Membranes Flashcards
Phospholipid Structure
- amphipathic molecules
- hydrophilic polar head
- head: choline, phosphate, glycerol
- hydrophobic non-polar fatty acid tails
3 Phospholipids
- phosphatidyl-ethanolamine
- phosphatidyl-serine
- sphingomyelin
Sphingomyelin
- built from sphingosine
- fatty acid attached to amino acid group and phosphocholine attached to a terminal hydroxyl group
- free -OH can form H-bonds
Sterol
- polar head group with rigid steroid ring structure and non polar hydrocarbon tail
- affects phospholipid compacting in membrane
Glycolipids
- molecules modified via addition of sugars
eg. galactoserebroside and ganglioside
Bilayer Formation
- 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
Membrane Properties
- 5-8 nm thick
- appear trilaminar
- fluid
- impermeable to large polar solutes + permeable to nonpolar small solutes
Fluid Mosaic Model
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
Sonication
- 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
Photobleaching
- 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
Lipid Movement
- 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
Membrane fluidity + composition
- 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
Cholesterol
- 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
Lipid Domains
- certain lipid mixtures cause formation of transient domains
eg. sphingomyelin, cholesterol
Lipid Rafts
- cholesterol + sphingomyelin
- atomic force microscopy gives a contour map of the membrane to show these regions
Membrane Asymmetry
- 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
Membrane Proteins
- membrane proteins are asymmetric and this is absolute
- interact with membrane in many ways
- integral for biological function
- 50% of total membrane mass
Transmembrane Proteins
- a helices compose the TM domain and exterior soluble regions
- single or multipass proteins
- can be beta barrels as well
Peripheral Proteins
- monotopic, ie. only associate with one leaflet
- lipid modification acting as an anchor to one side
- form complexes with integral proteins
a helices
- 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
B barrels
- form from curved B strangs
- rigid structures acting as pores
- exclusively found in bacterial and mitochondrial membranes
Protein Asymmetry
- 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
Detergent Solubilisation
- 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
Membrane Channels
- hydrophilic pores across the membrane
- narrow and highly selective
- 100 million ions/second
Gating
- voltage gated
- ligand gated
- mechanically gated
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
Channel Roles
- fundamental to cell life
- rapid responses
- water transport
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
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
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
Transporters
- carriers/permeases
- bind solute on one side of membrane, rearrange, and release the solute on the other side
- high affinity solute binding site
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
Passive Transport
- facilitated diffusion
- electrochemical gradient needed for driving force
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
Ion Drive Pump
- uniport
cotransport: - symport or antiport
Coupled Transporters
- harvest energy shared in the electrochemical gradient of one solute
- drives transport of other solute
2 Types of Active Transport
- Primary = driven by ATP hydrolysis
- Secondary = couples downhill movement of solute with transport of another solute against its gradient
- ion gradient has to be generated
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
Enzyme Coupled Receptors
- ligand binding leads to dimerization of inactive receptor in activate a catalytic domain
- ion channel coupled receptors
- G protein coupled receptors
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
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
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
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