Ion channels Flashcards
How are ion channels gated?
Gating of Ion Channels
Various ion channel types gate differently, some open only transiently whereas others stay open as long as the stimuli exists.
Stimuli for ion channel activation are either:
1) A change in the membrane potential
2) A change in the concentration of extracellular ligands (neurotransmitters)
3) A change in the concentration of intracellular ligands (Ca2+, H+, cyclic nucleotides, G protein subunits)
4) Mechanical stimulation (stretch)
Explain the overall molecular structure of ion channels
For voltage-gated channels, the pore-forming subunits are called α-subunits, whereas the accessory subunits are called β, γ, or δ subunits. Moreover, it has become clear that ion channels are not solitary proteins but are part of larger signaling networks.
How is the structure of Voltage-Gated Ion Channels?
The pore-forming α-subunits of voltage-gated ion channels are all built upon common structural elements and come in four variants:
2-TM:
* Consists of four subunits that form the channel
* Each subunit consists of two transmembrane (TM) segments connected by a membrane-reentrant pore-loop with N- and C-termini on the inside
* Typical for Kir channels (inward rectifying potasium)
4-TM:
* Consists of two subunits that form the channel
* Each subunit consists of a chain with two 2-TM units
* K2P channel (two-pore potassium)
6-TM:
* Consists of four subunits
* Each subunit consists of a 2-TM unit with four extra membrane-spanning N-terminal domains hence 6-TM unit. Transmembrane segments are named (S1-S6)
* A voltage sensor have been added to the basic 2-TM pore unit
* KV channel, KCa channel (calcium-activated), cyclic nucleotide-gated channel (CNG), hyperpolarization-gated channel (HCN), cation channel sperm-associated protein (CatSper) and transient receptor potential channel (TRP)
24-TM:
* Is made by a chain with four homologous 6-TM repeat domains and six transmembrane segments (S1–S6) in each domain – this makes up a 24-TM subunit that forms the channel alone
* Nav and Cav (Voltage-gated Na+ and Ca2+ channels)
Three different parts of the channel are responsible for the functions: ion permeation, pore gating, and regulation.
How does the selectivity filter of Kv channels differ from Na+ channels?
Selectivity filter
The narrow part of the pore and is responsible for the selectivity mechanism.
The K+ channel selectivity filter:
The residues in the pore loop line and their peptide backbone carbonyl groups act as surrogate-water molecules, preserving the chemical energy of dehydrated K+ ions entering the pore – meaning that the energy state of the K+ ions remain the same when they enter the pore despite losing their water molecules. This design ensures both high selectivity and permeability of K+ ions as they pass through in a single-file manner. Even though Na+ ions are smaller, they won’t enter the K+ pore since it is energetically unfavorable.
The Na+ channel selectivity filter:
Is larger than K+. At the extracellular end of the filter, four glutamate residues with negative charges interact with Na+ ions, partially removing their hydration. Following this, two ion coordination sites are formed, comprised of peptide carbonyls aligned to bind Na+ with four planar waters of hydration. This facilitates the conduction of Na+ ions as hydrated entities through the channel.
How does the voltage sensing domain work?
The voltage-sensing domain senses changes in the voltage and regulates the gating mechanism.
The membrane potential creates an electrical field across the membrane. Charged amino acids (usually arginine) are found in the fourth transmembrane segment (S4) of the channel and move according to changes in the electrical field. This movement, facilitated by the S4-S5 linker, results in the bending of the S6 segment (inner helix gate) and the subsequent opening of the pore.
Which families of voltage-Gated Potassium Channels are there?
2-TM Kir channel family:
Has 6 subtypes that play diverse roles in the body.
6-TM KV channel family:
Has 12 subtypes. It can be composed of four different subunits from the same subfamily giving large variations.
6-TM KCa Channels:
Are divided into three families based on single-channel conductance.
- Ca2+ Gating: Controlled by Ca2+ binding directly to the channel or indirectly to constitutively bound calmodulin.
- Cell Regulation: Typically involved in reducing a cell’s activity by inducing hyperpolarization when internal Ca2+ levels increase.
- Molecular Brake: Act as a regulatory mechanism, slowing down cellular processes when necessary.
TRP Channels, Transient Receptor Potential (TRP) channels:
Consist of 6-TM and a cation-permeable pore loop located between S5 and S6.
- Diverse Subtypes: There are 28 TRP subtypes, some selective to Na+/K+, Mg2+, or Ca2+. They can be regulated by G-protein-coupled receptors, kinases, and phospholipases.
- Six Mammalian Subfamilies: Mammalian TRP channels are categorized into six subfamilies: TRPA, TRPC, TRPM, TRPP, TRPML, and TRPV.
Which families of voltage-Gated Potassium Channels are there and how are they different from each other?
Functionally CaVs are closed at RMP (-50 - -80 mV) but activated by depolarization. Based on this and pharmacological properties the 10-cloned α-subunits can be grouped in three families:
- CaV1.x (L-type): high-voltage-activated dihydropyridine-sensitive calcium channels. Require MP of -20 to +10 mV to activate
- CaV2.x: high-voltage-activated dihydropyridine-insensitive channels
- CaV3.x (T-type): low-voltage-activated.
