Ion channels and Transporters Flashcards
2 major subclasses of ion channels
- Voltage-gated
- Ligand Gated
Currents carried by Na+ are
Inward at potentials more negative than ENa and reverse their polarity above ENa
Properties of single Na+ channels
The amplitude of current depends
on Na+ concentration
Properties of single Na+ channels
Time course of opening, closing and inactivation matches
macroscopic current
macroscopic current
stochastic events averaged many times.
Properties of single Na+ channe
Opening and closing of channels are
voltage-dependent
Properties of single Na+ channels
Tetrodotoxin blocks
both microscopic and macroscopic Na+ currents.
properties of single channel K+
single channel K+ currents reflect
macroscopic currents
single channel K+ currents are…(Inward or Outward)?
Outward currents
during
brief depolarizations, single channel K+ channels…
Do not inactivate
single channel K+ channels are
voltage
-dependent
single channel K+ channels
Depolarization (increases or decreases) probability of opening
increase
single channel K+ channels
Hyperpolarization (increases or decreases) probability of closing
Increases
single channel K+ channels
single channel K+ channels are blocked by drugs that….
affect the macroscopic
current
Voltage-gated Ion Channels
Voltage-gated Ion Channels show…
ion selectivity
Voltage-gated Ion Channels
voltage-sensor
depolarization increases open
probability, while hyperpolarization
closes them
Voltage-gated Ion Channels
Which channel has a mechanism for inactivation?
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Na+
Levels of protein structure
primary structure
The properties of a protein are determined by its
amino acid sequence
Levels of protein structure
secondary structure
Active proteins require the folding of polypeptide
chains into precise 3
-dimensional conformations (linked via hydrogen bonds).
Depending on the nature and arrangement of the
amino acids present
The 3D
structure is the thermodynamically most stable
configuration.
Levels of protein structure
alpha helices
Secondary structure in the shape of a coil
Levels of protein structure
beta sheets
Secondary structure with a flat, folded shape
Levels of protein structure
tertiary structure
Further folding and reorganization within the
molecule results in higher order
Occurs when ertain attractions are present between alpha helicies and beta/pleated sheets
Levels of protein structure
Quaternary structure
A question consisting of more han one amino acid chain
X-ray crystallography
A beam of X-rays strikes a crystal/protein and scatters into many different directions
From the angles and intensities of these scattered beams, one can produce a three-dimensional picture of the density of electrons within the crystal/protein
X-ray crystallography determines…
the arrangement of atoms within a crystal.
the mean positions of the
atoms in the crystal can be determined, as well as theirchemical bonds.
X-ray crystallography is used to…
determine how a drug interacts with its protein target and how this interaction can be improved
X-ray crystallography of membrane proteins is challenging because
it requires detergents to solubilize them in
isolation and such detergents often interfere with crystallization.
Cryogenic-electron microscopy
In Cryogenic
-electron microscopy
a biological sample is flash frozen
(vitrified), sliced, and then imaged using an
electron microscope
Cryogenic-electron microscopy has allowed for
the
determination of biomolecular
structures at near -atomic
resolution (~1.25
-ångström)
Molecular Structure of Ion Channels
Hetero-oligomers
constructed from
distinct subunits
Molecular Structure of Ion Channels
Homo-oligomers
constructed from
a single type of subunits
Molecular Structure of Ion Channels
single polypeptide chain
organized into repeating
motifs, each motif
functioning like a subunit
Molecular Structure of Ion Channels
auxiliary subunits (β or γ)
modulate the gating characteristics of
the central core
Molecular Structure of (typical) voltage-gated Ion Channels
The pore-forming subunits
(α-subunit) of the voltage-gated
Na+, Ca2+, and K+ channels are
composed of a common repeated domain contains
* 6 alpha-helical regions (S1-S6) and a
* P region (“Pore loop”) that goes in and out of the membrane.
* P region confers ion selectivity
* S4 is positively charged
and represents the voltage sensor
________ K+ channel subunits form a channel
Four
The Na+ channel
The Na+ channel consists of…
A pore-forming α subunit
associated with auxiliary β subunits
The Na+ channel
The α subunits are organized in
four homologous domains (I–IV), which each contain six transmembrane alpha helices (S1–S6)
and an additional pore loop located between the S5 and S6 segments.
