Signal Transduction — MCQs

Signal Transduction Indian Medical PG Practice Questions and MCQs

Question 21: Which of the following statements about Pseudohypoparathyroidism is true?

A. Caused by a 'gain of function' inherited mutation in the Gsa subunit
B. Decreased formation of cyclic GMP is observed
C. Decreased formation of inositol triphosphate is observed
D. Decreased formation of cyclic AMP is observed (Correct Answer)

Explanation: **Explanation:** **Pseudohypoparathyroidism (PHP)** is a clinical syndrome characterized by **end-organ resistance** to Parathyroid Hormone (PTH). 1. **Why Option D is correct:** PTH normally binds to its receptor on target cells (kidney and bone), which is coupled to a **Stimulatory G-protein (Gsα)**. This activation stimulates **Adenylate Cyclase**, converting ATP into **cyclic AMP (cAMP)**. In PHP (specifically Type 1a/Albright Hereditary Osteodystrophy), there is a **'loss of function' mutation** in the *GNAS1* gene encoding the Gsα subunit. Consequently, even when PTH levels are high, the G-protein cannot activate Adenylate Cyclase, leading to **decreased formation of cAMP**. 2. **Why other options are incorrect:** * **Option A:** It is caused by a **'loss of function'** mutation. A 'gain of function' mutation in Gsα is seen in **McCune-Albright Syndrome**, leading to constitutive activation. * **Option B & C:** The PTH receptor primarily utilizes the **Gs-cAMP pathway**, not the cGMP or IP3/DAG (Phospholipase C) pathways. Therefore, the biochemical defect is specific to cAMP signaling. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Profile:** Low Calcium, High Phosphate (mimicking hypoparathyroidism), but **Elevated PTH** (due to resistance). * **Albright Hereditary Osteodystrophy (AHO):** Clinical phenotype of PHP Type 1a including short stature, round face, obesity, and **short 4th/5th metacarpals**. * **Ellsworth-Howard Test:** Administration of exogenous PTH fails to increase urinary cAMP levels in PHP patients, serving as a diagnostic tool. * **Pseudopseudohypoparathyroidism (PPHP):** Phenotypic features of AHO are present, but PTH levels and cAMP signaling are **normal** (paternal inheritance).

Question 22: Which of the following functions as second messengers?

A. cAMP
B. DAG
C. IP3
D. All of the above (Correct Answer)

Explanation: **Explanation:** Signal transduction is the process by which an extracellular signal (first messenger, like a hormone) is converted into a cellular response. Since many hormones cannot cross the lipid bilayer, they bind to surface receptors, triggering the release of **second messengers** inside the cell to amplify the signal. 1. **cAMP (Cyclic Adenosine Monophosphate):** Produced from ATP by the enzyme Adenylyl Cyclase (activated by Gs proteins). It primarily activates Protein Kinase A (PKA). It is the second messenger for hormones like Glucagon, ACTH, and PTH. 2. **DAG (Diacylglycerol) & IP3 (Inositol Trisphosphate):** These are generated simultaneously when Phospholipase C (PLC) cleaves PIP2 (Phosphatidylinositol 4,5-bisphosphate). * **IP3** is water-soluble and diffuses to the endoplasmic reticulum to release **Calcium** (another vital second messenger). * **DAG** remains in the membrane to activate **Protein Kinase C (PKC)**. **Why "All of the above" is correct:** All three molecules (cAMP, DAG, and IP3) are classic examples of intracellular signaling molecules that relay signals from receptors to target effector proteins. **High-Yield Clinical Pearls for NEET-PG:** * **cGMP:** Acts as a second messenger for Nitric Oxide (NO) and ANP, leading to vasodilation. * **Calcium:** Often considered a "third messenger" in the IP3 pathway, but generally classified as a second messenger. * **Receptor Tyrosine Kinases (e.g., Insulin):** Do not use cAMP; they utilize phosphorylation cascades (IRS-1, PI3K pathway). * **Mnemonic for Gq-coupled receptors (using IP3/DAG/Ca2+):** **HAV 1 M&M** (H1, Alpha-1, V1, M1, M3).

