- Bacteriorhodopsin
Bacteriorhodopsin is a protein used by
archaea , most notablyhalobacteria . It acts as aproton pump , i.e. it captures light energy and uses it to moveproton s across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy.Bacteriorhodopsin is an
integral membrane protein usually 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 ofretinal buried deep within, the typical structure forretinylidene protein s. It is the retinal molecule that changes its conformation when absorbing aphoton , resulting in aconformational change of the surrounding protein and the proton pumping action. It is covalently linked to Lys216 in the chomosphore bySchiff base action. 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).
The three-dimensional
tertiary structure of bacteriorhodopsin resembles that ofvertebrate rhodopsin s, thepigment s that sense light in theretina . Rhodopsins also contain retinal, however the functions of rhodopsin and bacteriorhodopsin are different and there is only slight homology in theiramino acid sequences. Both rhodopsin and bacteriorhodopsin belong to the7TM receptor family of proteins, but rhodopsin is aG protein coupled receptor and bacteriorhodopsin is not. In the first use ofelectron crystallography to obtain an atomic-levelprotein structure , the structure of bacteriorhodopsin was resolved in1990 . It was then used as a template to build models of other G protein-coupled receptors before crystallographic structures were also available for theseprotein s.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 likechannelrhodopsin .All other photosynthetic systems in bacteria, algae and plants use
chlorophyll s orbacteriochlorophyll s rather than bacteriorhodopsin. These also produce a proton gradient, but in a quite different and more indirect way involving anelectron transfer chain consisting 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 tocarbon fixation (the incorporation ofcarbon dioxide into 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.See also
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Protein-coated disc - theoretical data storage capacity of 50terabyte sExternal links
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* - Calculated spatial positions of bacteriorhodopsin-like proteins in membrane
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