G-Protein Coupled Receptors Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for G-Protein Coupled Receptors. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
G-Protein Coupled Receptors Indian Medical PG Question 1: All these hormones primarily use cyclic adenosine monophosphate (cAMP) as their main second messenger pathway, except:
- A. Dopamine (Correct Answer)
- B. Glucagon
- C. vasopressin
- D. Corticotropin
G-Protein Coupled Receptors Explanation: ***Dopamine***
- **Dopamine** has dual signaling mechanisms depending on receptor subtype, making it unique among the listed hormones.
- **D1-like receptors** (D1, D5) couple to Gs proteins and **increase cAMP** levels.
- **D2-like receptors** (D2, D3, D4) couple to Gi proteins and **decrease/inhibit cAMP** production.
- Since dopamine's effects are mediated through both cAMP-increasing and cAMP-decreasing pathways with significant physiological roles for both, it does **not primarily use cAMP** as a straightforward second messenger like the other hormones listed.
- Therefore, dopamine is the exception as it has mixed cAMP signaling rather than primarily activating the cAMP pathway.
*Corticotropin (ACTH)*
- **Corticotropin** (ACTH) binds to melanocortin-2 receptors (MC2R) on the adrenal cortex and **primarily utilizes the cAMP pathway**.
- Activation of adenylyl cyclase leads to increased intracellular cAMP, which activates protein kinase A (PKA).
- This stimulates the synthesis and release of glucocorticoids (primarily cortisol).
*Glucagon*
- **Glucagon** binds to its G-protein coupled receptors on hepatocytes, leading to activation of adenylyl cyclase and increased intracellular **cAMP**.
- The cAMP then activates protein kinase A, mediating glucagon's metabolic effects including **glycogenolysis and gluconeogenesis**.
- This is a classic example of cAMP-mediated hormone action.
*Vasopressin*
- **Vasopressin** (ADH) primarily acts through **V2 receptors** in the renal collecting ducts, which use the **cAMP pathway** to increase water reabsorption (its primary physiological function).
- V1 receptors (vasoconstriction) use the IP3/DAG pathway, but this is a secondary effect.
- Since vasopressin's main clinical action is via cAMP-mediated V2 receptors, it primarily uses cAMP as its second messenger.
G-Protein Coupled Receptors Indian Medical PG Question 2: G-protein coupled receptor that does not act through opening of potassium channels is:
- A. Dopamine D2 receptor
- B. Muscarinic M2 receptor
- C. Serotonin 5 HT 1 receptor
- D. Angiotensin 1 receptor (Correct Answer)
G-Protein Coupled Receptors Explanation: ***Angiotensin 1 receptor***
- The **angiotensin 1 receptor (AT1R)** is a **Gq-coupled receptor** that primarily activates the **phospholipase C (PLC)** pathway, leading to increased intracellular **calcium** and **IP3/DAG** signaling.
- Its activation mediates vasoconstriction, aldosterone release, and cardiac hypertrophy, none of which involve direct opening of potassium channels.
*Dopamine D2 receptor*
- **Dopamine D2 receptors** are **Gi/o-coupled receptors** that inhibit adenylyl cyclase and **open potassium channels**, leading to **hyperpolarization** and reduced neuronal excitability.
- This action contributes to its **antipsychotic** and **motor control** effects.
*Muscarinic M2 receptor*
- **Muscarinic M2 receptors** are **Gi/o-coupled receptors** found in the heart that cause **bradycardia** by activating **acetylcholine-gated inwardly rectifying potassium (GIRK) channels**, leading to hyperpolarization.
- They also inhibit adenylyl cyclase, reducing cAMP levels and decreasing heart rate and contractility.
*Serotonin 5 HT 1 receptor*
- **Serotonin 5-HT1 receptors** (e.g., 5-HT1A) are **Gi/o-coupled receptors** that, upon activation, **increase potassium conductance** (hyperpolarization) and inhibit adenylyl cyclase.
- This leads to a reduction in neuronal firing and is implicated in the anxiolytic and antidepressant effects of these receptors.
G-Protein Coupled Receptors Indian Medical PG Question 3: Which of the following stimulate adenylate cyclase with G-protein coupled action ?
