membranes part 2 Flashcards
what are the stages of intracelluclar memrbane traffic, what does it mediate
stages:
– Budding (fission) of the vesicle from the parent membrane
– Transport of the vesicle, along cytoskeleton
– Tethering/docking at target membrane (recognition)
– Fusion of vesicle and target membranes
Mediates:
– reorganization of membrane-bound compartments
– exchange of membrane and “cargo” between compartments
– internalization/recycling/degradation of material from plasma membrane
what is the role of SNARES
Mediate membrane fusion during key cellulat processes:
– insulin secretion
– up-regulation of glucose transporters
– transport between ER and Golgi
– phagocytosis
– neurotransmitter release
what does SNARE stand for/ what are thy
Soluble N- ethylmaleimide-sensitive factor attachment protein receptor (dont care about the full name)
*Vsnare on vesicle, targt membrane has t snare
-v-SNARE and t-SNARE are single-spanning transmembrane proteins, whole thing is alphahelicle but large parts stick out
They also have an extended helical domains (approx. 60 amino acids)
These helical domains can interact to form a coiled- coil structure with SNAP25
*specific Vsnare recognizes specific tsnare SNAP25 is an ex for a facillitator protein
how do SNAREs mediate neurotransmitter release
- secretory vesile w/ vsnare and t snare interact on plasma membrane and bidn to eachother and “zip up” from amino terimal
*alpha helicies are twisting around each other facilitated by snap 25
- this twisting beinds to membranes in close proximity, get hemi fusion (outer portion of liplayer fuse but inner do not)
- when inner leaflets come into contact get a fusion pore, membrane of vesicle becomes part of plasma membrane and contents of vesible and dumped into extracullular space
explain memrbane permeability
ions and polar molecules cannot cross (essentially impermeable)
e. g. Na+, Cl-, sugars, amino acids
- small, uncharged molecules cross slowly e.g. glycerol, ethanol
- hydrophobic molecules, gases cros squickly e.g. steroid hormones, O2, CO2, N2
explain ion gradients across membranes
The ionic composition of the cytosol is different from the extracellular environment
There are ion concentration gradients across the plasma membrane and across organelle membranes
These ion gradients are actively maintained by cell, at the cost of ATP
These ion gradients can do work
*
explain memrbane transport
-simple diffusion: non polar comp only go down concentration gradeint (steroid hormones, so few and far between will just move in
- facillitated diffusion
goes down an electrochemical gradient (uses a channel that recognizes)
- primary active transport
against electrochemical gradient, driven by atp
- secondary active transport:
against electrochemical gradietn, driven by moving an ion down its gradient, ATP not required uses generated ionic gradient
- membrane transprot via ion channel:
down electrochemical gradient, may be gated by a ligand or ion, closed until signal arrives to open channel
Ionophore mediated ion transport:
down electrochemical gradient, may be protein channel, ionophore can pass thru membrane in both direction but usually doesnt do so until its bound by a particular ion
explain simple diffusion to transport olute across membranes
- free energy of solution (concentration = c)
ΔG=ΔGo +RTlnc
ΔG when molecule moves from c1 (high) to c2 (low)
ΔG = RTlnc2 –RTlnc1 = RTln(c2/c1)
if c1 > c2, ln (c2/c1) is -ve, so ΔG is -ve
- diffusion occurs spontaneously from high to low concentration
- c1 = c2 at equilibrium
explain Movement of a neutral solute across a permeable membrane
- note neutral comp so no charge impolcation
- befor equilibrium there is a net flux bc C1>>>C2 at equilibrium theres no ent flux
explain transport of hydrophilic solutes
- water is surrounding hydrophilic molecule, it takes energy to pull this water off bc it wont pass thru the lipid bilayer
- when comes out other side has to reassociate with water
*decent change in free energy to be able to do this
- mol will probably travel thru transporter so the transport itself doesnt need energy but the stripping off the water itself does
composition of membrane channels
- Doughnut-like pore spans bilayer
