Signal Transduction — MCQs

Signal Transduction Indian Medical PG Practice Questions and MCQs

Question 91: Many signaling pathways involve the generation of inositol trisphosphate (IP3) and diacylglycerol (DAG). What is the origin of these molecules?

A. They are first messengers
B. They activate phospholipase C
C. They are derived from PIP2 (Correct Answer)
D. They cannot activate calcium calmodulin-dependent protein kinases directly or indirectly

Explanation: **Explanation:** The Phosphoinositide signaling pathway is a major mechanism for hormones like Oxytocin, GnRH, and TRH. The correct answer is **C** because IP3 and DAG are generated through the enzymatic cleavage of **Phosphatidylinositol 4,5-bisphosphate (PIP2)**, a minor phospholipid component of the inner leaflet of the plasma membrane. When a ligand binds to a Gq-protein-coupled receptor (GPCR), it activates the enzyme **Phospholipase C (PLC)**. PLC then hydrolyzes PIP2 into two distinct second messengers: 1. **Inositol trisphosphate (IP3):** A water-soluble molecule that diffuses into the cytosol to trigger calcium release from the endoplasmic reticulum. 2. **Diacylglycerol (DAG):** A lipid-soluble molecule that remains in the membrane to activate Protein Kinase C (PKC). **Analysis of Incorrect Options:** * **Option A:** They are **second messengers**. First messengers are the extracellular ligands (hormones/neurotransmitters) that bind to the receptor. * **Option B:** They are the **products** of Phospholipase C activity, not its activators. PLC is activated by the α-subunit of the **Gq protein**. * **Option D:** They **can** activate these kinases. IP3 increases intracellular calcium, which binds to Calmodulin. The Ca²⁺-Calmodulin complex then activates various Ca²⁺-Calmodulin-dependent protein kinases (CaM-Kinases). **High-Yield NEET-PG Pearls:** * **Mnemonic for Gq-coupled receptors:** "H1, Alpha-1, V1, M1, M3" (The "1s and 3s"). * **Lithium Connection:** Lithium inhibits the recycling of inositol by blocking inositol monophosphatase, which is a key aspect of its use in Bipolar Disorder (Inositol Depletion Hypothesis). * **DAG** specifically requires **Calcium** and **Phosphatidylserine** as cofactors to fully activate Protein Kinase C.

Question 92: All of the following serve as intracellular second messengers in receptor-mediated signal transduction except?

A. Cyclic adenosine monophosphate (cAMP)
B. Inositol trisphosphate (IP3)
C. Diacylglycerol (DAG)
D. G proteins (Correct Answer)

Explanation: ### Explanation **Why G proteins are the correct answer:** In signal transduction, **G proteins** (Guanine nucleotide-binding proteins) function as **transducers** or "molecular switches," not second messengers. They reside on the inner surface of the plasma membrane and couple the extracellular signal (first messenger) from a G-protein coupled receptor (GPCR) to an effector enzyme (like Adenylyl cyclase or Phospholipase C). Second messengers are small, non-protein molecules produced *downstream* of these effector enzymes to amplify the signal within the cell. **Analysis of incorrect options (Second Messengers):** * **Cyclic AMP (cAMP):** Produced by Adenylyl cyclase from ATP. It primarily activates Protein Kinase A (PKA). * **Inositol trisphosphate (IP3):** Generated by the cleavage of PIP2 by Phospholipase C. It is water-soluble and diffuses to the endoplasmic reticulum to trigger the release of **Calcium** (another vital second messenger). * **Diacylglycerol (DAG):** Also generated from PIP2 cleavage. It remains membrane-bound and activates Protein Kinase C (PKC) in the presence of Calcium. **High-Yield Clinical Pearls for NEET-PG:** * **G-protein Structure:** They are heterotrimeric (α, β, γ subunits). The **α-subunit** possesses intrinsic GTPase activity, which acts as a self-terminating mechanism. * **Cholera Toxin:** Inhibits the GTPase activity of **Gs**, leading to constitutive activation of Adenylyl cyclase and permanent elevation of cAMP. * **Pertussis Toxin:** Inhibits **Gi**, preventing the inhibition of Adenylyl cyclase. * **cGMP:** Another important second messenger involved in Nitric Oxide (NO) signaling and the visual cycle (Phototransduction).

