G-Protein Coupled Receptors — MCQs
All these hormones primarily use cyclic adenosine monophosphate (cAMP) as their main second messenger pathway, except:
G-protein coupled receptor that does not act through opening of potassium channels is:
Which of the following stimulate adenylate cyclase with G-protein coupled action ?
Which of the following statements about G protein-coupled receptors (GPCRs) is correct?
Beta 2 receptors act via which of the following secondary messenger systems
Which of the following statements about G protein-coupled receptors (GPCRs) is true?
All are true regarding G protein Except?
Which of the following act through G protein coupled receptors?
In response to changes in Ca2+ concentration, which of the following Ca2+ binding proteins can modify the activity of many enzymes & proteins?
Agent that acts through tyrosine kinase receptor is
G-Protein Coupled Receptors Indian Medical PG Practice Questions and MCQs
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
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.
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)
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.
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
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.
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.
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.
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
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**.
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.
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.
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)
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.
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
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.
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
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.
Question 10: Agent that acts through tyrosine kinase receptor is
- A. Insulin (Correct Answer)
- B. MSH
- C. TSH
- D. TRH
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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