1. The Membrane Bilayer

Question 1

What are the THREE variations of membrane phospholipids?

Answer 1

1. TYPICAL Phospholipids: glycerol-based with 2 ester-linked fatty acids
2. PLASMALOGENS: 1 vinyl ether linked fatty alcohol
3. SPHINGOMYELIN: not glycerol based

Question 2

What are glycolipids? Name the two subtypes.

Answer 2

Same phospholipid structure but the “phosphate + head group” are replaced with a sugar.

a) Cerebrosides: head group = sugar monomer
b) Gangliosides: head group = sugar multimer

Question 3

Cholesterol makes up 45% of membrane lipid. How does cholesterol stabilise the bilayer?

Answer 3

Rigid planar conjugated ring intercalates in the bilayer.

Reduces fluidity - hydrogen bond with fatty acid carbonyl group reduces chain motion.

Increases fluidity - insertion in bilayer reduces phospholipid packing.

Question 4

Compare lipid and protein dynamics in the membrane.

Answer 4

Both display:

• lateral diffusion
• axial rotation

Lipids also display:

• transverse diffusion (flip flop)
• intrachain motion i.e. kink

Proteins also display:
- Conformational change

Question 5

What does osmotic haemolysis of the erythrocyte teach us about its membrane proteins?

Answer 5

Most membrane proteins are:

a) PERIPHERAL - released by pH changes or ionic strength
b) CYTOPLASMIC - subject to proteolysis when cytoplasmic face is accessible.

Question 6

Describe the structure of the erythrocyte cytoskeleton.

Answer 6

Actin and Spectrin form the cytoskeleton.

SPECTRIN: long heterotetramer (a2b2) molecules
ACTIN: ~14 actin monomers cross link spectrin rods

The Spectrin cytoskeleton is attached to the membrane by adapter proteins:

BAND 4.1: attaches to GLYCOPHORIN A
ANKYRIN: attaches to BAND 3

Glycophorin A and Band 3 are integral membrane proteins.

Question 7

What are the FOUR main functions of biological membranes?

Answer 7

1. Selective barrier
2. Control of enclosed chemical environment
3. Communication and Recognition
4. Signal generation

Other specific functions e.g. mitochondrial membranes - oxidative phosphorylation

Question 7

What are the differences between the two types of haemolytic anaemias?

Answer 7

HEREDITARY SPHEROCYTOSIS
Spectrin depletion (cells become rounded)

HEREDITARY ELLIPTOCYTOSIS
Spectrin defect (cells fragile and elliptoid)

Question 9

Describe how membrane proteins are transported to the membrane.

Answer 9

1. SIGNAL RECOGNITION PARTICLE binds to hydrophobic signal. This halts translation.
2. SRP bind to SRP RECEPTOR on the RER.
3. SRP is released (requires GTP hydrolysis)
4. SIGNAL SEQUENCE RECEPTOR binds the signal.
5. This directs protein synthesis into the RER lumen via the PROTEIN TRANSLOCATOR COMPLEX.
6. STOP TRANSFER SIGNAL remains embedded in the membrane.
7. SIGNAL PEPTIDASE removes initial signal.
8. LATERAL GATING MECHANISM releases protein from translocator.

Question 10

Describe the structure and function of the different signal sequences in a membrane protein.

Answer 10

a) 18-30 aa hydrophobic signal. This binds to:
- Signal receptor particle - halts translation and transports to RER
- Signal sequence receptor (on protein translocator) directs translation into RER lumen

b) Stop transfer signal (18-22 aa hydrophobic signal followed by charged amino acids)
- Remains embedded in the membrane

Question 11

What are luminal binding proteins?

Answer 11

Luminal binding proteins (BiP) are proteins related to chaperones but which assist in stabilising partially folded polypeptides. (important for multiple transmembrane proteins)