Cell Surface Receptors: Types and Functions

Overview - Receptor Rendezvous

• Definition: Proteins, usually on cell surface, binding specific ligands (e.g., hormones, neurotransmitters).
• Location: Embedded in plasma membrane.
• General Mechanism:
  • Ligand binding (extracellular).
  • Receptor conformational change.
  • Intracellular signal cascade activation.
• Key Properties:
  • Specificity: High affinity for particular ligands.
  • Saturation: Finite receptors; response can plateau.
  • Signal Amplification: Single ligand binding activates multiple downstream effectors, magnifying the signal.

⭐ Cell surface receptors are crucial for hydrophilic signaling molecules that cannot cross the plasma membrane.

GPCRs - G-Force Masters

Largest receptor family; crucial drug targets. Structure: 7 transmembrane (7-TM) α-helices. Ligand binding activates heterotrimeric G-proteins (α, β, γ subunits).

• G-Protein Subtypes & Signaling: 📌 Mnemonic: "QISS" (Gq → PLC; Gi → AC ↓; Gs → AC ↑)
  • Gs (Stimulatory): Activates Adenylyl Cyclase (AC) → ↑cAMP. Effectors: AC. 2nd Messengers: cAMP. Examples: Glucagon, β-Adrenergic receptors.
  • Gi (Inhibitory): Inhibits Adenylyl Cyclase (AC) → ↓cAMP. Effectors: AC. 2nd Messengers: cAMP. Examples: Opioid, α2-Adrenergic receptors.
  • Gq: Activates Phospholipase C (PLC) → IP3 & DAG. Effectors: PLC. 2nd Messengers: IP3, DAG, Ca²⁺. Examples: α1-Adrenergic receptors.

⭐ Cholera toxin constitutively activates Gsα by ADP-ribosylation, leading to persistent cAMP production.

Enzyme-Linked Receptors - Catalytic Crew

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Ion Channel Receptors - Gate Keepers

• A.k.a.: Ligand-gated ion channels, ionotropic receptors.
• Function: Convert chemical signals directly into electrical signals. Crucial for rapid synaptic transmission in nerve and muscle cells.
• Structure: Multisubunit transmembrane proteins forming a central, ion-selective pore.
• Mechanism: Ligand binding (e.g., neurotransmitter) → conformational change → channel opens → specific ions (e.g., $Na⁺$, $K⁺$, $Ca²⁺$, $Cl⁻$) flow down electrochemical gradient → alters membrane potential.
• Key Examples:
  • Nicotinic ACh Receptor: $Na⁺$/$K⁺$ channel; neuromuscular junction, autonomic ganglia.
  • GABA-A Receptor: $Cl⁻$ channel; major inhibitory receptor in CNS.
  • NMDA Receptor (Glutamate): $Ca²⁺$ channel; vital for learning, memory.

⭐ Myasthenia gravis is caused by antibodies against nicotinic acetylcholine receptors at the neuromuscular junction, leading to muscle weakness.

High‑Yield Points - ⚡ Biggest Takeaways

• GPCRs: Largest family, 7 transmembrane domains, activate G proteins (α, β, γ).
• RTKs: Dimerize, autophosphorylate tyrosine; e.g., Insulin receptor, EGFR.
• Ligand-gated ion channels: Rapid ion flux; e.g., Nicotinic AChR, GABA-A receptor.
• Cytokine receptors: Use JAK-STAT pathway via associated tyrosine kinases.
• Second messengers like cAMP, IP3, DAG, Ca²⁺ amplify intracellular signals.
• Gs ↑adenylyl cyclase; Gi ↓adenylyl cyclase; Gq ↑phospholipase C.