Signal Transduction Practice Questions

Q: All of the following belong to the steroid receptor superfamily EXCEPT:

Epinephrine receptor. **Explanation:** The **Steroid Receptor Superfamily** consists of intracellular receptors (located in the cytoplasm or nucleus) that act as **ligand-activated transcription factors**. These receptors share a common structural domain, including a highly conserved Zinc-finger DNA-binding domain. **Why Option D is Correct:** The **Epinephrine receptor** (Adrenergic receptor) is a **G-Protein Coupled Receptor (GPCR)**. Unlike steroid hormones, epinephrine is a catecholamine (water-soluble) that cannot cross the lipid bilayer. It binds to receptors on the **cell surface**, triggering a second messenger cascade (cAMP), rather than directly entering the nucleus to modulate transcription. **Why the other options are Incorrect:** * **Vitamin D3, Thyroid (T3/T4), and Retinoid (Vitamin A) receptors** all belong to the Type II subclass of the steroid receptor superfamily. * Despite their different chemical structures, these hormones are lipophilic and cross the plasma membrane to bind to receptors already located in the **nucleus**. * They typically form **heterodimers** with the Retinoid X Receptor (RXR) to bind to Hormone Response Elements (HRE) on DNA. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Steroid receptors (Estrogen, Progesterone, Glucocorticoids) are primarily cytoplasmic, while Thyroid and Retinoid receptors are always nuclear. * **Zinc Fingers:** The DNA-binding domain of this superfamily contains "Zinc Finger" motifs (Cys2-Cys2 type), a frequent target for MCQ questions. * **Mechanism:** They regulate gene expression by recruiting co-activators or co-repressors to the promoter region of target genes.

Q: Fibroblast Growth Factor mediates its effect through which of the following receptors?

Tyrosine kinase. **Explanation:** **1. Why Tyrosine Kinase is Correct:** Fibroblast Growth Factor (FGF) belongs to a class of ligands that utilize **Receptor Tyrosine Kinases (RTKs)** for signal transduction. Upon FGF binding, the receptor undergoes dimerization, leading to the autophosphorylation of intracellular tyrosine residues. This triggers a downstream signaling cascade, most notably the **MAP Kinase (Ras-Raf-MEK-ERK) pathway**, which regulates cell proliferation, differentiation, and angiogenesis. Most growth factors (EGF, PDGF, IGF-1, Insulin) share this mechanism. **2. Why Other Options are Incorrect:** * **B. cAMP:** This is a second messenger for **G-Protein Coupled Receptors (GPCRs)** linked to Gs or Gi proteins. Hormones like Glucagon, ACTH, and PTH use this pathway, not growth factors. * **C. cGMP:** This pathway is utilized by **Atrial Natriuretic Peptide (ANP)** and **Nitric Oxide (NO)**. It involves guanylyl cyclase activation, leading to vasodilation and fluid balance regulation. * **D. Ca2+:** Calcium (and IP3/DAG) is the second messenger for GPCRs linked to the **Gq protein**. Examples include GnRH, Oxytocin, and Gastrin. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Achondroplasia:** Caused by a gain-of-function mutation in the **FGFR3** (FGF Receptor 3) gene, leading to inhibited bone growth. * **JAK-STAT Pathway:** Do not confuse RTKs with the JAK-STAT pathway. While both involve tyrosine phosphorylation, JAK-STAT is used by **Cytokines, Growth Hormone, and Prolactin** (receptors lack intrinsic kinase activity). * **SH2 Domains:** Proteins that bind to phosphorylated tyrosine residues on RTKs typically contain SH2 (Src Homology 2) domains.

Q: Adrenaline acts through which type of receptor?

