1. Institut de Biologie Structurale, CEA-CNRS-Université Joseph Fourier, UMR 5075, 41 rue Jules Horowitz, F-38027 Grenoble Cedex 1, France
2. European Synchrotron Radiation Facility , 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France
3. Department of Biochemistry, Uppsala University, Biomedical Centre, Box 576, S-751 23 Uppsala , Sweden
4. Biozentrum, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland
5. These authors contributed equally to this work.
6. Present address: Department of Physiology and Biophysics and Sealy Center for Structural Biology, University of Texas Medical Branch, 301 University Boulevard, Galveston , Texas 77555-0641, USA.

Correspondence to: Ehud M. Landau^5,6 Correspondence and requests for materials should be addressed to E.M.L. (e-mail: Email: emlandau@utmb.edu). Crystallographic co-ordinates and structure factor amplitudes have been deposited with the Protein Data Bank (accession code: 1E0P).

Abstract

A wide variety of mechanisms are used to generate a proton-motive potential across cell membranes, a function lying at the heart of bioenergetics. Bacteriorhodopsin, the simplest known proton pump¹, provides a paradigm for understanding this process. Here we report, at 2.1 Å resolution, the structural changes in bacteriorhodopsin immediately preceding the primary proton transfer event in its photocycle. The early structural rearrangements² propagate from the protein's core towards the extracellular surface, disrupting the network of hydrogen-bonded water molecules that stabilizes helix C in the ground state. Concomitantly, a bend of this helix enables the negatively charged³ primary proton acceptor, Asp 85, to approach closer to the positively charged primary proton donor, the Schiff base. The primary proton transfer event would then neutralize these two groups, cancelling their electrostatic attraction and facilitating a relaxation of helix C to a less strained geometry. Reprotonation of the Schiff base by Asp 85 would thereby be impeded, ensuring vectorial proton transport. Structural rearrangements also occur near the protein's surface, aiding proton release to the extracellular medium.

1. Institut de Biologie Structurale, CEA-CNRS-Université Joseph Fourier, UMR 5075, 41 rue Jules Horowitz, F-38027 Grenoble Cedex 1, France
2. European Synchrotron Radiation Facility , 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France
3. Department of Biochemistry, Uppsala University, Biomedical Centre, Box 576, S-751 23 Uppsala , Sweden
4. Biozentrum, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland
5. These authors contributed equally to this work.
6. Present address: Department of Physiology and Biophysics and Sealy Center for Structural Biology, University of Texas Medical Branch, 301 University Boulevard, Galveston , Texas 77555-0641, USA.

Correspondence to: Ehud M. Landau^5,6 Correspondence and requests for materials should be addressed to E.M.L. (e-mail: Email: emlandau@utmb.edu). Crystallographic co-ordinates and structure factor amplitudes have been deposited with the Protein Data Bank (accession code: 1E0P).