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Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved February 1, 2013 (received for review December 19, 2012)
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 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.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: waclab{at}purdue.edu.
Author contributions: W.A.C. designed research; S.S.H., E.Y., and D.B. performed research; W.A.C. contributed new reagents/analytic tools; S.S.H., E.Y., D.B., and W.A.C. analyzed data; and S.S.H. and W.A.C. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The atomic coordinates and structure factors have been deposited in the Protein Data Bank, www.pdb.org [PDB ID codes 4H44 (native cyt b6f structure), 4H13 (tridecyl-stigmatellin–containing structure), and 4H0L (2-nonyl-4-hydroxyquinoline N-oxide–containing structure)].
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1222248110/-/DCSupplemental.
Proton transfer by cytochrome b6f
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