:"The Q-cycle at 45: Insights from the plant cytochrome b6f complex”.

Rieske /cytochrome b -type complexes such as cytochrome bc₁ and cytochrome b₆f play a central role in energy conservation and transformation in bioenergetic membranes involved in photosynthesis and respiration. Electron transfer within these complexes involves oxidation of the two electron carrier quinol and the reduction of a one electron carrying soluble protein such as plastocyanin or cytochrome c. The second electron derived from quinol is recycled to a second quinone molecule within the complex in a series of reactions known as the ‘Q-cycle’, which doubles the number of protons transferred across the membrane per quinol molecule oxidised. However, both the oxidation of quinol and reduction of quinone can potentially lead to the formation of the highly reactive semiquinone intermediate, which can reduce molecular oxygen to superoxide causing damage to the cell. The existence of such reactions are particularly deleterious to oxygenic phototrophs, which are bathed in high concentrations of oxygen produced by photosystem II activity. Yet it remains unclear how Rieske /cytochrome b -type complexes avoid or minimise semiquinone formation. By purifying the higher plant cytochrome b₆f complex from spinach and solving its high-resolution structure by cryo-electron microscopy [1] we provide new insights into these questions. Moreover, our data provides a rationale for the presence of additional co-factors (chlorophyll and extra c-type haem) present in photosynthetic cytochrome b₆f but absent from respiratory cytochrome bc₁.