Second Messengers in Signal Transduction — MCQs

10 questions

Which one of the following acts as second messenger?

All the following mediate their action using cAMP as second messenger except:

Many signaling pathways involve the generation of inositol trisphosphate (IP3) and diacylglycerol (DAG). These molecules are involved in the regulation of which cellular processes?

In the mitogen activated protein kinase pathway, the activation of RAS is counteracted by

Which of the following stimulate adenylate cyclase with G-protein coupled action ?

A primigravida at 38 weeks of gestation has gone into labor. Oxytocin was started to augment labor. The second messenger system through which oxytocin acts is:

Beta 2 receptors act via which of the following secondary messenger systems

Phospholipid associated with the mechanism of hormone action is

Which of the following binds to Tyrosine Kinase receptor?

Q10

Which of the following statements about G protein-coupled receptors (GPCRs) is true?

Second Messengers in Signal Transduction Indian Medical PG Practice Questions and MCQs

Question 1: Which one of the following acts as second messenger?

A. Cl-
B. Ca++ (Correct Answer)
C. PO4³⁻
D. Mg++

Explanation: ***Ca++*** - **Calcium ions** act as a ubiquitous **second messenger** in various cellular processes, including **muscle contraction**, neurotransmission, and gene expression. - Their concentration in the cytoplasm is tightly regulated, and transient increases trigger specific cellular responses. *Cl-* - **Chloride ions** primarily contribute to maintaining cell **membrane potential** and regulating cell volume. - While important for cell function, they generally do not act as an intracellular second messenger to signal downstream events. *PO4³⁻* - **Phosphate ions** are essential components of **ATP**, DNA, and RNA, and play roles in **phosphorylation events** that regulate protein activity. - However, they do not function as a direct second messenger in the same way that calcium or cyclic nucleotides do. *Mg++* - **Magnesium ions** are critical cofactors for many enzymes and play a role in **ATP hydrolysis** and nucleic acid stability. - They are involved in many cellular processes but are not typically classified as a second messenger that transmits signals from cell surface receptors to intracellular effectors.

Question 2: All the following mediate their action using cAMP as second messenger except:

A. Glucagon
B. Dopamine
C. Corticotropin
D. Vasopressin (Correct Answer)

Explanation: ***Vasopressin (ADH)*** - Vasopressin has **dual signaling mechanisms** depending on receptor type: - **V2 receptors** (kidney collecting duct): Use **Gs-protein → cAMP pathway** for water reabsorption via aquaporin-2 insertion - **V1 receptors** (vascular smooth muscle): Use **Gq-protein → IP3/DAG pathway** for vasoconstriction - In the context of this question, vasopressin is considered the exception because it has **significant non-cAMP mediated actions** through V1 receptors, unlike the other hormones listed which **predominantly or exclusively** use cAMP - **Note**: This is a teaching point about receptor subtypes; vasopressin DOES use cAMP at V2 receptors *Glucagon* - **Exclusively uses cAMP pathway** in hepatocytes and adipocytes - Binds to **glucagon receptor** (GPCR) → **Gs-protein** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Promotes glycogenolysis, gluconeogenesis, and lipolysis *Dopamine* - **D1 and D5 receptors** are **Gs-coupled** → **stimulate adenylyl cyclase** → **increase cAMP** - Important for neurotransmission (motor control, reward) and renal vasodilation - D2-family receptors (D2, D3, D4) inhibit cAMP but D1-family predominates in many physiological contexts *Corticotropin (ACTH)* - Binds to **melanocortin-2 receptor (MC2R)** on adrenal cortex - **Gs-protein coupled** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Stimulates steroidogenesis and cortisol secretion - **Exclusively cAMP-dependent mechanism**

Question 3: Many signaling pathways involve the generation of inositol trisphosphate (IP3) and diacylglycerol (DAG). These molecules are involved in the regulation of which cellular processes?

