What are the functions of a biological mebrane?
- Continuous, highly selective permeablity barrier
- Control of the enclosed chemical environment
- Recognition - signalling moecules, adhesion protein and immune surveillance (e.g. presenting 'self' antigen so cells are not attacked by own immune system)
- Signal generation in response to stimuli (involving interactions with other cells) which can be electrical or chemical.
- Communication - control the flow of information between cells and theirenvironment
Are all membranes the same?
Different membanes have specialised functions e.g. mitochondrial membrane is specialised for ATP generaton by oxidaive phosphorylation.
What kind of functions can different regions of a biological membrane have?
Interaction with basement membrane
Interaction with adjacent cells
Absorption of body fluids
Synapses (nerve junctions allow communication between cells)
Electrical signal conduction
Changing shape may change the properties of a particular region.
What is the General Membrane Composition?
Varies with source of membrane (depends on function and location point on cell) but generally membranes contain approximately (dry weight):
sea of lipids packed with proteins
NB: membranes are hydrated structures so 20% of total weight is WATER
What is the major property of membrane lipids?
They are AMPHIPATHIC molecules (they contain both a hydrophilic and a hydrophobic moiety). Distribution varies depending on cell type.
Name the predominant membrane lipid. Give an example
Phospholipid: 3C (Glycerol) backbones with two fatty acid chains and a phosphate head group.
E.g. Phosphatidylcholine (choline head group attached to the phosphate group which is attached to the glycerol)
Give examples of phospholipid head groups
Describe the structure and properties of a phospholipid
- Wide range of head groups (choline, amines, amino acids, sugars) which are POLAR and HYDROPHILIC.
- Fatty acid chains (length between C14 and C24 but C16 and C18 most prevalent) resulting in all fatty acids being approximately the same length so membrane is approximately the same width.
- In unsaturated fatty acids, the cis double bond introduces a kink which REDUCES phospholipid packing.
What is Sphingomyelin?
- Plasmalogen (non-classical phospholipid)
- Only phospholipid not based on glycerol
- In the membrane, the conformation of Sphingomyelin resembles other phospholipids
Describe the structure of Glycolipids
- Replacing the phosphocholine moiety with a sugar makes a Glycolipid
- Sugar containing lipids (no phosphate head groups)
- Cerebrosides: head group conains a single sugar monomer
- Gangliosides: head group contains oligosaccharides (sugar multimers)
What structure do amphipatic molecules form in water and which structure is favoured for phospholipids and glycolipids?
- Form one of two structures in the water, Micelles and Bilayers
- Bilayers are the favoured structures for Phospholipids and Glycolipids
- Bilayer formation is spontaneous in water, driven by Van der Waals attractive forces between hydrophobic tails.
- The co-operative structure is stabilised by non-covalent forces: electrostatic and hydrogen bonding between hydrophilic moeties and interactions between hydrophilic groups and water.
- Pure lipid bilayers have a very low permeability to ions and most polar molecules.
Is the Phospholipid Bilayer dynamic?
Yes, highly so (moving around all the time)
Describe the influence of cis double bonds in bilayer structure
The kinks (caused by double bonds) in unsaturated fatty acid chains aid membrane dynamics by reducing phospholipid packing - they disrupt the hexagonal packing of phospholipids and so increase mmbrane fluidity.
What modes of mobility do lipid moecules possess in a lipid bilayer?
Membrnes are fluid structures
- Intra-chain motion (kink formation in the fatty acid chains - FLEXION)
- Fast axial rotation
- Fast lateral diffusion within the PLANE of the bilayer
- Flip-Flop: movement of lipid molecules from one half of the bilayer to the other on a one for one exchange basis.
- NOTE: Flip-flop is very rare
Describe the structure and property of cholesterol
Rigid Ring Structure
It is a plasma membrane lipid - 45% of total membrane lipid.
Distribution of different lipids is tissue specific and related to function
What effect does cholesterol have on the membrane bilayer?
- Maintains constant environment and integrity of lipid bilayer by maintaining fluidity.
- Stabilises the plasma membrane by hydrogen bonding to the fatty acid chains.
- THIS ABOLISHES THE ENDOTHERMIC PHASE TRANSITION OF PHOSPHOLIPID BILAER
What effect does cholesterol have at HIGH temperatures?
