23 - Fluidity and Membrane Proteins Flashcards
importance of membrane fluidity
- lipids need to diffuse laterally
- proteins not involved in membrane anchoring also diffuse
- proteins need to be able to move and transmit signals - and molecules need to be able to move/diffuse across membranes
- vesicles need to be able to bud off and fuse with the membrane
- otherwise all these processes would be very slow
measuring rate of lateral diffusion experiment
- use fluorophores
- membrane with fluorophores
- intense light (laser beam) bleaches fluorophores
- fluorophore membranes recover, then start to fluoresce
- rate of diffusion of fluorophores back to into the membrane area can be recorded
How fluid is the membrane - how often does it move
- constant movement of membranes within bilayer
- rotation and flexion occur at a high rate:
- lateral diffusion (within membrane) ~ 2 micrometres per second - very quick movement within the membrane
- transverse (flip one side to the other) - every three days
Temp effect on membrane fluidity
increase temp - melting transition - crystal - gel - fluid - so membrane more permeable as temp increases
- lipid molecules move faster
- membrane becomes more permeable
decrease temp:
- fluidity decreased as temp increases
- lipid molecules move more slowly: gel-like
- membrane becomes less permeable
membrane fluidity - lipid composition
- effects of different lipids on fluidity
- mixture of lipids in membrane
Increases fluidity:
- unsaturated lipids give kinks
- short chains allow fewer interactions between lipids
- higher temp (not relevant to humans, cholesterol)
decrease fluidity:
- saturated chains
- longer chains
- lower temp (not relevant to humans, cholesterol)
membrane adaptions of cold-blooded (poikilotherms)
- organisms regulate their lipid composition in their membranes depending on their habitat and surroundings
Low temp:
- shorter unsaturated fatty acids
- keep membrane more fluid to compensate for lower temps
High Temp:
- longer chain, more staurated fatty acids to compensate
How plants use changes in membrane fluidity to detect temperature changes
fluidity increases - temp increasing
opp also true
- allows plant to prepare for heat stress
What regulates membrane fluidity in mammals
cholesterol
How cholesterol works in membranes - simple
warmer temps:
- cholesterol restrains phospholipid movements at high temps
- so membrane is less fluid
cooler temps:
- cholesterol prevents close packing of lipids
- so that membrane is more fluid at lower temps
what organisms cholesterol found in
only animals
- not in plants and bacteria
cholesterol structure
- contains a polar head group and a nonpolar hydrocarbon tail
- hydrocarbon tail is ‘flexible’ - makes area more fluid
- steroid rings are ‘rigid’ - decrease fluidity in parts of membrane
Cholesterol conformaion in the membrane
- why is like this
- increases fluidity in middle of the membrane (flexible tail)
- decreases fluidity at edge of the membrane (steroid ring)
effect of ethanol on membranes
- ethanol increases membrane fluidity - one of the ways it is toxic - fatty acid molecules pack against each other - more fluid
- chronic alcoholics compensate by increasing cholesterol contents of membrane
- so when they sober up, membranes are less fluid (when not containing ethanol)
- anaesthetics (analgesiacs) increase membrane fluidity
lipid bilayer - asymmertric
- the two layers have different lipid composition
- transverse diffusion (flip-flop) once every 3 days (rare)
- proteins known as phospholipid translocators (flippases) catalyse the flip-flop event to maintain phospholipids in the correct monolayer
ER and Membrane synthesis
ER is involved in membrane synthesis and modififcation
- determines the asymmetric distribution of lipids, proteins and carbohydrates
- proteins destined for membranes are synthesised on one membrane of the ER (not in the ER)
- membrane proteins have a clear direction (cannot flip-flop/transverse travel)
- when membrane proteins are present on ER, carbs restricted to cell’s exterior
RER info
- contains ribosomes scattered on membrane
- ## involved in protein synthesis (on ribosomes)
SER info
synthesis and modification of lipids (also of membranes)
Different types of membrane proteins
- integral membrane proteins (a & B) - traverse across the membrane (at least once)
- Peripheral membrane proteins associate with a membrane face
- some proteins bind to the surface of integral proteins
- anchors - some covalently anchored to the membrane
Types of integral membrane proteins (3) - info about each
- integral proteins cross the membrane at least once
- single span hydrophobic a-helix
- either C - or N-Terminal can be intra-cellular
- multi-spanning containing a-helices
- 7 transmembrane helix proteins - big family of proteins
- elicit a signal inside the cell - can be mimicked 0useful in pharma
