Week 24 - drug targets, ion channels Flashcards
Most abundant ions in the human body - cations (+)
Sodium (Na⁺): Predominantly found in extracellular fluid, essential for maintaining fluid balance and nerve transmission
Potassium (K⁺): Major intracellular cation, crucial for muscle contractions and heart function
Calcium (Ca²⁺): Vital for bone structure, muscle contractions, and blood clotting
Most abundant ions in the human body - anions (-)
Chloride (Cl⁻): Key extracellular anion, important for maintaining osmotic balance and acid-base regulation
Phosphate (PO₄³⁻): Found in bones and cells, involved in energy storage (ATP) and acid-base buffering
Fluoride (F⁻): Found in small amounts, important for dental health and bone strength
Roles of ions
-Maintain osmotic pressure and hydration
-Facilitate nerve signal transmission and muscle function
-Support enzymatic activity and cellular processes
Key features and properties of ion channels
-Selective transmembrane pore (molecular sieve/filter)
-Specific sensors for gating (open and close)
-Regulatory mechanisms
Selective transmembrane pore (molecular sieve/filter)
-Ion channels act as selective filters, permitting the passage of specific ions based on their charge and size
->Sodium (Na⁺) channels: Do not permit potassium (K⁺) ions due to size and charge differences
->Potassium (K⁺) channels: Highly selective for K⁺ over Na⁺.
-Ensures precise ionic balance and function in cellular processes
Specific sensors for gating (open and close)
Ion channels possess gating mechanisms controlled by conformational changes in the channel proteins
->These changes determine when the channel is open or closed, regulating ion flow
Types of sensors or molecular switches:
-Voltage-gated ion channels
-Ligand-gated channels
-Mechanosensitive channels
Voltage-gated channels
Activated by changes in membrane potential (e.g., Na⁺ channels during action potentials)
Ligand-gated channels
Open upon binding of specific neurotransmitters or ligands (e.g., GABA or acetylcholine receptors)
Mechanosensitive channels
Respond to physical stimuli like
temperature or membrane stretch
Regulatory mechanisms
-Inactivation control (intrinsic):
Many ion channels have built-in mechanisms to switch to an
inactive state after prolonged activation
-Abundance and Location:
The number of ion channels and their placement, such as in post-
synaptic density, influences their activity
-Modulation by cellular components:
G-proteins, second messengers, and protein kinases can regulate
ion channel activity, affecting their gating and responsiveness
Physiological importance (regulatory mechanisms)
These regulatory mechanisms ensure proper ion flow, maintaining cellular homeostasis and preventing abnormal activities like over- excitation or prolonged inactivity
Conformational states of ion channels
Closed confirmation: The ion channel is not permitting ion flow, blocking passage between the inside and outside of the cell
Open-active confirmation: The ion channel is open, allowing ion
movement across the membrane
Open-inactive confirmation: The ion channel remains open but is unable to conduct ions, preventing further activity - this state is crucial for preventing overactivation
Voltage-gated ion channel - basic structure
(Sodium, potassium and calcium channels)
Basic structure: Composed of 4 subunits, which align together to form one functional ion
channel -> each subunit contains 6 transmembrane helices (S1 to S6)
Voltage-gated ion channel - key components
P-Loop: Forms the selectivity filter or molecular sieve -> aligns across subunits to create the transmembrane pore, allowing
only specific ions to pass
S4 Segment (Voltage Sensor):
Contains positively charged amino acids -> moves up or down in response to changes in membrane potential, enabling the channel to open or close
->N-Terminus (start of amino acid) and C-Terminus (end of amino acid) are located intracellularly (inside the cell)
Functional Assembly: The 4 aligned subunits form a complete channel with distinct
ion selectivity and gating properties
Importance: These channels regulate ion flow critical for processes such as nerve impulse transmission, muscle contraction, and cellular signalling
6 transmembrane helices -> Amino acid dips 6 times through the transmembrane (phospholipid membrane)
Voltage-gated ion channels: voltage-sensing
RESTING STATE:
-S4 segments (voltage sensors) are positioned in response to the resting membrane potential
->Outside of the membrane: Positively charged (+)
->Inside of the membrane: Negatively charged (-)
DEPOLARISATION:
Membrane polarity reverses during depolarisation:
->Inside becomes positively charged (+)
->Outside becomes negatively charged (-)
This change causes the S4 voltage sensors to shift, triggering the channel to open
ION FLOW:
Once the channel opens, sodium ions (Na⁺) flow through the channel from the outside to the inside of the cell -> this ion movement contributes to the action potential
Action potential
