ICPP S2 Membranes And Biological Function

Question 1

Give some functional evidence for proteins being present in membranes.

Answer 1

• Facilitated diffusion of things that would not normally diffuse through the bilayer. Proteins must be facilitating this.
• Ion gradients are maintained by pumps.
• Specificity - certain cell responses occur when specific molecules bind to complementary proteins.

Question 2

Give biochemical evidence for proteins being present in membranes.

Answer 2

• Membrane fractionation + gel electrophoresis - we can separate out molecules in the membrane based on their Mr.
• Freeze fracture - we can freeze a sample and cut it with a microtome and view through electron microscope. Gaps in the seperate lamellae of the bilayer will be present where integral proteins are embedded.

Question 3

What are the 3 modes of motions of membrane proteins?

Answer 3

• Conformational change
• Rotational
• Lateral diffusion

Question 4

Why do membrane proteins not ‘flip-flop’ aka transversely diffuse?

Answer 4

• Very large compared to phospholipids and require substantially more energy to flip flop.
• Movement of protein to opposing lamellae would be too disruptive to the bilayer.

Question 5

What are 3 ways in which membrane protein mobility can be restricted.

Answer 5

• Aggregation
• Tethering
• Interaction with other cells.

Question 6

Describe tethering.

Answer 6

Proteins being tethered via interactions to other molecules intra/extracellularly.

Question 7

Give 3 examples of interaction between cells, in terms of restricting protein mobility.

Answer 7

• Desmosomes
• Gap junctions
• Tight Junctions

Question 8

Give an example of aggregation.

Answer 8

• Lipid-mediated effect proteins tend to aggregate in fluid areas that lack cholesterol.

Question 9

What are the 2 types of membrane proteins?

Answer 9

Peripheral and Integral

Question 10

Describe the properties of peripheral proteins.

Answer 10

• Bound to cell surface
• Relatively weak interaction ie H bonds and electrostatic interactions.
• Easily removed by changes in pH and temperature.

Question 11

Describe properties of Integral membrane proteins.

Answer 11

• Completely span the phospholipid bilayer.
• Extensive interaction with the hydrophobic region of the bilayer.
• Not removed by change in pH because they are held in place by stronger bond.
• Require detergents and organic solvents to complete for the non-polar interactions they form.

Question 12

R groups of AA residues in the transmembrane domain are largely hydrophobic.

True or false?

Answer 12

True

Question 13

What conformation do transmembrane domains often take?

Answer 13

Alpha helical.

Question 14

Describe the key features of the erythrocyte skeleton.

Answer 14

• Heterodimers of alpha and beta spectrum from the majority of the internal cytoskeleton.
• 2 tethering sites of the spectrin to the membrane.
• One tethering site - band 3 anchored to spectrin via ankryin.
• Other tethering site - spectrin heterotetramer to the membrane. Involves actin, adducin, band 4.1 and glycoprotein

Question 15

What is anaemia?

Answer 15

Condition in which there is a deficiency of red blood cells or Haemoglobin in the blood.

Question 16

What is haemolytic anaemia?

Answer 16

Anaemia due to destruction of red blood cells - haemolysis.

Question 17

What is hereditary spherocytosis and how is it caused?

Answer 17

Is the loss of biconcavity in red blood cells, resulting them to form sphere shapes.
Genetic disorder in which spectrin is depleted by about 50%.
Erythrocytes round up and are less resistant to lysis.

Question 18

Why does hereditary spherocytosis lead to splenomegaly?

Answer 18

Abnormal, spherical erythrocytes are destroyed by spleen, increased spleen activity leads to its enlargement.

Question 19

What are some symptoms of anaemia?

Answer 19

Weakness
Shortness of breath
Feeling tired
Confusion
Loss of consciousness
Increased thirst

Question 20

What is hereditary eliptocytosis and how is it caused?

Answer 20

Refers to the erythrocytes being shaped elliptically as opposed to biconcave.
Occurs due to defect in spectrin molecules prevent heterotetramer formation.
Resulting cells are fragile.

Question 21

What are the steps involved in secreted protein biosynthesis?

Answer 21

1. Ribosome translates proteins 5’ to 3’.
2. N-terminus - contains signal sequence.
3. SRP - signal recognition particle - binds to the signal arresting translation.
4. SRP directs ribosome to ER membrane where the SRP binds to the SRP receptor.
5. Ribosome transferred to a protein translocator complex (translocon) and at the same time SRP and its receptor are released from the complex using energy from GTP hydrolysis.
6. Polypeptide translation is guided through the translocon into the ER lumen.
7. Signal sequence is cleaved by signal peptidase enzyme.

Question 22

What are the steps involved in membrane protein biosynthesis?

Answer 22

1. Polypeptide - translated with an internal signal-anchor sequence.
2. SRP binds to signal - SRP binds to receptor.
3. SRP aids in insertion of polypeptide into the translocon.
4. Internal signal sequence is uncleaved and remains inside the translocon in the transmembrane domain, whilst the growing C terminus passes into the ER lumen.
5. Once protein synthesis is complete the internal signal-anchor sequence leads the protein out into the bilayer - leaving the translocon.

Question 23

Why does the internal anchor sequence contain hydrophobic amino acid residues?

Answer 23

Because this is the region that will reside in the hydrophobic region of the bilayer.