Acid-Base Equilibria and the Proton Pump in Bacteriorhodopsin

The light-induced proton pump in bacteriorhodopsin is reviewed with emphasis on acid-base equilibria of protein residues and of the retinal Schiff base moiety. Pump mechanisms in bR and in some of its mutants are classified in terms of light-induced pK_a changes (class I) or light-induced exposure changes, in which the proton accessibility of the protein changes from the outside to the inside of the membrane (class II). A discussion of the theoretical basis of the factors which determine the pK_a of ionizable protein groups is followed by a review of the experimental phenomena associated with the titration of residues in both unphotolyzed bR and during its photocycle. The time-resolved titrations of the Schiff base and of the Asp-85 residue are discussed in terms of the accessibility of the two groups to external protons. Finally, the molecular aspects of the pH-dependent proton pump in native bR and in various mutants are analyzed, focusing on the mechanism of the initial proton release reaction and on the subsequent molecular switch which allows reprotonation from the inside of the cell. Special attention is devoted to the question of coupling between the photocycle intermediates (primary M formation and decay) and the transmembrane proton translocation. Recent work with bR mutants raise the question as to whether proton transfer from the Schiff base to Asp-85 at the M stage is directly responsible for proton translocation, as well as for the reprotonation switch.