The Na+ channel
The S5 and S6 segments
line the inner cavity and form the activation gate (confer ion selectivity)
The Na+ channel
S4 segments
Positively charged amino acid residues in the S4 segments serve as gating charges that move in response to depolarization.
The Na+ channel
The inactivation gate…
The short intracellular loop connecting homologous domains III and IV serves as the inactivation gate, folding into the channel
structure and blocking the pore from the inside during sustained depolarization.
The Na+ channel
β subunits
modulate the kinetics and voltage-dependence of channel gating, and they are involved in channel localization and
interaction with cell adhesion molecules, extracellular matrix and intracellular cytoskeleton.
Voltage sensor of the Na+ channel
S4 (red) = voltage sensor (positively charged amino acids)
depolarization causes conformational change in channel
Cycle of Na+ channel states
Rapid opening (activation) followed
by slower closing (inactivation)
Recovery of inactivation
of Na+ channels
Two-pulse voltage clamp protocols
test the kinetics of channel gating
During the intgerpulse interval, come channels recover from inactivation
2nd pulse determines what fraction have recovered in that time
The relative contribution of the persistent Na+
current (INaP) becomes more obvious at
depolarized potentials where the fast Na+ current is
inactivated
The late openings in single-channel recordings suggest that INaP is
generated by different kinetic modes of the same sodium channel,
with the same channel occasionally entering an open state that
lacks fast inactivation.
INaP is activated in
the subthreshold
voltage range
INaP serves to amplify
the response to
synaptic input and it
enhances repetitive firing capabilities.
INaP consists of
Only ~0.5–5% of the maximum
transient sodium current, but the
resulting current (5–200 pA) is
functionally very significant at
subthreshold voltages.
Most abundant Na+ channels - α subunits in adult CNS
Nav1.1, Nav1.2, and Nav1.6
Similar properties (subtle differences in voltage-dependence and activation/inactivation).
Their functions are non-overlapping. Knock-out of either is lethal.
Four major classes of K+ channels,
grouped by TransMembrane domains:
Tandem pore domain potassium channels (4TM)
Voltage-gated potassium channels (6TM)
Calcium-activated potassium channels (6 or 7TM)
[Inwardly rectifying potassium channels (2TM)]
Tandem pore domain potassium channels (4TM)
constitutively open or
possess high basal activation, such as the “resting potassium channels” or
“leak channels” that set the negative membrane potential of neurons.
Voltage-gated potassium channels (6TM) -
voltage-gated ion
channels that open or close in response to changes in
the transmembrane voltage
Three subtypes
1. delayed rectifyer
2. A type
3. KCa2+ (BK, SK, IK)
Calcium-activated potassium channels (6 or 7TM)
open in response to the
presence of calcium ions or other signaling molecules
Voltage-gated K+ channel
voltage gate S4; Depolarization…
pulls on the S4-S5 linker to open the pore
Kv
Voltage-gated K+ channels
Delayed rectifiers Inactivate**
slowly or not at all
Delayed rectifiers are further divided by…
by their activation kinetics
* Fast
* Slow
And their voltage-sensitivity
* High
* Low
A-type inactivates
Inactivate rapidly
AKA Transient currents
The classic A-type Kv
channels (Kv
1.4 and Kv
4) are
low-voltage activated
Mammals gave 17 voltage-gated
K+ channel (Kv) genes, within
4 subfamilies related to
Shaker (Kv1.1-Kv1.8),
* Shab (Kv2.1 and Kv2.2),
* Shaw (Kv3.1-Kv3.4), and
* Shal (Kv4.1-Kv4.3)
the first cloned K+ channel from a
mutant Drosophila fruit fly (1987)
Shaker
Cloning of Shaker gene (1987) allows
first identification
of amino acid sequence of a channel gate
(→ inactivation gate).
