Molecular mechanism of ion transport in bacteriorhodopsin: Insights from crystallographic, spectroscopic, kinetic, and mutational studies

In the past few years, a multidisciplinary approach to the study of bacteriorhodopsin has provided answers to the most important questions about the photochemical transport cycle and yielded detailed molecular descriptions of several of its steps. The free energy gain after absorption of a photon is found to consist of electrostatic changes at the retinal Schiff base and steric conflicts with protein residues and bound water, introduced by photoisomerization of the retinal. Relaxation of the retinal chain, initially constrained by its binding site, is seen after deprotonation of the Schiff base. Structural rearrangements in the extracellular region, initiated by protonation of Asp-85 and mediated by Arg-82, cause release of a proton to the surface. Structural rearrangements in the cytoplasmic region, initiated by;movement of the 13-methyl group of the retinal and mediated by Trp-182, then cause reprotonation of the Schiff base by Asp-96. The initial state is regained by reprotonation of Asp-96 from the cytoplasmic surface, reisomerization of the retinal, and deprotonation of Asp-85, that appear to utilize the same pathways of coupling.