΔpH sets lumen pH, which acts as a chemical signal for photoprotective switches on timescales of seconds to minutes.⁷ They name two control axes: excitation dissipation by NPQ, especially qE and qZ, and electron-transfer throttling at cytochrome b6f.⁷ On their account, Δψ dissipation through anion influx lets ΔpH rise rapidly, engaging qE and photosynthetic control before PSI acceptor-side limitation becomes severe.⁸ If g(H+) is very high or Δψ dissipation is weak, ΔpH expression is delayed and PSI becomes more vulnerable.⁸