An Anhydrous Proton Transfer Pathway In The Cytochrome B6f Complex

Cytochrome b6f, a hetero-oligomeric energy transducing membrane protein complex, catalyzes proton-coupled electron transfer reactions of the substrate quinone to generate as much as two-thirds of the total proton gradient used for ATP synthesis in oxygenic photosynthesis. Proton transfer pathways within b6f have remained unidentified due to limitations of crystallographic resolution1,2,3,4,5. using new crystallographic information based on a 2.70 Å crystal structure of the native complex, and structures obtained in the presence of quinone analogue inhibitors, tridecyl-stigmatellin (TDS) and 2-nonyl-4-hydroxyquinoline-N-oxide (NQNO) (resolution 3.07 Å and 3.25 Å respectively) seen in close proximity to heme cn on the electrochemically negative (n) side of the b6f complex, an anhydrous proton uptake pathway is defined that delivers protons from the n-side aqueous phase to plastoquinone bound at heme cn. A hydrated channel is identified on the electrochemically positive (p) side of the complex, that may provide a route for proton exit to the p-side aqueous phase. These n- and p-side pathways contribute to the first complete description of quinone-mediated trans-membrane proton transfer. 1. Kurisu et al. 2003, 2Stroebel et al. 2003, 3Yan et al. 2006, 4Yamashita et al. 2007, 5Baniulis et al. 2009