Cholesterol decreases vibration (reduces phopholipid chain motion) which can cause fractures in the membane or increasing protein conformation
What effect does cholesterol have on LOW temperatures?
Phospholipids can aggregate at low temperatures (crystallisation) which can cause fractures so cholesterol reduces phospholipid packing, increasing membrane fluidity and therefore crystallisation.
Paradoxical effects: cholesterol reduces phospholipid packing, increasing membrane fluidity (at low temps) but at high temps, it reduces phospholipid chain motion, decreasing membrane fluidity
What is the functional evidence for proteins in membranes?
They carry out distinctive functions:
- Ion channels
- Facilitatd Diffusion
- Ion gradients
- Specificity of Cell responses
Protein content can vary from ~18% in myelin to ~75% in mitochondria
What is the Biochemical Evidence for Proteins in Membranes
- Membrane fractionation + gel electrophoresis (SDS-Page)
- Freeze Fracture
What are the three modes of motion (of proteins) permitted?
- Conformational change (important for function)
NO FLIP FLOP BECAUSE THEY HAVE LARGE HYDROPHILIC MOIETIES AND LARGE AMOUNTS OF ENERGY WOULD BE REQUIRED TO PASS THROUGH THE HYDROPHOBIC REGION OF THE BILAYER
What are the restraints on membrane protein mobility?
Proteins are fixed in position
- Lipid mediated effects: proteins tend to separate out into the fluid phase or cholesterol poor regions
- Membrane protein associations (a lot of signallig proteins aggreggate within cholesterol rich regions)
- Association with extra-membranous proteins (periheral proteins) e.g. cytoskeleton
- Aggregates tend to lead to more sluggish movement
- Tethering e.g. to the basement membrane or cytoskeleton
- Interaction with other cells will tether adhesion molecules between the two cells.
Describe the structure of a lipid bilayer
Biological membranes are composed of a lipid bilayer with associated membrane proteins which may be deeply embedded in the bilayer (integral) or associated with the surface (peripheral)
Describe a Peripheral Membrane Protein
- Bound to surface
- Electrostatic and hydrogen bond interactions
- Removed by changes in pH or in ionic strength (can be 'washed off')
Describe an Integral Mmbrane Protein
- Interact extensively with hydrophobic domains of the lipid bilayer
- Cannot be removed by manipulatin of pH and ionic strength
- Are removed by agents that compete for non-polar interactions (therefore destroying the membrane) e.g. Detergents and organic solvents
Why is membrane asymmetry important?
- Asymmetrical orientation of proteins is important for function e.g. a receptor for a hydrophilic extracellular messenger molecule such as insulin must have its recognition site directed towards the extracellular space to be able to function.
Why is the plasma membrane described to be fluid?
- Contains hydrophobic integral component such as lipids and membrane proteins that move laterally through the membrane so the membrane is not solid - more fluid.
- The membrane is depicted as mosaic because it is made up of many different parts such as integral proteins, peripheral proteins, glycoproteins, phospholipids, cholesterol etc.
Describe Secretory Protein Synthesis / Protein Secretion Pathway
- Free ribosome initiates protein synthesis from mRNA molecule
- Hydrophobic N-terminal signal sequence is produced.
- Signal sequence of newly formed protein is recognised and bound to by the Signal Recognition Particle (SRP)
- Protein synthesis stops.
- GTP-bound SRP directs the ribosome synthesising the secretory protein to SRP receptors on the cytosolic face of the ER.
- SRP dissociates
- Protein synthesis continues and the newly formed polypeptide is fed into the ER via a pore in the membrane (peptide translocation complex)
- Signal sequence is removed by a signal peptidase once the entire protein has been synthesised.
- The ribosom dissociates and is recycled
What is a Hydropaty Plot?
Some membrane proteins have multiple (hydrophobic) transmembrane domains. Hydropathy plots can detect the number of hydrophobic transmembrane domains a protein has.
R groups of amino acid residues in transmembrane domains are largely hydrophobic. Transmembrane domains are often alpha-helical.
Glycophorin is a single transmembrane domain protein. Bacteriorhodopsin is a multiple transmembrane domain protein. Hydrohobic domains are orange (~20 amino acis are needed to span the domain)
How are all proteins of a particular type organised? Give an example
All proteins of a particular type are organised facing a particular way. For Glycophorin, the N-terminal is inside and the C-terminal is outside.