Demonstration of “ball-and-chain” mechanism that had
been first hypothesized for Na+ channels
* However, a different part of the protein is involved and
there are 4 inactivation gates instead of 1, because K+
cannels are oligomers (4 subunits)
Inactivation is modulated by N-terminus
Functions of Delayed Rectifiers
- Resting potential
- AP Threshold
- AP shape (repolarization and hyperpolarization)
- Membrane excitability
Functions of A-type channels
- Spike frequency coding
- Dendritic Processing
HVA Kv channels shape
action potentials and contribute to firing-related changes in
excitability
Located on the soma, nodes between myelin, presynaptic
LVA Kv channels
keep excitability in check (Kv1, Kv7)
*
Located on Axon nodes
Kv7 provide the “M-current” (muscarinic), which limits firing rate
A-type channels are expressed in
dendrites and are active at low membrane potentials
In dendrites, A-type currents limit depolarization to
active synapses, enabling synapse-specific
plasticity to occur.
A-type currents help to
slow down depolarization after an AP (but not too much since they
inactivate), enabling frequency adaptation of AP firing (→ “rate coding”)
A
-type currents
regulate dendritic excitability
IA increases in
distal dendrites and reduces EPSPs
* Counteracts (local) EPSP amplification by Ca++
currents, INaP, as well as bAPs
A type channels are blocked b y
4-AP
The intensity of signals is coded through
the frequency of APs
rate coding
Cells respond to an increase in inputs with an increase in AP firing
Neurons receive depolarizing inputs
at the
dendrites, the soma, and at the
axon hillock
Axons transmit APs that
result from
summed inputs
* Spike rate is a function of depolarization
* Higher input → higher firing rate
A-type currents
Kv4.x and Kv1.4 subtypes open
briefly only at relatively low membrane potentials
(i.e. during AHP between spikes)
A-type currents
A-type currents provides
a hyperpolarizing current that pulls membrane from AP threshold
* Lengthens time between spikes,
* But also allows more Na+ channels to recover from inactivation
With higher input
intensities,
membrane
potential does not
hyperpolarize as much
* Fewer KA channels recover
from inactivation
→ neuron can fire faster
High-voltage activated delayed
rectifiers open
on depolarization
* In the axon (e.g. Kv3) HVAs
contribute to shaping of the AP
* In the soma (e.g. Kv2) HVAs
regulate excitability
Low-voltage activated delayed
rectifiers are open
below AP
threshold (and above)
* In the axon (e.g. Kv1, Kv7) they
regulate excitability, AP shape, and
firing rate
β
-subunits can alter
inactivation * pharmacology * regulation (e.g. ATP sensitivity)
Calcium-activated potassium channels (6 or 7TM) has three subtypes:
BK – Big potassium channel (KCa1.1)
SK – Small potassium channel
IK – Intermediate potassium channel
SK current activates
slower than BK
SK
helps shape the afterhyperpolarization.
contributes little to the fast
repolarization of the action potential,
The duration of SK conductance reflects
the decay of intracellular free calcium
(>100 ms).
BK channels deactivate
far more quickly, since both depolarization
and high local intracellular calcium are
required for activation
BK channels (KCa1.1) –
large conductance Ca2+
-activated K+channels are sensitive to
TEA and charybdotoxin (CTX,
scorpion venom)
At the soma, BK
mediate rapid spike
repolarization and fast
afterhyperpolarization.
In dendrites, BK
regulate the duration of
dendritic calcium spikes and burst firing.
SK channels (KCa2+) - Ca2+-activated K+channels have a
Smaller conductance (10-20 pS) than BK channels, but more sensitive to Ca++
SK channels are nly (weakly or strongly) voltage-dependent
weakly
SK channels are sensitive to
apamin (bee venom)
Regulators of K+ conductance (RCK) domains encode “calcium bowls”
(Ca++ binding sites)
Only on BK channels
SK channels are modulated by
modulated by calmodulin
* Also activated by Ca++ spikes, or by Ca++ influx through NMDARs
SK channels contribute
to spike frequency adaptation.
All K+ channels discovered so far possess a core of alpha
subunits, each comprising
g either one or two copies of a
highly conserved pore loop domain (P-domain).
