Intro into Membrane Proteins

Question 1

Integral membrane proteins

Answer 1

Goes all the way through the bilayer in a single pass (goes through once)

Question 2

Transmembrane domains of alpha helixes (integral) + (aq)

Answer 2

The amino acid sequence is predominately hydrophobic as surrounded by hydrophobic fatty acids

Question 3

Transmembrane domains of beta sheets (integral) + (aq)

Answer 3

Tend to be hydrophobic
- Sheets organised into a barrel-shaped pore
=> important in transport of molecules like ions across membrane

Question 4

The uses and interpretation of hydrophophic plot

Answer 4

Can make predictions of membrane proteins based on amino acids

• A lot of hydrophobic amino acids = could be transmembrane
• A lot of hydrophilic could be on either side of membrane

Question 5

Peripheral membrane proteins

Answer 5

Still associated with membrane but the polypeptide chain isn’t inserted into the membrane

=> through protein-protein interactions with transmembrane integral proteins
e.g hydrophobic lipid tethers added post-translation to anchor a PMP

Question 6

Importance of membrane being fluid

Answer 6

• Allows membranes to fuse with others e.g. exocytosis of vesicles
• Ensures membranes are shared between daughter cells post-cell division
• Cell migration - can remodel membrane to go through tissues

Question 7

How movement in membrane lipids is shown experimentally

Answer 7

FRAP - Fluorescence Recovery After Photobleaching

1. Label lipid with fluorescent dye
2. Laser a region to bleach/destroy fluorescence of a region
3. When left alone, there is a recovery in the bleached area as neighbouring lipids migrate into the area

• Membrane lipids move sideways by lateral diffusion, spin on their axis + flip-flop rarely w/o translocators

Question 8

Pure phospholipid bilayer + change w/ temperature

Answer 8

Pure phospholipid bilayers experience rapid phase transition over a narrow temperature shift of 1-2 degrees

• Heat => disordered liquid-crystalline phase
• Cool => ordered gel phase

This is an issue for cells as they need to stay fluid over a wider range

Question 9

Phospholipid fatty acid chains’ role in membrane fluidity

Answer 9

The more double bonds + the longer the chain length, the lower the melting point + the more fluid the membrane

Question 10

Van der Waals, fatty acid chains and temperature decreased

Answer 10

Chain length decreases, Van der Waals decreases between neighbouring phospholipids

Double bonds increase, spacing between fatty acid chains increases, and van der Waals forces decrease

Makes membrane more fluid

Question 11

How do membranes withstand temperature change?

Answer 11

The phospholipids change e.g. increasing/decreasing chain length or double bonds

Question 12

Van der Waals, fatty acid chains and temperature increased

Answer 12

Chain length increases, Van der Waals increase between neighbouring phospholipids

Double bonds decreases, spacing between fatty acid chains decreases, and van der Waals forces increases

Makes membrane less fluid

Question 13

Cholesterol in membranes + body temperature (37 degrees)

Answer 13

• Makes membranes less fluid, stabilising interactions between neighbouring phospholipids

Question 14

Cholesterol in membranes + lower temperatures

Answer 14

• Phase transition prevented
  => stops fatty acid chains of phospholipids from interacting with each other

Question 15

Membrane protein moving in cells - an experiment

Answer 15

• Mouse cell (w/ fluorescein-labelled membrane proteins) + human cell (w/ rhodamine-labelled membrane proteins) are fused
• 40 minutes post fusion - the proteins had moved/mixed

However, many cells have restricted lateral movement of proteins