Wize University Biochemistry Textbook > Membrane Protein & Transporters
Membrane Protein Function - Specifics
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Bacteriorhodopsin (bR)
- A light fueled proton pump found in halobacterial membranes (consist of ~75% of membrane)
- A multi-spanning membrane protein
- Membrane proteins are hard to crystallize and study
- The detergents needed to remove them from the membrane make it too large for NMR
- The purity and abundance of bR in the membrane provided an opportunity to study the crystal structure of a membrane protein
https://commons.wikimedia.org/wiki/File:Bacteriorhodopsin_chemiosmosis.gif. Darekk2. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
Light activation of bR
- There are 7 transmembrane alpha-helices that surround a chromophore
- A retinal molecule (everything in trans conformation) is bound to a key Lys residue (Schiff base)
Step 0: light causes the C-13 to take on a cis conformation (resting state is trans conformation)
Step 1-2: the conformational change results in proton transfer to a nearby Asp (Asp85) and proton release from Glu194/Glu204 pair
Step 3: The Schiff base acquires a proton form Asp96
Step 4: Asp96 takes up a proton from the cytosol
Step 5: Asp85 re-protonates the Glu194/Glu204 pair resetting the system
- This "proton hopping" is faster than diffusing
https://commons.wikimedia.org/wiki/File:Bacteriorhodopsin_retinal.png. Darekk2. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
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Porins
- Consist of beta-barrels
- Located in outer-membranes of cells and organelles
- Small hydrophilic molecules pass through
- Amino acids in strands alternate between polar and non-polar
- polar amino acids face inwards
- non-polar amino acids face outwards
- Some bacteria can use porins to lyse host cells
- 7 identical beta sheet pairs (beta-turn-beta) are added into a host membrane creating a pore
- Cell contents spill out
https://commons.wikimedia.org/wiki/File:PDB_2prn_EBI.jpg. Jawahar Swaminathan. This image has been released into the public domain.
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Which Statement BEST describes the activity of Bacteriorhodopsin?
a) Light activates the transfer of hydrogen atoms form ATP into the proton pump
b) The retinal molecule is in trans-confomation initially and is bound to a Schiff base (Lys residue)
c) The hydrogen atoms are pumped form the extracellular space into the cytosol
d) Light activation of a retinal molecule results in a hydrogen atom being transferred between several different amino acids from the cytosol to the extracellular environment.
d) Light activation of a retinal molecule results in a hydrogen atom being transferred between several different amino acids from the cytosol to the extracellular environment.
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Which statement is FALSE about membrane proteins?
a) Membrane proteins are hard to solve structurally because they tend to be insoluble and the detergents needed to separate them from the membrane makes then to large for NMR structural determination
b) Porins can be used by invading bacteria to lyse host cells
c) Ion channels are highly specific for their respective ions and often have an open and closed state
d) Transport proteins are required to transport CO2 across the membrane
e) None of the above
d) CO2 can diffuse across the membrane un-facilitated along its concentration gradient