M&R S1 - Lipids, Proteins and Membrane Structure

Question 1

Fatty acids are amphipathic, what does this mean?

Answer 1

They contain both hydrophilic and hydrophobic moieties

Question 2

A phospholipid bilayer is composed of what?

Give percentages

Answer 2

Dry Weight:
40% Lipid
60% Protein
1-10% Carbohydrate

20% Water (when hydrated)

Question 3

What is the predominant type of lipid in a cell membrane?

Give an example

Answer 3

Phospholipids

Phosphatidylcholine

Question 4

What is a phospholipid composed of?

Answer 4

Glycerol backbone with:

2 fatty acid chains (can be identical or different)

A phosphate group w/ head group

Question 5

Give examples of head groups you may find in a phospholipid and give the physical property they all possess

Answer 5

Choline
Amines
Amino acids
Sugars

All polar

Question 6

Give two features of fatty acid chains commonly found in phospholipids

Answer 6

Enormous variety in chain length, commonly C16 or C18

Unsaturated fatty acid side chains in the cis formation introduce a ‘kink’ that reduces phospholipid packing and hence increases membrane fluidity

Question 7

What phospholipid doesn’t follow the traditional structure?

Briefly describe HOW it differs

Does this have any effect on its conformation in the membrane?

Answer 7

Shingomyelin

Not based on glycerol

Conformation in the membrane resembles other phospholipids

Question 8

What are glycolipids?

What types are there and how do they differ?

Answer 8

Suger containing lipids

Cerebrosides - Head group sugar monomers
Gangliosides - Head group sugar oligosaccharides

Question 9

What percentage of a cell membrane lipids are cholesterol?

Answer 9

45%

Question 10

What two structures will amphipathic molecules form in water?

Answer 10

Micelles

Bilayers

Question 11

How does formation of a bilayer occur?

What stabilises this structure?

Answer 11

Amphipathic molecules form a bilayer spontaneously in water

This is driven by Van der Waals forces between hydrophobic tails

Structure is stabilised by non-covalent forces:

Electrostatic and hydrogen bonds between hydrophilic moieties

Interactions between hydrophilic moieties and water

Question 12

What are the ways lipid molecules can move in a bilayer?

Give a short description of each

Answer 12

Intra-chain motion (Flexion) - Movement of the fatty acid chains

Fast axial rotation - Spinning of the molecule

Fast lateral diffusion - Movement of the molecules relative to each other in the plane of the bilayer i.e. molecules can travel around the bilayer

Flip-Flop (Transverse Diffusion) - One for one exchange of molecules from each layer of the bilayer

Question 13

What are some of the functions of membrane proteins?

Answer 13

Enzymes
Transporters
Pumps
Ion channels
Receptors
Energy transducers

Question 14

To what degree does protein content of a membrane vary?

Answer 14

From 18% (Myelin)

To 75% (Mitochondrial membranes)

Question 15

What evidence does membrane function provide for the existance of membrane proteins?

Answer 15

Facilitated diffusion
Ion gradients
Specificity of cell responses

These three functions are not performed by the bilipid membrane, so must be the domain of proteins.

Question 16

What Biochemical techniques can be used to verify the existance of membrane proteins?

Answer 16

Freeze fracture

Membrane fractionation and gel electrophoresis

Question 17

Membrane proteins have what 3 modes of motion?

Answer 17

Conformational change
Rotational
Lateral

Question 18

What mode of motion is available to lipids in a membrane but not proteins?

Why is this?

Answer 18

Flip-Flop / Transverse

Proteins have large hydrophilic moieties that would require a large amount of energy to pass through the hydrophobic region of the bilayer

Question 19

What factors might be involved in restriction of protein movement in a membrane?

Answer 19

Lipid mediated effects - Proteins will tend to separate out into the fluid phase or cholesterol poor regions

Membrane protein associations/aggregation

Association with extramembranous proteins
E.g. Cytoskeleton anchoring or adhesion to the basement membrane

Question 20

Describe 2 features of peripheral membrane proteins

Answer 20

Bound to the surface of the membrane non-covalently by electrostatic and hydrogen bonds

Can be removed by changing the pH or ionic strength

Question 21

In what 2 ways do integral membrane proteins differ from peripheral proteins?