- A. Shiga toxin
- B. Cholera toxin (Correct Answer)
- C. Diphtheria toxin
- D. Pseudomonas toxin
G-Protein Coupled Receptors Explanation: ***Cholera toxin***
- Cholera toxin is a **G-protein-activating toxin** that irreversibly activates **adenylate cyclase**.
- This leads to increased intracellular levels of **cAMP**, causing excessive fluid secretion into the intestinal lumen and severe diarrhea.
*Shiga toxin*
- Shiga toxin acts by inactivating the **60S ribosomal subunit**, thereby inhibiting protein synthesis in eukaryotic cells.
- Its primary effect is **cytotoxicity**, not direct stimulation of adenylate cyclase.
*Diphtheria toxin*
- Diphtheria toxin inhibits **protein synthesis** by inactivating **elongation factor-2 (EF-2)** through ADP-ribosylation.
- This toxin specifically targets host cells, leading to cellular death and tissue damage.
*Pseudomonas toxin*
- **Exotoxin A** produced by *Pseudomonas aeruginosa* also inhibits **protein synthesis** by ADP-ribosylating and inactivating **EF-2**, similar to diphtheria toxin.
- It does not directly affect adenylate cyclase activity.
G-Protein Coupled Receptors Indian Medical PG Question 4: Which of the following statements about G protein-coupled receptors (GPCRs) is correct?
- A. The alpha subunit of G proteins determines whether they are stimulatory or inhibitory. (Correct Answer)
- B. G proteins bind hormones directly before transmitting signals to receptors.
- C. G proteins are active when bound to GDP and inactive when bound to GTP.
- D. G proteins require the beta and gamma subunits to remain bound to the alpha subunit to transmit signals.
G-Protein Coupled Receptors Explanation: ***The alpha subunit of G proteins determines whether they are stimulatory or inhibitory.***
- The **alpha subunit** of a G protein determines its specific function, such as activating or inhibiting downstream enzymes like adenylyl cyclase, thereby classifying the G protein as Gs (stimulatory), Gi (inhibitory), or Gq.
- This specificity arises from the **alpha subunit's unique binding sites** for downstream effectors and its intrinsic GTPase activity, which regulates its activation state.
*G proteins require the beta and gamma subunits to remain bound to the alpha subunit to transmit signals.*
- This is **incorrect**. Upon activation, the **alpha subunit dissociates from the beta-gamma dimer**, and both components function **independently** to modulate downstream effectors.
- The **alpha subunit** regulates enzymes like adenylyl cyclase or phospholipase C, while the **beta-gamma complex** can independently modulate ion channels and other effector proteins.
*G proteins bind hormones directly before transmitting signals to receptors.*
- **G protein-coupled receptors (GPCRs)** are responsible for binding hormones (ligands) directly, which then causes a **conformational change in the receptor**.
- This conformational change is what then activates the associated G protein, which subsequently transmits the signal to intracellular effectors.
*G proteins are active when bound to GDP and inactive when bound to GTP.*
- **G proteins are generally active when bound to GTP** and inactive when bound to GDP.
- Upon activation, the G protein exchanges GDP for GTP, which leads to its conformational change and dissociation into active alpha and beta-gamma subunits.
G-Protein Coupled Receptors Indian Medical PG Question 5: Beta 2 receptors act via which of the following secondary messenger systems
- A. Adenylate Cyclase (Correct Answer)
- B. Phospholipase C
- C. Guanylate Cyclase
- D. Direct ion channel activation
- E. Tyrosine Kinase
G-Protein Coupled Receptors Explanation: ***Adenylate Cyclase***
- **Beta-2 adrenergic receptors** are G-protein coupled receptors that primarily activate the **Gs protein**.
- Activation of Gs protein leads to the stimulation of **adenylate cyclase**, which converts ATP to **cAMP**, a crucial secondary messenger for various cellular responses.
*Phospholipase C*
- **Phospholipase C** is typically activated by **Gq protein-coupled receptors**, such as alpha-1 adrenergic receptors or M1/M3 muscarinic receptors.
- Its activation leads to the production of **IP3** and **DAG**, which then trigger intracellular calcium release and protein kinase C activation, respectively.
