Signal Transduction — MCQs

Signal Transduction Indian Medical PG Practice Questions and MCQs

Question 11: What is the mechanism of action of glucagon?

A. Inositol Pathway
B. Arachidonic Acid Pathway
C. c-GMP Pathway
D. c-AMP Pathway (Correct Answer)

Explanation: **Explanation:** Glucagon is a peptide hormone produced by the alpha cells of the pancreas, primarily acting on the liver to maintain blood glucose levels during fasting. Its mechanism of action is a classic example of the **c-AMP (Cyclic Adenosine Monophosphate) Pathway**. 1. **Mechanism of the Correct Answer (D):** When glucagon binds to its specific **G-protein coupled receptor (GPCR)** on the cell membrane, it activates the stimulatory G-protein ($G_s$). This activates the enzyme **Adenylate Cyclase**, which converts ATP into **c-AMP**. c-AMP then acts as a second messenger to activate **Protein Kinase A (PKA)**, leading to the phosphorylation of key enzymes that trigger glycogenolysis and gluconeogenesis. 2. **Analysis of Incorrect Options:** * **A. Inositol Pathway ($IP_3/DAG$):** Utilized by hormones like Oxytocin, Vasopressin ($V_1$ receptors), and TRH. It involves Phospholipase C and calcium release. * **B. Arachidonic Acid Pathway:** This is the pathway for inflammatory mediators like prostaglandins and leukotrienes, not peptide hormones. * **C. c-GMP Pathway:** Used by Atrial Natriuretic Peptide (ANP) and Nitric Oxide (NO) to induce vasodilation. **NEET-PG High-Yield Pearls:** * **Hormones using c-AMP:** FLAT ChAMP (FSH, LH, ACTH, TSH, CRH, hCG, ADH ($V_2$), MSH, PTH) and Glucagon. * **Insulin vs. Glucagon:** While Glucagon uses c-AMP, **Insulin** uses the **Tyrosine Kinase** pathway. * **Clinical Correlation:** Glucagon is used as an antidote for **Beta-blocker overdose** because it increases c-AMP in cardiac tissue independently of beta-receptors, thereby increasing heart rate and contractility.

Question 12: Hormonal stimulation of the formation of second messenger inositol 1, 4, 5-triphosphate (IP3) quickly leads to the release of which other intracellular messenger?

A. Cyclic AMP
B. Prostaglandin
C. Calcium (Correct Answer)
D. Leukotriene

Explanation: **Explanation:** The correct answer is **Calcium**. This question tests the understanding of the **Phosphoinositide (Gq) signaling pathway**. **1. Why Calcium is Correct:** When a hormone (e.g., Vasopressin V1, Oxytocin, or Angiotensin II) binds to a Gq-protein-coupled receptor, it activates the enzyme **Phospholipase C (PLC)**. PLC cleaves the membrane phospholipid **PIP2** (Phosphatidylinositol 4,5-bisphosphate) into two second messengers: * **DAG (Diacylglycerol):** Remains in the membrane to activate Protein Kinase C. * **IP3 (Inositol 1,4,5-triphosphate):** Diffuses into the cytosol and binds to IP3-gated channels on the **Endoplasmic Reticulum (ER)**. This triggers the rapid release of sequestered **Calcium (Ca²⁺)** into the cytoplasm, which then mediates various cellular responses. **2. Why Other Options are Incorrect:** * **A. Cyclic AMP:** This is the second messenger for the **Gs/Gi** pathways (via Adenylyl Cyclase), not the IP3 pathway. * **B & D. Prostaglandins and Leukotrienes:** These are eicosanoids derived from **Arachidonic acid** via the COX and LOX pathways, respectively. While PLC can indirectly lead to arachidonic acid release, they are not the immediate intracellular messengers released by IP3. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Gq receptors:** "HAV 1 M&M" (**H**1, **A**lpha-1, **V**1, **M**1, **M**3). * **Lithium Connection:** Lithium inhibits the recycling of inositol by blocking inositol monophosphatase, which is a key mechanism in treating Bipolar Disorder. * **Calmodulin:** Once calcium is released by IP3, it often binds to **Calmodulin** to activate various protein kinases.

Question 13: Crystallization and storage of insulin requires which ion?

