Helix deformation is coupled to vectorial proton transport in the photocycle of bacteriorhodopsin

A wide variety of mechanisms are used to generate a protonmotive potential across cell membranes, a function lying at the heart of bioenergetics. Bact eriorhodopsin, the simplest known proton pump(1), provides a paradigm for u nderstanding this process. Here we report, at 2.1 Angstrom resolution, the structural changes in bacteriorhodopsin immediately preceding the primary p roton transfer event in its photocycle. The early structural rearrangements (2) propagate from the protein's core towards the extracellular surface, di srupting the network of hydrogen-bonded water molecules that stabilizes hel ix C in the ground state. Concomitantly, a bend of this helix enables the n egatively charged(3) 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 th eir electrostatic attraction and facilitating a relaxation of helix C to a less strained geometry. Reprotonation of the Schiff base by Asp 85 would th ereby be impeded, ensuring vectorial proton transport. Structural rearrange ments also occur near the protein's surface, aiding proton release to the e xtracellular medium.