G-Protein–Coupled Receptors (GPCRs)

GPCRs are membrane receptors that transmit signals via G-proteins, regulating many physiological processes.

Structure of GPCRs:

• Single polypeptide chain with 7 transmembrane α-helices
• Coupled to G-proteins (GTP-binding proteins) inside the cell

Mechanism of GPCRs:

1. Agonist binds to receptor (outside of cell)
2. Receptor undergoes conformational change
3. Activates G-protein by exchanging GDP for GTP
4. G-protein dissociates into α and βγ subunits
5. These subunits regulate target proteins such as:
   • Adenylyl cyclase → ↑ or ↓ cAMP
   • Phospholipase C → produces IP₃ and DAG
   • Ion channels

Second Messengers:

• cAMP → activates protein kinase A (PKA)
• IP₃ → increases intracellular Ca²⁺
• DAG → activates protein kinase C (PKC)

Response Time:

• Seconds to minutes

Examples:

• β-adrenergic receptors (↑ cAMP)
• Muscarinic M2 receptor (↓ cAMP)
• Dopamine receptors
• Histamine H1 receptor

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