PMID: 23440205

Authors:

Hasan SS, Yamashita E, Baniulis D, Cramer WA

Title:

Quinone-dependent proton transfer pathways in the photosynthetic cytochrome b6f complex.

Journal:

Proc Natl Acad Sci U S A. 2013 Mar 12;110(11):4297-302. doi:, 10.1073/pnas.1222248110. Epub 2013 Feb 25.

Abstract:

As much as two-thirds of the proton gradient used for transmembrane free energy storage in oxygenic photosynthesis is generated by the cytochrome bf 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-A crystal structure and in structures with quinone analog inhibitors at 3.07 A (tridecyl-stigmatellin) and 3.25-A (2-nonyl-4-hydroxyquinoline N-oxide) resolution. The simplest n-side proton pathway extends from the aqueous phase via Asp20 and Arg207 (cytochrome b subunit) to quinone bound axially to heme c. On the positive side, the heme-proximal Glu78 (subunit IV), which accepts protons from plastosemiquinone, 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.