Signal Transduction and G Protein-Coupled Receptors

Definition: The process by which cells respond to extracellular signalling molecules (hormones, neurotransmitters, growth factors) via specific receptors.
Receptor Locations:
- Intracellular: For lipid-soluble ligands like steroid and thyroid hormones.
- Cell-surface: For the majority of extracellular signals that cannot cross the plasma membrane.
Functional Goal: Ligand binding triggers "transduction" via intracellular components to produce responses like contraction, secretion, or differentiation.

Ligand-gated (receptor-operated) ion channels: Example: nicotinic acetylcholine receptors.
Intrinsic enzymatic activity: Example: receptor tyrosine kinases like the insulin receptor.
G protein-coupled (7TM) receptors (GPCRs): Example: muscarinic acetylcholine receptors.

Global Impact: Approximately $40\%$ of prescription drugs target GPCRs.
Drug Examples:
- Anti-asthma: SALBUTAMOL, SALMETEROL ( $\beta_2$ adrenoceptor agonists).
- Analgesia: MORPHINE, FENTANYL ( $\mu$ -opioid receptor agonists).
- Cardiovascular: PROPRANOLOL, ATENOLOL ( $\beta$ adrenoceptor antagonists for hypertension).
- Neuroleptics: HALOPERIDOL, SULPIRIDE ( $D_2$ dopamine receptor antagonists).
Pharmacology:
- Agonists: Bind and activate the receptor to trigger signal transduction.
- Antagonists: Bind but do not activate, blocking agonist effects.

Genomics: Over $800$ GPCRs identified in the human genome (> 2\% of genes).
Structure:
- Single polypeptide chain ( $300-1200$ amino acids).
- 7-transmembrane (7TM)-spanning regions.
- Extracellular N-terminal and intracellular C-terminal.
Ligand Binding Sites:
- Formed by 2-3 transmembrane (TM) domains for small molecules.
- Located at the N-terminal or other extracellular domains for others.
Stimuli Variety: GPCRs respond to ions ( $H^+$ , $Ca^{2+}$ ), neurotransmitters (Glutamate), peptide/non-peptide hormones (Glucagon, Adrenaline), glycoproteins (TSH), light (Rhodopsin), tastes, and odors.

Activation: Ligand binding causes a conformational (3D shape) change in the GPCR, facilitating interaction with a heterotrimeric G protein.
G Protein Structure: Composed of three subunits: $\alpha$ (alpha), $\beta$ (beta), and $\gamma$ (gamma).
The Signalling Cycle:
1. Exchange: Activated GPCR causes $GTP$ to exchange for $GDP$ on the $\alpha$ subunit.
2. Dissociation: The $\alpha$ - $GTP$ complex and free $\beta\gamma$ subunits separate to interact with intracellular effectors (enzymes or ion channels).
3. Termination: The intrinsic $GTPase$ activity of the $\alpha$ subunit hydrolyzes $GTP$ back to $GDP$ .
4. Reassociation: the inactivated $\alpha$ - $GDP$ and $\beta\gamma$ subunits reform the heterotrimeric complex.
Localization: All receptor-G protein-effector interactions occur at the plasma membrane.

Encoding: Human genome encodes $16$ $G\alpha$ , $5$ $G\beta$ , and $14$ $G\gamma$ proteins, allowing for > 1000 combinations.
Receptor Selection: Activated GPCRs preferentially interact with specific G proteins; the $\alpha$ subunit is the primary determinant.
Major G Protein Types:
- $G\alpha_s$ : Stimulates adenylyl cyclase (e.g., Adrenaline at $\beta$ -adrenoceptors).
- $G\alpha_i$ : Inhibits adenylyl cyclase (e.g., Adrenaline at $\alpha_2$ -adrenoceptors).
- $G\alpha_q$ : Stimulates phospholipase C (e.g., Adrenaline at $\alpha_1$ -adrenoceptors).

Cholera Toxin (CTx): Produced by Vibrio cholera. It prevents the termination of signalling in $G_s$ -preferring GPCRs by blocking $GTPase$ activity. Signaling remains chronically "ON," leading to severe diarrhea.
Pertussis Toxin (PTx): Produced by Bordetella pertussis. It "uncouples" $G_i$ -preferring GPCRs from signal transduction, keeping signalling chronically "OFF." It causes highly contagious whooping cough.