Lipids are not covalently anchored to other lipids
Non-covalent interactions among lipids allows individual lipids to migrate.
Membranes are highly flexible.
Gel Phase is the formation of a semisolid lipid bilayer which constrains the motion of individual lipid molecules. This is due to lack of energy from temperature to allow motions to occur.
Liquid-Disordered state, the interior of the bilayer is fluid and allows for rotation of lipid molecules and free migration, this occurs when there is enough energy from temperature to overcome intermolecular interactions.
Liquid-Ordered State is intermediate between liquid disordered and gel phase. It requires the presence of cholesterol. Acyl chains are ordered, but there is allowance for lateral movement of lipids.
Lipid Characteristics Which Influence Membrane Fluidity: Saturation
Saturated Fatty acids can pack more tightly together producing higher melting points and lower membrane fluidity. Unsaturated fatty acids have kinks in their hydrocarbon chains that disallow tight packing, increasing membrane fluidity
Lipid Characteristics Which Influence Membrane Fluidity: Hydrocarbon Length
Hydrocarbon chain length influences amount of hydrophobic interactions. Longer chains provide more interactions decreasing membrane fluidity and shorter chains have less interactions increasing membrane fluidity.
Lipid Characteristics Which Influence Membrane Fluidity: Presence of Sterols in Membrane
Sterols stabilize the liquid-ordered phase. They act as buffers to maintain membrane fluidity. Sterols reduce the freedom of acyl chains to move by rotation around their carbon-carbon bonds and force the chains into fully extended conformations, this results in less-tight packing, resulting in increased membrane fluidity.
Maintenance of Membrane Fluidity
Cells regulate lipid composition to maintain a constant membrane fluidity under different conditions to conserve a membrane which can provide proper transport.
Trans-bilayer Movement of Lipids
Flip-flop diffusion of a molecule, when a lipid molecule from one leaflet of the bilayer is transferred to the other. This occurs very rarely.
Lateral diffusion of a lipid molecule occurs more commonly.
Trans-bilayer movement - energetically unfavourable, but necessary
Flip-flop diffusion requires the polar head group to move through the hydrophobic interior of the bilayer. It has a large, positive free-energy change. ~ 40-50 kcal/mol
Flip-flop diffusion is necessary because some lipids are synthesized on one face of the membrane but are only functional on the other side of the membrane.
Flippases - Catalysis of flip-flop diffusion
Flippases catalyze the translocation of aminophospholipids: phosphatidylethanolamine and phosphatidylserine from the extracellular to the cytosolic leaflet of the lipid membrane.
Floppases - Catalysis of flip-flop diffusion
Floppases catalyze the translocation of phospholipids from the cytosolic leaflet to the outer leaflet of a membrane.
Scramblase - Catalysis of flip-flop diffusion
Scramblases move lipids in either direction to reach equilibrium. This enzyme is important because a lot of lipids are synthesized on the inner membrane, and a high concentration of these lipids is formed. This gradient allows scramblase to effectively transport lipids from the inner membrane to the outer membrane.
Observation of Lateral Diffusion of Lipids: FRAP
Method used: FRAP (Fluorescence Recovery After Photolysis)
Stain lipids with bleach, use a laser to beam bleached area, and observe how quickly the non-bleached area fills in. Take the area and time, and determine the rate of lateral diffusion of lipid molecules: ~ 1 μm/s
Observation of Lateral Diffusion of Lipids: Single Lipid Tracking
Method used: Lipid labelling, allow particle diffusion to be observed. Label lipids individually and observe under a microscope the path of their diffusion. Discovery of barriers which restrict overall random movement of lipid diffusion. Lipids appeared as though they moved around in a corralled region, then moved to another corralled region.
Possible Barriers of Lateral Diffusion
Actin and Spectrin are filamentous cytoskeletal proteins which tether proteins.
Proteins are tethered to spectrin and restrict lateral-diffusion of lipid molecules.
Glycophorin, chloride-bicarbonate exchanger both are immobile and are barriers to lateral diffusion.
Actin and Spectrin form junctional complexes.
Ankyrin is attached along Spectrin.
Cl- - HCO3- exchangers are stabilized by ankyrin.
Glycophorin is anchored by interactions directly with spectrin.
Cholesterol-sphingolipid microdomains are on the outer monolayer of the membrane.
Rafts are composed mainly of sphingolipids which have longer, more saturated acyl groups, and form more stable interactions with Cholesterol.
Physical Characteristics of Membrane Rafts
Membrane rafts are slightly thicker and more ordered (less fluid) than neighbouring micro domains.
Membrane rafts are more difficult to dissolve with nonionic detergents.
Enriched in 2 classes of integral membrane proteins: proteins with two lipid anchored saturated fatty acids, and GPI-anchored proteins
Functional Significance of Membrane Rafts
Some membrane proteins require there membrane proteins for functionality.
Membrane rafts contain certain subsets of proteins.
This allows for more collision of membrane proteins for functionality.
Caveolin, an integral membrane protein which has two globular domains, connected by hydrophobic domain. This domain helps attach it to the membrane.
3 Palmitoyl groups (on the carboxyl terminal of the globular domain) anchor it to the plasma membrane.
Caveolin binds cholesterol and forces inward curvature of a membrane.
Caveolae vs. Membrane Rafts
Caveolae are rafts which involve both leaflets of the plasma membrane, whereas normal membrane rafts do not involved both leaflets of the plasma membrane.
Caveolin is strongly associated because of lipid anchors.
Cholesterol interacts strongly with Caveolin, which allows for membrane rigidity even when the membrane is curved.
Caveolin is a dimer it has an alpha helix with a proline residue which turns it.
Amphiphilic - there are both polar and non-polar regions.
Protein with intrinsic curvature on its surface interacts strongly with a cured membrane surface, allowing both membrane and protein to achieve lowest energy.
Monomeric subunits form a superstructure to stabilize a curved membrane.
Protein with one or more amphipathic helices insert into one leaflet of the bilayer and crowds the lipids in that leaflet forcing the membrane to bend.
Structure of Integrins
Integrins are heterodimers: two-unlike subunits, α and β subunits.
Combine to form a specific binding site for extracellular proteins such as collagen and fibronectin. [They contain a common determinant of integrin binding (Arg-Gy-Asp)]
α and β subunits are anchored to the membrane via transmembrane helices.
Functions of Integrins
Surface adhesion proteins
Carry signals in both directions across plasma membranes.
Cadherins are also surface adhesion molecules which undergo homophilic interactions. They bind identical cadherins on the surface of neighbouring cells.
Selectins have extracellular domains that in the presence of Ca2+ bind specific polysaccharides on the surface of an adjacent cell. They are essential for blood clotting.
Requirements for fusion of membranes
Two membranes must recognize one another
Membrane surfaces requires removal of water molecules normally associated with polar head groups, to bring them close together.
Bilayer structures must become locally disrupted, fusion of outer leaflets.
Bilayers must fuse to form one continuous bilayer
Process must be triggered at the appropriate time
Neurotransmitter Release at Synapses: Example of Fusion