Question 93: Which of the following hormones does not act through c-AMP?

A. FSH
B. Progesterone
C. Estrogens
D. GH (Correct Answer)

Explanation: **Explanation:** The mechanism of hormone action is determined by the chemical nature of the hormone and its specific receptor. **1. Why GH is the Correct Answer:** Growth Hormone (GH) does not use the cAMP second messenger system. Instead, it binds to surface receptors that belong to the **Cytokine Receptor Superfamily**. These receptors lack intrinsic tyrosine kinase activity and instead signal through the **JAK-STAT pathway** (Janus Kinase - Signal Transducer and Activator of Transcription). Other hormones using this pathway include Prolactin, Erythropoietin, and Leptin. **2. Analysis of Incorrect Options:** * **FSH (Follicle Stimulating Hormone):** Like most glycoprotein hormones (LH, TSH, hCG) and Glucagon, FSH acts via **G-protein coupled receptors (GPCRs)** that activate Adenylate Cyclase, increasing intracellular **cAMP**. * **Estrogen & Progesterone:** These are steroid hormones. While they primarily act via **intracellular/nuclear receptors** to alter gene transcription, they are often grouped differently from peptide hormones. However, in the context of this specific question, GH is the classic "exception" among peptide hormones because it bypasses the cAMP system entirely in favor of JAK-STAT. **3. High-Yield Clinical Pearls for NEET-PG:** * **cAMP Users:** "FLAT ChAMP" mnemonic (FSH, LH, ACTH, TSH, CRH, hCG, ADH (V2), MSH, PTH, Glucagon). * **cGMP Users:** ANP, BNP, and Nitric Oxide (NO). * **IP3/DAG Users:** "GOAT" mnemonic (GnRH, Oxytocin, ADH (V1), TRH). * **Tyrosine Kinase (Intrinsic):** Insulin, IGF-1, and various Growth Factors (EGF, PDGF). * **JAK-STAT (Non-intrinsic):** PIG (Prolactin, Immunomodulators/Cytokines, GH).

Question 94: Which of the following factors acts in vesicle targeting?

A. Sec 12
B. Rab (Correct Answer)
C. Ras
D. SNARE

Explanation: ***Rab*** - **Rab GTPases** are small G proteins that regulate many steps of membrane trafficking, including vesicle formation, cargo selection, vesicle budding, uncoating, motility, and fusion. - They act as molecular switches, cycling between an active GTP-bound state and an inactive GDP-bound state, thereby coordinating the proper targeting of vesicles to their destination membranes. *Sec 12* - **Sec12** is a **GEF (guanine nucleotide exchange factor)** for **Sar1**, which is involved in COPII vesicle formation from the ER. - While it initiates a step in vesicle budding, it does not directly act as a targeting molecule to guide the vesicle to its destination. *Ras* - **Ras GTPases** are primarily involved in cell signaling pathways regulating cell proliferation, differentiation, and survival. - They are not directly involved in the process of **vesicle targeting** in membrane trafficking. *SNARE* - **SNARE proteins** (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) are crucial for the **fusion of vesicles** with their target membranes. - While essential for the later stages of trafficking, they mediate membrane fusion rather than initial vesicle targeting.

Question 95: 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 96: 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 97: Which of the following substances is a growth factor that is capable of inducing all the steps necessary for angiogenesis?

A. TGF -a
B. PDGF
C. EGF
D. Basic fibroblast growth factor (Correct Answer)

Explanation: ***Basic fibroblast growth factor*** - **Basic fibroblast growth factor (bFGF or FGF2)** is a potent inducer of **angiogenesis**, promoting endothelial cell proliferation, migration, and tube formation. - It plays a crucial role in **wound healing** and **tumor growth** by stimulating the development of new blood vessels. *TGF-α* - **TGF-alpha** primarily acts as a **mitogen** for epithelial cells and fibroblasts, promoting cell growth and differentiation. - While it can indirectly influence angiogenesis, it does not directly induce all steps of the process like bFGF. *PDGF* - **Platelet-derived growth factor (PDGF)** is a mitogen for **fibroblasts, smooth muscle cells, and glial cells**, involved in wound healing and tissue remodeling. - Its primary role in angiogenesis is thought to be in the **stabilization of new vessels** by recruiting pericytes and smooth muscle cells, rather than initiating new vessel formation. *EGF* - **Epidermal growth factor (EGF)** primarily stimulates the **proliferation and differentiation of epidermal and epithelial cells**. - While it contributes to tissue repair and cell growth, its direct role in inducing all stages of angiogenesis is less prominent compared to bFGF.

