Membranes

Question 1

List some common features of biological membranes.

Answer 1

• sheet like structures that form closed boundaries
• consist mainly of lipids and proteins (some carbs)
• hydrophilic and hydrophobic regions
• asymmetric
• form non-covalent assemblies
• fluid structure = both lipid and protein molecules diffuse rapidly in the plane of the membrane but do not rotate across the membrane
• electrically polarized (inside is negative)

Question 2

Define amphipathic.

Answer 2

Containing both a hydrophilic (polar head) and hydrophobic region (hydrocarbon tail)

Question 3

What are the two ways that membrane lipids can arrange themselves in a aqueous solution?

Answer 3

Micelle or lipid bilayer (bimolecular sheet)

Question 4

When is a Micelle formed?

Answer 4

Formed when a variety of molecules including soaps and detergents are added to water

Question 5

What is the favored structure for most phospholipids and glycolipids in aqueous media?

Answer 5

A bimolecular sheet (lipid bilayer)

Question 6

What are the major driving forces for the assembly of the lipid bilayer?

Answer 6

• hydrophobic interactions (major)
• Van der Waals attractive forces between hydrocarbon tails
• electrostatic and hydrogen-bonding attractions between head groups and water molecule

Question 7

How are lipid bilayer said primarily held together?

Answer 7

Hydrophobic interactions

Question 8

What are the three significant consequences of hydrophobic interactions in lipid bilayer formation?

Answer 8

1. An inherent tendency to be extensive (sheet)
2. Tend to close on themselves so that there are no edges with exposed hydrocarbon chains, and so they form compartment
3. Self-sealing (energetically favorable)

Question 9

What do lipid bilayers have a LOW permeability for?

Answer 9

Ions and most polar molecules

Water is able to pass easily because of small size, high concentration and lack of a complete charge

Question 10

All biological membranes are _______________________

Answer 10

Asymmetric

Question 11

Describe lateral diffusion

Answer 11

Rapid movement of molecules on the same side of the membrane

Question 12

Describe transverse or flip-flop diffusion

Answer 12

Very slow
Requires more energy
When molecules move across the membrane (from one side to the other)

Question 13

What is the fluid mosaic model?

Answer 13

• Proteins move laterally in the lipid bilayer
• proteins do not flip-flop
• phospholipid molecules flip-flops once every several hours

Question 14

What controls membrane fluidity?

Answer 14

1. Fatty acid composition:
   - length of fatty acid chain
   - saturation levels
   - position of double bonds (especially cis-position produces a bend which effects the T_m and provides more fluidity
2. Cholesterol content

Question 15

Describe an integral membrane protein.

Answer 15

• Interact with the hydrocarbon chains of membrane lipids
• can only be released by organic solvents and detergents
• span the lipid bilayer (go all the way through)

Question 16

Describe Bacteriorhodopsin structure (membrane protein structure)

Answer 16

• has a light driven proton pump
• built most entirely of alpha helices arranged perpendicularly to the bilayer plane
• membrane spanning alpha helices are the most common structure motif in membrane proteins

Question 17

Describe cyclooxygenase 1 (COX) also called prostaglandin H_2 synthase (PGHS) 1

Answer 17

• integral protein that binds to the luminal membrane of the ER
• homodimer that consists mostly of alpha helices
• not a membrane spanning protein

Question 18

Describe peripheral membrane proteins.

Answer 18

• Bound to membranes primarily by electrostatic and hydrogen-bond interactions
• dissociated from membranes by adding salt or pH change
  -bound to the surfaces of integral proteins either on
  The cytosol if or extracellular side of membrane
  Anchored to the lipid bilayer by a covalently attached hydrophobic chain such as a fatty acid

Question 19

What are the three types of lipid-linked protein modifications?

Answer 19

1. Palmitoylation
2. Farnesylation
3. Glycosylphosphatidylinositol

Question 20

How does palmitoylation work?

Answer 20

Thioester bond at a cysteine residue

Question 21

What is the mechanism of farnesylation?

Answer 21

• covalent attachment of a farnesyl (C15) unit to C-terminal tetrapeptide CAAX in which Cysteine is followed by 2 aliphatic residues
• after the farnesyl group is added to the protein by a this ester linkage with the cys residue, the AAX tripeptide is hydrolytically cleaved away
• FUNCTION: anchoring the protein to the membrane and facilitating protein to protein interaction

Question 22

Describe the Glycosyl phosphatidylinositol (GPI) anchor protein.

Answer 22

• anchors the proteins to the outer leaflet of the plasma membrane
• many cell to surface hydrolytic enzymes and adhesions are tethered to cells by a GPI unit

Question 23

Describe the two membrane/two compartment system in mitochondria.

Answer 23

1. Outer membrane: permeable to most small molecules and ions - mitochondrial porin (VDAC)
2. Inner membrane: impermeable to nearly all ions and polar molecules

• inter membrane space compartment:
  Where oxidative phosphorylation takes place in the inner mitochondrial membrane

-matrix:
The site of most of the TCA cycle and fatty acid oxidation