Seven pass receptor. **Explanation:** **1. Why the Correct Answer is Right:** Adrenaline (Epinephrine) acts via **G-Protein Coupled Receptors (GPCRs)**, specifically the alpha (α) and beta (β) adrenergic receptors. These receptors are structurally characterized by a single polypeptide chain that traverses the plasma membrane **seven times**. Consequently, they are known as **Seven-pass receptors**, Hepthelical receptors, or Serpentine receptors. When adrenaline binds to the extracellular domain, it induces a conformational change that activates intracellular G-proteins ($G_s$, $G_i$, or $G_q$), leading to the production of second messengers like cAMP or $IP_3$/DAG. **2. Why the Other Options are Incorrect:** * **Option A (Single pass):** These are typically **Enzyme-linked receptors** (e.g., Insulin receptor, Growth Factor receptors like EGFR). They cross the membrane only once and often possess intrinsic tyrosine kinase activity. * **Option B (Four pass):** This structure is characteristic of certain **Ionotropic receptors** (e.g., Nicotinic Acetylcholine receptors, $GABA_A$ receptors), where four transmembrane segments form a subunit of an ion channel. * **Option D (Ligand-gated channel):** These are receptors that act as pores for ions (e.g., $Na^+$, $K^+$, $Cl^-$) upon ligand binding. Adrenaline does not directly open ion channels; it works through metabolic signaling (metabotropic). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Largest Receptor Family:** GPCRs are the largest family of cell surface receptors. * **Second Messengers:** * $\beta_1, \beta_2, \beta_3$ receptors act via **$G_s$** (Increases cAMP). * $\alpha_1$ receptors act via **$G_q$** (Increases $IP_3$/DAG/Calcium). * $\alpha_2$ receptors act via **$G_i$** (Decreases cAMP). * **Rhodopsin:** The photoreceptor in the retina is also a classic example of a seven-pass GPCR.

Q: Which of the following hormones are synthesized as peptide precursors?

All of the above. **Explanation:** The synthesis of protein and peptide hormones follows a universal pathway: they are initially synthesized on ribosomes as large, inactive molecules called **Pre-prohormones**. These undergo post-translational modifications in the endoplasmic reticulum (removal of the signal peptide) to become **Prohormones**, and are further cleaved in the Golgi apparatus or secretory vesicles into the final active **Hormone**. * **Insulin (Option A):** Synthesized as *Preproinsulin*. The signal sequence is removed to form *Proinsulin*. In the secretory granules, Proinsulin is cleaved into active Insulin and C-peptide. * **Parathyroid Hormone (Option B):** Synthesized as *Prepro-PTH* (115 amino acids), which is cleaved to *Pro-PTH* (90 amino acids) and finally to the active PTH (84 amino acids). * **Renin (Option C):** Although often discussed as an enzyme, Renin is a peptide hormone synthesized as *Preprorenin*. It is processed into *Prorenin* and then into active Renin in the juxtaglomerular cells of the kidney. Since all three substances are peptide-based and follow this biosynthetic pathway, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **C-Peptide:** Since C-peptide and Insulin are secreted in equimolar amounts from Proinsulin, C-peptide levels are used as a marker of endogenous insulin production (useful in distinguishing Type 1 vs. Type 2 Diabetes or Factitious Hypoglycemia). * **Storage:** Unlike steroid hormones (which are synthesized on demand), peptide hormones are stored in **secretory vesicles** and released via exocytosis in response to a stimulus (usually involving Calcium influx). * **Receptors:** Peptide hormones are water-soluble and cannot cross the lipid bilayer; they bind to **cell surface receptors** (GPCRs or Enzyme-linked receptors).

Q: Which of the following is NOT true about G proteins?

G proteins are made up of 4 subunits.. **Explanation:** **Why Option C is correct:** G proteins (Guanine nucleotide-binding proteins) are **heterotrimeric**, meaning they are composed of **three subunits**: **Alpha (α), Beta (β), and Gamma (γ)** [1]. They are not made of 4 subunits. In the inactive state, the α-subunit binds GDP [2]. Upon ligand binding to a G-protein coupled receptor (GPCR), GDP is replaced by GTP, causing the α-subunit to dissociate from the βγ-complex to trigger downstream signaling [3]. **Analysis of other options:** * **Option A:** G proteins directly or indirectly regulate various ion channels (e.g., K+ channels in the heart via G_i), often referred to as G-protein-gated ion channels [3]. * **Option B:** G proteins (specifically G_s) activate Adenylyl Cyclase, increasing cAMP [4]. This activates Protein Kinase A, which phosphorylates **Phosphorylase Kinase**, leading to the formation of **Active Glycogen Phosphorylase** [4]. * **Option D:** The **Ras protein** is a member of the "Small GTPase" superfamily. These are "monomeric" G proteins (single subunit) that function similarly to the α-subunit of heterotrimeric G proteins and are critical in cell growth signaling; mutations in Ras are linked to various cancers [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Vibrio cholerae toxin:** Causes ADP-ribosylation of **G_s**, locking it in the "ON" state (persistent high cAMP) [5]. * **Bordetella pertussis toxin:** Causes ADP-ribosylation of **G_i**, locking it in the "OFF" state (preventing inhibition of cAMP) [5]. * **Second Messengers:** G_s and G_i act on Adenylyl Cyclase (cAMP), while **G_q** acts on Phospholipase C (IP3/DAG/Ca2+).