- Compared to transporters, solutes flow through rapidly (diffusion)
- Rate of transport α [substrate] – not saturable
- GATED: open and close in response to stimuli
Many types, highly selective e.g. Na+, K+, Cl- and H2O channels
*must be reg bc if it was just open everything would just pass through
what do membrane channels do
*recall they cant be saturated, conc gradient is what determines it
- Channels form pores and transport specific solutes down their concentration gradient
- Solutes flow through very rapidly (diffusion-like rates of ~108 s-1)
- Rate of transport is proportional to substrate concentration and is not saturable (no maximum rate of transport)
- Channels can be gated: i.e. they open and close in response to external stimuli
- There are many types, but all are highly selective – e.g. H2O glycerol urea Na+ K+ Cl-
what are aquaporins
- water channels, also transport glycerol and urea
- key amino acids line the pore the creates mechanisms allowing for specific items (like water) to pass
- there is size restriction, electrostatic repulsion, water dipole reorientation
*only water has atoms/electrons oriented in way to pass fomr key amino acids
how are membrane transporters classified
- uniport (tranports one mol in one direction)
- symport (capable of transporting 2 mol of different identity)
- antiport (two diff mol required
(symport and antiport are types of cotransport)
explain passive transporters
- Transport DOWN a concentration gradient (also known as facilitated diffusion)
- Highly selective (stereospecific)
- Not a continuous pore through membrane
– Transport one set of molecule(s) at a time
- Rate of transport is regulated, saturable number of binding site(s) for substrate
e. g. Passive glucose transporters: GLUT1 in erythrocytes imports glucose GLUT2 in liver, intestine exports glucose
explain GLUT1
passive transporter operon
1) Substrate binds on one side of membrane
2) Conformational change takes place
3) Site opens on other side of membrane & substrate is released
4) Conformational change takes place
*GLUT2 is this guy but inverted
*change back to oringial con occurs very quickly bc dont want glucose to flow back in
explain active transporters
Transport against a concentration gradient
Transporters often called “pumps”
Many are powered by ATP hydrolysis - “ion-pumping ATPase”
Generate ion gradients across membranes
explain ion gradients across cellular membranes
- Na+ concentrations are much higher outside (~150 mM) than inside the cell (~10 mM)
Cl- concentrations are also much higher outside (~110 mM) than inside the cell (~5 mM)
The opposite is true of potassium (~140mM inside the cell, ~5 mM outside)
explain Movement of a charged solute across a permeable membrane
- before membrane is permeable have ammebrane potential, more pos changed on one side and more neg on other
- add voltage geted ion channels, charges equalize to memrbane potential (Vm=0)
- For a charged solute, the energy of moving the solute in the chemical and electrical gradients is additive
ΔGt = RT ln (c2/c1) + zFΔΨ
(may generate chemical potential (conc ignoring charge)or electrical potenital)
what is memrbane potential
- When a charged molecule is moved across membrane, a charge imbalance results
Membrane potential, ΔΨ (in Volts)
- Typical plasma membrane ΔΨ = - 60 mV (-ve inside)
- Free energy of charged species is different on each side of membrane
ΔG = zFΔΨ F = faraday constant z = unit charge
how are ion gradients maintained using the Na+ ATPase
- Generates gradients of Na+ and K+
- Moves three Na+ ions out and two K+ ions in
- This results in a net negative charge inside the cell
- Hydrolysis of one ATP provides energy (one ATP for each 2 out and 2 in)
- Both ions move up their respective concentration gradients
- This ATPase uses 1⁄4 of your ATP when at rest
explain Na+K+-ATPase: an ion-pumping ATPase
- Generates gradients of Na+ and K+ - Control cell volume
- Drive active transport of other species
- Render nerve cells electrically excitable
- tetramer (α2β2); α performs transport
- 3 Na+ OUT and 2 K+ IN
- Both ions move UP concentration gradient - ATP hydrolysis provides energy
- Net +ve charge OUT
Generates membrane potential; -ve inside
how is ATP is generated by reversing F-type ATPases
