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

Question

Accepted Answer

B. GPCRs are ionotropic receptors that directly gate ion channels upon ligand binding. ## 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.

Answer

A. Activation of GPCRs leads to dissociation of Gα-GTP from Gβγ subunits, both of which can modulate downstream effectors

Answer

C. GPCRs contain seven transmembrane domains and couple to heterotrimeric G proteins

Answer

D. GPCRs undergo rapid desensitization through phosphorylation by G-protein receptor kinases (GRKs) followed by β-arrestin binding

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

Explanation

Classification of Receptors and GPCR Mechanism

Correct Statements About GPCRs

Why Option 4 Is Wrong

Correct Options Explained

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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