Answer 21

Interact extensively with the hydrophobic regions of the membrane, not just the surface

Requires agents (Detergents, Organic solvents) that compete for the non polar interactions with the bilayer to remove
i.e. Cannot be removed by changes in pH or ionic strength

Question 22

Describe the protein secretion pathway in full (MGD S6)

Answer 22

1. Free ribosome begins protein synthesis
2. Hydrophobic N-terminal signal sequence produced
3. Signal recognition particle (SRP) recognises signal sequence and binds
4. Protein synthesis stops
5. GTP-bound SRP directs ribosome to SRP receptors on RER cytostolic surface
6. SRP dissociates
7. Protein synthesis continues, feeding protein into RER via pore in membrane (peptide translocation complex)
8. Signal sequence is removed by signal peptidase once the entire protein is finished
9. The ribosome dissociates and is recycled

Question 23

How is orientation of a membrane protein determined?

Answer 23

Protein synthesis determines this

It does so with the addition of a highly hydrophobic stop transfer signal

When the protein is being translated and fed into the ER lumen the stop signal will then remain in the ER membrane once it has been translated

The rest of the protein is translated in the cytoplasm and the protein will span the membrane

Question 24

Describe the composition of a stop transfer signal

Answer 24

Normally 18-20 AAs long

Made up of hydrophobic, small or uncharged amino acids

Question 25

What is a hydropathy plot and what is it used for?

Answer 25

Hydropathy plots show the various hydrophobic and hydrophilic regions of a protein by showing the hydropathy index of each amino acid

Can be used to show how many transmembrane regions are contained within a protein

Question 26

What is indicated by an amino acids’s hydropathy index?

Describe the appearance of a hydropathy index i.e. What is plotted along each axis?

Answer 26

A positive hydropathy index indicates the amino acid is hydrophobic, negative indicates a hydrophilic amino acid

Hydropathy index is plotted along the Y axis

Amino acids in the protein are numbered and plotted along the X axis

Question 27

Why is asymmetry of membrane proteins important to function?

Give an example.

Answer 27

Important to specific functions of each protein

E.g. Hydrophilic hormone receptor must have its recognition site on the extracellular face to function properly

Question 28

What are the general functions of cholesterol in a phospholipid bilayer?

Answer 28

Stabilises the membrane by hydrogen bonding to the fatty acid chains

Abolishes the endothermic phase transition of membranes

Question 29

Cholesterol has paradoxical effects on membrane fluidity, explain.

Answer 29

Cholesterol’s large sterol ring structure reduces phospholipid packing and hence increases membrane fluidity

However, Cholesterol’s hydrophobic tail reduces phospholipid chain motion and hence reduces membrane fluidity

Question 30

Give a general description of cholesterol structure and briefly explain how this allows it to dissolve in the phospholipid bilayer

Answer 30

Hydrophilic head group interacts with phosphate heads of the phospholipids

Large sterol ring structure and the hydrophobic tail interact with the hydrophobic regions of the phospholipid bilayer

Question 31

Describe the Fluid Mosaic model

The name is extremely descriptive, and also a hell of a good basis for your description. Hint Hint.

Answer 31

Plasma membrane is said to be a fluid because its hydrophobic integral components can move laterally along the membrane as if moving through a fluid

The membrane is depicted as a mosaic as its made up of many different components (E.g. Proteins, phospholipids, cholesterol etc)

Question 32

What is the function of the erythrocyte cytoskeleton?

Where is it found?

How is it isolated for analysis?

Answer 32

Cytoskeleton holds the shape of the RBC

Found along the cytosolic face of the membrane

Can be removed from the membrane by a low strength ionic wash

Question 33

Describe the composition of the erythrocyte cytoskeleton.

Include details of its anchorage to the plasma membrane

Answer 33

Composed of Spectrin and Actin molecules
attached to the membrane by Ankyrin (to Band 3) and by Band 4.1 (to Glycophorin)

(Band 3 and Glycophorin are transmembrane proteins)

Question 34

What is the consequence of the attachment of the cytoskeleton to the membrane proteins?

Answer 34

Restricts lateral mobility of the membrane proteins

Question 35

What are the general features of haemolytic anaemias?

Answer 35

Erythrocyte cytoskeleton is improperly formed causing the RBCs to ‘round up’ and are lysed by shearing forces in the capillary beds and cleared by the spleen

Question 36

Describe ‘Hereditary Spherocytosis’

Answer 36

Genetic disease commonly found in dominant form

Characterised by the reduction of spectrin levels by 40-50%

The resulting ‘round up’ and increased lysis results in reduced RBC lifespan

The bone marrow cannot compensate, leading to haemolytic anaemia

Question 37

Describe ‘Hereditary Elliptosis’

Answer 37

Spectrin molecules are unable to form stable tetramers

Results in fragile elliptoid cells, which lead to haemolytic anaemia