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Bacteriorhodopsin

Structure

Bacteriorhodopsin is an integral membrane protein that functions as a proton pump. It has a primary structure that includes 248 amino acids with a molecular weight of 26,800.58 Da.^[1] The is seen in the seven alpha helices (deep pink) and two beta strands (yellow) that are antiparallel. Located in the center is the ligand, which is bonded to . The alpha helices contain specific charged polar sides that help carry the proton through the middle of the protein's hydrophilic membrane ^[2]. includes one domain and an alpha bundle motif. ^[3] The quaternary structure has C3 symmetry and is a homotrimer with three subunits.^[4]

Function

Bacteriorhodopsin functions as a proton pump that transports H+ across the gradient and is driven by green light(500nm-650nm).^[5] Hydrophobic lipid tails are able to interact with proteins' rigid surfaces while keeping a hydrophilic center that allows the movement of protons.^[6] 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. ^{[7] [8]} The first step of the catalytic cycle is the photoisomerization by retinol to cause all trans to configure into 13-cis. The change allows the Schiff base to transfer a proton to asp 85. Aspartic acid 96 then reprotonates the schiff base via the cytoplasmic channel, causing the retinol reverses the conformational change, returning to all trans^[9]. 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.^[10] 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.^[12] When substituted into glutamine, less than 10% of normal function will occur.^[13] If they are substituted or a mutation occurs, normal processes of bacteriorhodopsin will not occur due to the slow photocycle. is responsible for proton release for the bacteriorhodopsin. is responsible for the deprotonation and protonation of the Schiff Base.^[14] The is a catalytic ligand that is equidistant between Aspartic acids 85 and 212 and it contains three water molecules. It is a planar petagonal cluster and protonated, counteracting the negative charge of the Aspartic acids, and has one oxygen from each amino acid. Once light is absorbed, the photoisomerization step takes place, causing a conformational change from all-trans to 13-cis. The Schiff base then transfers a proton to Asp85, causing the downstream effect of proton transfer reactions. ^[15]

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.^[16]