23 - Fluidity and Membrane Proteins Flashcards

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1
Q

importance of membrane fluidity

A
  • 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
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2
Q

measuring rate of lateral diffusion experiment

A
  • 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
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3
Q

How fluid is the membrane - how often does it move

A
  • 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
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4
Q

Temp effect on membrane fluidity

A

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

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5
Q

membrane fluidity - lipid composition
- effects of different lipids on fluidity

A
  • 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)

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6
Q

membrane adaptions of cold-blooded (poikilotherms)

A
  • 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

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7
Q

How plants use changes in membrane fluidity to detect temperature changes

A

fluidity increases - temp increasing
opp also true

  • allows plant to prepare for heat stress
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8
Q

What regulates membrane fluidity in mammals

A

cholesterol

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9
Q

How cholesterol works in membranes - simple

A

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

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10
Q

what organisms cholesterol found in

A

only animals
- not in plants and bacteria

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11
Q

cholesterol structure

A
  • 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
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12
Q

Cholesterol conformaion in the membrane
- why is like this

A
  • increases fluidity in middle of the membrane (flexible tail)
  • decreases fluidity at edge of the membrane (steroid ring)
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13
Q

effect of ethanol on membranes

A
  • 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
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14
Q

lipid bilayer - asymmertric

A
  • 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
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15
Q

ER and Membrane synthesis

A

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

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16
Q

RER info

A
  • contains ribosomes scattered on membrane
  • ## involved in protein synthesis (on ribosomes)
17
Q

SER info

A

synthesis and modification of lipids (also of membranes)

18
Q

Different types of membrane proteins

A
  • 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
19
Q

Types of integral membrane proteins (3) - info about each

A
  • 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
20
Q

what is membrane topology

A

arrangement relative to the membrane: this does not change - e.g. proteins will stay where they are relative to the membrane

21
Q

Intergral membrane proteins - membrane topology

A

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
22
Q

Integral Membrane proteins - structure

A

23
Q

Why do single-spanning membrane proteins use a hydrophobic a-helix rather than a hydrophobic B-strand

A
  • 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
24
Q

difference between a-helices and B-pleated sheets/strands

A
  • 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
25
Q

Examples of integral membrane proteins

A
26
Q

ICAM - Integral membrane protein info

A
  • 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
27
Q

Bacteriorhodopsin - integral membrane protein example info

A
  • 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
28
Q

Porins - integral membrane protein example info

A
  • 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
29
Q

Peripheral membrane proteins info

A
  • 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
30
Q

Peripheral membrane proteins - anchors

A
  • proteins are anchored to the membrane through hydrocarbon groups
  • protein covalently attached to a hydrocarbon group
  • hydrophobic hydrocarbon group inserts into lipid bilayer
31
Q

what is anchorage/lipid-anchored proteins

A
  • proteins located on the surface of the cell membrane that are covalently attached to lipids embedded within the cell membrane
32
Q

membrane-associated proteins - ankyrin and spectrin info

A

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

33
Q

carbohydrates (glycans) on cell membranes
- what is glycoalyx

A
  • 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
34
Q

what is the glycoalyx

A

a network of glycoproteins with mucus like consistency

35
Q

function of carbs/glcyoalyx on exterior of cell membranes

A
  • 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.

36
Q

membrane carbs: function

A
  • 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
37
Q

membrane carbs: structure

A
  • 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