ACID-BASE EQUILIBRIA AND THE PROTON PUMP IN BACTERIORHODOPSIN

The light-induced proton pump in bacteriorhodopsin is reviewed with em phasis on acid-base equilibria of protein residues and of the retinal Schiff base moiety. Pump mechanisms in bR and in some of its mutants a re classified in terms of Light-induced pK(a) changes (class I) or lig ht-induced exposure changes, in which the proton accessibility of the protein changes from the outside to the inside of the membrane (class Il). A discussion of the theoretical basis of the factors which determ ine the pK(a) of ionizable protein groups is followed by a review of t he experimental phenomena associated with the titration of residues in both unphotolyzed bR and during its photocycle. The time-resolved tit rations of the Schiff base and of the Asp-85 residue are discussed in terms of the accessibility of the two groups to external protons. Fina lly, the molecular aspects of the pH-dependent proton pump in native b R and in various mutants are analyzed, focusing on the mechanism of th e initial proton release reaction and on the subsequent molecular swit ch which allows reprotonation from the inside of the cell. Special att ention is devoted to the question of coupling between the photocycle i ntermediates (primary M formation and decay) and the transmembrane pro ton translocation. Recent work with bR mutants raise the question as t o whether proton transfer from the Schiff base to Asp-85 at the M stag e is directly responsible for proton translocation, as well as for the reprotonation switch.