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Bacteriorhodopsin
Structure
Bacteriorhodopsin, a membrane protein, contains a sequence that has 248 amino acids, including 3 sets of 7 alpha helices and 3 sets of 2 antiparallel beta strands. There are 3 domains that form an alpha helix barrel. BR is a homotrimer with three subunits and has a C3 symmetry.
Function
Bacteriorhodopsin functions as a proton pump that transports H+ across the gradient and is driven by green light. The protons are used to create ATP which is a vital part of the haloarchaea's survival. Once bacteriorhodopsin absorbs a photon, catalysis is triggered, causing a conformational shift from trans to cis, a release of a proton, and a transfer of a proton. The catalytic cycle includes 6 steps of isomerization, accessibility change, and ion transport (8). Bacteriorhodopsin is a type three membrane protein. The side chains of the amino acids are hydrophobic, causing a highly hydrophobic membrane protein pump. Hydrophobia is very common in membrane proteins.
Relevance
Without bacteriorhodopsin, the light would not be converted into the energy that drives the proton pump, making it much harder for bacteria cells to produce the ATP needed to function normally. Bacteriorhodopsin is also essential in helping the bacteria cells create a chemical gradient for sodium, and assists in creating the energy needed for the cell to rotate its flagella. The energy it helps create helps normal cell function, such as the transport of amino acids, occur. A lack of bacteriorhodopsin would be detrimental to these bacterial cells (11).
Enzyme Mechanism
Aspartic Acids 96 and 85 play a very important role in the function of bacteriorhodopsin. When substituted into glutamine, less than 10% of normal function will occur. If they are substituted or a mutation occurs, normal processes of bacteriorhodopsin will not occur due to the slow photocycle(12). Aspartic acid 85 is responsible for proton release for the bacteriorhodopsin. Aspartic acid 96 is responsible for the deprotonation and protonation of the Schiff Base(1).
Interesting Findings
Halophilic archaea live in hypersaline environments such as salt lakes and are exposed to extremely strong sunlight. This increases the salinity so the haloarchaea depend on the proton gradient system through its photo-reactive proteins. Due to bacteriorhodopsin having low availability at a high price, studies have produced a BR recombinant protein called highly expressible bacteriorhodopsin (HEBR). HEBR may decrease the likelihood of cell proliferation and migration of lung cancer cells.
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References
Butt, H. J.; Fendler, K.; Bamberg, E.; Tittor, J.; Oesterhelt, D.
Aspartic acids 96 and 85 play a central role in the function of bacteriorhodopsin as a proton pump. EMBO. 1989, 8 (6), 1657-1663.
Edman, K.; Nollert, P.; Royant, A.; Belrhali, H.; Pebay-Peyroula, E.; Hajdu, J.; Neutze, R.; Landau, E. M. High resolution x-ray structure of an early intermediate in the bacteriorhodopsin photocycle. RSCB PDB. 1999, 401 (6755), 822-826.
Haupts, U.; Tittor, J.; Oesterhelt, D. Closing in on bacteriorhodopsin: progress in understanding the molecule. Annu. Rev. Biophys. Biomol. Struct. 1999, 28, 367-399.
Khorana, H. G.; Gerber, G. E.; Herlihy, W. C.; Gray, C. H.; Anderegg, R. J.; Nihei, K.; Biemann, K. Amino acid sequence of bacteriorhodopsin. Proc. Natl. Acad. Sci. USA 1997, 76 (10), 5046-5050.
Lanyi, J. K.; Varo, G. The photocycles of bacteriorhodopsin. Isr. J. Chem. 1995, 35 (3-4), 365-385.
Noort, J. Unraveling bacteriorhodopsin. Biophys. J. 2005, 88 (2), 763-764.
Ovchinnikov, Y. A.; Abdulaev, N. G.; Feigina, M. Y.; Kiselev, A. V.; Lobanov, N. A. The structural basis of the functioning of bacteriorhodopsin: an overview. ICHB. 1979, 100 (2), 219-224.
Ovichinnikov, Y. A.; Rhodopsin and bacteriorhodopsin structure--function relationships. IBCH. USSR 1982, 148 (2), 179-191.
Stoeckenius, W.; Bogomolni, R. A. Bacteriorhodopsin and related pigments of halobacteria. Ann. Rev. Biochem. 1982, 52, 587-616.
Wong, C. W.; Ko, L. N.; Huang, H. J.; Yang, C. S.; Hsu, S. H. Engineered bacteriorhodopsin may induce lung cancer cell cycle arrest and suppress their proliferation and migration. MDPI. 2021, 26 (23).
Kouyama, T.; Kinosita, K.; Ikegami, A. Structure and Function of Bacteriorhodopsin. Adv. Biophys. 1988, 24, 123–175.
Mogi, T.; Stern, L. J.; Marti, T.; Chao, B. H.; Khorana, H. G. Aspartic Acid Substitutions Affect Proton Translocation by Bacteriorhodopsin. Proc. Natl. Acad. Sci. USA. 1988, 85 (12), 4148–4152.
