Lipid Bilayer, Proteins and Membrane Structure

Question 1

Describe the composition of the plasma membrane

Answer 1

• 40% Lipid
• 60% Protein
• 1-10% Carbohydrate
• 20% Water (hydrated)

Question 2

What components may be associated with the hydrophilic region of a phospholipid?

Answer 2

• Choline
• Amines
• Amino acids
• Sugars

Question 3

What is the significance of unsaturated fatty acids in phospholipid molecules?

Answer 3

• Unsaturated fats have a DOUBLE C=C BOND which introduces a ‘kink’ in the fatty acid chain
• This REDUCES PACKING of phospholipids in the membrane to increase fluidity

Question 4

Describe the general structure of phospholipids

Answer 4

• GLYCEROL backbone
• Fatty acid chains attached to C1 and C2
• Phosphate group attached to C3

Question 5

What is ‘sphingomyelin’?

Answer 5

• Only phospholipid NOT BASED ON GLYCEROL

- Resembles other phospholipids in the membrane

Question 6

Describe the different types of glycolipids found in the membrane

Answer 6

• CEREBROSIDES - head group contains a SINGLE sugar monomer

- GANGLIOSIDES - head group contains many sugar OLIGOSACCHARIDES

Question 7

What is a ‘glycolipid’?

Answer 7

Lipids with a carbohydrate attached by a glycosidic bond

Question 8

What structure(s) can a lipid form in water?

Answer 8

MICELLE or BILAYER

Question 9

Briefly describe the formation of a lipid bilayer in water

Answer 9

• Bilayer forms SPONTANEOUSLY in water by Van der Waals forces between the hydrophobic tails
• Cooperative structure is stabilised by NON COVALENT electrostatic forces and H bonding between the hydrophilic moieties and the polar heads with water

Question 10

Name 4 ways in which lipids can move in the fluid bilayer

Answer 10

• FLEXION (intrachain motion)
• Fast axial ROTATION
• Fast LATERAL DIFFUSION within the plane of the bilayer
• FLIP FLOP

Question 11

What is meant by lipids being able to ‘flip flop’ in the membrane?

Answer 11

Movement of lipid molecules from one half of the bilayer to the other on a one-for-one exchange basis

Question 12

Why are there proteins embedded within the membrane?

Answer 12

• Proteins carry out a range of functions within the membrane - mainly SELECTIVE PERMEABILITY
• Can be transporters, enzymes, pumps, ion channels, receptors or energy transducers

Question 13

What biochemical evidence is there for the existence of membrane proteins?

Answer 13

• Membrane fractionation and gel electrophoresis

- Freeze fracture

Question 14

Describe the motion of membrane proteins within the membrane

Answer 14

• Conformational change (e.g. Receptors, carrier channels)
• Rotation
• Lateral diffusion within the bilayer
• NO FLIP FLOP

Question 15

Why do membrane proteins not flip flop?

Answer 15

Contain LARGE HYDROPHILIC MOIETIES that would require a large amount of energy to pass through the hydrophobic region of the bilayer (would be THERMODYNAMICALLY UNSTABLE)

Question 16

Describe 3 ways in which membrane protein movement is restricted

Answer 16

• AGGREGATES of membrane proteins in cholesterol-poor regions or due to lipid mediated effects
• INTERACTIONS WITH OTHER CELLS e.g. Synapses
• TETHERING extra-membranous association with peripheral proteins or cytoskeleton

Question 17

What is the difference between peripheral and integral membrane proteins?

Answer 17

• PERIPHERAL - bound non-covalently to the surface by H bonds and electrostatic attractions
• INTEGRAL - interact extensively with hydrophobic regions of bilayer

Question 18

How can integral proteins be removed from the bilayer?

Answer 18

• DETERGENTS or ORGANIC SOLVENTS

- Compete with proteins for the non polar interactions within the bilayer

Question 19

Why are phospholipids described as ‘amphipathic’?

Answer 19

Contain both hydrophobic AND hydrophilic regions

Question 20

How does cholesterol contribute to membrane stability at low and high temperatures?

Answer 20

• LOW - Reduces phospholipid packing to INCREASE fluidity

- HIGH - Reduces phospholipid chain motion to DECREASE fluidity

Question 21

What are the restraints on mobility of proteins within the membrane?