Question 23: Cyclic GMP acts on which of the following?

A. Insulin
B. Thyroxin
C. Atrial natriuretic peptide (Correct Answer)
D. Growth hormone

Explanation: **Explanation:** The correct answer is **Atrial Natriuretic Peptide (ANP)**. **1. Why ANP is correct:** Signal transduction via **Cyclic GMP (cGMP)** is utilized by a specific group of ligands, primarily ANP, Brain Natriuretic Peptide (BNP), and Nitric Oxide (NO). ANP binds to its transmembrane receptor (Guanylyl Cyclase-A), which possesses intrinsic enzymatic activity. Upon activation, it converts GTP into cGMP. The cGMP then activates **Protein Kinase G (PKG)**, leading to vasodilation and natriuresis. **2. Why the other options are incorrect:** * **Insulin:** Uses the **Tyrosine Kinase** receptor pathway. Binding triggers autophosphorylation of the receptor and activation of Insulin Receptor Substrates (IRS). * **Thyroxin (T4):** Being a lipid-soluble hormone, it uses **Intracellular/Nuclear receptors**. It acts as a transcription factor to alter gene expression directly. * **Growth Hormone:** Utilizes the **JAK-STAT pathway**. It binds to a receptor that lacks intrinsic kinase activity but recruits cytosolic Janus Kinases (JAKs). **3. NEET-PG High-Yield Pearls:** * **Two types of Guanylyl Cyclase:** * *Membrane-bound:* Activated by ANP/BNP. * *Soluble (cytosolic):* Activated by Nitric Oxide (NO). * **Phosphodiesterase-5 (PDE-5):** This enzyme breaks down cGMP. Inhibitors like Sildenafil (Viagra) increase cGMP levels, promoting prolonged vasodilation. * **Mnemonic for cGMP:** "**ANP** helps you **P**ee (cGMP)." * **Other pathways to remember:** Glucagon and ACTH use cAMP; Oxytocin and TRH use the IP3-DAG pathway.

Question 24: What are G protein-coupled receptors?

A. Intracellular membrane proteins that help to regulate movement within the cell.
B. Plasma membrane proteins that couple the extracellular binding of primary signaling molecules to activation of small G proteins. (Correct Answer)
C. Plasma membrane proteins that couple the extracellular binding of primary signaling molecules to the activation of heterotrimeric G proteins.
D. Intracellular proteins that couple the binding of primary messenger molecules with transcription.

Explanation: **Explanation:** **G Protein-Coupled Receptors (GPCRs)**, also known as 7-transmembrane or serpentine receptors, are the largest class of cell-surface receptors. They function by sensing extracellular signals (ligands) and converting them into intracellular responses. **Why Option B is Correct:** GPCRs are **plasma membrane proteins**. Upon binding an extracellular ligand (primary messenger), they undergo a conformational change that allows them to interact with and activate **G proteins**. While GPCRs traditionally interact with heterotrimeric G proteins, the broader definition in modern biochemistry (and the context of this specific question) emphasizes their role in activating G proteins—molecular switches that cycle between GDP-bound (inactive) and GTP-bound (active) states—to trigger downstream effectors like Adenylyl Cyclase or Phospholipase C. **Why Other Options are Incorrect:** * **Option A:** GPCRs are not primarily involved in physical intracellular movement; they are signaling transducers. * **Option C:** While GPCRs do activate heterotrimeric G proteins ($\alpha, \beta, \gamma$ subunits), Option B is often considered the more "functional" definition in competitive exams focusing on the activation mechanism of the G protein itself. *(Note: In many textbooks, C is also technically accurate, but B highlights the biochemical "coupling" action).* * **Option D:** This describes **Nuclear Receptors** (e.g., steroid receptors), which are intracellular and act as transcription factors. **High-Yield NEET-PG Pearls:** * **Structure:** 7-transmembrane $\alpha$-helices; N-terminus is extracellular, C-terminus is intracellular. * **Second Messengers:** Common pathways include **cAMP** (via $G_s$ or $G_i$) and **$IP_3/DAG$** (via $G_q$). * **Examples:** Adrenergic receptors, Rhodopsin, PTH receptors, and Glucagon receptors. * **Clinical Link:** Roughly 30-50% of modern drugs target GPCRs. **Vibrio cholerae** toxin acts by ADP-ribosylation of the $G_s$ subunit, keeping it permanently active.