A. Mn++
B. Zn++ (Correct Answer)
C. Cu++
D. Ca++

Explanation: **Explanation:** **Why Zn++ is the correct answer:** Insulin is synthesized in the pancreatic β-cells as a monomer, but for efficient storage within secretory granules, it must be stabilized. **Zinc (Zn++)** plays a critical role in this process. Two zinc ions coordinate with six insulin molecules to form a stable **hexameric structure**. This hexamerization allows for the dense packing and crystallization of insulin, protecting it from degradation and maintaining a concentrated reservoir ready for secretion. Once released into the bloodstream, the hexamer dissociates back into active monomers. **Analysis of Incorrect Options:** * **Mn++ (Manganese):** Primarily acts as a cofactor for enzymes like pyruvate carboxylase and superoxide dismutase; it has no structural role in insulin storage. * **Cu++ (Copper):** Essential for enzymes like cytochrome c oxidase and lysyl oxidase, but it is not involved in the hexameric stabilization of insulin. * **Ca++ (Calcium):** While calcium is crucial for the **exocytosis** (release) of insulin granules from the β-cell, it is not the ion responsible for the crystallization and internal storage structure of the hormone itself. **High-Yield Clinical Pearls for NEET-PG:** * **Hexameric vs. Monomeric:** Pharmaceutical insulin analogs are modified based on this principle. **Rapid-acting analogs** (Lispro, Aspart) are designed to resist hexamer formation for faster absorption, while **long-acting analogs** (Glargine) promote precipitation/crystallization at the injection site. * **C-Peptide:** Remember that insulin and C-peptide are secreted in equimolar amounts; C-peptide levels are used to differentiate endogenous insulin production from exogenous administration. * **Zinc Deficiency:** Can lead to impaired glucose tolerance due to decreased insulin storage efficiency.

Question 14: The RAS gene codes for which type of protein?

A. Growth factor receptor
B. Cell cycle regulator
C. Signal transduction protein (Correct Answer)
D. Nuclear transcription factor

Explanation: **Explanation:** The **RAS gene** encodes a family of small **GTP-binding proteins** (GTPases) that function as essential molecular switches in **signal transduction** pathways. 1. **Why Option C is Correct:** RAS proteins act as intermediaries between cell surface receptors (like Receptor Tyrosine Kinases) and intracellular signaling cascades (such as the **MAPK/ERK pathway**). When a growth factor binds to its receptor, RAS is activated by exchanging GDP for GTP. Once active, it triggers downstream signaling that promotes cell growth and differentiation. It is a classic example of a **signal transduction protein**. 2. **Why Other Options are Incorrect:** * **Option A (Growth factor receptor):** Examples include EGFR or HER2/neu. RAS is located on the inner surface of the plasma membrane, downstream of these receptors. * **Option B (Cell cycle regulator):** These include Cyclins and Cyclin-Dependent Kinases (CDKs). While RAS signaling eventually influences these, it is not a regulator itself. * **Option D (Nuclear transcription factor):** Examples include **MYC**, FOS, and JUN. These act inside the nucleus to alter gene expression, whereas RAS operates at the membrane level. **High-Yield Clinical Pearls for NEET-PG:** * **Point Mutation:** The most common mechanism of RAS oncogenic activation is a point mutation (usually at codons 12, 13, or 61), which impairs GTPase activity, leaving RAS "permanently on." * **Associations:** * **K-RAS:** Most common; associated with **Pancreatic** (90%), Colon, and Lung cancers. * **H-RAS:** Associated with Bladder and Kidney tumors. * **N-RAS:** Associated with Melanomas and Hematologic malignancies. * **GAP (GTPase Activating Proteins):** These act as "off-switches" for RAS. Mutations in **NF1** (Neurofibromatosis type 1) lead to loss of a GAP, causing constitutive RAS activity.

Question 15: What is the action of the alpha-subunit of a G protein?

