M&R S1 - Lipids, Proteins and Membrane Structure Flashcards
Fatty acids are amphipathic, what does this mean?
They contain both hydrophilic and hydrophobic moieties
A phospholipid bilayer is composed of what?
Give percentages
Dry Weight:
40% Lipid
60% Protein
1-10% Carbohydrate
20% Water (when hydrated)
What is the predominant type of lipid in a cell membrane?
Give an example
Phospholipids
Phosphatidylcholine
What is a phospholipid composed of?
Glycerol backbone with:
2 fatty acid chains (can be identical or different)
A phosphate group w/ head group
Give examples of head groups you may find in a phospholipid and give the physical property they all possess
Choline
Amines
Amino acids
Sugars
All polar
Give two features of fatty acid chains commonly found in phospholipids
Enormous variety in chain length, commonly C16 or C18
Unsaturated fatty acid side chains in the cis formation introduce a ‘kink’ that reduces phospholipid packing and hence increases membrane fluidity
What phospholipid doesn’t follow the traditional structure?
Briefly describe HOW it differs
Does this have any effect on its conformation in the membrane?
Shingomyelin
Not based on glycerol
Conformation in the membrane resembles other phospholipids
What are glycolipids?
What types are there and how do they differ?
Suger containing lipids
Cerebrosides - Head group sugar monomers
Gangliosides - Head group sugar oligosaccharides
What percentage of a cell membrane lipids are cholesterol?
45%
What two structures will amphipathic molecules form in water?
Micelles
Bilayers
How does formation of a bilayer occur?
What stabilises this structure?
Amphipathic molecules form a bilayer spontaneously in water
This is driven by Van der Waals forces between hydrophobic tails
Structure is stabilised by non-covalent forces:
Electrostatic and hydrogen bonds between hydrophilic moieties
Interactions between hydrophilic moieties and water
What are the ways lipid molecules can move in a bilayer?
Give a short description of each
Intra-chain motion (Flexion) - Movement of the fatty acid chains
Fast axial rotation - Spinning of the molecule
Fast lateral diffusion - Movement of the molecules relative to each other in the plane of the bilayer i.e. molecules can travel around the bilayer
Flip-Flop (Transverse Diffusion) - One for one exchange of molecules from each layer of the bilayer
What are some of the functions of membrane proteins?
Enzymes Transporters Pumps Ion channels Receptors Energy transducers
To what degree does protein content of a membrane vary?
From 18% (Myelin)
To 75% (Mitochondrial membranes)
What evidence does membrane function provide for the existance of membrane proteins?
Facilitated diffusion
Ion gradients
Specificity of cell responses
These three functions are not performed by the bilipid membrane, so must be the domain of proteins.
What Biochemical techniques can be used to verify the existance of membrane proteins?
Freeze fracture
Membrane fractionation and gel electrophoresis
Membrane proteins have what 3 modes of motion?
Conformational change
Rotational
Lateral
What mode of motion is available to lipids in a membrane but not proteins?
Why is this?
Flip-Flop / Transverse
Proteins have large hydrophilic moieties that would require a large amount of energy to pass through the hydrophobic region of the bilayer
What factors might be involved in restriction of protein movement in a membrane?
Lipid mediated effects - Proteins will tend to separate out into the fluid phase or cholesterol poor regions
Membrane protein associations/aggregation
Association with extramembranous proteins
E.g. Cytoskeleton anchoring or adhesion to the basement membrane
Describe 2 features of peripheral membrane proteins
Bound to the surface of the membrane non-covalently by electrostatic and hydrogen bonds
Can be removed by changing the pH or ionic strength
In what 2 ways do integral membrane proteins differ from peripheral proteins?