Question 25: Notch signaling in embryogenesis is an example of which type of intercellular signaling?

A. Autocrine
B. Paracrine (Correct Answer)
C. Endocrine
D. None of the above

Explanation: **Explanation:** **Intercellular signaling** is categorized based on the distance the signal travels to reach its target cell. **Why Paracrine is correct:** **Paracrine signaling** involves cells secreting local mediators that affect neighboring cells in the immediate environment. **Notch signaling** is a specialized form of paracrine signaling known as **Juxtacrine (contact-dependent) signaling**. In this pathway, both the ligand (e.g., Delta or Serrate) and the receptor (Notch) are transmembrane proteins. Signaling occurs only when two cells come into direct physical contact. Since it acts on immediate neighbors rather than distant sites, it is classified under the broader umbrella of paracrine signaling. It is crucial during embryogenesis for "lateral inhibition," ensuring neighboring cells differentiate into different cell types. **Why other options are incorrect:** * **Autocrine:** The cell secretes a hormone or chemical messenger that binds to receptors on the **same cell**, leading to self-stimulation (e.g., IL-2 in T-cell proliferation). * **Endocrine:** Signaling molecules (hormones) are secreted into the **bloodstream** to act on distant target cells (e.g., Insulin acting on muscle/adipose tissue). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Upon ligand binding, the Notch receptor undergoes proteolytic cleavage by **$\gamma$-secretase**, releasing the Notch Intracellular Domain (NICD) which translocates to the nucleus to regulate gene expression. * **Clinical Correlation:** Mutations in the Notch signaling pathway are associated with **Alagille syndrome** (congenital heart defects and bile duct paucity) and certain cancers like **T-ALL** (T-cell Acute Lymphoblastic Leukemia). * **Key Concept:** Remember that Notch signaling does **not** involve a second messenger; the receptor itself acts as the signal transducer to the nucleus.

Question 26: Which of the following molecules functions as a signaling molecule?

A. Carbon dioxide
B. Oxygen
C. Nitric oxide (Correct Answer)
D. Nitrogen

Explanation: **Explanation** **Correct Answer: C. Nitric Oxide (NO)** Nitric oxide is a unique gaseous signaling molecule (gasotransmitter) that plays a critical role in cardiovascular and neurological signaling. Unlike most hormones, it is highly lipophilic and diffuses directly across cell membranes. Its primary mechanism involves binding to and activating **soluble Guanylate Cyclase (sGC)**, which increases intracellular levels of **cyclic GMP (cGMP)**. This leads to the activation of Protein Kinase G (PKG), resulting in smooth muscle relaxation and vasodilation. **Why the other options are incorrect:** * **A. Carbon dioxide (CO₂):** While CO₂ is a metabolic byproduct that influences blood pH and respiratory drive via chemoreceptors, it is not classified as a primary signaling molecule in the context of signal transduction pathways. * **B. Oxygen (O₂):** Oxygen is the final electron acceptor in the electron transport chain. While its levels are "sensed" by the body (e.g., via HIF-1α), it does not function as a ligand or messenger for signal transduction. * **D. Nitrogen (N₂):** Nitrogen is an inert gas that makes up the majority of the atmosphere but has no physiological signaling role in the human body. **High-Yield Clinical Pearls for NEET-PG:** * **Synthesis:** NO is synthesized from **L-Arginine** by the enzyme **Nitric Oxide Synthase (NOS)**; NADPH and Oxygen are essential cofactors. * **Isoforms:** There are three types of NOS: nNOS (neuronal), eNOS (endothelial), and iNOS (inducible/macrophage-related). * **Pharmacology:** Nitroglycerin works by being converted into NO, leading to vasodilation in angina. Sildenafil (Viagra) works downstream by inhibiting **PDE-5**, the enzyme that breaks down cGMP. * **Half-life:** NO has an extremely short half-life (seconds), making it a potent paracrine (local) signaling agent.

