Week 7 - Membrane Proteins and Transport

Question 1

What are the

3 general categories of membrane proteins?

Answer 1

1. Integral membrane proteins
2. Peripheral membrane proteins
3. Lipid-Anchored membrane proteins

Question 2

Integral membrane proteins are…

Answer 2

amphipathic.

Question 3

What are the 2 domains that integral membrane proteins have?

Answer 3

1. Hydrophilic domains: amino acids side chains are polar

2. Hydrophobic domains: amino acid side chains are non-polar

Question 4

What are the 2 kinds of transmembrane proteins?

Answer 4

1. Single pass: a single alpha helix

2. Multipass: multiple alpha helixes (crossing membrane multiple times)

Question 5

How long is a typical membrane spanning alpha helix?

Answer 5

Typically 20-30 hydrophobic amino acids.

Question 6

Are integral membrane proteins only alpha helixes?

Answer 6

No! They can be other things, like a beta-sheet.

Question 7

What are 3 examples of functions of membrane proteins?

Answer 7

1. Receptors
2. Ion channels; do not form pores
3. Beta-barrel (rolled beta sheet); DOES form a pore

Question 8

Where can you find beta-barrels?

Answer 8

Some channels in bacteria, mitochondria, and chloroplasts.

Question 9

Do ion channels (form of membrane proteins) undergo conformational changes?

Answer 9

Ion channels undergo conformational changes to regulate permeability, unlike Beta-barrels, which are rigid.

Question 10

Do Beta-Barrels (form of membrane proteins) undergo conformational changes?

Answer 10

Beta-barrels do NOT undergo conformational changes. They are rigid.

Question 11

2 ways membrane protein structures are identified?

Answer 11

1. X-ray crystallography; 3D structure

2. Hydrophobicity (Hydropathy) plots; show the thresholds of hydrophobic regions

Question 12

Why do proteins anchored on the cytosolic face have to be an amphipathic alpha helix? (i.e. an integral membrane protein, not transmembrane)

Answer 12

The side of the protein facing the tails must be hydrophobic while the side face the polar heads must be hydrophilic.

Question 13

Where are Lipid-anchored membrane proteins synthesized, and where do they end up? (i.e. GPI-anchored protein)

Answer 13

They are synthesized in the endoplasmic reticulum lumen, and they end up on the cell’s surface.

Question 14

What do cytosolic enzymes do for lip-anchored membrane proteins?

Answer 14

They add to the anchor to direct the protein to the cytosolic face.

Question 15

How are peripheral membrane proteins bound to other proteins or lipids?

Answer 15

They are bound on EITHER face of the membrane by non-covalent interactions.

Question 16

What technique is used to extract peripheral membrane proteins?

Answer 16

Use gentle extraction that does not destroy the lipid bilayer; it changes the buffers

Question 17

What technique is used to extract Integral membrane proteins?

Answer 17

Use detergents that DO destroy the membrane; can also use to extract phospholipids and study only specifically what you want to study.

Question 18

What is Fluorescence Recovery After Photobleaching (FRAP) used for?

Answer 18

FRAP is used to study protein movement; can view lateral diffusion.

Question 19

What are the steps of FRAP?

Answer 19

1. Label proteins with fluorescent dye
2. Photo-bleach spot with a laser beam
3. Recovery; view how long it takes the destroyed region to fluoresce

Question 20

What is the lipid bilayer permeable to?

Answer 20

1. Hydrophobic molecules

2. Small, uncharges polar molecules (but not very common)

Question 21

How do molecules move via simple diffusion through the lipid bilayer?

Answer 21

They move down the concentration gradient; from an area of high concentration to low concentration.

Question 22

The more hydrophobic and non-polar the molecule…

Answer 22

the faster it diffuses across the lipid bilayer.

Question 23

What is the lipid bilayer impermeable to?

Answer 23

1. Large, uncharges polar molecules
2. Ions
   * These require membrane proteins for transport.*