The movement of Na⁺ ions results in a spike in the membrane potential, depicted in the action potential curve
->The channel will eventually return to its resting state once the depolarisation is complete
Functional significance:
Voltage-gated ion channels are critical for propagating electrical signals in excitable tissues such as neurons and muscles
Voltage-gated ion channels: inactivation loop
Regulatory Mechanisms: Inactivation is built-in: it is an intrinsic property of the ion
channel -> the inactivation loop ensures rapid channel closure to control ion flow and maintain proper cellular function
Resting (closed confirmation) -> inactivation loop
The ion channel is closed, maintaining the resting membrane potential:
->Outside of the membrane: positive charge (+)
->Inside of the membrane: negative charge (-)
->The inactivation loop (illustrated as the “ball and chain”) is not engaged
Depolarised (open-active confirmation) - inactivation loop
Depolarisation occurs:
->Inside becomes positively charged (+)
->Outside becomes negatively charged (-)
The channel opens, allowing ion flow (e.g., Na⁺) across the membrane
Inactivation (open-inactive confirmation) - inactivation loop
After a brief period of opening, the inactivation loop (ball and chain mechanism) blocks the channel pore from the intracellular side -> this prevents further ion flow, even if the channel remains open structurally
Voltage-gated ion channel function and drug action
INORGANIC IONS: Certain inorganic ions act as modulators by either enhancing or inhibiting
the ion channel’s activity
NEUROTOXINS: Toxins from venomous creatures like snakes, spiders, and others can directly target voltage-gated ion channels
->These toxins often:
-Block ion flow
-Alter channel gating
-Cause overactivation or suppression of neural activity
DRUGS: Synthetic drugs are designed to interact with voltage - gated ion channels for therapeutic purposes
e.g., Local anaesthetics block sodium channels to prevent pain signals // Calcium channel blockers (e.g., for hypertension) reduce calcium influx
Neurotoxin action
Neurotoxins block sodium channels in all conformational states -> closed, open, and inactivated
This blockade prevents sodium ions from flowing into the cell, leading to disrupted neural signalling and paralysis
Lidocaine mechanism
Lidocaine -> a local anaesthetic - prefers to bind to sodium channels in their open or inactivated states
->This mechanism is described as “use dependency”:
-Lidocaine’s action increases with higher frequency or repetitive channel activation
->It blocks the flow of sodium ions, disrupting the nerve signal
transmission
By targeting sodium channels during their active or inactivated states, lidocaine effectively blocks pain signal transmission
->This property makes it a widely used anaesthetic in clinical settings
Three types of calcium channels
(Differ in sensitivity and conductance)
1) T-Type Channels
2) N-Type Channels
3) L-Type Channels
T-type channels
-Found in pacemaker cells and peripheral nerves
-Associated with transient (short-term) calcium currents
-Therapeutic use includes epilepsy and neuropathic pain
N-type channels
-Primarily located in neuronal synapses
-Regulate neurotransmitter release
-Targeted for chronic neuropathic pain management
L-type channels
-Predominantly found in cardiac and smooth muscle cells
-Play a crucial role in muscle contraction and neurotransmitter release
-Therapeutic interventions include drugs for hypertension, arrhythmias, and angina
Voltage-gated ion channels: calcium channel
Drug example: ω-conotoxin; Synthetic Analogue: Ziconotide
Targets N-type calcium channels in the nervous system
Mechanism of Action:
Administered intrathecally (injected into the spinal canal).
Blocks nociceptive nerves (pain-transmitting nerves) in the spinal cord
Inhibits neurotransmitter release, effectively reducing pain signalling
Clinical Application:
Ziconotide is used for managing chronic severe pain, particularly in cases where other pain
medications are ineffective
Voltage-gated ion channels: potassium channel
Unlike sodium and calcium channels that initiate and propagate action potentials,
potassium channels are primarily involved in:
-Restoring membrane potential
-Controlling the duration of the action potential
Key Functions:
->Repolarisation: After the depolarisation phase of the action potential, potassium channels allow K⁺ efflux (movement out of the cell), helping the membrane potential return to its resting state
->Hyperpolarisation: The continued efflux of K⁺ ions can lead to more negative membrane potential, briefly surpassing the resting state
Features:
Depolarised state: During an action potential, sodium influx occurs -> potassium channels then open to counteract this and restore balance
Hyperpolarised state: The efflux of K⁺ ions moves the membrane potential below the resting level temporarily
Modulatory Mechanisms:
->Intracellular Ligands: Example: Calcium-activated potassium channels (regulated by intracellular Ca²⁺).