- B-barrel protein forming a pore - good transport proteins, amongst other things
what is membrane topology
arrangement relative to the membrane: this does not change - e.g. proteins will stay where they are relative to the membrane
Intergral membrane proteins - membrane topology
topology maintained by hydrophobic and electrostatic interactions:
- hydrophobic helices are hydrophobic
- multi-spanning helices are hydrophobic where they interact with the membrane (not in contact with exterior)
- hydrophobic has to interact with hydrophobic
- other interactions occur between the helices
- B-barrel pore proteins - hydrophobic where they interact with membrane hydrophilic environment in middle of pore
- +vely charged amino acids interact with -vely charged lipid head groups in phospholipid bilayer
Integral Membrane proteins - structure
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Why do single-spanning membrane proteins use a hydrophobic a-helix rather than a hydrophobic B-strand
- B-strands cannot form hydrogen bonds for the strand within the membrane, as they have intermolecular hydrogen bonds
- alpha-helices have backbones that can form hydrogen bonds
difference between a-helices and B-pleated sheets/strands
- a-helices have intramolecular hydrogen bonds, and can form bonds with different molecules due to their backbone - can form hydrogen bonds
- B-sheets/strands have intermolecular hydrogen bonds, and cannot form hydrogen bonds with other groups/domains
- hence why only alpha protein strands found in integral membrane proteins
Examples of integral membrane proteins
ICAM - Integral membrane protein info
- involved in cell adhesion
- expressed in cells of the immune system and endothelial cells
- upregulated during inflammation
- has 5 extracellular immunoglobulin domains
- single transmembrane spanning helix
- short cytoplasmic tail
Bacteriorhodopsin - integral membrane protein example info
- absorbs light
- light causes conformational change in retinal
- causes pumping of protons from cytosol to extracellular space
- produces proton gradient
- proton gradient used for photosynthesis
Porins - integral membrane protein example info
- forms a barrel shaped strcuture with a pore in the centre
- found mainly in bacteria
- hydrophobic exterior
- hydrophilic interior
- in bacteria, used to take up nuteients and substances in gram -ve bacteria
- in E.coli functions in adhesion, invasion, biofilm formation
Peripheral membrane proteins info
- dont interact with hydrophobic core of membrane
- can be cytoplasmic or exoplasmic - and does not change from either one of these
- interact with lipid head groups and integral membrane proteins
Peripheral membrane proteins - anchors
- proteins are anchored to the membrane through hydrocarbon groups
- protein covalently attached to a hydrocarbon group
- hydrophobic hydrocarbon group inserts into lipid bilayer
what is anchorage/lipid-anchored proteins
- proteins located on the surface of the cell membrane that are covalently attached to lipids embedded within the cell membrane
membrane-associated proteins - ankyrin and spectrin info
Spectrin:
- scpectrin cytoskeleton protein creates a scaffold on intra-cellular side of membrane - holds membrane in place inside the cell
Ankyrin:
- binds to several integral membrane proteins and to spectrin - can bind many (integral) proteins to spectrin scaffold
carbohydrates (glycans) on cell membranes
- what is glycoalyx
- carbs only found on exoplasmic side of membranes
- carbs attached to both lipids (glycolipids) and protein (glycoproteins)
- glycoalyx is network of glycoproteins with mucus like consistency
what is the glycoalyx
a network of glycoproteins with mucus like consistency
function of carbs/glcyoalyx on exterior of cell membranes
- glycoalyx and other carbs (glycolipids/glycoproteins) form mucus-like consistency
Functions:
- physical barrier (protect against viruses and bacteria)
- mechanosensing - responding to mechanical stimuli and changes (stress, strain, rigidity, adhesiveness, topology, etc.)
- possible roles in cell shape, maintaining it etc.
membrane carbs: function
- cell recognition, communication adhesion
- especially important in immune responses - targeting foreign cells
- distinguish self and non-self cells - useful in infection, transplantation (can recognise foreign transplanted organs/cells and target for immune response)
- Red blood cell types - diff carbs present on diff blood group antigens
- genes determine which enzymes we have, hence which blood group we have
membrane carbs: structure
- carbs are attached to proteins
- most proteins have at least one carb unit
- few lipids (~10%) have carb units
- exist as either oligosaccharide chains or single-sugar resiudes
- glycoproteins usually have oligosaccharide chains
- glycolipids usually have single sugar residues
