The local-access mechanism of proton transport by bacteriorhodopsin

Abstract

The recently proposed local-access model, based on spectroscopic data, explains the various modes of light-driven proton transport reported for wild-type bacteriorhodopsin and Asp-85 mutants, and reveals the nature of the causal interrelationships in the transport cycle of this retinal protein. In the metastable photoisomerized states, although not in the thermally stable isomers, the local access of the retinal Schiff base alternates rapidly between the extracellular and cytoplasmic directions. The direction of proton transfer, and therefore the ‘protonation switch’ of the pump, is decided by the proton acceptor and donor functions of Asp-85 and Asp-96, respectively. In the D85N/D96N mutant the acceptor and donor are absent, and the various observed transport modes are based on the same principles but a different combination of events. It is the result of the interplay between the differing proton conductivities between the Schiff base and the two membrane surfaces, and return to the obligate access to the extracellular side once the initial all- trans state is regained in the cycle. The causes and effects in this local-access model are explicit: the changes in the access of the Schiff base from one membrane surface to the other are direct consequences of the proton transfer reactions that precede them.