Structural Changes in Bacteriorhodopsin in Response to Alternate Illumination Observed by High-Speed Atomic Force Microscopy

摘要

The membrane protein bacteriorhodopsin (bR), found in Halobacterium salinarum, functions as a light-driven proton pump transferring protons across the membrane from the cytoplasmic side to the extracellular side. bR is comprised of seven transmembrane a helices (named A-G) surrounding the retinal chromophore covalently bound to Lys216 through a protonated Schiff base. bR assembles into trimers, which are packed into two-dimensional hexagonal lattices, the so-called purple membrane. Upon absorption of light, photoisomerization from the all-trans to the 13-cis conformation of retinal takes place, with subsequent primary proton transfer from the Schiff base to Asp85, which triggers a cascade of changes in bR structure. A series of intermediates designated J, K, L, M, N, and O have been defined by spectroscopy, and M (M_(410), having a light absorbance peak at 410 nm) is the only intermediate containing a deprotonated Schiff base.