*Guanylate Cyclase*
- **Guanylate cyclase** produces **cGMP** as a secondary messenger and is primarily associated with **nitric oxide signaling** (soluble guanylate cyclase) or **natriuretic peptide receptors** (particulate guanylate cyclase).
- This system is distinct from the adrenergic receptor pathways.
*Direct ion channel activation*
- **Direct ion channel activation** occurs in **ligand-gated ion channels**, where the binding of a neurotransmitter directly opens an ion pore without the involvement of G-proteins or secondary messengers.
- Examples include nicotinic acetylcholine receptors and GABA-A receptors, which are functionally different from the G-protein coupled **beta-2 receptors**.
*Tyrosine Kinase*
- **Tyrosine kinase** signaling is characteristic of **receptor tyrosine kinases (RTKs)**, such as insulin receptors and growth factor receptors (e.g., EGF, PDGF receptors).
- These receptors undergo autophosphorylation and initiate signaling cascades independent of G-proteins, making them distinct from **beta-2 adrenergic receptors**.
G-Protein Coupled Receptors Indian Medical PG Question 6: Which of the following statements about G protein-coupled receptors (GPCRs) is true?
- A. The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.
- B. G proteins can act as either inhibitory or excitatory based on the type of alpha subunit. (Correct Answer)
- C. G proteins bind directly to hormones to become activated.
- D. In the resting state, G proteins are bound to GTP.
G-Protein Coupled Receptors Explanation: ***G proteins can act as either inhibitory or excitatory based on the type of alpha subunit.***
- Different classes of Gα subunits (e.g., **Gαs**, **Gαi**, **Gαq**) couple to diverse downstream effectors, leading to either **stimulation** (excitatory) or **inhibition** of cellular processes.
- For example, **Gαs** activates adenylyl cyclase, while **Gαi** inhibits it, demonstrating their opposing roles.
*The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.*
- Upon activation, the **Gα subunit dissociates** from the **Gβγ dimer**, and both free units can then independently modulate effector molecules.
- For the G protein to function in signal transduction, the α subunit often separates from the βγ dimer to interact with its target enzyme or ion channel.
*G proteins bind directly to hormones to become activated.*
- **GPCRs** (the receptors themselves) bind to hormones or other ligands on the **extracellular side** of the membrane.
- The binding of the ligand to the GPCR induces a conformational change in the receptor, which then activates the associated G protein on the intracellular side.
*In the resting state, G proteins are bound to GTP.*
- In the **resting (inactive) state**, the Gα subunit of the trimeric G protein is bound to **GDP**.
- Activation occurs when the GPCR facilitates the exchange of **GDP for GTP** on the Gα subunit.
G-Protein Coupled Receptors Indian Medical PG Question 7: All are true regarding G protein Except?
- A. It has a trimeric structure
- B. Alpha subunit has GTPase activity
- C. It is activated by GTP binding
- D. Active conformation has all three subunits (Correct Answer)
G-Protein Coupled Receptors Explanation: ***Active conformation has all three subunits***
- The **active conformation** of a G protein typically involves the dissociation of the **alpha subunit** from the **beta-gamma dimer** upon GTP binding.
- The alpha subunit, now bound to GTP, and the beta-gamma dimer then activate downstream effector proteins independently; thus, the fully trimeric structure is not the active conformation.
*It has a trimeric structure*
- In its **inactive state**, a G protein is indeed a **heterotrimer** composed of alpha (α), beta (β), and gamma (γ) subunits, with GDP bound to the alpha subunit.
- This **trimeric structure** is a defining characteristic of G proteins before activation by a G protein-coupled receptor.
*Alpha subunit has GTPase activity*
- The alpha subunit of a G protein possesses intrinsic **GTPase activity**, which is crucial for terminating signaling.
- This activity allows it to **hydrolyze bound GTP back to GDP**, returning the G protein to its inactive state.
*It is activated by GTP binding*
- G proteins are activated when a **guanosine diphosphate (GDP)** molecule bound to the alpha subunit is exchanged for a **guanosine triphosphate (GTP)** molecule.
