63. Which one of the following is not a type of G-protein coupled receptors?
1. 𝐺𝑞
2. 𝐺𝑖
3. 𝐺𝑠
4. 𝐺p

Understanding G-Protein Coupled Receptors (GPCRs): Types and Functions

G-protein coupled receptors (GPCRs) are a large family of membrane proteins that play a crucial role in signal transduction. They mediate various physiological responses by interacting with intracellular G-proteins and activating downstream signaling pathways.

Correct Answer: Gp (Option 4) is NOT a Recognized GPCR Type

The commonly known G-proteins associated with GPCR signaling are Gq, Gi, and Gs. The term Gp does not correspond to any established class of G-proteins.

Types of G-Protein Coupled Receptors

GPCRs are classified based on their interaction with different types of G-proteins, which regulate distinct cellular pathways:

1. Gq-Coupled Receptors

• Activates phospholipase C (PLC), leading to the production of inositol triphosphate (IP3) and diacylglycerol (DAG).
• This signaling pathway increases intracellular calcium levels, activating various cellular functions.
• Example receptors: Muscarinic M1, M3, and M5 receptors, α1-adrenergic receptors.

2. Gi-Coupled Receptors

• Inhibits adenylyl cyclase, reducing cyclic AMP (cAMP) levels inside the cell.
• Leads to decreased protein kinase A (PKA) activation.
• Example receptors: Muscarinic M2 and M4 receptors, α2-adrenergic receptors, opioid receptors.

3. Gs-Coupled Receptors

• Stimulates adenylyl cyclase, increasing intracellular cAMP levels.
• Enhances protein kinase A (PKA) activity, which regulates various cellular processes.
• Example receptors: β-adrenergic receptors, dopamine D1 receptors, histamine H2 receptors.

4. Gp: Not a Recognized GPCR Type

• No established signaling pathway exists for a Gp protein.
• This option is incorrect in the given question.

Functions of GPCRs in the Human Body

GPCRs are involved in various physiological and pathological processes, including:

1. Neurotransmission

• GPCRs mediate the effects of neurotransmitters like dopamine, serotonin, and acetylcholine.
• Important for mood regulation, learning, and memory.

2. Hormonal Regulation

• GPCRs regulate endocrine signaling, including responses to adrenaline, glucagon, and thyroid hormones.
• Essential for metabolic control and homeostasis.

3. Sensory Perception

• Involved in vision (rhodopsin), smell (olfactory receptors), and taste perception.

4. Cardiovascular and Immune Function

• Regulate heart rate, blood pressure, and immune cell signaling.

Clinical Significance of GPCRs

• Drug Targets: Nearly 40% of modern pharmaceuticals target GPCRs.
• Diseases Associated: GPCR dysfunction is linked to diseases like hypertension, diabetes, depression, and cancer.
• Therapeutic Applications: Beta-blockers, antihistamines, and opioid analgesics act through GPCR pathways.

Conclusion

The correct answer to the question is Gp (Option 4), as it is not a recognized type of G-protein coupled receptor. Understanding GPCR classification and signaling pathways is essential in physiology and drug development, given their pivotal role in cellular communication and disease treatment.