Answer 21

• LIPID MEDIATED EFFECTS where proteins separate out in fluid phase or cholesterol poor regions
• MEMBRANE PROTEIN ASSOCIATIONS e.g. sugars
• ASSOCIATIONS WITH EXTRA-MEMBRANOUS PROTEINS (peripheral proteins e.g. cytoskeleton)

Question 22

What is the percentage of protein in a lipid bilayer of an average cell?

Answer 22

~60% dry weight

Question 23

Give 2 ways in which peripheral proteins can be removed

Answer 23

• Change in pH

- Change in ionic strength

Question 24

Describe how integral proteins can be removed from the bilayer

Answer 24

• Detergents or organic solvents

- Compete for the non-polar interactions within the bilayer

Question 25

Why is the asymmetric orientation of proteins important?

Answer 25

- RECEPTORS for hydrophilic messengers e.g. Insulin | - Recognition site must be directed towards extracellular space in order to detect signals

Question 26

The majority of proteins within the erythrocyte membrane are peripheral. How do we know that they are on the cytoplasmic face?

Answer 26

The peripheral proteins are susceptible to proteolysis only when the cytoplasmic face is accessible

Question 27

What are the only integral proteins in the erythrocyte membrane?

Answer 27

- BAND 3 | - Glycophorin A (band 7)

Question 28

What is the function of carbohydrate chains on membrane proteins?

Answer 28

- Extracellular carbohydrates are HIGHLY HYDROPHILIC - Lock orientation of the protein and PREVENT FLIP-FLOP - Role in cellular recognition

Question 29

Describe the composition of the erythrocyte cytoskeleton

Answer 29

- Composed of SPECTRIN and ACTIN molecules - Heterotetramers of spectrin are cross linked by short actin protofilaments - Band 4.1 and Adductin form interactions towards the end of spectrin rods

Question 30

Explain how erythrocyte membrane proteins associate with the cytoskeleton

Answer 30

- Attached by ADAPTOR PROTEINS - Ankyrin (band 4.9) links spectrin to band 3 membrane protein - Band 4.1 links spectrin to Glycophorin A

Question 31

What two proteins link spectrin rods together in the cytoskeleton of RBC?

Answer 31

Adductin and Band 4.1

Question 32

Describe the structure of spectrin

Answer 32

- α and β subunits wind together to form a HETERODIMER - Two heterodimers form a head-to-head association forming a HETEROTETRAMER of α2β2 - Spectrin rods are cross linked by short actin protofilaments

Question 33

What is the main role of the cytoplasm in erythrocytes?

Answer 33

Allows FLEXIBILITY and DEFORMABILITY of membrane so cells can pass through small capillaries without lysis

Question 34

Describe how dominant hereditary spherocytosis can lead to haemolytic anaemia

Answer 34

- Autosomal dominant condition whereby spectrin levels of RBC are depleted by 40-50% - Cells round up so cannot pass through capillaries, therefore undergo lysis in spleen - Insufficient production of RBC by bone marrow leads to HAEMOLYTIC ANAEMIA

Question 35

Explain the pathophysiology and treatment of hereditary elliptocytosis

Answer 35

- Genetic defect in spectrin so that it cannot form heterotetramers - Results in fragile ellipsoid RBC - Treated with CYTOCHALASIN drugs which can alter the deformability of RBC by capping polymerising actin filaments

Question 36

Describe how membrane proteins are embedded in the ER membrane

Answer 36

- N terminal signal sequence recognised by SRP and binds to protein translocation complex on ER membrane - Protein is threaded through membrane until a STOP TRANSFER SEQUENCE is reached (hydrophobic region of 18-22aa) - STS spans the hydrophobic region of membrane and forms transmembraneous protein

Question 36

How does the embedding of a transmembrane protein determine its orientation?

Answer 36

- Signal sequence on N terminus remains bound to PTC as protein is threaded through membrane - Resulting orientation is N terminus on lumenal side and C terminus in cytoplasmic side

Question 37

Give an example of a multiple spanning transmembrane protein

Answer 37

G protein coupled receptor

Question 38

Describe how a transmembrane protein in the ER membrane is transported and embedded in the plasma membrane

Answer 38

- Transmembrane protein travels from ER to Golgi and is processed and packaged into vesicles - Vesicles fuse with plasma membrane and embed protein in its correct orientation