Lecture 6: Membrane Protein Structure Flashcards
How many different membrane proteins are known?
around 700 from different organisms
What do 50% of modern drugs act upon?
GPCRs
Where is bacteriohodopsin found?
the purple membrane
What is the structure of bacteriohodopsin from EM studies?
7 transmembrane domains
What is the chemiosmosis in bacteriohodopsin?
Light hits it, causes protons to move into lumen. this activates a mitochondrial ATPsynthase so ADP and Pi make ATP with release of a proton.
Retinal undergoes what and is bound to what?
trans-cis conversion. Lysine216.
How was the proton path in BR elucidated and what is it?
X-ray structure. Hight hits BR. change in conformation of retinal. Release of proton from schiff base to Asp85. Proton released on extracellular side. Schiff base reprotonated by Asp96 which is reprotonated from the cytoplasm. Proton transferred from Asp85 to Glu204 or 194.
What passes the proton along?
Continual flipping of conformations
What was the first membrane protein structure solved at high resolution?
Photosynthetic reaction centre of B.viridis.
What was the model built up for B.viridis?
light harvesting complexes and a reaction centre.
Why was the structure of B.viridis significant?
gave a model of how photosynthesis could work in bacteria.
Describe the structure of K+ channels and explain why they are 10,000x more selective for K+ over Na+.
4 identical subunits (heterotetrameric). Each subunit contains 2 TM domains. Ions have hydration shells and when they enter the selectivity filter they are dehydrated. Binding the carbonyl oxygens on the peptide backbone compensates for the energetic cost of dehydration so K+ can pass through. Na+ is too small so can’t bind carbonyl oxygens.
How can we get an idea of the mechanism of action?
By solving structures in different conformational states.
What often exists in different conformations but what can only be solved in one conformation?
Gated ion channels. The protein structure.
What is important in the mitochondrial cytochromebC1 complex?
The spacial arrangement of cofactors.
What are found in the outer membrane of gram negative bacteria eg E.coli?
Porins
What does the outer membrane of bacteria do and what does the porin do?
Protects the bacteria in the intestine from harmful agents eg proteases, antibiotics. Porins provide channels for the passage of disaccharides, phosphate etc.
What is the AA sequence of porins like?
Predominantly polar, no long hydrophobic segments like alpha helical domains.
What does X-ray chrystalography show about porin structure?
Porins are trimers of identical subunits and in each subunit 16 B strands form a barrel structure with a pore at the centre. In a B strand, half the amino acid side chains point one way, half the other.
What happens with the outward facing side groups on each of the B strands?
They are hydrophobic and interact with lipids or other porins
What happens with the side groups facing inwards?
They are hydrophilic and line the pore which allows small, water soluble molecules to cross the membrane.
