Bacteriorhodopsin is a light-driven H+ ion transporter found in some Haloarchaea, most notablyHalobacterium salinarum (formerly known as syn.H. halobium). Theproton-motive force generated by the protein is used byATP synthase to generateadenosine triphosphate (ATP). By expressing Bacteriorhodopsin, the archaea cells are able to synthesise ATP in the absence of a carbon source.[4][5]
A bacteriorhodopsintrimer, showing the approximate positions of the extracellular and cytoplasmic sides of the membrane (red and blue lines respectively)
Bacteriorhodopsin is a 27kDaintegral membrane protein usually found in two-dimensional crystalline patches known as "purple membrane", which can occupy almost 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.[6] Each monomer has seventransmembrane alpha helices and an extracellular-facing, two-strandedbeta sheet.[7][8]
Bacteriorhodopsin molecule is purple and is most efficient at absorbing green light (in the wavelength range 500-650nm). In the native membrane, the protein has a maximum absorbance at 553 nm, however addition of detergent disrupts the trimeric form, leading a loss ofexciton coupling between the chromophores, and the monomeric form consequently has an absorption maximum of 568 nm.[13][14]
Bacteriorhodopsin has a broad excitation spectrum. For a detection wavelength between 700 and 800 nm, it has an appreciable detected emission for excitation wavelengths between 470 nm and 650 nm (with a peak at 570 nm).[15]When pumped at 633 nm, the emission spectrum has appreciable intensity between 650 nm and 850 nm.[16]
Bacteriorhodopsin is a light-driven proton pump. It is the retinal molecule that changes its isomerization state from all-trans to 13-cis when it absorbs aphoton. The surrounding protein responds to the change in the chromophore shape, by undergoing an ordered sequence ofconformational changes (collectively known as the photocycle).[17] The conformational changes alter thepKa values of conserved amino acids in the core of the protein, including Asp85, Asp96 and the Schiff base N atom (Lys216). These sequential changes in acid dissociation constant, result in the transfer of one proton from the intracellular side to the extracellular side of the membrane for each photon absorbed by the chromophore.
The bacteriorhodopsin photocycle consists of nine distinct stages, starting from the ground or resting state, which is denoted 'bR'. The intermediates are identified by single letters and may be distinguished by theirabsorption spectra.[18] The nine stages are:
bR + photon → K ⇌ L ⇌ M1 ⇌ M2 ⇌ M2' ⇌ N ⇌ N' ⇌ O ⇌ bR[18]
Conformational change, pairedstereogram. The orange molecule is all-trans retinal and the red molecule is 13-cis retinal.
Bacteriorhodopsin in the ground state absorbs a photon and the retinal changes isomerization from all-trans 15-anti to the strained 13-cis 15-anti in the K state. The isomerisation reaction is fast and occurs in less than 1 ps. The retinal adopts a less strained conformation to form the L intermediate.
Bacteriorhodopsin is similar tovertebraterhodopsins, thepigments that sense light in theretina. Rhodopsins also contain retinal; however, the functions of rhodopsin and bacteriorhodopsin are different, and there is limitedsimilarity 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 in 1990.[22] It was then used as a template to build models of G protein-coupled receptors beforecrystallographic structures were also available for theseproteins. It has been excessively studied on both mica[23][24] and glass substrates usingAtomic force microscopy and Femtosecond crystallography.[25]
All otherphototrophic systems in bacteria, algae, and plants usechlorophylls orbacteriochlorophylls 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. It is possible that phototrophy independently evolved at least twice, once in bacteria and once in archaea.
^Pebay-Peroua E, Rummel G, Rosenbusch JP, Landau EM (1997). "X-ray structure of bacteriorhodopsin at 2.5 Å from microcrystals grown in lipidic cubic phases".Science.277 (5332):1676–1681.doi:10.1126/science.277.5332.1676.PMID9287223.
^Luecke H, Schobert B, Richter HT, Cartailler JP, Lanyi JK (1999). "Structure of bacteriorhodopsin at 1.55 Å resolution".Journal of Molecular Biology.291 (4):899–911.doi:10.1006/jmbi.1999.3027.PMID10452895.
^Oesterhelt, Dieter (1982). "[3] Reconstitution of the retinal proteins bacteriorhodopsin and halorhodopsin".Reconstitution of the retinal proteins bacteriorhodopsin and halorhodopsin. Methods in Enzymology. Vol. 88. pp. 10–17.doi:10.1016/0076-6879(82)88006-3.ISBN9780121819880.
^Pescitelli G, Woody RW (2012). "The Exciton Origin of the Visible Circular Dichroism Spectrum of Bacteriorhodopsin".Journal of Physical Chemistry B.116 (23):6751–6763.doi:10.1021/jp212166k.PMID22329810.
^Schenkl, Selma; Zgrablic, Goran; Portuondo-Campa, Erwin; Haacke, Stefan; Chergui, Majed (2007). "On the excitation wavelength dependence of the fluorescence of bacteriorhodopsin".Chemical Physics Letters.441 (4–6):322–326.Bibcode:2007CPL...441..322S.doi:10.1016/j.cplett.2007.04.086.
^Ohtani, H.; Tsukamoto, Y.; Sakoda, Y.; Hamaguchi, H. (1995). "Fluorescence spectra of bacteriorhodopsin and the intermediates O and Q at room temperature".FEBS Lett.359 (1):65–68.doi:10.1016/0014-5793(94)01440-c.PMID7851532.
^Dioumaev, A. K.; Richter, H. T.; Brown, L. S.; Tanio, M.; Tuzi, S.; Saito, H.; Kimura, Y.; Needleman, R.; Lanyi, J. K. (1998). "Existence of a proton transfer chain in bacteriorhodopsin: Participation of Glu-194 in the release of protons to the extracellular surface".Biochemistry.37 (8):2496–2906.doi:10.1021/bi971842m.PMID9485398.
^Balashov, S. P.; Lu, M.; Imasheva, E. S.; Govindjee, R.; Ebrey, T. G.; Othersen b, 3rd; Chen, Y.; Crouch, R. K.; Menick, D. R. (1999). "The proton release group of bacteriorhodopsin controls the rate of the final step of its photocycle at low pH".Biochemistry.38 (7):2026–2039.doi:10.1021/bi981926a.PMID10026285.{{cite journal}}: CS1 maint: numeric names: authors list (link)
^Henderson R, Baldwin JM, Ceska TA, Zemlin F, Beckmann E, Downing KH (1990). "Model for the structure of bacteriorhodopsin based on high-resolution electron cryo-microscopy".J Mol Biol.213 (4):899–929.doi:10.1016/S0022-2836(05)80271-2.PMID2359127.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^abNishikawa, T.; Murakami, M. (2005). "Crystal structure of the 13-cis isomer of bacteriorhodopsin in the dark-adapted state".J. Mol. Biol.352 (2):319–328.doi:10.1016/j.jmb.2005.07.021.PMID16084526. PDB ID: 1X0S.
^abImage created withRasTop (Molecular Visualization Software).
![Bacteriorhodopsin single monomer with retinal molecule between 7 vertical alpha helixes (PDB ID: 1X0S [26][27][28]). One more small helix is light blue, beta sheet yellow.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aBacteriorhodopsin_subunit_1X0S_large.gif)
![Bacteriorhodopsin trimer with one retinal molecule in each subunit seen from the extracellular side EC (PDB ID: 1X0S [26][27][28])](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aBacteriorhodopsin_trimer_1X0S_large.gif)
