Crystal structure bacteriorhodopsin

Essen L., Siegert R, Lehmann WD, Oesterhelt D (1998) Lipid patches in membrane protein oligomers crystal structure of the bacteriorhodopsin-lipid complex. Proc. Natl. Acad. Sci. USA 95(20) 11673-11678... 

Lanyi JK, Schobert B. Local-global conformational coupling in a heptahelical membrane protein transport mechanism from crystal structures of the nine states in the bacteriorhodopsin photocycle. Biochemistry 2004 43 3-8. 

Kouyama T, Nishikawa T, Tokuhisa T, Okumura H. Crystal structure of the L intermediate of bacteriorhodopsin evidence for vertical translocation of a water molecule during the proton pumping cycle. J. Mol. Biol. 2004 335 531-546. 

A complex multi-chromophoric system comprises the purple membrane patches from Halobacterium salinarium. These patches are composed of about 3000 bacter-iorhodopsin proteins. The hyperpolarizability of solubilized monomeric bacterio-rhodopsin was measured by HRS and found to be 2100 x 10 esu at 1064 nm. This high value is due to the presence of a chromophore in the protein, the proto-nated Schiff base of retinal. A purple membrane patch can be treated as a two-dimensional crystal of bacteriorhodopsin proteins, and its structure is known in considerable detail. The analysis of the purple membrane tensor was performed by adding the hyperpolarizabilities of the individual proteins in the purple membrane. From (depolarized) HRS measurements on purple membrane suspensions, the structure of the purple membrane patches, and an average membrane size measured by atomic force microscopy, a fi value of 2200 x 10 esu was calculated for bacteriorhodopsin [22]. The organization of the dipolar protonated Schiff base chro-mophores in the membranes was found to be predominantly octopolar. 

Notwithstanding this complexity, the need for three-dimensional, structural information at the atomic level of resolution is central and indispensable to biomembrane science. X-ray, and to a lesser extent neutron-diffraction, as the most important sources for such information have, therefore, been widely used in this field (for reviews, see Refs. 1-4). The success of this approach, however, has generally been less spectacular than for instance in the cases of protein or nucleic acid structure. The reasons for this lie in the very nature of biological membranes with few, notable exceptions (such as the purple membrane of halobacterium halobium, which can be viewed essentially as a two-dimensional crystal of bacteriorhodopsin with only little lipid. Refs. 5, 6,25) biological membranes are characterized by highly complex and variable molecular compositions, and by the structural dynamics, fluidity , which is in many cases essential for enzymatic, or other, functions of membranes. As a reflection of this most natural membranes do not crystallize, and a full, three-dimensional atomic structure analysis seems out of reach. 

Bacteriorhodopsin is the quintessential transmembrane ion pun ). It consists of a small, seven-helix protein where proton transport across the membrane is driven by photoisomerization of retinal from the all trans to the 13-cis,l5-anti configuration. A number of high-resolution crystal structures of the protein and its photointermediates have been used to propose several competing mechanisms describing proton translocation to fhe extracellular surface. Unresolved issues include understanding how conformational changes couple to proton transfer and the role played by water molecules in the proton transfer process. ... 

Essen, L., Siegert, R., Lehmann, W.D., Oesterhelt, D. 1998. Lipid patches in membrane protein oligomers crystal structure of the bacteriorhodopsin-Iipid complex. Proc. Natl. Acad. Sci. USA 95 11673-11678. Ferguson, A.D., Welte, W., Hofmann, E., Lindner, B., Holst, O., Coulton, J.W., et al. 2000. A conserved structural motif for lipopolysaccharide recognition by procaryotic and eucaryotic proteins. Structure Fold. Des. 8 585-592. 

At the molecular level, it is the light-induced trans-to-cis isomerization of the chromophore retinal that drives the vectorial proton transport. A detailed understanding of the molecular events leading to proton transport was greatly enhanced by elucidation of the crystal structures of ground-state bacteriorhodopsin and several intermediates of the photocycle. The detailed picture is gradually emerging but is still in a state of flux. Periodic reviews were provided by Lanyi. - ... 

Figure 12.3 Two-dimensional crystals of the protein bacteriorhodopsin were used to pioneer three-dimensional high-resolution structure determination from electron micrographs. An electron density map to 7 A resolution (a) was obtained and interpreted in terms of seven transmembrane helices (b).

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