Molecular structures:  Pores - Channels - Transport Archaeal Rhodopsins

Deutsch (nur bR)

UHH > Faculty of Biology > Teaching Stuff > Highlights of Biochemistry > Pores - Channels - Transport > Active Transport > Rhodopsins	search

View into the cytolpasm on trimeric bacteriorhodopsin from Halobacterium salinarum

Bacteriorhodopsin from Halobacterium salinarum

Halorohodopsin from Halobacterium salinarum

Habitats with extreme environmental conditions are often populated by archaebacteria. Those specialized to live in water near salt saturation (halobacteria) also specialized to utilize solar energy. Light of distinct wavelenghts is used to fuel primary transport of both protons and chloride ions as well as for phototaxis.

Both transport and phototaxis perform energy transduction by rhodopsins, which despite their different tasks share very similar molecular structures. The light receptor is a retinal molecule coupled to a conserved lysin in form of a Schiff base, with the supporting protein inserted into the cytoplasmic membrane with seven transmembrane helices. The arrangement of these seven transmembrane helices pertained through evolution in the form of G-protein coupled receptors not only for visual pigments but for other kinds of sensory systems too. A photoinduced change of the retinal's stereochemical conformation is amplified by the protein either to translocate solutes across the membrane or to trigger signal transduction to associated proteins.

In Halobacterium salinarum (recently renamed from Halobacterium halobium or Halobacterium salinarium) four distinct rhodopsins are found. Besides the pumps transporting proteins to the outside or chloride ions into the cytoplasm there are two sensory systems directing movement away from blue-green light (phoborhodopsin) or towards orange light by associated transducer proteins which regulate the flagellar motor switch.

Structural investigations on the proton pumping bacteriorhodopsin were done on different states of the protein: electron diffraction on single layers of the purple membrane of the bacteria led to a first molecular model (which is shown here in the refinement of 1996). Later X-ray experiments with crystals of the protein in different reaction states gave more insight into the reaction mechanism. Both methods show the trimeric native state of the pump (with maybe some limitations due to packing effects in the crystals). NMR methods (or magnetic relaxation dispersion) reveal details of solubilized single molecules trapped in micelles of nonionic detergent. The crystal structure of the chloride pump halorhodopsin shows how the same construction principle is used for a reverse mass transport.

Above a crucial step of the photoreaction in bacteriorhodopsin is shown: upon isomerization of the retinal from trans to cis and deprotonation of the Schiff base the Nz of the lysine flips its conformation, too. The orbital previously occupied by the proton (indicated by blue dots) points to a different direction thus preventing the transported proton to return.

In all following scripts the orientation of the molecules is as in other proteins of this project: top is outside, down is to the cytoplasm. In the original literature on rhodopsins the reverse orientation is used in figures.