Bacteriorhodopsin is a protein used by
archaea, most notably halobacteria. It acts as a proton pump, i.e. it captures light energy and uses it to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy.
Bacteriorhodopsin is an
integral membrane proteinusually found in two-dimensional crystalline patches known as "purple membrane", which can occupy up to nearly 50% of the surface area of the archaeal cell. The repeating element of the hexagonal lattice is composed of three identical protein chains, each rotated by 120 degrees relative to the others. Each chain has seven transmembrane alpha helices and contains one molecule of retinalburied deep within, the typical structure for retinylidene proteins. It is the retinal molecule that changes its conformation when absorbing a photon, resulting in a conformational changeof the surrounding protein and the proton pumping action. It is covalently linked to Lys216 in the chomosphore by Schiff baseaction. After photoisomerization of the retinal molecule, Asp85 becomes a proton acceptor of the donor proton from the retinal molecule. This releases a proton from a "holding site" into the extracellular side (EC) of the membrane. Reprotonation of the retinal molecule by Asp96 restores its original isomerized form. This results in a second proton being released to the EC side. Asp85 releases its proton into the "holding site" where a new cycle may begin.
The bacteriorhodopsin molecule is purple and is most efficient at absorbing green light (wavelength 500-650 nm, with the absorption maximum at 568 nm).
tertiary structureof bacteriorhodopsin resembles that of vertebrate rhodopsins, the pigments that sense light in the retina. Rhodopsins also contain retinal, however the functions of rhodopsin and bacteriorhodopsin are different and there is only slight homology in their amino acidsequences. Both rhodopsin and bacteriorhodopsin belong to the 7TM receptorfamily of proteins, but rhodopsin is a G protein coupled receptorand bacteriorhodopsin is not. In the first use of electron crystallographyto obtain an atomic-level protein structure, the structure of bacteriorhodopsin was resolved in 1990. It was then used as a template to build models of other G protein-coupled receptors before crystallographic structures were also available for these proteins.
Many molecules have homology to bacteriorhodopsin, including the light-driven chloride pump
halorhodopsin(for whom the crystal structure is also known), and some directly light-activated channels like channelrhodopsin.
All other photosynthetic systems in bacteria, algae and plants use
chlorophylls or bacteriochlorophylls rather than bacteriorhodopsin. These also produce a proton gradient, but in a quite different and more indirect way involving an electron transfer chainconsisting of several other proteins. Furthermore, chlorophylls are aided in capturing light energy by other pigments known as "antennas"; these are not present in bacteriorhodopsin based systems. Lastly, chlorophyll-based photosynthesis is coupled to carbon fixation(the incorporation of carbon dioxideinto larger organic molecules); this is not true for bacteriorhodopsin-based system. It is thus likely that photosynthesis independently evolved at least twice, once in bacteria and once in archaea.
Protein-coated disc- theoretical data storage capacity of 50 terabytes
* - Calculated spatial positions of bacteriorhodopsin-like proteins in membrane
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