Paper I — 2020 February (Supplementary) (2010 Scheme) — Q3

Paper I

2020 February (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

G-protein coupled receptors

Q33 marksShort Notes★✓

Answer

G-protein coupled receptors (GPCRs), also called metabotropic receptors, are seven-transmembrane-domain receptors coupled to intracellular guanine nucleotide-binding proteins (G-proteins).

Mechanism

• Ligand binding causes a conformational change in the receptor, activating the associated G-protein (which exchanges GDP for GTP)
• The activated G-protein (Gs, Gi, or Gq, depending on receptor type) modulates effector enzymes:
  • Gs: activates adenylyl cyclase → increases cAMP
  • Gi: inhibits adenylyl cyclase → decreases cAMP
  • Gq: activates phospholipase C → generates IP3 and DAG (second messengers), releasing intracellular calcium
• Response occurs over seconds, slower than ligand-gated ion channels but faster than nuclear receptors

Examples

• β-adrenergic receptors (Gs), muscarinic M2 receptors (Gi), α1-adrenergic receptors (Gq)

Clinical significance

• Major drug target class — many important drugs act via GPCRs (beta-blockers, opioids, antihistamines)

Related Concepts

1. 1Pharmacodynamics

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