Ion channels Flashcards
Ion Channels:
Membrane Proteins that open and close (“gate”).
When ion channels are open, they
only allow certain substances to pass (“selectivity”)
Ion channels are present
both in the plasma membrane and in membranes of intracellular organelles.
Their gating is controlled by a vast array of stimuli including:
temperature (hot/cold)
mechanical deformation
membrane potential
extracellular agents (taste, olfaction, neurotransmitters) intracellular agents (ATP, cAMP, Ca2+)
Some channels respond to multiple stimuli
e.g., both hot and capsaicin, voltage and Ca2+)
Gating stimulus:
ACh receptors, GABAA receptors.
Selectivity
Na+ channels, K+ channels, aquaporins
Muscle
contraction/relaxation, cardiac pacemaking
Neurons
sensory transduction, signal propagation, neurotransmitter release, postsynaptic responses, plasticity
T lymphocytes
activation
Pancreatic β cells
insulin release
Ion channels are important therapeutic targets
pain, arrhythmia, hypertension, epilepsy
conus geographus
a-conotoxin GVIA
Ca2+ channels
Sea krait
a-bungarotoxin
nicotinic AChRs
Ryania speciosa
ryanodine RyRs
Fugu
puffer fish
tetrodotoxin
Na+ channels
KV, NaV and CaV
4 membrane-spanning domains
Kv separate polypeptides
NaV, CaV linked together as 4 repeats (I, II, III, and IV)
KV, NaV and CaV
each of 4 domains
Each of the 4 domains contains 6 α-helices (S1-S6)
how do Kv Nav Cav sense voltage
S4 helices have positively charged residues (Lys/arg) at every third position and are the structures that sense voltage
s4 helices have
have positively charged residues (lys or arg) at
every third position and are the structures that “sense” voltage.
S5 and S6 helices, and the connecting “P loop”, assemble to form
the ion conducting pathway and “selectivity filter.”
selectivity filter
S5 and S6 helices and P loop form conduction pathway
Pentamer ligand-gated channels includes
(GABAAR, GlyR, nAChR, 5-HT3R)
Pentamer ligand-gated channels are:
- Heteropentamers. 4 transmembrane α-helices (M1-M4) per subunit.
- M2 helices surround a central, ion-conducting pathway.
- Selective for Cl- or cations with slight preference for Na+ over K+.
Tetrameric ligand gated channels. examples
Ionotropic glutamate receptors
Tetrameric ligand gated channels are:
- 4 subunits w/ 3 a-helices each
2. In NMDA receptors, two subunits bind glutamate, two bind glycine.
CLC chloride chanels
- Dimer
- Each subunit has an independently-gate pore
- Another gate controls both pores simultaneously
- Some ClC’s are H+/Cl- exchangers
aquaporins are
- tetramers
- each subunit contains a water pore
- water pores exclude al ions including H+
- in addition to water pores, a gated, central ion pore
Channel selectivity depends on
- size
- charge
- dehydration
- multiple binding sites
Selectivity varies examples
KV – K+:Na+ is 10,000:1;
CaV – Ca2+:Na+ is 3000:1; NaV – Na+:K+ is 12:1;
nAChR – Na+:K+ is 1.3:1
Selectivity Size:
ions that are too large are rejected. However ionic crystal radius of Na+ < K+ yet rejected by KV
Selectivity charge
Sign is important (cations vs anions). Valence also.
Selectivity dehydration
Waters are removed and dehydrated ion interacts with protein environment within pore. Cannot bind too tightly (107 ions/sec per channel)
Where are the activation and inactivation gates?
- S4 helices (4/channel) are the voltage sensors.
- The activation gate(s) likely represent the inner ends of the S6 helices swinging hinge-like motion around conserved glycines
The inactivation gate of NaV channels is formed by ______
the cytoplasmic III-IV linker.
Inactivation occurs when the
III-IV linker folds over the inner end of the ion-conducting pathway.
Neurotransmitter receptors are either directly
ionotropic receptors
metabotropic receptor
ionotropic receptors
coupled to ion channels (i.e. the receptor and channel are part of the same protein),
metabotropic
activate second messenger pathways which can affect physically separate ion channels.
A given neurotransmitter typically is able to activate both
ionotropic and metabotropic receptors.
