High-resolution cryo-EM structures of plant cytochrome b 6 f at work.

Plants use solar energy to power cellular metabolism. The oxidation of plastoquinol and reduction of plastocyanin by cytochrome b 6 f (Cyt b 6 f) is known as one of the key steps of photosynthesis, but the catalytic mechanism in the plastoquinone oxidation site (Q p ) remains elusive. Here, we describe two high-resolution cryo-EM structures of the spinach Cyt b 6 f homodimer with endogenous plastoquinones and in complex with plastocyanin. Three plastoquinones are visible and line up one after another head to tail near Q p in both monomers, indicating the existence of a channel in each monomer. Therefore, quinones appear to flow through Cyt b 6 f in one direction, transiently exposing the redox-active ring of quinone during catalysis. Our work proposes an unprecedented one-way traffic model that explains efficient quinol oxidation during photosynthesis and respiration.