Following activation, CaVs inactivate in the presence of sustained membrane depolarization.
Describe the structure and molecular biology of Cavs
The CaVs consist of a large α-subunit, a 24-TM unit – appr. 2000 amino acid residues.
The positive voltage-sensing domain is in the S4 segments, and the pore loops are at S5-S6 with the pore loops and S6 segments believed to line the channel lumen.
CaVs are 1000-fold selective for Ca2+ ions over Na+ and K+. The Selectivity is created by a ring of four negatively charged glutamic acid residues projecting into the ion channel pore – one residue by each of the four pore loops.
Subunits:
- CaVs consisting of only the α1 subunit forms a functional ion channel
- Native CaVs form multisubunit complexes with cytoplasmic β subunit, an extracellular membrane leaflet anchored α2δ subunit, and a 4-TM spanning γ subunit.
The role of the subunits is to regulate surface expression, gating, and pharmacological properties.
What is the physiological role of Cavs?
Under resting conditions, the intracellular [Ca2+] < 100 nM and extracellular [Ca2+] is 1–2 mM, creating a 10,000-fold concentration gradient. Many homeostatic mechanisms operate to keep this gradient.
The Ca2+-equilibrium potential is > +100 mV so Ca2+ always flows into a cell, when CaVs are activated by depolarization.
Voltage-gated Ca2+ channels are “gatekeepers” of calcium entry into excitable cells. Thereby, they constitute the principal entrance for calcium influx in nerves, endocrine, and muscle cells.
- In muscle tissue: Ca2+ binds to protein troponin C allows myosin-mediated sliding of actin filaments –> leads to the shortening of muscle fibers.
- In cardiac and smooth muscle. Indirect Ca2+-influx through CaV –> Muscle contraction.
- In nerves: Ca2+-influx through CaV2.1and CaV2.2 triggers neurotransmitter release from synaptic nerve terminals –> The CaV2.1 and CaV2.2 subunits bind directly to proteins of the protein machinery involved in membrane fusion of neurotransmitter-containing vesicles.
How is the pharmacology of Cavs?
Cadmium, Cd2+, produces nonselective inhibition of all type of CaVs –> Cd2+ binds to the ring of four glutamates in the selectivity filter, blocking the pore.
CaV1 (L type)
- Modulated by organic calcium blockers which bind with high affinity and selectivity to the α1 subunit: phenylalkylamines (verapamil), benzothiaziepine (diltiazem) and dihydropyrdines (nifedipine)
CaV2 (N-, P/Q- and R type)
- N-type channel is inhibited by peptide toxins from fish-eating marine snails.
CaV3 (T type)
- Moderate selective blocking by vasodialiting compound mibefradil.
Auxiliary subunits
- Gabapentin and pregabalin binds to α2δ subunit of CaVs –> decreasing CaV cell surface expression –> Decreases the amplitude of calcium currents without producing complete blockage
Descibe the functionality of Navs
Functionally NaVs are closed at RMP and open when the membrane becomes depolarized, activation requiring membrane potentials of -70 to -30 mV.
Most NaVs inactivate within ~1–10 ms in the presence of sustained depolarization.
How is the structure of Navs?
NaVs are composed of a large α-subunit – appr. 2000 amino acid residues) with structural similarity to the α1-subunit of CaVs.
Rapid inactivation of most NaVs is due to the cytoplasmic domain III-IV linker which functions as a “hinged lid” that swings in to occlude the intracellular opening of the pore.
Subunits
- NaVs consisting of only the α-subunit forms a functional ion channel
- Native NaVs form protein complexes composed by a α-subunit and auxiliary subunits
- Only one subunit, β-subunit, has been identified, and it is composed of a large extracellular part which interacts with the α-subunit and a small C-terminal portion consisting of a single TM segment.
- The function of the β-subunits can be divided into
1) Modulation of functional properties of NaVs
2) Enhancement of membrane expression
3) Mediating interactions between NaVs and extracellular matrix proteins as well as various signal transduction molecules.
What is the difference between desensitization and deactivation?
Desensitization:
- Definition: Desensitization refers to a process in which the responsiveness of an ion channel to its ligand (activating molecule) decreases over time, leading to a reduced ability of the channel to open in response to the same stimulus.
- Example: In the context of neurotransmitter-gated ion channels, such as the glutamate receptors, desensitization occurs when the receptor becomes less responsive to repeated exposure to neurotransmitter molecules. This phenomenon is crucial for preventing excessive and prolonged signaling.
Deactivation:
- Definition: Deactivation, on the other hand, refers to the process by which an ion channel returns to its resting (closed) state after being open. It is the termination of the channel’s conductive state.
- Example: In voltage-gated ion channels, such as those involved in action potentials in neurons, deactivation occurs when the channels close after being opened in response to a change in membrane potential. This closure is essential for repolarization and restoring the resting membrane potential.
