6. 25 Drug Targets – Ligand-Gated Ion Channels & G-Protein-Coupled Receptors

Voltage-Gated Ion Channels (Brief Recap)

• Previously examined three families: potassium, sodium, calcium.
• Voltage-gated potassium channels are tetramers; each subunit ≈ one domain of sodium & calcium channels.
• Voltage-gated sodium & calcium channels each possess an “inactivation gate” in addition to the activation gate.
• Calcium channels classified as LVA (low-voltage activated) & HVA (high-voltage activated).

Historical Foundations of “Receptors”

• Observation (late $19^{th}$ century): Some drugs / hormones produce effects without entering the cell.
• John Langley (early $1900$ s): Proposed a “receptive substance” on cell surface.
• Paul Ehrlich ($1909$): Coined the term receptor.
• Operational definition – A receptor is:
• A macromolecular complex that binds an endogenous ligand (hormone, neurotransmitter) or drug.
• Ligand-induced conformational change → downstream signalling → physiological response.
• Analogy: key (ligand) fits lock (receptor) → lock twists (conformation) → door opens (effectors activated).

Categories of Drug-Relevant Receptors Covered in Lecture

• Ligand-gated ion channels (LGICs, ionotropic).
• G-protein-coupled receptors (GPCRs, metabotropic).
• Transmembrane receptors linked to intracellular enzymes (to be covered next session).
• Receptors stimulating cyclic GMP synthesis (future).
• Nuclear hormone receptors / transcription factors (future).

Ligand-Gated Ion Channels (LGICs)

Core Characteristics

• Alternate names: channel-linked receptors, ionotropic receptors.
• Coupled “directly” to ion flux; gating determined by ligand binding—not membrane potential.
• Pore diameter larger than voltage-gated channels → poor ionic selectivity; electro-chemical gradients decide ion direction.
• Functional speed: “fast receptors” – response within $\text{milliseconds}$.
• Physiological locales: neurotransmission (CNS & PNS), cardiac conduction, skeletal muscle contraction.

Structural Blueprint

• Most LGICs are pentameric (oligomeric) – $5$ subunits circled around a central pore.
• Stoichiometry example (neuromuscular nicotinic receptor): $2\,\alpha + 1\,\beta + 1\,\gamma/\varepsilon + 1\,\delta$.
• Each subunit contains $4$ trans-membrane (TM) $\alpha$-helices (TM$1–4$).
• TM$2$ helices from all $5$ subunits line the pore & form the activation gate via "kinks" that occlude or open.
• Two $\alpha$-subunits harbor ligand-binding sites exhibiting positive cooperativity:
• Binding of first ligand facilitates binding of the second.
• Both sites must be occupied for channel opening.

Opening Mechanism (Conformational Switch)

1. Resting (closed): TM$2$ helices kink inward; side-chains obstruct pore.

2. Ligand(s) bind $2$ $\alpha$ sites → conformational change.

3. TM$2$ helices rotate / splay outward → pore dilates.

4. Ions flow per gradient – typically $Na^+$ influx dominates; limited $K^+$ efflux ⇒ depolarization.

5. In inhibitory LGICs (e.g., GABA$_A$) $Cl^-$ influx → membrane hyperpolarization.

Functional Labels

• Excitatory LGICs: increase action-potential frequency.
• Nicotinic acetylcholine receptor (nAChR).
• Ionotropic glutamate receptors (NMDA subtype highlighted).
• Inhibitory LGICs: decrease action-potential frequency.
• GABA$_A$ receptor.
• Glycine receptor.

Quantitative Review Questions (lecture polling)

• “How many TM domains in a single LGIC complex?” $5\text{ subunits}\times4=20$.
• “Number of agonist molecules needed to open channel?” $2$ (both $\alpha$ sites).

Detailed Exemplars

Nicotinic Acetylcholine Receptor (nAChR)

• Locations: CNS synapses, neuromuscular junction (NMJ), autonomic ganglia.
• Stoichiometry differences:
• NMJ (adult): $\alpha<em>2\beta\delta\varepsilon$ (neonate uses $\gamma$ instead of $\varepsilon$). • Autonomic ganglia: $\alpha</em>2\beta<em>3$ (multiple $\alpha$ isoforms $\alpha</em>2–\alpha<em>{10}$, $\beta</em>2–\beta_4$).
• Pharmacological consequence: subtype selectivity → design of NMJ blockers with minimal autonomic side effects.

