BIOCH200 Lecture Notes - Lecture 11: Van Der Waals Force, Alpha Helix, Lipid Bilayer
1 May 2018
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Membranes
❏TAG’s can not form Lipid bilayers → not amphipathic
❏2 acyl chains are required to form a perfect bilayer → glycerophospholipids.
❏Bilayers vary depending on lipid composition (acyl chain and polar headgroups) and non-covalent
assembly (fluid yet stable)
❏Can not interact covalently with one another, and are stabilized because of hydrophobic
and van der waals
Fluidity of Lipid Bilayers
❏Cool temperature (below transition temperature) is an ordered gel phase → pack together in van
der waals contact
❏Heat → more disordered liquid crystalline phase → above transition temperature → move freely +
rapidly
❏Fluidity is a function of temperature and lipid composition (chain length + degree of saturation)
❏Transition temperature of artificial membranes are sharp, but biological membranes have a high
variety → no sharp change.
Membrane Rigidity
❏Cholesterol increases rigidity bc planar and rigid → limits rotation and increases van der waals
interactions.
❏Lipids move laterally (lateral diffusion)
❏Do not undergo transverse diffusion (flip flop) at fast rates
❏Enzymes (Flippases) increase rate of transverse diffusion → significant energy barrier
associated with dissolving a polar headgroup to move through hydrophobic bilayer.
Types of Membrane Proteins
❏Integral (Transmembrane or intrinsic proteins) → embedded within the membrane with a
hydrophobic core to stabilize the part within the membrane.
❏Can be separated from membrane by pH (change in charge), or salt.
❏Peripheral → Outside of membrane → hydrogen bonds and ion pairs
❏Lipid-linked proteins → outside of membrane → hydrophobic interactions (weak) and H-bonds
and ion pairs
K+ Channel
Alpha Helices and beta barrels
❏Alpha Helices → everytime a protein crosses membrane one alpha helices forms.
❏Crosses 12 times → 12 alpha helices.
❏Generally hydrophobic amino acids interacting with membrane.
❏Beta Barrels → create a whole in middle of membrane to allow ions to go through.
❏Non-polar exterior to interact with acyl chains.
❏Mitochondrial outer membranes.
Fluid Mosaic Model
❏Mixture of various lipids, carbohydrates and proteins
❏Dynamic, non-covalent, complex assembly.
❏Most important is non-covalent
❏Proteins bound to cytoskeleton → less movement, if loose they move more easily.
Small molecules can cross lipid bilayer by simple diffusion
❏Large polar and charged molecules need active transport (transporters) to be able to diffuse.
❏Simple diffusion → no energy or protein needed,
Rate of simple unmediated diffusion depends on
Document Summary
Tag"s can not form lipid bilayers not amphipathic. 2 acyl chains are required to form a perfect bilayer glycerophospholipids. Bilayers vary depending on lipid composition (acyl chain and polar headgroups) and non-covalent. Can not interact covalently with one another, and are stabilized because of hydrophobic assembly (fluid yet stable) and van der waals. Fluidity of lipid bilayers der waals contact rapidly variety no sharp change. Cool temperature (below transition temperature) is an ordered gel phase pack together in van. Heat more disordered liquid crystalline phase above transition temperature move freely + Fluidity is a function of temperature and lipid composition (chain length + degree of saturation) Transition temperature of artificial membranes are sharp, but biological membranes have a high. Cholesterol increases rigidity bc planar and rigid limits rotation and increases van der waals interactions. Do not undergo transverse diffusion (flip flop) at fast rates.
