Ion Channel Structure and Function Flashcards
(46 cards)
why do we need ion channels?
important for human survival and for quick decisions
ion channels as signaling devices
channels open/close on a millisecond timescale –> conduct millions of ions down electrochemical gradient –> rapid changes in membrane potential and ion []s (Ca) –> trigger downstream effects (AP, muscle contraction, secretion, gene expression)
ion channels
-membrane proteins that form pore to allow flowing of specific ions
-not only within membranes but also in organelles
ion channels vs transporters
- transport speed- channels can transport millions of ions/second, while transporters are 1000x slower
-when channels open, it’s free flow but transporters have alternate accessibility - channels are always passive b/c ions can move in both directions and determined by [] and electrical gradients but transporters directly/indirectly use the gradient to do work
2 basic properties of ion channels: ion selectivity
-channels when they open they completely open and they have selectivity filter to decide which ions will flow
-Na, Ca, K, Cl
-Na will always go into cells and Ca as well
-K always goes out and Cl can go both ways
how is high selectivity for K over Na achieved? selectivity filter of a K channel
-solved structure of one prototype bacteria K channel- found that the selectivity was aligned in such a way that it’s just like how K resides in water- forms bonds with O2 groups
-selectivity filter where the side chain of O2 groups align like K water but smaller ion like Na cannot go through b/c you would need to shave off water
types of ion channels by selectivity
-many channels not very selective- nonselective anion channels- Cl, glutamate, ATP4-
-channels which can conduct large anions
-aquaporin- channels that conduct water
aquaporin: CHIP28 (channel integral membrane protein of 28 kDa)
-membrane protein- thought it was water channel since RBCs are permeable to water
-cRNA to oocytes to express the protein and study the function
-when they put CHIP28 into oocytes then put hypoosmotic solution, the oocyte swells very slowly
-for controls, could be any channels that when you pass ion they bring water
-inject GABA channel- Cl channel activated when GABA binds it- when added to hypoosmotic environment doesn’t swell
-water specific channel
-oocytes are quite resistant to water b/c frogs will lay eggs in water so the eggs would break
2 basic properties of ion channels: they are gated
-receive signals to open
-voltage-gated- when voltage changes it opens or closes
-ligand-gated- Ca, IP3, Cgap, proton, neuronal- gated by binding of ligand
-mechanical force and cell volume- MscS, MscL, Piezo, Swell1
-temperature and chemicals
channel gate
KcsA- bacterial K channel- selective filter and when closed you cannot pass and when they open they open this gae
patch clamp electrophysiology
-microscope and you put a cover slip then you have the cell grow then took polished pipette with opening of micromolar
-people initially took large pipette and stuck it in cells to record APs but thought to use small pipette to form a sphere (tight seal) to record single proteins’ currents through these channels
-break the membrane you have access to all the channels (whole cell recording)
patch clamp recording of a single ion channel
-2 states: open or closed
-conductance is feature of channel- sometimes small or big
-gating is probability function- proteins are moving
-all these open/closed states are protons moving from one state to another state- correlates with physical movement of protein
Na/K-ATPase pumps Na out and K into the cell and maintains their [] gradient
-for the nerves to conduct, they make gradients of Na and K- Na/K pump utilize ATP and pump Na out and K in
-Na/K pumps consume ~25% of total ATP in resting humans and ~70% of total in neurons
–> drop of ATP from blood loss and you lose gradient immediately then cells swell and die (ischemia)
equilbirum potential for K: -90 mV
-equilibrium potential for K is ~-90 mV- voltage where K stopped its flow for either direction
-when channel is open, K goes out and down its gradient and makes cell more negative b/c positive charge is left
-set up this gradient- K will go out until membrane potential reaches -90 and this is the point where K cannot move anymore since there’s a force repelling it back into the cell
-reversal potential for K is very negative
equilbirum potential for Na: +66 mV
always wants to go inside the cell and stops going in when membrane potential is very positive b/c there’s enough resistance to stop it
resting membrane potential (-70 mV) depends mainly on K leak channels
-main channels open at this time are slow and nonselective but mainly a K leak channel
-K leaks out of cells so membrane potential gets close to reversal potential of K, which is -90 but not -90 since there are leaks of other kinds
-leak is mainly K dominant
reaching threshold potential of -55 mV
-resting membrane potential at -70 mV and stimuli comes to depolarize it then threshold occurs when voltage-gated Na channels are triggered by activator then it’s an all or none response where these channels open triggers AP
-could also have cases where initial depolarization is not big enough- neurons have a lot of dendrites that receive info from other cells and depends on strength of input if neuron will fire
depolarization: opening of voltage-gated Na channel
-when depolarization happens, it is mainly Na ions come in first b/c the Na channels are sensitive to membrane potential change but also faster than K channels
-upstroke will go to Na reversal potential at 66 mV
repolarization: inactivation of voltage-gated Na channel and delayed opening of voltage-gated K channel
-ball and chain model for inactivation- shaker channel and you have nice current at different voltages and people realized that it has N terminus that may confer inactivation property
-use protease in pipette to digest protein- truncation of N terminus means the channels no longer inactivate and stay open
-add a little peptide into pipette and reconstituted inactivation
propagation of AP along the axon
-inactivation dictates where the APs are going
-depolarization and keeps moving in one direction and inactivation keeps it from back propagating b/c those channels are in the refractory period and cannot open
AP: hyperpolarization and return to resting state
-opening of K voltage gated channels- slower than Na channels but when Kv channels open, they will repolarize the membrane more towards to reversal potential of K
-these will also be inactivated and neuron goes back to resting state when it’s reset by the Na/K ATPase pump and K channels
cardiac AP
-one phase that comprises broader shoulder is opening of voltage gated Ca channels b/c when muscles contract, they need Ca to mediate their contraction
-main difference between muscles and neurons
equilibrium potential for Ca is 130 mV
-why is Ca low inside the cell? when life first developed it’s made by peptides and phosphate groups but phosphate group precipitates with Ca
-when life first developed on earth, important for cells to exclude CA b/c it precipitated with organic molecules –> cells have a lot of Ca pumps to remove it from cells
-later on cells realize that with low Ca inside the cells it’s a great 2nd messenger- when Ca channels open, Ca always goes in
4 typical components of a voltage-gated ion channel
- pore domain- selectivity filter
- pore domain- gate
- voltage-sensing domain
- inactivation domain
