Session 1 - Lipid, Proteins and Memb structure Flashcards
What is meant by plasma membranes being amphipathic?
-Has both hydrophillic and hydrophobic moieties
What are the main functions of plasma membranes?
- Highly selective permeable barrier
- Compartmentalisation
- Communication
- Immune surveillance
What is the general composition of a plasma membrane?
- 40% Lipid
- 60% Protein
- 1-10% Carbohydrate
- 20% Water
What is the predominant lipid type in PM?
-Phospholipid
What is the general structure of a phospholipid?
- 1 Gylcerol
- 2 FAs
- Phosphate head group
Are phosphate head groups of phospholipids polar or non-polar?
-Polar
What are the 4 types of possible head group?
- Cholines
- Amines
- a’a
- Sugars
What are the possible lengths of the FA chains in phospholipids?
-C14 to C24
What are the predominant FA chain lengths of a phospholipid?
-C16 and C18
What cis double bonds introduce to FA chains?
-Kinks
What is the effect on the membrane of introducing a kink to a phospholipid FA chain?
-Reduces phospholipid packaging in the membrane which increases membrane fluidity
Can cis double bonds be produced naturally?
-No, they have to be dietary
What is the main difference between sphingomyelin and a phospholipid?
-Sphingomyelins are not based on glycerol backbone (sphingoid base backbone instead)
What is usually the head group of sphingomyelin?
-Phosphocholine
What is a glycolipid?
-Sugar-containing lipid (lipid with carbohydrate group attached)
Do glycolipids have a phosphate head group?
-No, the carbohydrate attaches straight to the FA chain
Are glycolipids more similar to phospholipids or sphingomyelin?
-Sphingomyelin (replace phosphocholine with a sugar=glycolipid)
What is a cerebroside?
-Glycolipid with a sugar monomer head group
What is a ganglioside?
-Glycolipid with an oligosaccharide head group
Approxmately, what % does chlesterol produce to the total membrane lipid?
-45%
What makes cholesterol soluble in plasma membranes?
-The hydrophilic head binds to the C=O of a FA
What two effects can cholesterol have on a membrane?
- Prevents aggregation of FA chains (reduces packing) and thus maintains membrane integrity and increases fluidity
- Reduces phospholipid chain motion to stabilise the membrane and decrease fluidity
Why is a lipid bilayer described as fluid mosaic?
- Fluid -> hydrophobic integral compounds (such as lipids and proteins) can move laterally
- Mosaic -> many different components and parts to the membrane
What two conformations can plasma membranes take?
- Micelles
- Bilayers
What conformation do plasma membranes form spontaneously in water?
-Lipid bilayer
What drives the spontaneous formation of a lipid bilayer in water?
-Van Der Waal forces between hydrophobic tails
What stabilises the bilayer formation of plasma membranes?
-Non-covalent bonds such as H bonding and electrostatic interactions
What are the 4 modes of movement of phospholipids?
- Flexion
- Fast-axial rotation
- Lateral diffusion
- Flip Flop
What is flexion of phospholipids?
-Intra-chain motion where kinks form in the FA tail
What is flip flop of phospholipids?
-1:1 exchange of phospholipids on opposite lamella
Why does flip flop not often occur?
-Requires a very high amount of energy
How do bilayers maintain there structure?
-By using energy to ensure the correct conformation is maintained
What is the functional evidence of protein existence in plasma membranes?
- Facilitated diffusion
- Ion gradients
What two biochemical ways can proteins be proved to exist in plasma membranes?
- Membrane fractionation followed by SDS PAGE
- Freeze Fracture then snowdrift
What 3 ways can proteins move within plasma membranes?
- Rotation
- Lateral diffusion
- Conformational Change
Why is it not possible for proteins to flip flop?
-They are large hydrophobic molecules and the energy required is too great. If the energy was acquired the action would tear the membrane apart
How is protein movement restricted?
- Aggregations of proteins slows movement
- Associations with the membrane restricts movement eg tethering proteins
- Interactions with other cells restricts movement
- Interactions with peripheral proteins restricts movement
Why would proteins aggregate in plasma membranes?