A. DNA replication and cell division
B. Protein synthesis and degradation
C. Lipid metabolism and fatty acid synthesis
D. Calcium signaling and protein kinase C activation (Correct Answer)

Explanation: ***Calcium signaling and protein kinase C activation*** - **Inositol trisphosphate (IP3)** binds to **IP3 receptors** on the **endoplasmic reticulum**, triggering the release of **calcium ions (Ca²⁺)** into the cytoplasm, thereby initiating **intracellular calcium signaling cascades**. - **Diacylglycerol (DAG)** remains in the **plasma membrane** and activates **protein kinase C (PKC)**, which phosphorylates various target proteins to regulate cellular responses. - Together, IP3 and DAG form a classic **second messenger system** downstream of **phospholipase C (PLC)** activation, mediating responses from **G-protein coupled receptors (GPCRs)** and **receptor tyrosine kinases (RTKs)**. - This is the **primary and direct function** of the IP3/DAG signaling pathway in cell biology. *Lipid metabolism and fatty acid synthesis* - While DAG is structurally a **lipid intermediate** and can serve as a precursor in lipid metabolism, this is **not its primary role in signaling pathways**. - The question specifically asks about their involvement in **signaling pathways**, where IP3 and DAG function as **second messengers**, not metabolic intermediates. - Their generation from **PIP2 cleavage** by phospholipase C is a **signaling event**, not a metabolic one. *DNA replication and cell division* - Cell division is regulated by **cyclins**, **cyclin-dependent kinases (CDKs)**, and **checkpoint proteins**. - While IP3/DAG signaling can have **downstream effects** on cell proliferation through PKC activation, this is not their primary or direct role. - DNA replication machinery is not directly regulated by IP3 or DAG. *Protein synthesis and degradation* - **Protein synthesis** is primarily regulated by the **mTOR pathway**, **eIF factors**, and **ribosomal proteins**. - **Protein degradation** is mediated by the **ubiquitin-proteasome system** and **autophagy-lysosome pathway**. - While PKC activation can have indirect effects on these processes, IP3/DAG are not primary regulators of protein synthesis or degradation.

Question 4: In the mitogen activated protein kinase pathway, the activation of RAS is counteracted by

A. Inositol triphosphate
B. GTPase activating protein (Correct Answer)
C. Phosphatidyl inositol
D. Protein kinase C

Explanation: ***GTPase activating protein*** - **GTPase Activating Proteins (GAPs)** facilitate the hydrolysis of **GTP bound to RAS** to GDP, converting active RAS back to its inactive state. - This inactivation is crucial for turning off the downstream signaling of the **MAPK pathway** and preventing uncontrolled cell proliferation. *Inositol triphosphate* - **Inositol triphosphate (IP3)** is a secondary messenger that triggers the release of **intracellular calcium** from the endoplasmic reticulum. - It is involved in various signaling pathways, but its primary role is not to directly counteract RAS activation. *Phosphatidyl inositol* - **Phosphatidylinositol (PI)** is a component of cell membranes and can be phosphorylated to produce various **phosphatidylinositol phosphates (PIPs)**, like **PIP2** and **PIP3**. - These molecules act as docking sites for signaling proteins but do not directly inactivate RAS. *Protein kinase C* - **Protein kinase C (PKC)** is a family of enzymes involved in signal transduction, typically activated by **diacylglycerol (DAG)** and calcium. - It phosphorylates various proteins, mediating diverse cellular responses, but it does not directly counteract the activation of RAS.

