module 2 lectures 1+2 Flashcards
function of lipids
Structural functions
– Membrane components
– Protein modification
Metabolic functions
– Energy storage
Other functions
– Cellular signalling, including hormones
– Enzyme cofators
– Electron carriers
– Pigments
effects of chain length
longer chain length = increase melting point, decreased solubility in H20
effect of double bonds
decrease melting point (the more the worse)
Glycerophospholipids
fatty acids (often unsaturated) - glycerol - phosphate - alcohol (head group)
Glycerophospholipid head groups
serine
glycerol
inositol
choline
ethanolamine
(SG ICE)
Sphingolipids
Major membrane components
Derivatives of the amino alcohol sphingosine
N-acyl fatty-acyl derivatives of sphingosine are called ceramides
Steroids
cholesterol is main one - important in membranes
Lipid aggregates
Micelle
Bilayer
Vesicle
Stabilisation of bilayers
Ionic bonds between head groups and
hydrogen bonds with water
van der Waals interactions between fatty acid tails
Lipid mobility in phospholipid bilayers
Lipids have two main types of motion:
* Spinning without changing location
– Rotation around their long axis
* Lateral diffusion – movement with the
same leaflet (VERY FAST!)
viscosity like olive oil
they could possible flip flop but it requires a lot of energy and uncatalysed it takes a loooong time t1/2
FRAP experiments
show lateral diffusion
Fluorescence recovery after photobleaching: shows that lipids can easily move laterally in a lipid bilayer
effect of heat on membranes
disorders them and change them from gel to fluid
effect of lipids on bilayers
- Long chain fatty acids aggregate extensively to give low fluidity (gel-like
state) - Short chain fatty acids have less surface area to aggregate and increase fluidity
- Unsaturated fatty acids also aggregate less extensively and increase fluidity
effects of sphingomyelin and cholesterol on membrane
Sphingomyelin (SM) associates into a thicker, more gel-like bilayer than phospholipids
Cholesterol increases thickness by ordering fatty acid tails, and stabilises head group interactions
Leaflet composition
Exoplasmic leaflet – rich in sphingolipids + PC (less fluid)
Cytosolic leaflet – rich in PE/PS/PI (more
fluid)
Cholesterol is relatively evenly distributed in both leaflets
How does asymmetry arise in leaflets?
Specific enzymes can catalyse
translocations
e.g. Sphingomyelin is synthesised in the exoplasmic face of the Golgi which becomes the exoplasmic face of the plasma membrane
– Glycerophospholipids are synthesised on the cytosolic face of the ER which becomes the cytosolic face of the plasma membrane
– PC arrives at the plasma membrane on the cytosolic side but is transported to the other leaflet by “flippase” enzymes which require energy from ATP hydrolysis.
Flippase
flip-flops/ moves phospholipids from outer leaflet to inner
Floppose
flip-flops/ moves phospholipids from inner leaflet to outer
Scramblase
moves phospholipids either direction toward equilibrium (flip/flop)
lipid rafts
Stable associations of sphingolipids and cholesterol
Can be disrupted by methyl-3-b-cyclodextrin (removes cholesterol from membranes) or the antibiotic filipin (sequesters cholesterol)
Types of membrane proteins
Integral (= transmembrane, intrinsic)
– Firmly associated with the membrane
(need detergents to release)
– Span membrane
hydrophobic and hydrophilic surfaces
Lipid-anchored
– Protein covalently linked to one or more lipid molecules
– The lipid is embedded in a membrane leaflet to anchor the protein
– The polypeptide does not enter the bilayer (released using phosholipase C)
Peripheral (= extrinsic)
(released with milder treatment)
* Carbonate at high pH; Ca2+-chelating agent; high salt
– Does not contact the hydrophobic core of the bilayer
– Forms hydrophilic interactions with membrane surface or other membrane
proteins