Q: Slow-reacting substance of anaphylaxis (SRS-A) is?

Leukotriene. **Explanation:** The correct answer is **Leukotriene**. **Slow-reacting substance of anaphylaxis (SRS-A)** is a mixture of specific cysteinyl leukotrienes, primarily **LTC4, LTD4, and LTE4**. These are synthesized via the **Lipoxygenase (LOX) pathway** from arachidonic acid. They are called "slow-reacting" because, unlike histamine which causes immediate bronchoconstriction, SRS-A induces a slower, more prolonged, and significantly more potent contraction of smooth muscle in the airways. **Why other options are incorrect:** * **Prostaglandins:** These are products of the Cyclooxygenase (COX) pathway. While they mediate inflammation and pain (e.g., PGD2 causes bronchoconstriction), they do not constitute SRS-A. * **Serotonin:** A monoamine neurotransmitter and vasoactive amine. While it plays a role in platelet aggregation and GI motility, it is not a component of SRS-A. * **Histamine:** This is a pre-formed mediator stored in mast cell granules. It is responsible for the **immediate** (Type I) hypersensitivity reaction but is rapidly metabolized, unlike the sustained action of SRS-A. **High-Yield Clinical Pearls for NEET-PG:** * **Potency:** Cysteinyl leukotrienes (SRS-A) are **100 to 1000 times more potent** than histamine in inducing bronchoconstriction and increasing vascular permeability. * **Aspirin-Exacerbated Respiratory Disease (AERD):** Inhibiting the COX pathway with Aspirin shunts arachidonic acid toward the LOX pathway, increasing leukotriene production and triggering asthma. * **Pharmacology Link:** **Montelukast and Zafirlukast** are leukotriene receptor antagonists (LTRA) used in chronic asthma management to block the effects of SRS-A. * **LTB4** is primarily involved in **chemotaxis** (neutrophil recruitment) and is not part of SRS-A.

Q: In the context of G-protein signaling, which of the following is considered a second messenger?

Calcium. **Explanation:** In G-protein coupled receptor (GPCR) signaling, **Calcium ($Ca^{2+}$)** is a classic second messenger. When a ligand binds to a $G_q$-coupled receptor, it activates Phospholipase C (PLC), which cleaves $PIP_2$ into $IP_3$ and DAG. $IP_3$ then binds to receptors on the endoplasmic reticulum, triggering the release of $Ca^{2+}$ into the cytosol. This rise in intracellular calcium activates various downstream proteins, such as Protein Kinase C (PKC) and Calmodulin. **Analysis of Options:** * **A. cAMP:** While cAMP is a major second messenger (produced via $G_s$ signaling), it is often categorized alongside $Ca^{2+}$. However, in the context of specific exam questions where multiple options might seem correct, $Ca^{2+}$ is frequently highlighted for its unique role in the $IP_3/DAG$ pathway. *Note: If this were a "multiple correct" format, both A and B would be right; however, in single-best-answer formats, $Ca^{2+}$ is a high-yield focus for $G_q$ pathways.* * **C & D. CO and NO:** Carbon Monoxide (CO) and Nitric Oxide (NO) are categorized as **gasotransmitters** or "third messengers." They are paracrine signaling molecules that can diffuse across membranes to activate intracellular targets (like guanylyl cyclase), rather than being traditional second messengers generated directly by GPCR activation. **High-Yield Clinical Pearls for NEET-PG:** * **$G_q$ Pathway:** Remember the mnemonic **"HAV 1 M&M"** (Histamine-$H_1$, Adrenergic-$\alpha_1$, Vasopressin-$V_1$, Muscarinic-$M_1$ and $M_3$)—all use the $IP_3/Ca^{2+}$ second messenger system. * **Calmodulin:** Calcium often acts by binding to Calmodulin; this complex then activates Myosin Light Chain Kinase (MLCK), essential for smooth muscle contraction. * **Amplification:** The primary purpose of second messengers is the intracellular amplification of the extracellular signal.

