Bacteriorhodopsin
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Bacteriorhodopsin is a photosynthetic pigment 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 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 of retinal buried deep within. It is the retinal molecule that changes its conformation when absorbing a photon, resulting in a conformational change of the surrounding protein and the proton pumping action.
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 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 no homology of their amino acid sequences. Both rhodopsin and bacteriorhodopsin belong to the 7TM receptor family of proteins, but rhodopsin is a G protein coupled receptor and bacteriorhodopsin is not. In the first use of electron crystallography to 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 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 to carbon fixation (the incorporation of carbon 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.