Ions must
shed water before they can pass through channels
Na+
is a smaller molecule than K+,
but its effective size is
is larger than
K+
, because it attracts a larger
sphere of water
The selectivity filter in a NA+ channel
is just large enough to
accommodate one Na+
ion contacting one water
molecule.
This involves transient binding and stabilizing of the
positive Na+
ion with a negatively charged amino acid
in the wall of the pore.Cations that are larger in diameter (e.g. K+
) cannot pass
through.
on-selectivity of channels - K+ channel
As K+ passes through the pore, interactions between K+
ions and water molecules are
prevented and K+
is stabilized by interacting with specific components of 8 amino acids that in
all K+ channels include the sequence TxxTxGxG (the K
+
selectivity sequence)
Ion-selectivity of channels - K
+ channel
The walls of a K+ channel are
too far apart to stabilize a dehydrated Na+ ion
Ion-selectivity of channels - K+ channel
The Negatively charged pore helix
Strips water from K+
ions
so they fit through the filter
Ca++ channels and α-subunits
α-subunits consist of
Single polypeptide chain. 4 repeats of a domain that contains
6 alpha-helical regions (S1-S6) and a P region,
plus 3 ancillary subunits α2, β, and γ.
Ca++ channels and α-subunits
Three genes for α subunits (Cav1-Cav3)
High-threshold channels (HVA)
* Cav1.x (L-type channels),
* Cav2.1 (P/Q-type channels), and
* Cav2.2 (N-type channels) show little inactivation.
Functionally similar, but distinguished by pharmacology (sensitivity to omega Ѡ-conotoxins)
Low-threshold channels (LVA)
Cav3.x (T-type channels)
inactivate rapidly
The L-, N-, and P-type Ca++ channels
are
high
-threshold channels (HVA)
L-type Ca++ channel
large, long
-lasting
N-type Ca++ channel
Neuronal
P-type Ca++ channel
Purkinje cell
The probability of the L type channels being in the open state
inreases with depol so that they overlap and lead to a suden increase in corresponding macroscopic current.
T-type Ca++ channels
are
low-threshold channels (LVA)
Tiny conductance, transient activation
T-type Ca++ channels become
de-inactivated during
cell membrane hyperpolarization, and
then open again to depolarization.
T-type channels are important for
rhythmic firing patterns in cardiac
muscle cells and thalamic neurons.
T-type channels are activated at
very
negative potentials (
→LVA) where
there is a large driving force for
calcium going into the cell.
T-type channels fast voltage-dependent inactivation
also allows for more
frequent depolarization.
Optogenetics
Light-gated channels
Two types of Light-gated channels
- Channelrhodopsin (ChR)
- Halorhodopsin (HR)
Channelrhodopsin (ChR)
Are not ion selective
Activated by blue light
Excitability (ChR2)
Halorhodopsin (HR)
Allow only choride
Activated by yellow light
inhibition (NpHR, or Arch)
Active Transporters
create and maintain Ion Gradients
Active Transporters require
energy (ATP, or electrochemical gradient of other ions)
Kinetics of Active Transporters
slow (e.g. several ms for 3 Na+
ions vs. >1000 Na+
ions per ms
through Na+ channel during AP.
Two subtypes of Active Transporters
- P-type ATPases
- Ion exchangers(use electrochemical gradient):
Two types of P-type ATPases
- Na+/K+ ATPase pump
- Ca++ pump
Three types of Ion Exchangers
- Na+/Ca2+ exchanger
- Na+/K+/Cl- co-transporter
- K+/Cl- co-transporter
The sodium-potassium pump (Na+/K+ATPase)
3 Na+
ions are carried out for
every 2 K+
ions brought in
→ net loss of 1 positive ion
inside (pump is electrogenic)
→ hyperpolarizes cell
The sodium-potassium pump (Na+/K+ ATPase) accounts for
20-40% of brain’s energy consumption
Uses about 3 million ATPs per second (in rod photoreceptors, which have high resting Na+
permeability the number is 60 million ATPs per second)
Antiporters
Typically coupled to Na+ movement
Ca++ and H+(pH regulation) are
exported from cells by antiporters which couple their export to the energetically favorable import of Na+