Question 27: Growth factors promote cell growth by acting on which of the following?

A. Cyclic AMP (cAMP)
B. Tyrosine Kinase (Correct Answer)
C. Cyclic GMP (cGMP)
D. TRM

Explanation: **Explanation:** **1. Why Tyrosine Kinase is Correct:** Most growth factors (e.g., Insulin, IGF-1, EGF, PDGF, and FGF) exert their effects through **Receptor Tyrosine Kinases (RTKs)**. These are transmembrane proteins with an extracellular ligand-binding domain and an intracellular catalytic domain. Upon ligand binding, the receptors undergo dimerization and **autophosphorylation** of tyrosine residues. This triggers a signaling cascade, most notably the **Ras-MAPK pathway**, which regulates gene expression, protein synthesis, and cell cycle progression, ultimately promoting cell growth and proliferation. **2. Why the Other Options are Incorrect:** * **Cyclic AMP (cAMP):** This is a second messenger for G-Protein Coupled Receptors (GPCRs) linked to $G_s$ or $G_i$ proteins. It primarily mediates hormonal actions (e.g., Glucagon, ACTH, PTH) rather than growth factor-induced mitogenesis. * **Cyclic GMP (cGMP):** This serves as a second messenger for Atrial Natriuretic Peptide (ANP) and Nitric Oxide (NO), primarily involved in vasodilation and fluid balance, not primary cell growth signaling. * **TRM:** This is not a recognized standard signaling molecule or pathway in the context of growth factor signal transduction. **3. High-Yield Clinical Pearls for NEET-PG:** * **JAK-STAT Pathway:** Remember that while most growth factors use RTKs, **Growth Hormone (GH)**, **Prolactin**, and **Erythropoietin** use the JAK-STAT pathway (Non-receptor Tyrosine Kinase). * **Oncogenes:** Mutations in RTKs or their downstream signaling proteins (like **Ras**) are frequently implicated in human cancers due to constitutive activation of growth signals. * **Insulin Exception:** Insulin is the most famous metabolic hormone that utilizes a Tyrosine Kinase receptor.

Question 28: All of the following hormones stimulate cAMP, EXCEPT:

A. Glucagon
B. Follicle stimulating hormone
C. Luteinizing hormone
D. Acetylcholine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Acetylcholine**. This question tests your knowledge of hormone signaling pathways, specifically those utilizing the **cAMP (Cyclic Adenosine Monophosphate)** second messenger system. **1. Why Acetylcholine is the correct answer:** Acetylcholine (ACh) primarily acts through two types of receptors: **Nicotinic** (ligand-gated ion channels) and **Muscarinic** (G-protein coupled). Muscarinic receptors utilize different pathways: * **M1, M3, M5:** Act via the **Gq pathway** (IP3/DAG/Calcium). * **M2, M4:** Act via the **Gi pathway**, which **inhibits** Adenylate Cyclase, thereby decreasing cAMP levels rather than stimulating them. **2. Why the other options are incorrect:** Options A, B, and C all utilize the **Gs-protein coupled receptor** pathway. When these hormones bind to their receptors, they activate Adenylate Cyclase, which converts ATP to cAMP: * **Glucagon:** Essential for glycogenolysis and gluconeogenesis in the liver via cAMP. * **FSH & LH:** These gonadotropins utilize the cAMP pathway to regulate gametogenesis and steroidogenesis in the gonads. **High-Yield Clinical Pearls for NEET-PG:** * **FLAT ChAMP:** A popular mnemonic for hormones using cAMP: **F**SH, **L**H, **A**CTH, **T**SH, **C**RH, **h**CG, **A**DH (V2 receptor), **M**SH, **P**TH, and Glucagon. * **Gq Pathway Mnemonic:** "**GOAT HAG**" — **G**nRH, **O**xytocin, **A**DH (V1 receptor), **T**RH, **H**istamine (H1), **A**ngiotensin II, and **G**astrin. * **Insulin & Growth Factor:** These typically use **Receptor Tyrosine Kinase** pathways, not cAMP.