A. Breakdown of GTP to GDP (Correct Answer)
B. Conversion of GDP to GTP
C. Internalization of receptors
D. Binding of agonist

Explanation: **Explanation:** The G-protein complex is a heterotrimer consisting of **α, β, and γ subunits**. The **α-subunit** is the functional engine of this complex because it possesses intrinsic **GTPase activity**. 1. **Why Option A is correct:** In its resting state, the α-subunit is bound to GDP. When a ligand binds to the G-protein coupled receptor (GPCR), GDP is released and replaced by GTP, activating the α-subunit. To terminate the signal and prevent overstimulation, the α-subunit acts as an enzyme to **hydrolyze (break down) GTP into GDP**. This "internal clock" mechanism returns the G-protein to its inactive trimeric state. 2. **Why other options are incorrect:** * **Option B:** The conversion of GDP to GTP (activation) is facilitated by the receptor acting as a Guanine Nucleotide Exchange Factor (GEF), not by the α-subunit’s enzymatic action. * **Option C:** Internalization of receptors (downregulation) is mediated by **β-arrestins** and clathrin-coated pits, usually following phosphorylation by G-protein coupled receptor kinases (GRKs). * **Option D:** Binding of the agonist occurs at the **extracellular domain** of the GPCR, not the intracellular G-protein. **High-Yield Clinical Pearls for NEET-PG:** * **Cholera Toxin:** Inhibits the GTPase activity of **Gsα**, leading to constitutive activation of Adenylate Cyclase, high cAMP, and "rice-water" diarrhea. * **Pertussis Toxin:** Inhibits **Giα** by ADP-ribosylation, preventing it from inhibiting Adenylate Cyclase, also leading to increased cAMP levels. * **G-protein types:** **Gs** (stimulates Adenylate Cyclase), **Gi** (inhibits Adenylate Cyclase), and **Gq** (activates Phospholipase C → IP3/DAG pathway).

Question 16: Which of the following is NOT a second messenger?

A. Guanylyl cyclase (Correct Answer)
B. cAMP
C. IP3
D. DAG

Explanation: **Explanation:** The core concept in signal transduction is distinguishing between **enzymes (effectors)** and **second messengers**. **1. Why Guanylyl Cyclase is the correct answer:** Guanylyl cyclase is an **enzyme (effector)**, not a second messenger. It catalyzes the conversion of GTP into cyclic GMP (cGMP). In the signaling cascade, a ligand (like Nitric Oxide or ANP) activates the enzyme, which then produces the second messenger. Therefore, Guanylyl cyclase is the *generator* of the signal, while cGMP is the actual messenger. **2. Analysis of Incorrect Options:** * **cAMP (Cyclic AMP):** The most common second messenger, produced by Adenylyl cyclase. It primarily activates Protein Kinase A (PKA). * **IP3 (Inositol triphosphate):** Produced by the action of Phospholipase C on PIP2. It binds to receptors on the Endoplasmic Reticulum to release intracellular **Calcium** (another vital second messenger). * **DAG (Diacylglycerol):** Also produced from PIP2 by Phospholipase C. It remains membrane-bound and activates Protein Kinase C (PKC). **High-Yield Clinical Pearls for NEET-PG:** * **Calcium** is often considered the "third messenger" in some contexts but is functionally a second messenger. * **Nitric Oxide (NO)** acts via **Soluble Guanylyl Cyclase** to increase cGMP, leading to vasodilation (mechanism of Nitroglycerin). * **Atrial Natriuretic Peptide (ANP)** acts via **Membrane-bound Guanylyl cyclase**. * **Receptor Tyrosine Kinases** (e.g., Insulin receptor) do not typically use traditional small-molecule second messengers; they use protein phosphorylation cascades (SH2 domains).

Question 17: Which of the following most accurately describes the transmembrane signaling process involved in steroid hormone action?

A. Action on a membrane-spanning tyrosine kinase
B. Action of a G protein which activates or inhibits adenylyl cyclase
C. Diffusion across a membrane and binding to an intracellular receptor (Correct Answer)
D. Opening of transmembrane ion channels

