Lecture 6: Membrane Protein Structure

Question 1

How many different membrane proteins are known?

Answer 1

around 700 from different organisms

Question 2

What do 50% of modern drugs act upon?

Answer 2

GPCRs

Question 3

Where is bacteriohodopsin found?

Answer 3

the purple membrane

Question 4

What is the structure of bacteriohodopsin from EM studies?

Answer 4

7 transmembrane domains

Question 5

What is the chemiosmosis in bacteriohodopsin?

Answer 5

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.

Question 6

Retinal undergoes what and is bound to what?

Answer 6

trans-cis conversion. Lysine216.

Question 7

How was the proton path in BR elucidated and what is it?

Answer 7

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.

Question 8

What passes the proton along?

Answer 8

Continual flipping of conformations

Question 9

What was the first membrane protein structure solved at high resolution?

Answer 9

Photosynthetic reaction centre of B.viridis.

Question 10

What was the model built up for B.viridis?

Answer 10

light harvesting complexes and a reaction centre.

Question 11

Why was the structure of B.viridis significant?

Answer 11

gave a model of how photosynthesis could work in bacteria.

Question 12

Describe the structure of K+ channels and explain why they are 10,000x more selective for K+ over Na+.

Answer 12

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.

Question 13

How can we get an idea of the mechanism of action?

Answer 13

By solving structures in different conformational states.

Question 14

What often exists in different conformations but what can only be solved in one conformation?

Answer 14

Gated ion channels. The protein structure.

Question 15

What is important in the mitochondrial cytochromebC1 complex?

Answer 15

The spacial arrangement of cofactors.

Question 16

What are found in the outer membrane of gram negative bacteria eg E.coli?

Answer 16

Porins

Question 17

What does the outer membrane of bacteria do and what does the porin do?

Answer 17

Protects the bacteria in the intestine from harmful agents eg proteases, antibiotics. Porins provide channels for the passage of disaccharides, phosphate etc.

Question 18

What is the AA sequence of porins like?

Answer 18

Predominantly polar, no long hydrophobic segments like alpha helical domains.

Question 19

What does X-ray chrystalography show about porin structure?

Answer 19

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.

Question 20

What happens with the outward facing side groups on each of the B strands?

Answer 20

They are hydrophobic and interact with lipids or other porins

Question 21

What happens with the side groups facing inwards?

Answer 21

They are hydrophilic and line the pore which allows small, water soluble molecules to cross the membrane.