BACTERIORHODOPSIN - A PARADIGM FOR PROTON PUMPS

Recent studies of the photochemistry of wild type and mutant bacterior hodopsins, their proton release and uptake kinetics, and their X-ray d iffraction structure have suggested a hypothesis for the way energy is coupled in this light-driven proton pump. The first and critical step in converting light energy to a vectorial proton potential is the tra nsfer of the Schiff base proton to D85 which causes dissociation of th e Schiff base-counterion complex. Removal of this primarily coulombic interaction destabilizes the protein structure, and results in transit ion to an alternative conformation in which the two proton conduction pathways between the active site and the membrane surfaces are reorgan ized. Recovery of the initial charge state of the Schiff base and D85 must therefore occur through a series of unidirectional proton transfe rs that create a transmembrane electrochemical proton gradient. Passag e of the transported proton through the two peripheral protein domains appears to utilize hydrogen bonded networks containing aspartate, arg inine and bound water. This kind of mutual interaction between the act ive site and the protein conformation that determines the conductive p athways to the two membrane surfaces may have relevance to ion pumps i n general.