Structure Images
Bacteriorhodopsin (BR) is a photon-driven ion pump. BR is a small (26 kD) protein consisting of seven transmembrane helices, A through G, and short extramembrane segments; it has retinal as a co-factor. It is found in the bacterium H. salinarium where it converts light to a proton gradient which in turn is used by a second membrane protein called ATP synthase to generate chemical energy in the form of ATP. ATP is then used by the cell to drive a multitude of vital processes.
The pump is energized by the all-trans to 13-cis photoisomerization of the retinal chromophore.The photocycle is initiated by the absorption of a photon by the all-trans retinal co-factor which is linked to the protein by a protonated Schiff base at Lys216. The structure of the unphotolyzed (BR) state has revealed a three-dimensional interconnected hydrogen-bonded network of protein residues and water molecules between the centrally located protonated retinal Schiff base and the extracellular surface and provided clues about how release of a proton to the extracellular membrane surface might be coupled to deprotonation of the Schiff base. The overall structure of the protein and the sequence of proton transfers that add up to the full translocation of a proton from one side of the membrane to the other are published here
Active transport of ions against their electrochemical potentials across
cell and organelle membranes is carried out by proteins that couple an
energy-yielding reaction, such as hydrolysis of adenosine triphosphate,
electron transfer, or isomerization of retinal, to the transmembrane movement
of ions. The mechanism of this kind of transport is a major, still unsolved,
problem of membrane bioenergetics. A satisfactory mechanism will explain
how the free energy gain from the driving reaction is used to change, in
an ordered way, the affinities of binding site(s) for the transported ion
and how the alternating accessibility of these ion binding site(s) to the
two membrane surfaces is mediated. Crystallographic structures are available
for only a few transport proteins, and much effort is expended to improve
their resolutions to give unambiguous answers to these questions.
CATH classification for domain 1C8RA0
Class 1: Mainly Alpha
Architecture: Up-down Bundle
Topology: Photosynthetic Reaction Center, subunit M, domain 1
Homologous Superfamily: Membrane spanning alpha-helix pairs
Sequence Family: PROTON TRANSPORT
Non-Identical Relatives: PROTON TRANSPORT
Identical Relatives: ION TRANSPORT (Only
1C8R is in the last classification level of CATH).
Experimental Details about Structure
Molecule: Bacteriorhodopsin ("Br" State Intermediate)
Chain: A; Engineered: Yes; Mutation: D96N
Biological_Unit: Homotrimer
Other_Details: Schiff Base Linkage Between Lys 216 (Nz) and
Ret 301 (C15) Diether Lipid Bilayer
Authors: H. Luecke
Exp. Method: X-ray Diffraction
Primary Citation: Luecke, H., Schobert, B., Richter, H.-T.,
Cartailler, J.-P., Lanyi, J. K.: Structural Changes in Bacteriorhodopsin
During Ion
Transport at 2 A Resolution Science 286 pp. 255 (1999)
Deposition Date: 29-Jul-1999
Release Date: 20-Oct-1999
Resolution [Å]: 1.80
R-Value: 0.126
Space Group: P 63
Unit Cell: dim [Å]: a 60.63
b 60.63 c 108.16
Angles [¯]: alpha 90.00 beta
90.00 gamma 120.00