36. Which one of the following is not a G-protein coupled receptor?
(1) Epinephrine receptor
(2) Transferrin receptor
(3) Glucagon receptor
(4) Thyroid stimulating hormone receptor

Introduction:

G-protein coupled receptors (GPCRs) are a vast and diverse family of cell surface receptors that play essential roles in transmitting signals from the external environment to the interior of the cell. They are involved in numerous physiological processes and are major targets for therapeutic drugs. Despite their importance, not all cell surface receptors belong to the GPCR family.

This article aims to clarify the distinguishing features of GPCRs and identify which receptor, among the options provided, is not a GPCR.

What Are G-Protein Coupled Receptors?

• Structural Characteristics: GPCRs share a common structural architecture comprising seven transmembrane α-helices. These seven transmembrane domains form a ligand-binding pocket and are accessible to external ligands.

• Activation Mechanism: Ligand binding induces a conformational change in the receptor, leading to activation of the associated G protein.

• G-Protein Coupling: GPCRs primarily couple to heterotrimeric G proteins composed of α, β, and γ subunits. Upon activation, the Gα subunit exchanges GDP for GTP and modulates downstream signaling pathways.

• Physiological Roles: GPCRs detect a wide range of molecules, including hormones, neurotransmitters, odorants, and photons, and activate cellular responses like cAMP production, Ca2+ mobilization, and kinase cascades.

Common GPCR Families

• Class A (Rhodopsin-like): The largest group, includes receptors for hormones and neurotransmitters.

• Class B (Secretin family): Include receptors for peptide hormones like secretin and glucagon.

• Class C (Metabotropic glutamate): Characterized by large extracellular domains.

• Others: D (Fungal pheromone receptors), E (cAMP receptors), F (Frizzled/Smoothened).

The Receptors Mentioned in the Question

1. Epinephrine receptor: A classic GPCR (β-adrenergic receptor). Correctly a GPCR.

2. Transferrin receptor: Incidentally, this is a not GPCR. It is a type I transmembrane protein but belongs to the receptor superfamily known as receptor tyrosine kinases (RTKs), which signal through phosphorylation cascades, not G proteins.

3. Glucagon receptor: A GPCR (Class B), involved in glucose metabolism.

4. Thyroid stimulating hormone (TSH) receptor: Also a GPCR (Class A), involved in thyroid hormone synthesis.

Conclusion

The transferrin receptor is not a GPCR. It belongs to the receptor tyrosine kinase family, which transduces signals via phosphorylation rather than G-protein-mediated mechanisms.

Correct answer: (2) Transferrin receptor.