Within the ionotropic category are the
pentameric ligand gated channels,
also called the Cys-loop family of neurotransmitter receptor channels,
including the GABAARs, GlyRs, nAChRs and 5-HT3Rs
Ions are energetically stabilized in solution by .
waters of hydration, which make the ions effectively larger in size
Multiple binding sites can increase
selectivity
If an ion interacts with multiple sites while traversing the channel pore, even relatively slight differences in the strength of interaction between
preferred and non-preferred ions at each site can result in a significant enhancement of overall selectivity for the preferred ion.
Furthermore, the waters of hydration essentially mask
small differences in the size of the ions.
Thus, ions must be substantially ______ before they pass through the channel pore,
de-hydrated
To compensate for this dehydration, which is energetically ______, the ion is stabilized within the pore by _______
unfavorable
energetic interactions with the amino acids forming the pore (but not too much, or the ion would stay “stuck” in the pore).
The energetic interactions of the ion with the pore can occur with
amino acid side chains (positive/basic residues: lysine or arginine; negative/acidic residues glutamate or aspartate),
with backbone carbonyls (negative) or
alpha helix dipoles (N-terminal: positive; C-teminal: negative).
These and other interactions depend on the sign, valence and size of the ion.
“activation gate”, which can
rotate around a center pivot point. This rotation is controlled by a “voltage-sensing” charge, indicated by the “+” sign on the activation gate.
When the inside of the cell is made negative the gate rotates ______ position and the current _____.
back to the closed
decays away
The gating of NaV channels is more complicated because they have both
an activation gate and an inactivation gate.
When the inside of the cell has a negative potential with respect to the outside, the gate is held in its _____ position and the current is ____.
closed
zero
When the inside is made positive, the gate rotates to its ____ position and K+ ions flow ______.
open
out of the cell (upward deflection in the current)
activation
when gate is open and K+ ions flow out of the cell and the current increases
deactivation
the inside of the cell is made negative the gate rotates back to closed position and the current decays away.
NaV activation gate, At negative potentials it is
closed and making the inside of the cell positive causes the NaV activation gate to swing open (“activate”) and sodium ions to flow into the cell (INa)
The second NaV gate is the
the inactivation gate, is open at the resting potential because the activation gate occludes access to a site within the inner end of the pore at which the inactivation gate can bind.
However, after the activation gate opens, the inactivation gate _____, a process called “____,” causing the current to _______.
closes
inactivation
decay to zero during a maintained depolarization
Note that inactivation is not the same process as _____.
deactivation
Also, the reversal of inactivation is called
“removal of inactivation.
Selectivity occurs within a
central, ion conducting pathway formed by the four KV subunits or four repeats of NaV, where this central pathway is surrounded by S5 and S6 helices and connecting P loop contributed by the each of the four subunits or repeats (see above for the structures of NaV and KV).
Voltage sensing is accomplished
by the S4 helices. These helices contain positively-charged Lys or Arg residues at every third position and translocate in response to changes in voltage across the membrane.
when the activation gates are closed, they
occlude access to an enlarged space (“vestibule”) through which the ions pass before/after transiting a narrower constriction (the “selectivity filter” nearer to the extracellular side.
The inactivation gate of NaV channels is formed by the: ________, When this cytoplasmic III-IV linker folds over the inner end of the central ion-conducting pathway, the channels is in a ______ state.
cytoplasmic loop which connects repeats III and IV
closed/inactivated
The location of the selectivity filter is near the _____
the extracellular side,
tetrodoxin (TTX) is a charged molecule that
cannot cross the membrane.
When TTX is added to the extracellular side, it:
binds within the entrance of the pore, just above the selectivity filter of NaV
The binding of TTX is
independent of the position of the activation/inactivation gates; TTX has no effect when added intracellularly.
Lidocaine is a
tertiary amine and equilibrates between de-protonated (neutral) and protonated (positively charged) forms.
The protonated form of lidocaine is dominant at
physiological pH and cannot cross the membrane, whereas the de-protonated form can.
Protonated lidocaine has no effect on NaV from the ______ side but can block the channel from the
extracellular
intracellular side (thereby producing local anesthesia).
The lidocaine block from the intracellular side can only occur if ______, which requires that the ______
the protonated lidocaine can access the vestibule
activation gate be open and that the inactivation gate not be closed.
the lidocaine block is
“state-dependent.”
if lidocaine is at its binding site within the vestibule, it can be trapped there if the
activation or inactivation gates are closed.
