Session 1-Lipids, Proteins and Membrane Structure

Question 1

What is the rough composition of a plasma membrane?

Answer 1

60% protein

40% lipid

Question 2

How may lipid molecules move within the bilayer?

Answer 2

Kink formation
Rotation
Lateral diffusion
Flip-flop

Question 3

List the main types of lipids:

think 5

Answer 3

1. ) phospholipids
2. ) plasmalogens
3. ) sphingomyelin
4. ) glycolipids
5. ) cholesterol

Question 4

How may membrane proteins move?

Answer 4

Conformal change
Rotation
Lateral diffusion
NOT FLIP FLOP (too high energy)

Question 5

What functional evidence is there for membrane proteins?

Answer 5

Facilitated diffusion
Ion gradients
Specific cell responses

Question 6

What biochemical evidence is there for membrane proteins?

Answer 6

Membrane fractionation/gel electrophoresis

Freeze fracture

Question 7

How do peripheral membrane proteins interact with the membrane?

Answer 7

Electrostatic interaction

Hydrogen bonds

Question 8

How do integral proteins interact with the membrane?

Answer 8

Interact extensively with hydrophobic regions

So removal requires agents (e.g. Detergents) to compete with the hydrophobic interactions

Question 9

What is a hydropathy plot? What information on a protein may it give?

Answer 9

It is a plot of the relative hydropathy index of the amino acids in a protein. It can show how many transmembrane domains there are in a protein.

Question 10

Provide a brief overview of the secretory pathway:

Hint: 8 steps

Answer 10

Free ribosome initiates protein synthesis from mRNA
Hydrophobic n-terminal sequence is produced
Signal sequence recognised by SRP
SRP directs complex to SRP receptors on the ER
Protein synthesis continues into ER through peptide translocation complex
Signal sequence removed by signal peptides em and the ribosome is recycled

Question 11

What is the function of the stop transfer signal?

Answer 11

It is a hydrophobic sequence that spans the bilayer. It stops translation through the ER membrane. The rest of the protein is then produced outside the ER in the cytoplasm.

Question 12

What is the function of cholesterol in membranes?

Answer 12

It mediates fluidity and eliminates the endothermic phase transition

Question 13

How does cholesterol increase fluidity?

Answer 13

It reduces phospholipid packing

Question 14

How does cholesterol decrease membrane fluidity?

Answer 14

It binds to and reduces phospholipid chain motion

Question 15

What are the restrictions of membrane protein mobility?

Answer 15

Aggregation-proteins associate with one another

Association with extra membranous of infra membranous proteins; e.g. basement membrane or cytoskeleton

Question 16

What are the two main proteins of the RBC cytoskeleton?

Answer 16

It is a network of actin and spectrin

Question 17

How is the RBC cytoskeleton attached to the membrane?

Answer 17

Adapter proteins; ankyrin and glycophorin binding to bands 3 and 4.1

Question 18

What is the important of the RBC cytoskeleton?

Answer 18

It maintains the overall shape of the cells-without it they tend to round up and there is increased lysis. This may lead to a haemolytic anaemia.

Question 19

What is hereditary spherocytosis?

Answer 19

Most common form = spectrin depleted by 40-50%

Results in rounding up of cells and lysis

Question 20

What is hereditary elliptocytosis?

Answer 20

The spectrin molecules are unable to form hetereotetramers which results in fragile eliptoid cells leading to lysis