Signal Transduction Practice Questions

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

They are derived from PIP2. **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.

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

G proteins. ### 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).

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

GH. **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).

Q: Which of the following hormones does not utilize the adenylyl cyclase-cAMP pathway as a second messenger system?

Insulin. **Explanation:** The correct answer is **Insulin**. The mechanism of action of a hormone depends on its chemical nature and the type of receptor it binds to. **1. Why Insulin is the correct answer:** Insulin (along with IGF-1 and Growth Hormone) does not use the cAMP pathway. Instead, it binds to a **Receptor Tyrosine Kinase (RTK)**, which is a transmembrane receptor with intrinsic enzymatic activity. Upon binding, the receptor undergoes autophosphorylation, activating the **MAP kinase** pathway (for growth/gene expression) and the **PI3K/Akt** pathway (for metabolic effects like glucose transport via GLUT-4). **2. Why the other options are incorrect:** * **TSH (Thyroid Stimulating Hormone):** As a glycoprotein hormone, it binds to G-protein coupled receptors (GPCRs) that activate the **Gs protein**, stimulating adenylyl cyclase to increase intracellular cAMP. * **Epinephrine:** When acting on **$\beta$-adrenergic receptors** ($\beta_1, \beta_2, \beta_3$), epinephrine activates the Gs-adenylyl cyclase-cAMP pathway. (Note: While it uses the $IP_3/DAG$ pathway via $\alpha_1$ receptors, it is a classic example of a cAMP-utilizing hormone). * **Glucagon:** This is the "prototypical" cAMP-mediated hormone. It binds to GPCRs in the liver to activate adenylyl cyclase, leading to glycogenolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for cAMP hormones:** "FLAT ChAMP" (FSH, LH, ACTH, TSH, CRH, hCG, ADH (V2), MSH, PTH, Glucagon, Calcitonin). * **ANP and Nitric Oxide (NO)** utilize **cGMP** as a second messenger. * **Steroid and Thyroid hormones** are lipophilic and utilize **intracellular/nuclear receptors** to directly alter gene transcription.

Q: JAK-STAT pathway is seen in which of the following?

Leptin. ***Leptin*** - **Leptin** binding to its receptor activates the **JAK-STAT pathway**, regulating appetite and metabolism. - This pathway involves the phosphorylation of **STAT proteins**, which then translocate to the nucleus to induce gene expression. *Calcitonin* - **Calcitonin** activates **G protein-coupled receptors**, leading to an increase in intracellular cyclic AMP (cAMP). - Its primary role is in **calcium homeostasis**, lowering blood calcium levels. *Aldosterone* - **Aldosterone** is a steroid hormone that binds to **intracellular mineralocorticoid receptors**. - This complex then acts as a **transcription factor**, affecting gene expression in the kidneys to regulate sodium and potassium balance. *Vasopressin* - **Vasopressin** (ADH) binds to **G protein-coupled receptors** (V1 and V2 receptors). - V2 receptor activation in the kidney leads to increased **cAMP** and insertion of aquaporins, regulating water reabsorption.

Q: Which hormone does not act through cAMP?

Insulin. ***Insulin*** - Insulin primarily acts through a **tyrosine kinase receptor**, which, upon ligand binding, autophosphorylates and triggers downstream signaling cascades involving various protein kinases and phosphatases. - This mechanism allows insulin to mediate its diverse metabolic effects, such as glucose uptake and storage, independent of the **cAMP pathway**. *TSH (Thyroid-stimulating hormone)* - TSH binds to its receptor on thyroid follicular cells, activating **G protein-coupled receptors** that stimulate **adenylyl cyclase**. - This leads to an increase in intracellular **cAMP**, which then activates protein kinase A (PKA) to promote thyroid hormone synthesis and release. *ACTH (Adrenocorticotropic hormone)* - ACTH acts on the **adrenal cortex** through G protein-coupled receptors, stimulating **adenylyl cyclase** and raising intracellular **cAMP levels**. - Increased cAMP then activates PKA, leading to the synthesis and secretion of **cortisol** and other adrenal steroids. *Adrenaline (Epinephrine)* - Adrenaline can act through various **adrenergic receptors (alpha and beta)**; **beta-adrenergic receptors** are Gs protein-coupled and activate **adenylyl cyclase**. - This increases **cAMP** levels, mediating effects such as bronchodilation, increased heart rate, and glycogenolysis.

Q: Which one of the following acts as second messenger?

Ca++. ***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.

Q: In response to changes in Ca2+ concentration, which of the following Ca2+ binding proteins can modify the activity of many enzymes & proteins?

Calmodulin. ***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.

Q: Which of the following is an amine hormone?

T4. ***T4*** - **Thyroxine (T4)** is a **thyroid hormone** derived from the amino acid **tyrosine**. - It is classified as an **amine hormone** (also called **amino acid-derived hormone**) because it is synthesized from a single amino acid and contains an **amine group**. - Other amine hormones include **T3, epinephrine, norepinephrine, and dopamine**. *FSH* - **Follicle-stimulating hormone (FSH)** is a **glycoprotein hormone**, not an amine hormone. - It is composed of **alpha and beta subunits** and plays a role in **gonad development** and **reproductive function**. *TSH* - **Thyroid-stimulating hormone (TSH)** is also a **glycoprotein hormone**. - It stimulates the **thyroid gland** to produce thyroid hormones (T3 and T4). *Insulin* - **Insulin** is a **peptide hormone**, consisting of **amino acid chains** (51 amino acids in two chains) linked by **disulfide bonds**. - It is critical for **glucose regulation** and is significantly larger and more complex than amine hormones.

Q: Which hormone activates the phospholipase C pathway?

GnRH. ***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 1

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

Accepted Answer

They are derived from PIP2

Answer

They are first messengers

Answer

They activate phospholipase C

Answer

They cannot activate calcium calmodulin-dependent protein kinases directly or indirectly

Question 2

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

Accepted Answer

G proteins

Answer

Cyclic adenosine monophosphate (cAMP)

Answer

Inositol trisphosphate (IP3)

Answer

Diacylglycerol (DAG)

Question 3

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

Accepted Answer

GH

Answer

FSH

Answer

Progesterone

Answer

Estrogens

Question 4

Which of the following hormones does not utilize the adenylyl cyclase-cAMP pathway as a second messenger system?

Accepted Answer

Insulin

Answer

TSH

Answer

Epinephrine

Answer

Glucagon

Question 5

JAK-STAT pathway is seen in which of the following?

Accepted Answer

Leptin

Answer

Calcitonin

Answer

Aldosterone

Answer

Vasopressin

Question 6

Which hormone does not act through cAMP?

Accepted Answer

Insulin

Answer

TSH

Answer

ACTH

Answer

Adrenaline

Question 7

Which one of the following acts as second messenger?

Accepted Answer

Ca++

Answer

Cl-

Answer

PO4³⁻

Answer

Mg++

Question 8

In response to changes in Ca2+ concentration, which of the following Ca2+ binding proteins can modify the activity of many enzymes & proteins?

Accepted Answer

Calmodulin

Answer

Collagen

Answer

Kinesin

Answer

Elastin

Question 9

Which of the following is an amine hormone?

Accepted Answer

T4

Answer

FSH

Answer

TSH

Answer

Insulin

Question 10

Which hormone activates the phospholipase C pathway?

Accepted Answer

GnRH

Answer

FSH

Answer

LH

Answer

TSH

Signal Transduction — MCQs

Signal Transduction — MCQs

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