GABA$_A$ Receptor

• Pentamer: typically $\alpha<em>2\beta</em>2\gamma$.
• Agonist = GABA binds $\alpha$ sites → $Cl^-$ influx → hyperpolarization → CNS inhibition.
• Allosteric modulators:
• Barbiturates bind $\beta$ subunits (both if [drug] high).
• Benzodiazepines bind $\gamma$ subunit.
• Effect: left-shift GABA dose–response curve (↑ potency) ⇒ anxiolytic, anticonvulsant, sedative actions.

G-Protein-Coupled Receptors (GPCRs)

Essential Properties

• Also called metabotropic receptors (responses mediated via metabolic/enzymatic cascades).
• Speed: “relatively fast” – onset $\text{seconds}$ to $\text{minutes}$ (slower than LGICs but faster than genomic pathways).
• Ubiquitous; >$800$ distinct genes in human genome.
• Representative families to know:
• Muscarinic ACh receptors ($M<em>1–M</em>5$) – activated by muscarine.
• Biogenic amine receptors – $\beta/\alpha$-adrenergic, dopaminergic, serotonergic, histaminergic.
• GABA$B$ receptors (note contrast with GABA$A$).
• Opioid receptors, peptide hormone receptors (e.g., angiotensin, vasopressin, etc.).
• Physiological roles: sensory perception, autonomic regulation (heart, smooth muscle), glandular secretion, CNS signalling.

Structural Hallmarks

• Single polypeptide traverses membrane $7$ times → "heptahelical".
• Segments TM$3–6$ furnish ligand-binding pocket (illustrated by darker residues in schematic).
• Third intracellular loop couples to heterotrimeric G-protein.
• "G" = guanine nucleotide binding.

Heterotrimeric G-Proteins

• Composition: $\alpha$, $\beta$, $\gamma$ subunits (distinct from LGIC nomenclature).
• Resting state: $\alpha$ bound to GDP; $\beta\gamma$ dimer associated.
• Activation cycle:

1. Agonist → GPCR conformational shift.

2. GDP released; GTP binds $\alpha$.

3. $\alpha$(GTP) dissociates from $\beta\gamma$.

4. Both entities modulate effectors (enzymes, ion channels).

5. Intrinsic $\alpha$ GTPase (accelerated by RGS proteins) hydrolyses GTP → GDP.

6. Trimer reassembles; cycle repeats while agonist present.

$\alpha$-Subunit Classes & Canonical Effectors

$\beta\gamma$ Dimer Contributions

• Modulates certain $K^+$ & $Ca^{2+}$ channels.
• Can activate PI3-kinase (PI$3$K).

Second-Messenger Pathways

cAMP Cascade ($G<em>s$ vs $G</em>i$)

1. GPCR → $G_s$ activation → adenylate cyclase converts ATP → cAMP.

2. cAMP activates protein kinase A (PKA).

3. PKA phosphorylates substrate proteins → altered cellular activity.

4. $G_i$ does the inverse (inhibits enzyme → ↓ cAMP).

PLC / DAG / IP$<em>3$ Cascade ($G</em>q$)

1. $G_q$ activates phospholipase C (PLC).

2. PLC cleaves PIP$<em>2$ → DAG + IP$</em>3$.

3. DAG remains membrane-bound → activates protein kinase C (PKC).

4. IP$_3$ diffuses to ER → triggers $Ca^{2+}$ release.

5. PKC + elevated $Ca^{2+}$ → protein phosphorylation → cellular response.

Why Phosphorylation Works

• Phosphate group carries $3$ negative charges → drastic conformational & electrostatic alteration of proteins.
• Dynamic control via opposing enzyme classes:
• Kinases add $PO<em>4^{3-}$. • Phosphatases remove $PO</em>4^{3-}$.

Specificity Determinants (How >800 GPCRs yield distinct effects)

• Ligand selectivity (e.g., epinephrine vs norepinephrine affinity for $\beta$ vs $\alpha$ receptors).
• Tissue-specific receptor expression levels.
• Different G-protein coupling (same receptor family may couple to $G<em>s$ in one tissue, $G</em>i$ in another).
• Receptor regulation (desensitization, phosphorylation, internalization).
• GPCR dimerization – homo- or hetero-dimers create unique signalling profiles.

Practical Drug-Design Implications

• Allosteric modulators (e.g., benzodiazepines) fine-tune endogenous signalling rather than mimic or block it outright.

