Membrane Asymmtry

Question 1

What are the functions of integral membrane proteins?

Answer 1

Enzymes
Carriers
Channels
Receptors
Cell-cell recognition

Question 2

What are the functions of peripheral membrane proteins?

Answer 2

Cell shape determination
Communication with environments
Intracellular transport
Cell signalling

Question 3

What happens to RBCs in a hypertonic solution?

Answer 3

Become crenelated - water leaves (shrivel)

Question 4

What happens to RBCs in a hypotonic solution?

Answer 4

Rupture due to water entry

Question 5

How are RBCs used in studying membranes?

Answer 5

Red blood cell ghosts - Hb removed
Solubilised in detergent (SDS)
Proteins can be analysed by gel electrophoresis
Allows proteins to be separated
Then dyed

Question 6

What is spectrin?

Answer 6

• Peripheral protein
• Forms dimers between alpha and beta chains
• Helical shape
• Responsible for biconcave shape of RBC
• Mutations cause certain types of haemolytic anaemia
• Can cause change of shape in RBC

Question 7

How is the RBC cytoskeleton structured?

Answer 7

• Linear spectrin connected to actin filaments: provides strength
• Spectrin connected to ankyrin
• Biconcave shape maintained by mesh of proteins on inner surface:
  
  • Peripheral proteins =
    spectrin, actin, ankyrin
  • Integral proteins =
    glycophorin and Band 3

Question 8

What is glycocalyx?

Answer 8

The cell coat

• Glycolipids and glycoproteins only on outside of cell
• Sugar groups also on outside

Question 9

What is blood group determined by?

Answer 9

The composition of oligosaccharides attached to sphingomyelin as well as sugars attached to transmembrane and peripheral proteins

Question 10

What are the different blood types and what is special about O and AB?

Answer 10

O, A, B, AB

O = universal donor
AB = universal acceptor

Question 11

What is the O antigen made up of?

Answer 11

Galactose, N-acetylglucosamine, galactose, fructose

Question 12

Structure of A antigen?

Answer 12

Same as O with addition of N-acetlygalactosamine

Question 13

Structure of B antigen?

Answer 13

Same as O, with addition of another galactose

Question 14

Why are membranes asymmetric?

Answer 14

Proteins:

• Membrane bound enzymes orientated to take up from one side and deliver to other
• Transport proteins work in one direction
• Receptors orientated so can bind extracellular ligands

Phospholipids:

• Different charge distribution
• Two leaflets have different fluidity
• Some proteins in signalling recognise lipid head groups in cytosolic monolayer
• Regulated transfer of some phospholipids

Question 15

How is membrane asymmetry maintained?

Answer 15

• Aminophospholipid translocase transfers phosphotidylserine from outer to inner leaflet
• Scramblase has reverse effect, abolishes asymmetry
• Equilibrium favours translocase

Question 16

Why is membrane asymmetry important in coagulation?

Answer 16

• Phosphotidylserine on platelets provides nucleation site
• Translocase activity inhibited in response to tissue damage
• Phosphotidylserine therefore present in extracellular leaflet

Question 17

What is Scott’s disease?

Answer 17

Scramblase deficiency, phosphatidylserine not moved sufficiently to EC leaflet, inhibiting coagulation

Question 18

Why is membrane asymmetry important in cell recognition/apoptosis?

Answer 18

• Macrophages remove dead cells by phagocytosis
• Macrophage plasma membrane contains receptors that recognise aminophospholipids, phosphatidylserine and phosphatidyehtanolamine
• Old RBC, platelets and apoptotic cells express aminophospholipids in outer leaftlet
• Aminophospholipids act as signal for phagocytosis