Explanation: ### Explanation **Correct Answer: C. Diffusion across a membrane and binding to an intracellular receptor** **Mechanism of Action:** Steroid hormones (e.g., Cortisol, Aldosterone, Estrogen, Testosterone) are **lipophilic** (hydrophobic) molecules derived from cholesterol. Because of their lipid solubility, they can easily diffuse through the lipid bilayer of the plasma membrane. Once inside the cell, they bind to specific **intracellular receptors** located either in the cytoplasm (e.g., Glucocorticoids) or the nucleus (e.g., Estrogen, Thyroid hormones). The hormone-receptor complex then acts as a **ligand-activated transcription factor**, binding to specific DNA sequences called **Hormone Response Elements (HREs)** to regulate gene expression and protein synthesis. **Why Other Options are Incorrect:** * **Option A (Tyrosine Kinase):** This is the mechanism for **Insulin** and various growth factors (IGF-1, EGF). It involves autophosphorylation of the receptor's intracellular domain. * **Option B (G-protein/Adenylyl Cyclase):** This is the mechanism for **peptide hormones** (e.g., Glucagon, ACTH, PTH, TSH) and catecholamines. These hormones are water-soluble and cannot cross the membrane, thus requiring second messengers like cAMP. * **Option D (Ion Channels):** This mechanism is typical for **neurotransmitters** (e.g., Acetylcholine at nicotinic receptors, GABA) which cause rapid changes in membrane potential. **High-Yield Clinical Pearls for NEET-PG:** * **Exceptions:** While most steroid receptors are intracellular, **Thyroid hormones (T3/T4)**—though not steroids—also use intracellular (nuclear) receptors because they are lipophilic. * **Speed of Action:** Steroid hormone signaling is **slow** (hours to days) because it requires transcription and translation, unlike G-protein signaling which is near-instantaneous. * **Zinc Fingers:** The DNA-binding domain of steroid receptors typically contains "Zinc Finger" motifs, a common point of inquiry in biochemistry exams.

Question 18: Hydrocortisone acts as an anti-inflammatory agent due to the induction of the synthesis of which of the following proteins?

A. Heat shock protein 90
B. Inhibin
C. Transcortin
D. Lipocortin (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Lipocortin** **Mechanism of Action:** Hydrocortisone (a glucocorticoid) acts via intracellular receptors that function as ligand-activated transcription factors. Upon binding, the glucocorticoid-receptor complex translocates to the nucleus and induces the transcription of specific genes. One of the most critical proteins synthesized is **Lipocortin-1 (also known as Annexin A1)**. Lipocortin-1 exerts its anti-inflammatory effect by **inhibiting Phospholipase A2 (PLA2)**. Since PLA2 is the enzyme responsible for releasing arachidonic acid from membrane phospholipids, its inhibition prevents the synthesis of potent inflammatory mediators, including prostaglandins, leukotrienes, and thromboxanes. --- ### Analysis of Incorrect Options: * **A. Heat shock protein 90 (Hsp90):** This is a chaperone protein that binds to the glucocorticoid receptor in its *inactive* state in the cytosol. Hydrocortisone causes the *release* of Hsp90 to allow receptor activation; it does not induce its synthesis. * **B. Inhibin:** This is a peptide hormone produced by the gonads (Sertoli/Granulosa cells) that inhibits the secretion of Follicle-Stimulating Hormone (FSH). It is unrelated to glucocorticoid signaling. * **C. Transcortin:** Also known as Corticosteroid-Binding Globulin (CBG), this is the transport protein for cortisol in the blood. It is synthesized by the liver, not induced by hydrocortisone as part of the inflammatory response. --- ### NEET-PG High-Yield Pearls: * **Genomic vs. Non-genomic:** Glucocorticoids primarily act via the **genomic pathway** (taking hours to show effects due to protein synthesis). * **Dual Action:** Glucocorticoids not only induce anti-inflammatory proteins (Lipocortin) but also **repress** the expression of pro-inflammatory genes like COX-2 and various cytokines (IL-1, TNF-α). * **Annexin A1:** Remember "Lipocortin" and "Annexin A1" interchangeably for exam questions.

Question 19: Which of the following statements is true about second messengers?

