The proton transfers in the cytoplasmic domain of bacteriorhodopsin are facilitated by a cluster of interacting residues.
The stepwise internal proton transfer reactions across the membrane, and the release and uptake at the surface, are the elementary steps that together constitute the transport mechanism in a proton pump. Although the proton donor and acceptor residues can be usually identified, the directionality and the energetics of the proton transfer must be determined to a large extent also by interactions of these with neighboring groups. We have examined the roles of residues D96, T46 and R227 in proton transfers during the photocycle of bacteriorhodopsin near its cytoplasmic surface, and in general the relationship between the reprotonation of the Schiff base and the subsequent proton uptake from the cytoplasmic side. The phenotypes of single and double mutants suggest close functional interaction among D96, T46, R227, and probably internal bound water. Measurements of the free energies of activation indicate that mechanistic interpretation of the rates changed by residue replacements is hindered by a general tendency toward lowered activation enthalpies in the mutated proteins. There is less ambiguity in the free energy levels of the photointermediates. It appears from these that the inhibitory and stimulatory influences of T46 and R227, respectively, on D96 as a proton donor compensate one another and ensure the effective reprotonation of the Schiff base. T46 and D96 mediate, in turn, proton uptake at the cytoplasmic surface. Although ultimately this will reprotonate D96, the observation of proton uptake from the bulk in R82Q without reprotonation of the aspartate residue suggests that the direct proton acceptor is not D96. The results thus indicate that the passage of the proton from the surface to the Schiff base is facilitated by multiple residue and water interactions in the cytoplasmic domain.