Bacteriorhodopsin as a model for proton pumps.

According to a long-standing hypothesis, membrane pumps function by flip-flopping between two protein conformations that allow alternative access of the ion binding site to the two membrane surfaces. Site-specific mutagenesis, time-resolved spectroscopy and X-ray diffraction confirm this mechanism for bacteriorhodopsin, and implicate change of electrostatic interaction at the active site as the trigger for the global protein conformation change during the proton transport cycle.