Question 29: Which of the following hormones utilize intracellular receptors?

A. ACTH
B. TSH
C. Glucocorticoids (Correct Answer)
D. Insulin

Explanation: **Explanation:** Hormones are classified based on their solubility and the location of their receptors. The correct answer is **Glucocorticoids** because they are lipophilic (steroid) hormones. **1. Why Glucocorticoids are correct:** Lipid-soluble hormones can easily diffuse across the phospholipid bilayer of the plasma membrane. Once inside the cell, they bind to **intracellular receptors** (either in the cytoplasm or nucleus). Glucocorticoids bind to cytoplasmic receptors, which then translocate to the nucleus to act as transcription factors, altering gene expression. **2. Analysis of Incorrect Options:** * **ACTH (Adrenocorticotropic Hormone):** A peptide hormone that binds to G-protein coupled receptors (GPCR) on the cell surface, specifically stimulating the Gs-adenylyl cyclase pathway. * **TSH (Thyroid Stimulating Hormone):** A glycoprotein hormone that also utilizes cell surface GPCRs to activate the cAMP second messenger system. * **Insulin:** A peptide hormone that binds to a **Receptor Tyrosine Kinase (RTK)**, a transmembrane receptor with intrinsic enzymatic activity. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Intracellular Receptors:** "VET PRATS" – **V**itamin D, **E**strogen, **T**estosterone, **P**rogesterone, **R**etinoic acid, **A**ldosterone, **T**hyroid hormones (T3/T4), and **S**teroids (Glucocorticoids). * **Thyroid Exception:** Unlike most intracellular receptors that start in the cytosol, Thyroid hormone receptors are typically already bound to DNA in the nucleus. * **Second Messengers:** Remember that water-soluble hormones (Peptides/Catecholamines) require second messengers (cAMP, IP3/DAG, cGMP), whereas lipid-soluble hormones generally do not.

Question 30: Which of the structural domains of mammalian regulatory factors may be called intracellular receptors?

A. Response elements
B. Antirepressor domains
C. Transcription-activating domains
D. Ligand-binding domains (Correct Answer)

Explanation: **Explanation:** The question refers to the mechanism of **Intracellular (Nuclear) Receptors**, which act as ligand-activated transcription factors. These receptors (e.g., for steroid hormones, thyroid hormones, and Vitamin D) are unique because they function as both receptors and signal transducers. **1. Why Ligand-binding domains (LBD) is correct:** Mammalian regulatory factors (nuclear receptors) are composed of distinct functional domains. The **Ligand-binding domain** is the specific region that recognizes and binds to the signaling molecule (the "first messenger"). Because this domain resides inside the cell (cytoplasm or nucleus) and directly captures the hormone to initiate a cellular response, it is functionally defined as the **intracellular receptor**. Once the ligand binds, it induces a conformational change that allows the receptor to bind to DNA and regulate gene expression. **2. Why other options are incorrect:** * **Response elements (A):** These are specific **DNA sequences** (e.g., HRE - Hormone Response Elements) located in the promoter region of target genes, not structural domains of the protein factor itself. * **Antirepressor domains (B):** These are regions that function to counteract transcriptional repression, often by recruiting co-activators or modifying chromatin, but they do not "receive" the initial signal. * **Transcription-activating domains (C):** These domains interact with the basal transcription machinery (like RNA Polymerase II) to initiate mRNA synthesis. While essential for the receptor's function, they do not perform the "receptor" role of ligand recognition. **High-Yield Clinical Pearls for NEET-PG:** * **Zinc Fingers:** The DNA-binding domain (DBD) of these intracellular receptors typically contains "Zinc Finger" motifs, which are high-yield topics in molecular biology. * **Type I vs. Type II:** Type I receptors (Steroids) are usually cytoplasmic and bound to **Heat Shock Proteins (HSP70/90)**; Type II (Thyroid/Retinoic acid) are constitutively located in the nucleus. * **Mechanism:** These receptors bypass second messengers (like cAMP) and act directly on the genome, leading to a slower but more prolonged physiological response.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free