- This **GTP binding** is triggered by the activation of a G protein-coupled receptor (GPCR) by its ligand, leading to conformational changes that facilitate nucleotide exchange.
G-Protein Coupled Receptors Indian Medical PG Question 8: Which of the following act through G protein coupled receptors?
- A. Ach Muscarinic receptors (Correct Answer)
- B. Insulin receptors
- C. Ach Nicotinic receptors
- D. GABA-A receptors
G-Protein Coupled Receptors Explanation: ***Ach Muscarinic receptors***
- All five **muscarinic acetylcholine receptors (M1-M5)** are **G protein-coupled receptors (GPCRs)** that mediate the parasympathetic nervous system's effects.
- Activation of these receptors leads to downstream signaling through various G proteins, influencing cellular functions like heart rate and smooth muscle contraction.
*Insulin receptors*
- Insulin receptors are **receptor tyrosine kinases (RTKs)**, not GPCRs.
- Upon insulin binding, they undergo autophosphorylation and activate intracellular signaling cascades involving **IRS proteins**, leading to glucose uptake.
*Ach Nicotinic receptors*
- Nicotinic acetylcholine receptors are **ligand-gated ion channels**, not GPCRs.
- They open an ion pore in response to acetylcholine binding, allowing ions like sodium to flow through, resulting in rapid depolarization.
*GABA-A receptors*
- GABA-A receptors are also **ligand-gated ion channels**, specifically anion channels that are permeable to chloride ions.
- When GABA binds, they open, allowing chloride influx, which typically hyperpolarizes the neuron and inhibits neural activity.
G-Protein Coupled Receptors Indian Medical PG Question 9: In response to changes in Ca2+ concentration, which of the following Ca2+ binding proteins can modify the activity of many enzymes & proteins?
- A. Collagen
- B. Calmodulin (Correct Answer)
- C. Kinesin
- D. Elastin
G-Protein Coupled Receptors Explanation: ***Calmodulin***
- **Calmodulin** is a highly conserved, 148-amino acid protein with four **calcium-binding EF-hand motifs**.
- Upon binding to **calcium ions (Ca2+)**, it undergoes a conformational change that enables it to interact with and regulate the activity of a wide variety of enzymes and proteins, including **kinases, phosphatases, and ion channels**, mediating many Ca2+-dependent cellular processes.
*Collagen*
- **Collagen** is a major structural protein in the extracellular matrix, providing **tensile strength** to tissues.
- Its primary function is structural support, rather than acting as a calcium-sensing regulatory protein for enzyme activity.
*Kinesin*
- **Kinesin** is a **motor protein** involved in intracellular transport, moving cargo along microtubules.
- While its activity can be modulated, it is not primarily known as a calcium-binding protein that directly regulates a broad range of enzymes in response to calcium concentration changes.
*Elastin*
- **Elastin** is a highly elastic protein found in connective tissue, allowing tissues to **recoil after stretching**.
- Like collagen, its main role is structural, contributing to the elasticity of tissues, rather than signaling or enzyme regulation via calcium binding.
G-Protein Coupled Receptors Indian Medical PG Question 10: Agent that acts through tyrosine kinase receptor is
- A. Insulin (Correct Answer)
- B. MSH
- C. TSH
- D. TRH
G-Protein Coupled Receptors Explanation: ***Insulin***
- **Insulin** binds to its receptor, which is a **tyrosine kinase receptor**, leading to autophosphorylation and the activation of intracellular signaling pathways.
- This activation is crucial for glucose uptake and metabolism by various cells in the body.
*MSH*
- **Melanocyte-stimulating hormone (MSH)** acts primarily through **G protein-coupled receptors**, specifically melanocortin receptors.
- These receptors activate adenylyl cyclase, leading to an increase in intracellular cAMP.
*TSH*
- **Thyroid-stimulating hormone (TSH)** also acts via a **G protein-coupled receptor** on thyroid follicular cells.
- Its binding stimulates adenylyl cyclase, increasing cAMP and thus thyroid hormone synthesis and release.
*TRH*
- **Thyrotropin-releasing hormone (TRH)** binds to **G protein-coupled receptors** on pituitary thyrotrophs.
- This interaction activates the phospholipase C pathway, leading to the release of TSH.
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