Q: Insulin receptors are:

Tyrosine Kinase receptors. **Explanation:** **1. Why Tyrosine Kinase Receptors is correct:** The insulin receptor is a classic example of a **Receptor Tyrosine Kinase (RTK)**. It is a heterotetrameric glycoprotein consisting of two extracellular $\alpha$-subunits (which bind insulin) and two transmembrane $\beta$-subunits. Upon insulin binding, the $\beta$-subunits undergo **autophosphorylation** on tyrosine residues. This triggers a signaling cascade by recruiting **Insulin Receptor Substrates (IRS 1-4)**, which subsequently activate the PI3K (Phosphoinositide 3-kinase) pathway for metabolic effects (like GLUT4 translocation) and the MAPK pathway for growth and gene expression. **2. Why other options are incorrect:** * **Phosphodiesterase (B):** This is an enzyme that breaks down cyclic nucleotides (cAMP/cGMP). While insulin *activates* phosphodiesterase to lower cAMP levels (antagonizing glucagon), the receptor itself is not a phosphodiesterase. * **Calcium-Calmodulin (C):** This is a second messenger system typically used by G-protein coupled receptors (GPCRs) linked to the $G_q$ protein (e.g., Oxytocin, $H_1$ receptors). Insulin does not primarily signal through this mechanism. * **Lipoprotein (D):** Lipoproteins (like LDL, HDL) are transport vehicles for lipids in the blood; they are not signaling receptors. **Clinical Pearls for NEET-PG:** * **GLUT4:** The only insulin-dependent glucose transporter, found primarily in skeletal muscle and adipose tissue. * **SH2 Domains:** Proteins involved in the insulin cascade (like PI3K) bind to phosphorylated tyrosine via their SH2 domains. * **Other RTKs:** Receptors for IGF-1, EGF, PDGF, and FGF also utilize the Tyrosine Kinase mechanism. * **Downregulation:** Chronic high insulin levels lead to receptor internalisation and downregulation, a key feature in Type 2 Diabetes/Insulin Resistance.

Q: What is the second messenger for oxytocin?

IP3-DAG. **Explanation:** The mechanism of action for hormones depends on their receptor type. Oxytocin acts via the **Gq protein-coupled receptor (GPCR)** pathway. When oxytocin binds to its receptor on the uterine or mammary smooth muscle, it activates **Phospholipase C (PLC)**. PLC cleaves membrane-bound Phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: **Inositol triphosphate (IP3)** and **Diacylglycerol (DAG)**. IP3 triggers the release of calcium from the sarcoplasmic reticulum, while DAG activates Protein Kinase C, leading to muscle contraction (labor and milk let-down). **Analysis of Options:** * **A. cAMP:** This is the second messenger for hormones acting via Gs/Gi pathways, such as Glucagon, ACTH, and ADH (V2 receptors). * **B. cGMP:** Utilized by Atrial Natriuretic Peptide (ANP) and Nitric Oxide (NO) via guanylyl cyclase activation. * **D. NO (Nitric Oxide):** This is a paracrine signaling molecule (gas) that acts as a primary messenger to stimulate cGMP production; it is not the second messenger for oxytocin. **High-Yield NEET-PG Pearls:** * **Gq Pathway Mnemonic:** Remember **"GOAT HAG"** for hormones using IP3-DAG: **G**nRH, **O**xytocin, **A**DH (V1 receptor), **T**RH, **H**istamine (H1), **A**ngiotensin II, and **G**astrin. * **Clinical Correlation:** Synthetic oxytocin (Pitocin) is used for induction of labor and management of postpartum hemorrhage (PPH) due to its potent contractile effect via the Gq pathway. * **ADH Contrast:** ADH uses **V1 receptors (Gq/IP3-DAG)** for vasoconstriction but **V2 receptors (Gs/cAMP)** for water reabsorption in the kidney.

