Coupling photoisomerization of retinal to directional transport in bacteriorhodopsin

In order to understand how isomerization of the retinal drives unidirection al transmembrane ion transport in bacteriorhodopsin, we determined the atom ic structures of the BR state and M photointermediate of the E204Q mutant, to 1.7 and 1.8 Angstrom resolution, respectively. Comparison of this M, in which proton release to the extracellular surface is blocked, with the prev iously determined M in the D96N mutant indicates that the changes in the ex tracellular region are initiated by changes in the electrostatic interactio ns of the retinal Schiff base with Asp85 and Asp212, but those on the cytop lasmic side originate from steric conflict of the 13-methyl retinal group w ith Trp182 and distortion of the pi-bulge of helix G. The structural change s suggest that protonation of Asp85 initiates a cascade of atomic displacem ents in the extracellular region that cause release of a proton to the surf ace. The progressive relaxation of the strained 13-cis retinal. chain with deprotonated Schiff base, in turn, initiates atomic displacements in the cy toplasmic region that cause the intercalation of a hydrogen-bonded water mo lecule between Thr46 and Asp96. This accounts for the lowering of the pK(a) of Asp96, which then reprotonates the Schiff base via a newly formed chain of water molecules that is extending toward the Schiff base. (C) 2000 Acad emic Press.