->G Proteins: Example: M2 muscarinic receptors (Gβγ-activated IK channels in cardiac pacemaker cells)
Ligand-gated ion channels - structure
->Composed of five subunits forming a pentamer
->Subunits include 2α, 1β, 1δ, and 1ε (or γ in foetal receptors)
->N-terminal and C-terminal regions are located extracellularly
(ionotrophic)
Ligand-gated ion channels - ligand binding site
Found extracellularly at the interface of two α subunits
->Activation requires the binding of two acetylcholine (Ach) molecules
Ligand-gated ion channels - pore formation
The S2 transmembrane domain forms the pore lining
->The receptor transitions between closed and open states based on ligand binding
Ligand-gated ion channels - functional properties
Activation: Upon binding of Ach, the receptor undergoes a conformational change,
opening the ion channel ->allows Na⁺ and K⁺ ions to pass through, leading to membrane depolarisation
Nicotinic Ach receptors are critical for neuromuscular transmission in skeletal muscles and synaptic signalling in the central and peripheral nervous systems
Examples of neurotransmitters and their channels
1.Acetylcholine: Channel: Nicotinic Ach Receptor (nAchR).
2.ATP: Channel: P2X.
3.5-HT (Serotonin): Channel: 5HT-3.
4.Glutamate: Channels: AMPA, NMDA, Kainate.
5.GABA (Gamma-Aminobutyric Acid): Channel: GABA-A receptors
Types of ligand-gated ion channels
Cation Channels (Excitatory): Conduct positively charged ions like sodium (Na+) through the membrane -> Effect: This
leads to depolarisation of the membrane, causing excitatory effects
Anion Channels (Inhibitory):
Conduct negatively charged ions like chloride (Cl−) -> Effect: Leads to hyperpolarisation, causing inhibitory effects on neuronal activity
Cation Channels: Stimulate action potentials and neural excitation
Anion Channels: Suppress overactivity by stabilising or reducing membrane excitability
Ligand-gated cation channel
Glutamate NMDA receptor: the NMDA receptor is a type of glutamate receptor that allows the flow of sodium (Na⁺) and calcium (Ca²⁺) ions into the cell upon activation by glutamate
->It has a glutamate binding site that triggers channel opening
Ion Flow: When glutamate binds, the channel opens, allowing Na⁺ and Ca²⁺ to flow into the cell, contributing to neuronal depolarisation and excitatory signalling
Non-Competitive Antagonism:
Non-competitive antagonists like memantine bind to a site different from the glutamate binding site (allosteric site)
This binding modulates receptor activity by blocking ion flow, even if glutamate is bound, thus inhibiting excessive excitatory signalling
Memantine is used to manage conditions like Alzheimer’s disease, where NMDA receptor overactivation contributes to neurotoxicity
Ligand-gated anion channels
ABA_A receptors are ionotropic receptors that mediate inhibitory neurotransmission
->Binding of gamma aminobutyric acid (GABA) opens the channel, allowing chloride
ions (Cl⁻) to enter the cell
Inhibitory Mechanism: Influx of Cl⁻ ions leads to hyperpolarisation of the postsynaptic membrane, reducing neuronal excitability and making it less likely to fire an action potential
Allosteric Modulation: The receptor has allosteric binding sites for various drugs that enhance or modulate GABA’s inhibitory effect
Examples of modulators:
Benzodiazepines (e.g., diazepam) bind to a specific allosteric site, enhancing GABA’s effect (increased frequency of channel opening)
Other modulators include anaesthetics, steroids, and alcohol
Therapeutic Implications:
Benzodiazepines: Used for anxiety, epilepsy, and as sedatives
Barbiturates: Used in anaesthesia and seizure management
Key advantages of allosteric drugs
Pharmacological “Fine-Tuning”: Allosteric drugs provide a novel mechanism for modulating receptor activity -> instead of fully activating or inhibiting receptors like orthosteric drugs, they adjust receptor response to meet specific physiological needs
Enhancement of weakened signals: These drugs can amplify a weakened endogenous hormone or neurotransmitter signal. This is particularly useful in localised deficits where receptor activity is suboptimal
Improved Clinical Safety:
Allosteric modulators are often more selective for their target receptors, reducing off-target effects -> they lower the risk of receptor tolerance and desensitisation compared to orthosteric drugs, offering a better safety profile for long-term use
Allosteric drugs are being explored in treating conditions like neurological disorders metabolic disorders, and immune dysregulation -> by modulating receptor responses without fully activating or blocking the receptor, these drugs create therapeutic opportunities with fewer side effects
