ABSTRACT    
   As much as two-thirds of the proton gradient used for transmembrane free   
   energy storage in oxygenic photosynthesis is generated by the cytochrome b6f   
   complex. The proton uptake pathway from the electrochemically negative (n)   
   aqueous phase to the n-side    
   quinone binding site of the complex, and a probable route for proton exit to   
   the positive phase resulting from quinol oxidation, are defined in a 2.70-Å   
   crystal structure and in structures with quinone analog inhibitors at 3.07 Å   
   (tridecyl-stigmatellin)    
   and 3.25 Å (2-nonyl-4-hydroxyquinoline N-oxide) resolution. The simplest   
   n-side proton pathway extends from the aqueous phase via Asp20 and Arg207   
   (cytochrome b6 subunit) to quinone bound axially to heme cn. On the positive   
   side, the heme-proximal Glu78 (   
   subunit IV), which accepts protons from pastosemiquinone, defines a route for   
   H+ transfer to the aqueous phase. These pathways provide a structure-based   
   description of the quinone-mediated proton transfer responsible for generation   
   of the transmembrane    
   electrochemical potential gradient in oxygenic photosynthesis   
      
   http://phys.org/news/2013-04-team-proton-pathway-photosynthesis.html   
      
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