Question 98: Which hormone activates the phospholipase C pathway?

A. FSH
B. GnRH (Correct Answer)
C. LH
D. TSH

Explanation: ***GnRH*** - **Gonadotropin-releasing hormone (GnRH)** binds to its receptor on pituitary cells, activating a **Gq protein**. - **Gq protein** then activates **phospholipase C (PLC)**, leading to the production of **inositol trisphosphate (IP3)** and **diacylglycerol (DAG)**, which ultimately increases intracellular calcium and triggers the release of LH and FSH. *FSH* - **Follicle-stimulating hormone (FSH)** primarily signals through the **adenylyl cyclase pathway**, increasing **cAMP** levels. - While it plays a crucial role in reproduction, its direct signaling cascade does not typically involve phospholipase C activation. *LH* - **Luteinizing hormone (LH)** also primarily acts via the **adenylyl cyclase pathway**, similar to FSH. - This pathway leads to increased **cAMP** and subsequent protein kinase A activation in target cells. *TSH* - **Thyroid-stimulating hormone (TSH)** primarily activates the **adenylyl cyclase pathway** in thyroid follicular cells. - Activation of this pathway leads to increased **cAMP** and stimulates thyroid hormone synthesis and release.

Question 99: cGMP acts as second messenger for ?

A. Nitric oxide (Correct Answer)
B. Dopamine
C. Calcitonin
D. FSH

Explanation: ***Nitric oxide*** - **Nitric oxide (NO)** activates **guanylyl cyclase**, which then converts GTP to **cGMP**. - **cGMP** mediates many of NO's biological effects, including **vasodilation**. *Dopamine* - **Dopamine** primarily acts through **G-protein coupled receptors** that affect adenylate cyclase activity, leading to changes in **cAMP** levels, not cGMP. - Its effects are often mediated by changes in neuronal excitability and neurotransmission. *Calcitonin* - **Calcitonin** is a hormone that primarily acts to lower **blood calcium levels**. - Its receptors are typically coupled to **G-proteins** that activate **adenylate cyclase** to produce **cAMP**. *FSH* - **Follicle-stimulating hormone (FSH)** binds to receptors on target cells in the gonads. - This binding generally activates **adenylate cyclase**, leading to the production of **cAMP**, which acts as a second messenger.

Question 100: Which of the following hormones stimulates the production of cAMP?

A. Somatostatin
B. Acetylcholine
C. Angiotensin II
D. Calcitonin (Correct Answer)

Explanation: ***Calcitonin*** - Calcitonin binds to its receptor, which is a **G-protein coupled receptor (GPCR)**, stimulating **adenylyl cyclase**. - This activation of adenylyl cyclase leads to the production of **cyclic AMP (cAMP)** from ATP, which then acts as a second messenger. *Somatostatin* - Somatostatin typically binds to **Gi-protein-coupled receptors**, which **inhibit adenylyl cyclase** activity. - This action leads to a **decrease in cAMP production**, thereby reducing the release of various hormones. *Acetylcholine* - Acetylcholine can act through both **nicotinic** and **muscarinic receptors**. Muscarinic receptors are GPCRs. - While some muscarinic receptors can inhibit adenylyl cyclase (M2, M4), others primarily activate **phospholipase C**, leading to increased **IP3 and DAG** rather than cAMP. *Angiotensin II* - Angiotensin II primarily acts through **GPCRs (AT1 and AT2 receptors)**, predominantly activating the **phospholipase C pathway**. - This results in the production of **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, which increase intracellular calcium, rather than stimulating cAMP production.

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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