-Most proteins prefer cholesterol poor regions so will aggregate there (scaffold proteins prefer cholesterol rich)
Describe how proteins are inserted into membranes
1) Free ribosome initiates translation from a mature mRNA molecule
2) Hydrophobic N-terminal sequence is produced and protrudes out of ribosome
3) Signal sequence recognised and bound by signal recognition particle
4) Protein synthesis stops
5) SRP directs the ribosome towards the SRP receptors on the cytosolic face of the ER
6) SRP binds to SRP receptor and the newly formed polypeptide is partially translocated into ER lumen;SRP dissociates
7) Protein synthesis continues, a hydrophobic stop transfer spanning region detected in the membrane and locks the protein into place
8) Protein sysnthesis continues in the cytosol
9) Signal sequence is removed by peptidase once the polypeptide synthesised and released into cytosol due to orientation of protein
10) Ribosome dissociates and is recycled
11) The inserted membrane protein is enveloped in an inside out vesicle through the golgi and is translocated to pm where is is inserted into the membrane through vesicle fusing
What are the two types of membrane proteins?
- Peripheral
- Integral
Where are peripheral proteins within the membrane?
-Bound to the surface
How are peripheral proteins bound to a membrane?
-Through H bonding and electrostatic interactions
How are peripheral proteins removed from a membrane?
-Through manipulation of pH or ionic strength through washing
Where are integral proteins within a membrane?
-Embedded within the membrane
How are integral proteins bound to a membrane?
-Through interactions with the hydrophobic regions
Are integral proteins completely embedded?
-It is rare, they are often only partially embedded or they are transmembrane
How are integral proteins removed?
-By using a detergent or organic solvent which competes for non-polar interactions and destroys the membrane
What conformation are transmembrane proteins, usually?
-a-helices
What types of amino acid make up transmembrane proteins?
- Small hydrophobic a’a
- Occasionaly polar uncharged a’a
- NEVER hydrophilic, acidic or basic
Do proteins always have 1 TMD?
-No they can have multiple
How does a hydropathy plot determine TM domains?
- Measures the hydrophobicity/hydrophilicity of the membrane protein
- > multiple hydrophobic regions must mean multiple TM domains
Why is asymmetrical orientation of a protein important?
-Important for correct functioning of proteins eg receptors
How is the cytoskeleton of a cell exposed?
-Wash off all peripheral proteins
What are the main components of an erythrocyte cytoskeleton?
-Spectrins and actin which form a network
How is the spectrin-actin network anchored to the membrane?
-Adaptor proteins bind to the spectrin-actin and to integral proteins
What are the two main adaptor proteins in an erythrocyte cytoskeleton?
- Ankyrin
- Band 4.1
Which integral protein does ankyrin bind to in an erythrocyte membrane?
-Band 3
What integral protein does band 4.1 bind to in an eythrocyte membrane?
-Glycophorin A
What makes up a junctional complex?
- Actin
- Band 4.1
- Glycophorin A
What effect does attachment of the cytoskeleton to integral proteins have?
-Restricts lateral mobility of proteins
Hereditary Spherocytosis and Hereditary Elliptocytosis are forms of…
…Haemolytic anaemias
What causes hereditary spherocytosis?
-Depletion of spectrin (upto 40-50% in most common form)
What happens morphologically to RBCs with depleted spectrin?
-Loose bioconcave shape (‘Round up’)
Why do RBCs become lysed in hereditary spherocytosis?
-They are lysed by the sheering forces exerted by the capillary bed due to loss of elasticity and flexibility
What effect does the spleen have in hereditary spherocytosis?
-Increases clearance of defective RBCs, reducing their lifespan
Why is there resulting anaemia in hereditary spherocytosis?
-The bone marrow cannot compensate leading to anaemia
How is hereditary spherocytosis treated?
-Blood transfusion resulting in improved performance for approximately 120 days (normal RBC life span)
What causes hereditary elliptocytosis?
-The inability of spectrin to form heterotetramers
What results from the inability to form heterotetramers in hereditary elliptocytosis?
-Elliptoid-shaped cells with a weakened lattice which are less resistant to lysis
Apart from spectrin, what other cytoskeleton protein defect can cause haemolytic anaemias?
-Ankyrin
What is the number of a’a needed to span a membrane?
-18 to 20 a’as
What determines membrane protein orientation?
-The location of the hydrophobic signal sequence
How does an N terminal signal sequence ensure that the N terminal is within the membrane?
1) N-terminal signal sequence binds and remains in hydrophobic region
2) Protein loops through into ER
3) Signal sequence is cleaved and N terminal released into lumen
4) Protein sysnthesis continues until membrane spanning domain reached and protein locked into place
5) protein synthesis continues in the cytoplasm
How is a protein with multiple transmembrane spanning domains synthesised into the membrane?
-1st TMD synthesised and locked into place within the membrane, the others are synthesised in the cytosol and brought into the membrane upon completion by hsp