A. They bind to the first messenger.
B. They are integral proteins.
C. They are hormones secreted by stimulation of other hormones.
D. They are substances that increase or decrease cellular function. (Correct Answer)

Explanation: ### Explanation **Correct Option: D. They are substances that increase or decrease cellular function.** Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules (the "first messengers," such as hormones or neurotransmitters). Their primary role is to **transduce and amplify** the signal from the cell surface to the interior, ultimately triggering a physiological response. This response can involve activating enzymes, opening ion channels, or altering gene expression, thereby increasing or decreasing specific cellular functions. **Analysis of Incorrect Options:** * **Option A:** Second messengers do not bind to the first messenger. Instead, the first messenger binds to a **cell-surface receptor**, which then triggers the production or release of the second messenger inside the cytoplasm. * **Option B:** Second messengers are typically small, diffusible molecules (e.g., **cAMP, IP3, DAG, Ca²⁺**) or ions. They are not integral proteins; however, the receptors and enzymes that generate them (like Adenylyl cyclase) often are. * **Option C:** This describes a **tropic hormone** (e.g., TSH stimulating the release of T4), not a second messenger. Second messengers are non-protein intracellular molecules, not secreted hormones. **High-Yield Clinical Pearls for NEET-PG:** * **cAMP:** The most common second messenger; inactivated by **Phosphodiesterase (PDE)**. Caffeine inhibits PDE, maintaining high cAMP levels. * **IP3 & DAG:** Derived from membrane phospholipid PIP2 via **Phospholipase C**. IP3 increases intracellular **Calcium** (from the ER), while DAG activates **Protein Kinase C**. * **cGMP:** Acts as a second messenger for **Nitric Oxide (NO)** and Atrial Natriuretic Peptide (ANP), leading to smooth muscle relaxation. * **Receptor Tyrosine Kinases:** Unlike G-protein coupled receptors, these (e.g., Insulin receptor) often use a phosphorylation cascade rather than traditional small-molecule second messengers.

Question 20: Which cells secrete IFN-β?

A. Leukocytes
B. Fibroblasts (Correct Answer)
C. NK cells
D. Macrophages

Explanation: ### Explanation **Correct Option: B. Fibroblasts** Interferons (IFNs) are a group of signaling proteins (cytokines) released by host cells in response to pathogens, particularly viruses. They are classified into Type I, Type II, and Type III. **Type I Interferons** include **IFN-α** and **IFN-β**. While both are involved in the innate antiviral response, they are produced by different primary cell types: * **IFN-α** is primarily produced by **Leukocytes** (specifically plasmacytoid dendritic cells). * **IFN-β** is primarily produced by **Fibroblasts** and epithelial cells. The underlying mechanism involves the detection of viral double-stranded RNA (dsRNA) by pattern recognition receptors (like TLR-3), which triggers the fibroblast to secrete IFN-β. This induces an "antiviral state" in neighboring cells by inhibiting protein synthesis and degrading viral RNA. **Analysis of Incorrect Options:** * **A. Leukocytes:** These are the primary source of **IFN-α** (Type I). * **C. NK cells:** Along with T-lymphocytes (Th1 cells), NK cells are the primary source of **IFN-γ** (Type II Interferon), which is crucial for activating macrophages and MHC expression. * **D. Macrophages:** While macrophages can produce some Type I IFNs, they are more significantly involved in secreting pro-inflammatory cytokines (IL-1, IL-6, TNF-α) and are targets for activation by IFN-γ. **High-Yield Clinical Pearls for NEET-PG:** * **IFN-β Clinical Use:** Used therapeutically in the management of **Multiple Sclerosis (MS)** to reduce the frequency of relapses. * **IFN-α Clinical Use:** Used in Chronic Hepatitis B and C, Kaposi Sarcoma, and Hairy Cell Leukemia. * **IFN-γ Clinical Use:** Used in **Chronic Granulomatous Disease (CGD)** to enhance phagocytic activity. * **Mnemonic:** **A**lpha = **A**ny (Leukocytes); **B**eta = **B**last (Fibroblasts); **G**amma = **G**ranulocytes/T-cells.

Practice by Chapter

Cell Surface Receptors: Types and Functions

Practice Questions

G-Protein Coupled Receptors

Practice Questions

Enzyme-Linked Receptors

Practice Questions

Second Messengers in Signal Transduction

Practice Questions

cAMP and cGMP Signaling

Practice Questions

Calcium as Second Messenger

Practice Questions

Inositol Phosphate Pathway

Practice Questions

MAP Kinase Cascades

Practice Questions

JAK-STAT Signaling Pathway

Practice Questions

Insulin Signaling Pathway

Practice Questions

Nuclear Receptors and Gene Regulation

Practice Questions

Defects in Signal Transduction and Disease

Practice Questions

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