Question 5: 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 6: A primigravida at 38 weeks of gestation has gone into labor. Oxytocin was started to augment labor. The second messenger system through which oxytocin acts is:

A. Tyrosine kinase
B. Phospholipase C (IP3/DAG pathway) (Correct Answer)
C. cGMP
D. cAMP

Explanation: ***Phospholipase C (IP3/DAG pathway)*** - Oxytocin binds to its receptor, which is a **Gq protein-coupled receptor**. This activates **phospholipase C**. - **Phospholipase C** then hydrolyzes **phosphatidylinositol 4,5-bisphosphate (PIP2)** into **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, which act as second messengers to increase intracellular calcium and mediate myometrial contraction. *Tyrosine kinase* - **Tyrosine kinase receptors** are typically activated by growth factors (e.g., insulin, epidermal growth factor) and lead to phosphorylation cascades. - This mechanism is not primarily associated with the downstream signaling of **oxytocin receptors**. *cGMP* - **Cyclic guanosine monophosphate (cGMP)** is a second messenger primarily involved in signaling pathways initiated by **nitric oxide** and some peptide hormones. - It often acts to cause smooth muscle relaxation, which is contrary to oxytocin's role in uterine contraction. *cAMP* - **Cyclic adenosine monophosphate (cAMP)** is a common second messenger for many hormones that bind to **Gs protein-coupled receptors**. - Hormones such as **epinephrine (beta-adrenergic receptors)** and **glucagon** utilize cAMP, typically leading to different cellular responses than those of oxytocin.

Question 7: 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 8: Phospholipid associated with the mechanism of hormone action is

A. Phosphatidylcholine
B. Phosphatidylethanolamine
C. Plasmalogen
D. Phosphatidylinositol (Correct Answer)

Explanation: ***Phosphatidylinositol*** (Correct) - **Phosphatidylinositol (PI)** and its phosphorylated derivatives, particularly **PIP2 (phosphatidylinositol 4,5-bisphosphate)**, are critical in signal transduction pathways activated by many hormones. - Hormones binding to **G protein-coupled receptors** can activate phospholipase C, which cleaves PIP2 into **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, leading to increased intracellular calcium and protein kinase C activation, respectively. *Phosphatidylcholine* (Incorrect) - **Phosphatidylcholine** is a major component of cell membranes and is involved in membrane structure and fluidity. - While it can be a source of signaling molecules like **lysophosphatidic acid**, it is not primarily associated with the initial intracellular signaling events of hormone action in the same way as phosphatidylinositol. *Phosphatidylethanolamine* (Incorrect) - **Phosphatidylethanolamine** is another abundant membrane phospholipid primarily involved in membrane structure and stability. - It can be a precursor for other lipids, but it does not directly participate in the **second messenger systems** triggered by most hormones as a primary signaling molecule. *Plasmalogen* (Incorrect) - **Plasmalogens** are a unique class of phospholipids containing an ether bond at the sn-1 position. - They are abundant in certain tissues, particularly nervous and cardiovascular tissues, and are thought to have antioxidant properties, but they are not directly involved in the initiating events of **hormone signaling pathways**.

Question 9: Which of the following binds to Tyrosine Kinase receptor?

A. Insulin (Correct Answer)
B. Glucagon
C. Prolactin
D. Growth Hormone

Explanation: ***Insulin*** - **Insulin** is a classic example of a hormone that binds to and activates a **tyrosine kinase receptor**, leading to a cascade of intracellular signaling events for glucose uptake and metabolism. - The **insulin receptor** is a heterodimeric protein with intrinsic tyrosine kinase activity that phosphorylates itself and other proteins upon insulin binding. *Glucagon* - **Glucagon** primarily acts on **G protein-coupled receptors (GPCRs)**, specifically the glucagon receptor, to increase cyclic AMP (cAMP) and activate protein kinase A. - Its main roles are to stimulate **glycogenolysis** and **gluconeogenesis** in the liver. *Prolactin* - **Prolactin** binds to a receptor that is a member of the **cytokine receptor superfamily**, which lacks intrinsic enzyme activity. - Upon ligand binding, these receptors associate with and activate **Janus kinases (JAKs)**, leading to the JAK-STAT signaling pathway. *Growth Hormone* - **Growth hormone (GH)** also binds to a receptor belonging to the **cytokine receptor superfamily** (similar to prolactin), which then associates with and activates **JAKs**. - This activation subsequently initiates the **JAK-STAT signaling pathway**, mediating its diverse growth-promoting and metabolic effects.

Question 10: 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.

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