Membranes Flashcards
(43 cards)
1
Q
Phospholipid Structure
A
- amphipathic molecules
- hydrophilic polar head
- head: choline, phosphate, glycerol
- hydrophobic non-polar fatty acid tails
2
Q
3 Phospholipids
A
- phosphatidyl-ethanolamine
- phosphatidyl-serine
- sphingomyelin
3
Q
Sphingomyelin
A
- built from sphingosine
- fatty acid attached to amino acid group and phosphocholine attached to a terminal hydroxyl group
- free -OH can form H-bonds
4
Q
Sterol
A
- polar head group with rigid steroid ring structure and non polar hydrocarbon tail
- affects phospholipid compacting in membrane
5
Q
Glycolipids
A
- molecules modified via addition of sugars
eg. galactoserebroside and ganglioside
6
Q
Bilayer Formation
A
- polar molecules are hydrophilic and interact with water
- hydrophobic molecules force energetically unfavorable rearrangements of the water molecules
- energetic cost is reduced by hydrophobic molecule packing
7
Q
Membrane Properties
A
- 5-8 nm thick
- appear trilaminar
- fluid
- impermeable to large polar solutes + permeable to nonpolar small solutes
8
Q
Fluid Mosaic Model
A
Singer & Nicholson 1972
- individual lipid molecules are able to diffuse freely within bilayers
- researched using liposomes
liposomes = phospholipids in water form multilaminar vesicles with onion like bilayer arrangement
9
Q
Sonication
A
- applies high frequency sound energy and the structures in multilalamellar vesicles rearrange to make liposomes
- liposomes are stable, closed self-sealing solvent filled vesicles
- used to study membrane fluidity and individual movement of molecules in bilayer
10
Q
Photobleaching
A
- Fluorescence recovery after photobleaching (FRAP) in which fluorescent molecules or gold particles are attached to lipids (polar head groups)
- green fluorescent protein (gfp) emits green light when exposed to blue light that emits at 509 nm
11
Q
Lipid Movement
A
- lateral diffusion
- rotation
- flexion
- flip flop (not common)
- rapid lateral movement within one monolayer/leaflet
- diffusion coefficient of 10^-8 cm2/sec
- noncovalent interactions between lipid/protein molecules make movement rapid/easy
12
Q
Membrane fluidity + composition
A
- synthetic bilayers (one type of phospholipid) can have an induced change in physical condition into a 2D rigid crystalline gel state
- Phase Transition
- affected by tail length and double bond frequency
13
Q
Cholesterol
A
- interacts with regions of the fatty acid tails closest to the polar head group
- looser packing maintains fluidity at low temperature
- amount can be varied
- amount to which polar head goes into the tail region determines packing and therefore fluidity
14
Q
Lipid Domains
A
- certain lipid mixtures cause formation of transient domains
eg. sphingomyelin, cholesterol
15
Q
Lipid Rafts
A
- cholesterol + sphingomyelin
- atomic force microscopy gives a contour map of the membrane to show these regions
16
Q
Membrane Asymmetry
A
- some lipids and proteins found predominantly in one leaflet
- asymmetry is not absolute
- important functional consequences
- apoptosis is triggered by movement of phosphotidylserine into the outer leaflet
17
Q
Membrane Proteins
A
- membrane proteins are asymmetric and this is absolute
- interact with membrane in many ways
- integral for biological function
- 50% of total membrane mass
18
Q
Transmembrane Proteins
A
- a helices compose the TM domain and exterior soluble regions
- single or multipass proteins
- can be beta barrels as well
19
Q
Peripheral Proteins
A
- monotopic, ie. only associate with one leaflet
- lipid modification acting as an anchor to one side
- form complexes with integral proteins
20
Q
a helices
A
- H bonds between residue N and residue N+4 (ie. bonds between carboxyl and amino groups) stabilise regular twisting structure
- maximises use of bond donors and acceptors
- all H bonds are intrahelical
- transmembrane a helical domains are hydrophobic
- specific amino acid residues: ile, leu, val, met
- 20 needed to make TM domain
21
Q
B barrels
A
- form from curved B strangs
- rigid structures acting as pores
- exclusively found in bacterial and mitochondrial membranes
22
Q
Protein Asymmetry
A
- specific orientation relative to membrane
- proteins only go one way up
eg. B adrenogenic receptors binding epinephrine only function if ligand binding site is correctly oriented
