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Bacteriorhodopsin

Copyright: Garland Science 11 4 Bacteriorhodopsin Proton Pump Bacteriorhodopsin is an abundant light-driven proton pump found in the membrane of Halobacter halobium, a purple archeon that lives in salt marshes in the San Francisco Bay Area. Bacteriorhodopsin is a multipass membrane protein that traverses the plasma membrane of the cell with seven long a helices. The helices surround a chromophore, retinal, that is covalently attached to the polypeptide chain and gives the protein and cells their characteristic purple color. Retinal is a long, unsaturated hydrocarbon chain that is covalently attached to a lysine side chain of the protein. When retinal absorbs a photon of light, one of its double bonds isomerizes from a trans to a cis configuration, thus changing the shape of the molecule. The change in retinal's shape causes conformational rearrangements in the surrounding protein. The light-induced isomerization of retinal is the key event in proton pumping. In the excited state, retinal is positioned so that it can transfer a proton to an aspartate side chain, aspartate 85, that is positioned towards the extracellular side of the protein. Aspartate 85 quickly hands off the proton to the extracellular space via a bucket brigade of water molecules. The now negatively-charged retinal takes up a proton from another aspartate, aspartate 96; this one is positioned towards the cytosolic face of the protein. Upon re-protonation, the retinal returns to the ground state. Aspartate 96 replenishes its lost proton from the cytosol, and the cycle can repeat. The net result: for each photon absorbed, one proton is pumped out of the cell. -Retinal is also used to detect light in our eyes and is attached to a similar protein structure to bacteriorhodopsin( Luecke et. al 1999) http://www.chemistry.wustl.edu/~edude... https://en.wikipedia.org/wiki/Rhodopsin https://en.wikipedia.org/wiki/Melanopsin http://photobiology.info/Crouch.html https://www.ncbi.nlm.nih.gov/pubmed/2... http://garlandscience.com/garlandscie...