Interact extensively with the hydrophobic regions of the membrane, not just the surface
Requires agents (Detergents, Organic solvents) that compete for the non polar interactions with the bilayer to remove i.e. Cannot be removed by changes in pH or ionic strength
Describe the protein secretion pathway in full (MGD S6)
- Free ribosome begins protein synthesis
- Hydrophobic N-terminal signal sequence produced
- Signal recognition particle (SRP) recognises signal sequence and binds
- Protein synthesis stops
- GTP-bound SRP directs ribosome to SRP receptors on RER cytostolic surface
- SRP dissociates
- Protein synthesis continues, feeding protein into RER via pore in membrane (peptide translocation complex)
- Signal sequence is removed by signal peptidase once the entire protein is finished
- The ribosome dissociates and is recycled
How is orientation of a membrane protein determined?
Protein synthesis determines this
It does so with the addition of a highly hydrophobic stop transfer signal
When the protein is being translated and fed into the ER lumen the stop signal will then remain in the ER membrane once it has been translated
The rest of the protein is translated in the cytoplasm and the protein will span the membrane
Describe the composition of a stop transfer signal
Normally 18-20 AAs long
Made up of hydrophobic, small or uncharged amino acids
What is a hydropathy plot and what is it used for?
Hydropathy plots show the various hydrophobic and hydrophilic regions of a protein by showing the hydropathy index of each amino acid
Can be used to show how many transmembrane regions are contained within a protein
What is indicated by an amino acids’s hydropathy index?
Describe the appearance of a hydropathy index i.e. What is plotted along each axis?
A positive hydropathy index indicates the amino acid is hydrophobic, negative indicates a hydrophilic amino acid
Hydropathy index is plotted along the Y axis
Amino acids in the protein are numbered and plotted along the X axis
Why is asymmetry of membrane proteins important to function?
Give an example.
Important to specific functions of each protein
E.g. Hydrophilic hormone receptor must have its recognition site on the extracellular face to function properly
What are the general functions of cholesterol in a phospholipid bilayer?
Stabilises the membrane by hydrogen bonding to the fatty acid chains
Abolishes the endothermic phase transition of membranes
Cholesterol has paradoxical effects on membrane fluidity, explain.
Cholesterol’s large sterol ring structure reduces phospholipid packing and hence increases membrane fluidity
However, Cholesterol’s hydrophobic tail reduces phospholipid chain motion and hence reduces membrane fluidity
Give a general description of cholesterol structure and briefly explain how this allows it to dissolve in the phospholipid bilayer
Hydrophilic head group interacts with phosphate heads of the phospholipids
Large sterol ring structure and the hydrophobic tail interact with the hydrophobic regions of the phospholipid bilayer
Describe the Fluid Mosaic model
The name is extremely descriptive, and also a hell of a good basis for your description. Hint Hint.
Plasma membrane is said to be a fluid because its hydrophobic integral components can move laterally along the membrane as if moving through a fluid
The membrane is depicted as a mosaic as its made up of many different components (E.g. Proteins, phospholipids, cholesterol etc)
What is the function of the erythrocyte cytoskeleton?
Where is it found?
How is it isolated for analysis?
Cytoskeleton holds the shape of the RBC
Found along the cytosolic face of the membrane
Can be removed from the membrane by a low strength ionic wash
Describe the composition of the erythrocyte cytoskeleton.
Include details of its anchorage to the plasma membrane
Composed of Spectrin and Actin molecules
attached to the membrane by Ankyrin (to Band 3) and by Band 4.1 (to Glycophorin)
(Band 3 and Glycophorin are transmembrane proteins)
What is the consequence of the attachment of the cytoskeleton to the membrane proteins?
Restricts lateral mobility of the membrane proteins
What are the general features of haemolytic anaemias?
Erythrocyte cytoskeleton is improperly formed causing the RBCs to ‘round up’ and are lysed by shearing forces in the capillary beds and cleared by the spleen
Describe ‘Hereditary Spherocytosis’
Genetic disease commonly found in dominant form
Characterised by the reduction of spectrin levels by 40-50%
The resulting ‘round up’ and increased lysis results in reduced RBC lifespan
The bone marrow cannot compensate, leading to haemolytic anaemia
Describe ‘Hereditary Elliptosis’
Spectrin molecules are unable to form stable tetramers
Results in fragile elliptoid cells, which lead to haemolytic anaemia
