M&R Flashcards
Describe the dry composition of biological membranes
40% lipids
60% protein
1-10% carbohydrate
What are the main functions of biological membranes? (6)
Communication
Recognition
Signal generation in response to stimuli
Highly selective permeability barrier
Allows control of enclosed chemical environment
Energy conservation by oxidative phosphorylation
What percentage of a hydrated bilayer is made of water?
20%
Describe the structure of a phospholipid
Head group - polar e.g. choline, amines, amino acids, sugars
Fatty acid chains:
- varied length (C16 and C18 most common)
What is the effect of an unsaturated fatty acid side chain in the cis conformation?
Produces a kink in the chain, which reduces phospholipid packing thus increases membrane fluidity
What is sphingomyelin?
The only phospholipid not based on glycerol, but its conformation still resembles other phospholipids
What is a glycolipid?
Sugar-containing lipid
Where phosphate head is replaced with a sugar
What are the two types of glycolipid?
Cerebrosides - head group is a sugar monomer
Gangliosides - head group is an oligosaccharide
What type of structure does a fluid membrane have and which forces stabilise it?
Co-operative structure
Non-covalent forces, electrostatic interactions and H bonds between hydrophilic moieties
What are the 4 modes of mobility within a lipid bilayer?
Flexion - kink formation in fatty acid chain
Axial rotation
Lateral diffusion - within plane of bilayer
Flip-flop - movement of molcs from one half of bilayer to the other (1-for-1 exchange)
How does cholesterol contribute to membrane stability?
Hydrogen bonds to fatty acid chains
Abolishes endothermic phase transition
Reduces phospholipid packing (increases memb fluidity)
BUT reduces phospholipid chain motion (decreases memb fluidity)
Explain the functional evidence for membrane proteins
Facilitated diffusion
Ion gradients
Specificity of cell responses
Explain the biochemical evidence for membrane proteins
Membrane fractionation and gel electrophoresis (SDS-page)
Freeze fracture
Describe how integral proteins associate with the lipid bilayer
Deeply imbedded
Interact with hydrophobic regions
Require detergent/organic solvent to be removed (competes for non-polar interactions)
Describe how peripheral proteins interact with the lipid bilayer
Associated with surface
Interact via electrostatic interactions and hydrogen bonds
Removed by changes in pH or ionic strength
Name the three modes of membrane protein movement
Conformational change
Rotation
Lateral diffusion
CANNOT FLIP-FLOP - have large hydrophilic regions so large amounts of energy would be needed for them to pass through hydrophobic part of bilayer
What restricts membrane protein mobility?
Lipid-mediated effects - proteins tend to separate out into fluid phase or cholesterol-poor regions
Membrane protein associations
Association with peripheral/extra-membranous proteins (e.g. cytoskeleton)
Describe how membrane proteins are inserted into membranes (i.e the secretory pathway)
Protein synthesis on free ribosomes
Signal sequence produced at N-terminal
SRP recognises and binds to signal sequence
Protein synthesis stops
SRP (GTP-bound) directs protein to SRP receptors on cytosolic face of ER
SRP dissociates, GDP and Pi released
Protein synthesis continues as new polypeptide is fed into ER lumen via a pore in the membrane (peptide translocation complex)
Signal sequence cleaved by signal peptidase
Dissociation of ribosome
Protein folds spontaneously, disulphide bonds form
N-linked glycosylation via dolichol
What is the difference between synthesis of membrane proteins and secretory proteins?
Membrane proteins need to span the membrane of the vesicle rather than be contained within in
How is correct orientation of membrane proteins ensured?
Directed by membrane synthesis
Addition of a highly hydrophobic stop transfer signal (18-20aas long - distance to span a phospholipid bilayer)
When membrane protein is being translocated into ER lumen, ribosome comes across stop transfer signal
Stop transfer signal remains in ER membrane and rest of protein is translated outside of ER in cytoplasm
Therefore protein spans membrane
How are erythrocyte membranes prepared for analysis
Ghosts prepared by osmotic haemolysis
Run on gel to show >10 major proteins
Most are peripheral (released by PH or ionic change)
Found on cytosolic face of membrane
Which two components make up the cytoskeleton?
Spectrin and actin
Describe the structure of spectrin
Long rod-like molecule
Alpha and beta subunits wound together as a heterodimer
Two heterodimers form a heterotetramer