M&R Session 1 Flashcards
Membrane composition?
60% protein
40% lipid
1-10% carbohydrate
5 main functions of biological membranes?
- Selectively permeable barrier
- Control chemical environment
- Communication
- Recognition
- Signal generation
Structure of phospholipids?
Glycerol
2 FA chains
Phosphate head group
What are cerebrosides?
Glycolipids with sugar monomer heads
What are gangliosides?
Glycolipids with oligosaccharide head groups
What kind of molecules are membrane lipids?
Amphipathic
Four methods of lipid mobility?
- Intra-chain motion
- Fast-axial rotation
- Fast lateral diffusion
- Flip-flop
Three modes of permitted mobility of proteins in biological membranes?
- Conformational change
- Rotational
- Lateral
Restraints on protein mobility in membranes?
Flip flop too energetic
Aggregates - movement w/in membrane slower
Tethering - cannot physically move
Interaction w/other cells - adhesion proteins fixed
Lipid mediated effects - prefer cholesterol poor regions
Evidence for membrane proteins?
Facilitated diffusion
Ion gradients
Specificity of cell responses
Membrane fracture
Amphipathic molecule arrangement in water?
Micelle or bilayer
How are liposomes formed?
Enclosure by lipid bilayer in aqueous media
Structure of cholesterol?
Polar head group
Rigid planar steroid ring
Non-polar hydrocarbon tail
What is the function of cholesterol in the phospholipid bilayer?
Abolishes endothermic phase transition (reduces melting temperature)
Reduces phospholipid chain motion (reduces fluidity)
How does cholesterol reduce fluidity in the CSM?
Beta-OH locks onto adjacent phospholipid - rigid ring obstructs packing
Is the flexible tail motion of a phospholipid bilayer affected by cholesterol?
No
What is the overall result of cholesterol on a phospholipid bilayer?
Homogenise properties to keep membrane constant over a greater range of temperatures
Why is there no flip-flop of membrane proteins?
Hydrophilic structure too large to cross hydrophobic domain - too much energy and resultant disruption to ion gradients too great
Classifying features of peripheral membrane proteins?
Bound to surface (can be washed off)
Electrostatic and H bond interactions
Removed by changes in pH/ionic strength
What type of membrane protein prefers cholesterol-rich areas?
Transducers of protein-protein associates
Features of integral membrane proteins?
Interact extensively w/hydrophobic domains
Only removed by non-polar interaction competitors (detergents/organic solvents)
Typical length of transmembrane polypeptides?
18-22 a.a.
Typical structure of transmembrane polypeptides?
Hydrophobic R groups
Alpha-helical transmembrane domains
How do glycoproteins orientate?
Oligosaccharides outside
Disulphide bonds outside
Transmembrane helix inside
Sulphylhydryl groups inside
Where are dolichol phosphate-linked p.p. attached to CSM?
On membrane lipids on extracellular side
What causes FA of lipids to reinsert into the bilayer?
What is the effect?
Post-translational modification
Restricts protein movement
Are peripheral protein associations extracellular or in the cytosol?
Both - Hah!
Why is asymmetrical orientation of membrane proteins important?
Receptor needs recognition site towards extracellular space
What is the structure of spectrin?
Alpha and beta subunits wind together –> antiparallel heterodimer
Long, floppy, rod-like (tee hee)
Heterotetramer formed by head-to-head association of 2 molecules
Which molecules restrict lateral mobility of membrane proteins?
Ankyrin
Band 4.1
Give two examples of haemolytic anaemias.
Hereditary spherocytosis
Hereditary elliptocytosis
Give an overview of hereditary spherocytosis.
Relatively rare
Spectrin decreased by 40-50%
Erythrocytes round up and shear through narrow lumen
RBCs less resistant to lysis –> easily fatiguable
Tx: adapt lifestyle and blood transfusion
Give a brief overview of hereditary elliptocytosis.
Defect in spectrin molecule
Can’t form heterotetramers –> no mesh
Fragile, elliptoid cells
What defines the orientation of a membrane protein?
Positioning of positively charged residues at N or C-terminal end of start-transfer sequence during protein synthesis
How is a membrane protein oriented with C terminus into the ER lumen?
+ve residue at N terminus of start-transfer sequence
What directs the N terminus of a membrane protein into the ER lumen?
+ve residue at C-terminal end of start-transfer sequence
What is the function of the stop codon during membrane protein synthesis?
Clamp protein in the membrane
What stabilises the partially folded growing p.p. in multiple transmembrane domain protein synthesis?
Lumenal binding proteins (related to heat-shock proteins)
Give a one sentence description of how G-protein coupled receptors are formed.
Spanning domains made outside and inserted into the membrane
Is protein orientation maintained during exocytosis?
Yes
Describe the vesicle resulting from endocytosis.
Sealed inside-out vesicle with reversed protein orientation
Describe the vesicle resulting from exocytosis.
Sealed right-side out vesicle
What property do the precursory vesicles of both exocytosis and endocytosis share?
They are leaky before they become sealed
