Primary charge motions and light-energy transduction in bacteriorhodopsin

The bacteriorhodopsin protein (bR) in the cell membrane of Halobacterium halobium is a light driven proton pump. Many details are known about its structure and the molecular mechanism of proton translocation. The events may be characterized by: (1) the changes in light absorption after photon excitation (the photocycle); (2) the charge motion cycle inside the protein: the steps taken by the proton during translocation; (3) the retinal cycle. changes in isomerization and protonation; and (4) the opsin cycle: alterations of protonation of different amino acids in the apoprotein. From a review of existing data a more or less concise picture of the parallelism of the above four cycles emerges, which may be valuable as a model for understanding other types of molecular pumps.