How to avoid a traffic jam in photosynthetic and respiratory electron transfer chains at the Q-cycle roundabout

Orateur : Daniel PICOT (Laboratoire de Biologie Physico-Chimique des Protéines Membranaires, Institut de Biologie Physico-Chimique, Paris)

Photosynthesis and respiration couple electron transfer with translocation of protons through the membrane to provide energy to drive cellular processes. The efficiency of the electron transfer chain is increased by the insertion of the Rieske/cytochrome b complex, called b6f in chloroplasts and bc1 in mitochondria ; however, it occurs at the cost of introducing a rate-limiting step able to produce oxygen reactive species. An elegant mechanism, called Q-cycle that postulates a bifurcated pathway for electrons had been proposed in the 70s by Peter Mitchell. While the concept of bifurcation has recently been extended to other enzymatic reactions, the Q-cycle has been challenged, sometimes awkwardly, by new structural and functional data. We will confront those with our own findings.

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