23
Q
Detergent Solubilisation
A
- membrane proteins are difficult to study
- analysis requires soluble protein and membrane proteins are insoluble
- addition of a detergent creates membrane protein in bilayer and detergent micelles
- this creates a water soluble complex able to be isolated and studied
24
Q
Membrane Channels
A
- hydrophilic pores across the membrane
- narrow and highly selective
- 100 million ions/second
25
Gating
1. voltage gated
2. ligand gated
3. mechanically gated
26
Selectivity
- channel allows some ions/molecules to pass
- pores are narrow and often charged
- ions/molecules of appropriate size/charge can pass
- ions lose associated water molecules
- selectivity filter
27
Channel Roles
1. fundamental to cell life
2. rapid responses
3. water transport
28
Potassium Channels
- 4 monomers associated to form a homoquaternary protein
- each monomer has 2 TM regions and one half helix
- selective for K ions but not Na ions
- it is more favorable for the ion to pass to the selectivity filter and shed the hydration shell
- K ion is a perfect fit while the Na ion is too small for optimal interaction
29
Mammalian K channel
- membrane potential sensed by voltage sensor domain containing positive amino acid residues
- pulled down to the negative cytosolic side of the membrane in a closed position
- if the outside is negative, the domain is pulled up and its connected loop is pulled down to open the pore
30
Aquaporins
- water channels
- responsible for water secretion and reabsorption (kidney)
- flow rate = 10^9
- directionality determined by osmotic gradient
- 6 helices and 2.5 helices per monomer
- Positive charge of arginine prevent hydronium ion (+) from entering
- to prevent proton hopping and loss of membrane potential, 2 asparagine residues at the end of the half helices, each water forms transient interactions with both residues to prevent H+ addition
31
Transporters
- carriers/permeases
- bind solute on one side of membrane, rearrange, and release the solute on the other side
- high affinity solute binding site
32
Channels
- much weaker interaction with solute
- no directionality
- form aqeous pores across the membrane
- pores open and allow solute movement across membrane
- much faster transport
33
Passive Transport
- facilitated diffusion
| - electrochemical gradient needed for driving force
34
Active Transport
- transport against gradient using pumps and ATP
- unidirectional solute movement
- coupling to source of metabolic energy
- coupled transporter - uses energy stored in ion gradient
- atp driven conformational change
35
Ion Drive Pump
- uniport
cotransport:
- symport or antiport
36
Coupled Transporters
- harvest energy shared in the electrochemical gradient of one solute
- drives transport of other solute
37
2 Types of Active Transport
1. Primary = driven by ATP hydrolysis
2. Secondary = couples downhill movement of solute with transport of another solute against its gradient
- ion gradient has to be generated
38
Lactose Permease
- 2 domains
- lactose and H= ions bind in cleft causing conformational change so molecule faces the other way
- ligand binding causes formation of a new salt bridge that forces opening a different way
39
Enzyme Coupled Receptors
- ligand binding leads to dimerization of inactive receptor in activate a catalytic domain
- ion channel coupled receptors
- G protein coupled receptors
40
GPCR
- helical bundle with specific ligand binding site
- high affinity interactions
- induced conformational change activates intracellular G protein
- a, B, y subunits with a,y anchored to membrane
41
G Protein Activation
- inactive form, a binds GDP
- ligand binding catalyses GDP - GTP exchange
- a subunit conformational change so it no longer has high affinity for the complex and dissociates
42
Downstream Signalling
- complex binds to adenylate cyclase that catalyses activation of cyclic AMP
- cyclic AMP activates protein kinase A which goes on to affect many cellular processes including protein expression
43
G Protein Reset
- GTP is hydrolysed and complex dissociates from the adenyl cyclase to reform a trimer
- receptor ligand dissociates
- active form is phosphorylated leads to B-arrestin binding to physically block further GCPR binding
