THE LOCAL-ACCESS MECHANISM OF PROTON TRANSPORT BY BACTERIORHODOPSIN

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 na ture of the causal interrelationships in the transport cycle of this r etinal protein. In the metastable photoisomerized states, although not in the thermally stable isomers, the local access of the retinal Schi ff base alternates rapidly between the extracellular and cytoplasmic d irections. The direction of proton transfer, and therefore the 'proton ation 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 proto n 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 effe cts 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. (C) 1 998 Elsevier Science B.V.