Structural changes underlying proton pumping by Bacteriorhodopsin

Protons are released from the extracellular surface and subsequently taken up from the cytoplasmic side of the bacteriorhodopsin molecule during its photocycle. A number of spectroscopically distinct intermediates have been characterised by a wide variety of techniques. The structure of most of these intermediates can be examined directly by electron diffraction of two-dimensional crystals provided the intermediate can be trapped by rapidly freezing the specimens to liquid nitrogen temperature. In addition, a variety of mutant bacteriorhodopsin molecules have perturbed photocycles which provide additional opportunities to examine the behaviour of the structure following a light flash. Proton release on the extracellular side and cytoplasmic uptake require opposite vectoriality in the Schiff base accessibility. The nature and size of the structure change required to provide this vectorial change in accessibility, the "gating" of the channels, will be described.