PTW - Cholinergic System III Flashcards
(15 cards)
Q: What structural insights were gained from M2 and M3 mAChR crystal structures? (3)
- Crystallised with antagonists QNB or Tiotropium
- Reveal how subtypes couple to different G-proteins, aiding subtype-selective drug design
- Help explain structural discrimination in ligand and G-protein binding
Q: What key structural features do M2 and M3 mAChRs share? (3)
- 7 transmembrane domains arranged in a 3+4 bundle
- An 8th amphipathic helix
- Large intracellular loops with poorly defined structure, important for G-protein coupling
Q: How does ligand binding differ between M2 and M3 receptors? (2)
- M3 has a larger binding site
- Ligands show similar spatial arrangement but interact differently due to subtle pocket variations
Q: What did Cryo-EM studies reveal about mAChR–G protein complexes? (3)
Q: What determines G-protein specificity in mAChRs? (2)
A:
- Interaction between C-terminal helix of G-protein and TM5 + TM6
- Conserved residues distinguish receptors that couple to Gi/Go vs Gq/G11
Q: What did molecular dynamics simulations reveal about mAChRs? (3)
- Identified an extracellular vestibule, previously thought to be only allosteric
- This region contributes to kinetic selectivity by raising the activation barrier
- Affects drug binding and dissociation rates in M3
Q: What key points summarise mAChR structure and function? (5)
- GPCR activated by ACh
- Ionic, cation-π, and hydrophobic interactions mediate agonist binding
- Binding pocket varies subtly between subtypes → allows subtype targeting
- Activation involves G-protein C-terminal helix, ICLs, and TM5/6
- Mutagenesis and SAR studies helped map ligand interaction sites
Q: What determines G-protein specificity in mAChRs? (2)
- Interaction between C-terminal helix of G-protein and TM5 + TM6
- Conserved residues distinguish receptors that couple to Gi/Go vs Gq/G11
Q: Compare nAChRs and mAChRs based on structure and signalling. (5)
nAChRs:
- Ligand-gated ion channels (LGICs)
- Cation-selective (Na⁺, K⁺, Ca²⁺)
- ACh binds via cation-π and sometimes H-bonding
- Enables fast signalling via direct ion flow
mAChRs:
- GPCRs
- ACh binds via ionic, hydrophobic, and H-bonding interactions
- Slow signalling through secondary messengers
- Coupled to Gi/Go or Gq/G11
Q: What are the selectivity profiles of β-adrenergic receptors? (2)
- β₁-AR: Isoprenaline > noradrenaline = adrenaline
- β₂-AR: Isoprenaline > adrenaline»_space; noradrenaline
Q: What are the main signalling pathways of β₁ and β₂ adrenergic receptors? (2)
- β₁-AR: ↑ cAMP, activates PKA, promotes muscle contraction
- β₂-AR: ↑ MAPK and cPLA₂, also leads to contraction, but via alternative pathways
Q: What are the α-adrenergic receptor subtypes, and how do they signal? (2)
- α₁-AR: Selective for noradrenaline; activates PLC → IP₃/DAG, ↑ [Ca²⁺]
- α₂-AR: Selective for adrenaline; inhibits adenylate cyclase, ↓ cAMP
Q: Why are β-adrenergic receptors (β-ARs) important in receptor research? (4)
- First GPCRs to be characterised via radioligand binding
- Avian β₁ and hamster β₂ receptors were cloned; human β-AR isolated later
- Structure solved via X-ray crystallography (2007)
- Serve as model systems for studying GPCR structure, signalling, and drug action
Q: What is the clinical relevance of β-adrenergic receptors? (2)
- GPCRs are targets for \~15% of all drugs
- β-AR agonists (e.g. asthma) and antagonists (e.g. arrhythmia) are widely used therapeutics
Q: What are key structural features of the β₂-adrenergic receptor? (3)
- Contains transmembrane domains, loops, and intracellular signalling sites
- Glycosylation sites important for folding/stability
- Disulfide bond stabilises extracellular structure
Q: What does the hydropathy plot of β₂-AR show? (2)
- Peaks = hydrophobic TM regions
- Troughs = extracellular/intracellular hydrophilic regions (Kyte-Doolittle scale)