- Protons can be pumped against the concentration gradient at the expense of ATP
- Alternatively, ATP can be generated at the expense of protons flowing down their concentration gradient
*this phosphorylates ADP to ATP
how do ion gradients power transport
Ion gradients serve the cell as reservoir of available free energy
Transport of one ion down its concentration gradient can drive transport of another solute up its gradient
This is secondary active transport
e.g. Na+-glucose transporter
explain Na+-glucose symporter
- moving two sodium ions at the same time as one glucose
- this get glucose into epithelial cell, then goes into glucose uniporter GLUT2 (gets glucose out)
- the sodium that went in via symporter is pumped into circulatory system, two potassiums come in
*symport can continuously bring in glucose bc sodium is moving down conc gradient not building up
explain ion channels
- Present in plasma membranes of all cells
- With ion pumps, define the permeability of membranes to ions
- Rapid movement of ions across membranes (107-108 ions/s per channel molecule)
ex: voltage gated K+ channel
Differences between channels and transporters:
- Rate of flux
- Saturability
- Channels are gated
explain the voltage gated K+ channel
- e.g. Streptomyces lividans
- tetramer: each subunit contains 2 transmembrane helices and a shorter helix (selectivity filter)
*need to ensure that ion channel is specific to ion its designed to transport
- outer helices in each subunit interacts with bilayer
- inner helices in each subunit contributes to inner pore - 10,000 fold more selective for K+ vs. Na+
explain K+ channel specificity
- potassium is hydrated with water molecules, but transport in dehydrated state
- potassium sticks in water filled pocket, 4 sites that potassium interacts with but can only every have 2 potassium mol at once in the pore
- a helix dipole stablizes the potassium once its dehydrated and occupying first potassium binding site
explain Selectivity of the K+ channel
(i) size
(ii) partial negative charges on C=O Gly-Tyr-Gly-Val-Thr
- carbonyl oxgen coordinate with unhydrated K+
explain gated ion channels
Channels are generally “gated”
This means that:
- By default they are closed, and let nothing past
- They open in response to a specific stimulus
- An inbuilt timer closes them again after a short delay, even if the stimulus is still present
The type of stimulus needed to open divides channels into:
i. ligand-gated: e.g. acetylcholine receptor ion channel
ii. voltage-gated: respond to changes in the voltage potential across the membrane
explain Voltage gated Na+ channel organization
Na+ channels differ from K+ channels predominantly in having a narrower specificity pore (as Na+ is smaller)
- α-helix 6 is the pore forming helix, 5 faces the membrane
- The voltage gating mechanism requires the addition of four additional α-helices (1-4)
- The four separate chains are all one protein, serving as domains I-IV
*linker of 3-4 is the inactivation part
explain structure of Voltage gated Na+ channel
The four pore-forming helices are arranged around the pore
The voltage sensing helix 4 (blue) can move in response to changing membrane voltage
The pore lining helix (6) is also called the activation gate
The inactivation gate is a small soluble domain (green) that connects domains III and IV
*total of 24 thansmembrane domains but also has short sectrions that are not full transmmebrane alpha helicies with hydrophobic region that sticks to membrane (selectivity filter)
explain opening the voltage gated Na+ channel
Helix 4 has a high net positive charge, and is sensitive to membrane voltage
The net negative charge inside the cell pulls it inward
When membrane is depolarized helix 4 relaxes, and moves towards outside
Coupled movements in helix 6 (lining the pore) opens the channel
After opening, channel is quickly blocked by the inactivation loop, stopping ions from passing
explain the realtionship of ion channels in disease and toxicity
Na+ channel in muscle
– Channel defects result in diseases where muscles are paralyzed or stiff
– Many toxins target ion channels as the effects are fast acting and very debilitating
Na+ channel in neurons
– Tetrodotoxin produced from puffer fish (fugu) binds to Na+ channels of neurons