Metaphors & Illustrative Scenarios Emphasised by Lecturer

• Key–lock analogy (ligand fits receptor; turning key = conformational change).
• Classroom exit metaphor for ion flux: Massive $Na^+$ influx impedes $K^+$ exit much like crowded hallway impedes re-entry.
• Row-boat & wind analogy for positive allosteric modulation: Benzodiazepine “wind” helps GABA “rower.”

Ethical / Clinical Relevance Briefly Mentioned

• Selective neuromuscular blockers designed to spare autonomic ganglia reduce systemic side effects.
• CNS-depressant drugs exploiting GABA$_A$ modulation treat anxiety & seizures, but require caution (sedation, dependency).

Quick Reference: Numerical & Formula Highlights

• Pentameric LGIC total TM segments: $5\times4=20$.
• GPCR helix count: $7$.
• GTP → GDP + $P<em>i$ (hydrolysis step governed by intrinsic GTPase / RGS). • PLC reaction: $PIP</em>2 \rightarrow DAG + IP<em>3$. • Adenylate cyclase reaction: $ATP \rightarrow cAMP + PP</em>i$.

Anticipated Next Topics (teaser)

• Enzyme-linked receptors (e.g., receptor tyrosine kinases, guanylyl cyclase-linked).
• Nuclear receptors & transcriptional control.

ADDITIONAL

8. Why Phosphorylation Works

• Phosphate groups (PO₄³⁻) are negatively charged.

• When added to proteins, they change shape and function.

• Controlled by:

  • Kinases (add phosphate).

  • Phosphatases (remove phosphate).

9. How Can 800+ GPCRs Do Different Things?

• Each GPCR can be unique because of:

  • Which ligand (signal molecule) it binds.

  • Where it's found in the body.

  • Which G-protein it couples with.

  • How it’s regulated (can be turned off, pulled inside the cell).

  • Combining with other receptors (dimerization).

10. Why This Matters in Medicine (Drug Design)

• Designing drugs that target specific receptor subtypes can reduce side effects.

• Allosteric modulators (like benzodiazepines) help natural signals work better—don’t act like an on/off switch.

11. Helpful Analogies from Class

• Key and Lock → Ligand = key; GPCR = lock.

• Crowded Hallway → Na⁺ floods in; hard for K⁺ to get out.

• Rowboat and Wind → GABA is rowing; benzos are the wind helping it move better (positive allosteric modulator).

12. Real-World Relevance

• Drugs that block muscle movement without affecting the heart are safer.

• GABA_A modulators treat anxiety/seizures—but can cause drowsiness or dependence.

13. Quick Math & Formulas to Know

• LGIC: 5 subunits × 4 segments = 20 transmembrane segments.

• GPCR: 7 helices in membrane.

• GTP → GDP + Pi (hydrolysis step).

• PLC: PIP₂ → DAG + IP₃.

• Adenylate cyclase: ATP → cAMP + PPi.

Drug targets, primarily receptors, produce effects through a process initiated by ligand binding. According to the operational definition of a receptor, it is a macromolecular complex that binds an endogenous ligand (like a hormone or neurotransmitter) or a drug. This binding induces a conformational change in the receptor, which then triggers downstream signaling pathways that ultimately lead to a physiological response.

For example, Ligand-Gated Ion Channels (LGICs) undergo a conformational switch when a ligand binds. This causes the channel's pore to dilate, allowing ions to flow across the membrane, which can lead to depolarization (e.g., $Na^+$ influx in excitatory LGICs) or hyperpolarization (e.g., $Cl^-$ influx in inhibitory LGICs). This ion flux directly alters the cell's electrical potential, influencing processes like neurotransmission or muscle contraction.

G-Protein-Coupled Receptors (GPCRs), also known as metabotropic receptors, operate by coupling to heterotrimeric G-proteins. When an agonist binds to a GPCR, it causes the G-protein to exchange GDP for GTP and dissociate into $\alpha$ (GTP) and $\beta\gamma$ subunits. Both of these entities then modulate various effectors, such as enzymes (like adenylate cyclase or phospholipase C) or ion channels. This leads to the production of second messengers (e.g., cAMP, DAG, $IP_3$, $Ca^{2+}$) that activate protein kinases (PKA, PKC) which phosphorylate substrate proteins, thereby altering cellular activity. These responses are slower than LGICs but still relatively fast compared to genomic pathways.