Membrane proteins

Question 1

Membrane proteins have 3 structures

Answer 1

alpha-helix
beta-barrel
amphipathic helix

Question 2

What can membrane proteins be attached to ?

Answer 2

Other proteins or lipid anchored

Question 3

Hydropathy plots

Answer 3

Indicate the hydrophobicity of protein segments - used to predict trans-membrane domains.

Question 4

What do trans-membrane proteins act as?

Answer 4

Transporters
Receptors
Anchors
Enzymes

Question 5

Two ways that proteins can interact with membranes

Answer 5

single pass

multi pass

Question 6

4 examples of peripheral membrane proteins

Answer 6

Lipid associated
Lipid modified anchor protein
GPI anchored
Proteins attached to integral proteins

Question 7

Why is the alpha helix a common structure for trans-membrane proteins

Answer 7

H bonds are stable in the backbone due to a lack of water

Question 8

Hydropathy

Answer 8

A calculated measure of the energy needed to pass a segment of polypeptide into water from a solvent.

Question 9

Multi pass transmembrane proteins

Answer 9

Some proteins have multiple transmembrane alpha helices

Gly is common in alpha helices of multipass transmembrane domains and can allow for flexibility and tight packing.

Question 10

Describe Beta-Barrel

Answer 10

Some proteins have beta-barrel structures which consist of antiparallel beta strands. Hydrogen bonds occur between barrels. Pore is in the middle with hydrophilic side chains in the middle and hydrophobic side chains facing outwards.

Question 11

Studying trans-membrane domains of SER & RER

Answer 11

Obtain cell membrane compartment, homogenise then place in test tube with gradient of increasing sucrose solution.
Smooth microsomes have a low density so stop at a low sucrose solution.
Rough microsomes have a high density so stop sedimenting at a high sucrose solution.

Question 12

What is the process of characterising trans-membrane proteins?

Answer 12

Solubilisation = detergents
Isolation = chromatography 
Characterisation = gel electrophoresis
Functional/structural = incorporation into artificial membranes

Question 13

How can you experimentally confirm trans-membrane proteins?

Answer 13

Fractionate cell to confirm that the protein is in a membrane fraction.
Wash with salt e.g. sodium chloride
Physically break the membrane open by grinding up after freezing- soluble content released from the cell
Solubilise protein with detergent

Question 14

What are detergents?

Answer 14

Amphipathic molecules that incorporate into a membrane to disrupt the structure.

Question 15

What shape do detergents form?

Answer 15

Micelles in bilayers

Question 16

How do detergents exist at low and high concentrations?

Answer 16

Low = monomers and insert into a membrane
High = form micelles and disrupt the membrane

Question 17

What are three examples of cytosolic lipid modifications?

Answer 17

Myristoyl anchor - amide link between terminal amino group and myristic acid
Palmitoyl anchor - thioester linkage between cysteine and palmitic group
Farnesyl anchor - thiether link between cysteine and prenyl group

Question 18

Ras protein

Answer 18

Signalling protein that is a small GTPase. It is switched on by incoming signals and switches on other proteins involved in cell growth, differentiation and survival.