All of the following statements regarding G-protein coupled receptors (GPCRs) are correct EXCEPT:

Explanation

## Classification of Receptors and GPCR Mechanism **Key Point:** GPCRs are metabotropic (not ionotropic) receptors — they signal through intracellular second messengers and G proteins, not by directly gating ion channels. ### Correct Statements About GPCRs | Feature | Details | | --- | --- | | **Structure** | 7 transmembrane α-helical domains; extracellular N-terminus; intracellular C-terminus | | **G-protein coupling** | Heterotrimeric G proteins (Gα, Gβ, Gγ); ligand binding causes GDP→GTP exchange on Gα | | **Downstream signaling** | Both Gα-GTP and free Gβγ dimers activate effectors (adenylyl cyclase, phospholipase C, ion channels) | | **Desensitization** | GRK phosphorylates activated GPCR → β-arrestin binding → uncoupling from G protein → internalization | ### Why Option 4 Is Wrong **High-Yield:** Ionotropic receptors (ligand-gated ion channels) directly gate ions upon ligand binding — examples: nicotinic acetylcholine receptor, GABA~A~, NMDA. GPCRs are **metabotropic** and work indirectly via second messengers. **Clinical Pearl:** This distinction is critical in pharmacology: agonists at ionotropic receptors produce fast, direct effects (milliseconds), while GPCR agonists produce slower, amplified responses (seconds to minutes) via signal cascades. ### Correct Options Explained - **Option 1:** Seven transmembrane topology is the hallmark of GPCRs; all couple to heterotrimeric G proteins. ✓ - **Option 2:** Gα-GTP activates some effectors (e.g., adenylyl cyclase); Gβγ activates others (e.g., phospholipase C, K^+^ channels). ✓ - **Option 3:** GRK-mediated phosphorylation and β-arrestin recruitment are the primary desensitization mechanism; leads to receptor internalization and downregulation. ✓ **Mnemonic:** **GPCR = G-Protein Coupled, Metabotropic, Receptor** — not ionotropic.