Lecture 12: Membranes Flashcards
1
Q
Name the common features of biological membranes
A
Sheet like structures that form closed boundaries between different compartments.
- Mainly, consist of lipids and proteins. Membranes also contain carbohydrates that are linked to lipids and protein.
- Membrane lipids have both a hydrophilic and a hydrophobic unit.
- Specific proteins are embedded in lipid bilayers and mediate distinct functions.
2
Q
Name the 3 features of the non-covalent assemblies that membranes can form
A
- Asymetric: The two faces of the biological membranes are always different from each other (They are? How?)
- Fluid Structure: Both lipid and protein membranes diffuse rapidly in the plane of the membrane but they do not rotate across the membrane
- Electrically Polarized: With the Inside Negative
3
Q
Describe the amphipathic properties of the membrane
A
Membrane lipids are amphipathic (amphiphilic), because they contain:
- Hydrophilic unit – polar head group, represented by a circle.
- Hydrocarbon tail – depicted by straight or wavy lines.
- Oil forms a mono molecular layer on the surface of water
- polar heads are in contact with water
- nonpolar hydrophobic lipid tails project into air
4
Q
List the two possible forms a hydrophobic lipid can take in an aqueous solution
A
Micelles
And Lipid Bilayers (or Bimolecular Sheets)
5
Q
Describe a micelle
A
- A micelle is a limited structure
- Usually less than 20 nm in diameters*
- A micelle is formed when a variety of molecules including soaps and detergents are added to water
6
Q
Describe lipid bilayers
A
- (Favored form) A bimolecular sheet can have macroscopic dimensions, such as 106 nm (1 mm)
- Self-assembled by Hydrophobic interaction (major), Van der Waals attractive forces between hydrocarbon tails, and Electrostatic and hydrogen-bonding attractions between polar head groups and water molecules
- Hydrophobic bonds have: 1. An inherent tendency to be extensive, 2. Tend to close on themselves so that there are no edges with exposed hydrocarbon chains, and so they form compartment, and 3. Self-sealing
- have a very low permeability for ions and most polar molecules, but Water can traverse membrane easily because Its small size, high concentration and lack of a complete charge.
7
Q
Describe Membrane Assymetry
A
- All biological membranes are asymmetric
- Lipid molecules are also asymmetrically distributed
- The biological composition of inner and outer membrane is different
- Membrane proteins have unique orientations
- Large amounts of CHOLESTEROL are present in both leaflets
8
Q
Define the Fluid Mosaic Model
A
- Proteins move laterally in lipid bilayer
- No flip-flops (switching of heads to other side of bilayer, except…)
- Phospholipid molecule flip-flops once in a several hours
- Membrane asymmetry can be preserved
- To restate, internal proteins don’t “flip-flop”, phospholipid bilayer can
9
Q
Describe Membrane Fluidity
A
Controlled by:
- Fatty acid composition
- Length of fatty acid chain
- Saturation levels (Tm, rigidity)
- Position of double bands, especially cis-position produces a bend, effects Tm, provides more fluidity (why cis is better)
- Cholesterol content (not equally distributed throughout the membrane)
- Provides an environment where signal transduction and molecule transportation take place
10
Q
Describe Integral Membrane Proteins
A
- Interact extensively with the hydrocarbon chains of membrane lipids.
- Can be released only by agents that compete for these nonpolar interaction, such as organic solvents and detergents.
- Span the lipid bilayer
- They are responsible for most of the dynamic processes carried out by membranes
- Membranes differ in their protein content
- The protein components can be visualized by Sodiumdodecylsulfate polyacrylamide gel electrophoresis(SDS/PAGE)
11
Q
What is BR in relation to a membrane protein?
A
- Membrane Protein Bacteriorhodopsin
- A light-driven proton pump, converts the energy of light into trans membrane proton gradient that is used to synthesize ATP
- The protein is built almost entirely of a helices (with most of nonpolar amino acids) arranged almost perpendicularly to the bilayer plane
- Membrane-spanning a helices are the most common structure motif in membrane proteins.
12
Q
What does COX and PGHS stand for?
A
- Cyclooxygenase and Prostaglandin H2 Synthase
- An integral protein and binds to the luminal leaflet of the ER.
- A homodimer that consists of primarily a helices.
- Is NOT a membrane spanning protein.
13
Q
Describe peripheral membrane proteins
A
- Bound to membranes primarily by electrostatic and interactions hydrogen-bond
- Dissociated from membranes by adding salt or pH changes
- Bound to the surfaces of integral proteins either on
- the cytosolic or extracellular side of membrane
- anchored to the lipid bilayer by a covalently attached hydrophobic chain, such as a fatty acid
14
Q
Define lipid-linked proteins
A
- Lipids are covalently associated to the proteins.
- Lipids anchor their attached proteins to membrane and mediate protein-protein interactions.
15
Q
What are the 3 types of lipid-linked protein modification
A
- Palmitoylation of cysteine residues by a thioester bond. (Read up carefully on this one)
- Farnesylation of cysteine residues at the Cterminus (And this one)
- Glycosylphosphatidylinositol-link to the carboxyl terminus. (And this one) (Don’t need to know specific structure, but know the names of structure, and what it does and all that stuff)
