Membrane Fluidity and Proteins

Question 1

Movement Of Phospholipids In The Bilayer

Answer 1

• Can rotate/ exchange in the lateral plane
• Move v. slowly from one leaflet to the other (flip-flop)

Question 2

Fluidity Of Bilayer Depends On It’s Composition

Answer 2

• No. of double bonds & C atoms in fatty acid chains of phospholipids determines fluidity
• Greater no. of double bonds & shorter acyl chains = less tightly packed molecules = greater fluidity
• All mebranes are fluid at temp of organism

-so plants & cold-blooded animals (ambient temp) have more double bonds &/ acyl chains than warm-blooded.

Question 3

Effects Of Cholesterol On Membrane

Answer 3

• Makes membranes less permeable
• Packs between phospholipids & makes membrane less permeable at surface
• Doesn’t make overall membrane more rigid
• At high conc. stops membrane becoming crystalline

Question 4

Structural Organisation Of Biological Membranes

Answer 4

Fluid-Mosaic Model

Question 5

Basics Of Protein Structure

Answer 5

• Polypeptides
• made of linear chains of amino acids (100-500) linked by amide bonds
• Primary structure: sequence of amino acids
• Secondary structure: folded polypeptide chain (alpha-helix, beta-sheet, random coil)
• Tertiary structure: combination of folded domains

Question 6

Protein Associations With Membrane

Answer 6

• Integral membrane proteins - directly insert into membrane by a hydrophobic domain ( eg B-barrel, GPCRs-span membrane 7 times)
• Peripheral membrane proteins - either have covalently bound lipids which insert into membrane or associate with integral proteins

Question 7

How Are Lipid Anchored Proteins Attached To The Membrane?

Answer 7

• Via covalently bound fatty acid (myristic or palmitric)
• Or prenyl group
• Or via phosphatidylinositol in outer leaflet of bilayer

Question 8

What Are The Adv.s Lipid Anchoring Confers On Membrane Proteins?

Answer 8

• Mobility at cell surface
  
  • Phospholipids move over 10x faster than proteins in 2D plane
  • Phosphatidylinositol linked proteins move realtively rapidly
• Rapid realease into extracellular space
  
  • specific phosphatidylinositol lipases release proteins anchored outside the cell
• Regulates binding of proteins to membranes & release from membranes
  
  • eg G proteins anchored by palmitate/ myrystate are converted to active form at membrane (inactive in cytosol)
  • G proteins are involved in cell signalling & by this mechanism can go through activation/ inactivation cycles (eg Ras)

Question 9

Transmembrane Domains

Answer 9

• Usually form alpha-helix with high conc of hydrophobic amino acids
• Multiple alpha-helices can interact to form a channel
• Can also form from B-strands, which interact to form a B-barrel
  
  • Form pores and function as receptors

Question 10

Rafts

Answer 10

• Cholesterol and sphingolipids can form microdomains (rafts)
• Membrane slightly thicker at raft

Question 11

Detergents

Answer 11

• Amphipathic molecules
• Used to study membrane proteins
• Act like lisosomes & can extract membrane proteins