Q: G-proteins function primarily as:

Signal transducers. ### Explanation **Why "Signal Transducers" is Correct:** G-proteins (Guanine nucleotide-binding proteins) act as molecular switches that link extracellular signals (first messengers) to intracellular effectors. When a ligand binds to a **G-Protein Coupled Receptor (GPCR)**, the receptor undergoes a conformational change, activating the heterotrimeric G-protein ($\alpha, \beta, \gamma$ subunits). The G-protein exchanges GDP for GTP, dissociates, and "transduces" the signal to enzymes like Adenylate Cyclase or Phospholipase C. Thus, they serve as the essential bridge or **transducer** between the receptor and the cellular response. **Analysis of Incorrect Options:** * **A. Hormone carriers:** These are proteins like Albumin or Sex Hormone Binding Globulin (SHBG) that transport hydrophobic hormones through the bloodstream. * **B. Hormone receptors:** These are the initial docking sites (e.g., GPCRs, Tyrosine Kinase receptors) that recognize the ligand. G-proteins are peripheral membrane proteins associated with these receptors, not the receptors themselves. * **C. Second messengers:** These are small intracellular molecules (e.g., cAMP, $IP_3$, $DAG$, $Ca^{2+}$) produced *after* G-protein activation. G-proteins are the "messengers' delivery system," not the message itself. **High-Yield Clinical Pearls for NEET-PG:** * **G-protein Activity:** They possess intrinsic **GTPase activity** (primarily in the $\alpha$-subunit), which hydrolyzes GTP to GDP to terminate the signal. * **Cholera Toxin:** Inhibits GTPase activity of $G_s$, leading to constitutive activation of Adenylate Cyclase and permanent rise in cAMP (causing secretory diarrhea). * **Pertussis Toxin:** Inhibits $G_i$ (the inhibitory G-protein), also leading to increased cAMP levels. * **Nobel Prize Fact:** Robert Lefkowitz and Brian Kobilka won the 2012 Nobel Prize in Chemistry for their work on GPCRs.

Question 1

All of the following belong to the steroid receptor superfamily EXCEPT:

Accepted Answer

Epinephrine receptor

Answer

Vitamin D3 receptor

Answer

Thyroid receptor

Answer

Retinoid receptor

Question 2

Fibroblast Growth Factor mediates its effect through which of the following receptors?

Accepted Answer

Tyrosine kinase

Answer

cAMP

Answer

cGMP

Answer

Ca2+

Question 3

Adrenaline acts through which type of receptor?

Accepted Answer

Seven pass receptor

Answer

Single pass receptor

Answer

Four pass receptor

Answer

Ligand gated channel

Question 4

Which of the following hormones are synthesized as peptide precursors?

Accepted Answer

All of the above

Answer

Insulin

Answer

Parathyroid hormone (PTH)

Answer

Renin

Question 5

Which of the following is NOT true about G proteins?

Accepted Answer

G proteins are made up of 4 subunits.

Answer

G channels are regulated by G proteins.

Answer

G proteins are involved in phosphorylase formation.

Answer

G proteins are related to the Ras oncogene.

Question 6

Slow-reacting substance of anaphylaxis (SRS-A) is?

Accepted Answer

Leukotriene

Answer

Prostaglandin

Answer

Serotonin

Answer

Histamine

Question 7

In the context of G-protein signaling, which of the following is considered a second messenger?

Accepted Answer

Calcium

Answer

cAMP

Answer

CO

Answer

NO

Question 8

Insulin receptors are:

Accepted Answer

Tyrosine Kinase receptors

Answer

Phosphodiesterase

Answer

Calcium calmodulin

Answer

Lipoprotein

Question 9

What is the second messenger for oxytocin?

Accepted Answer

IP3-DAG

Answer

cAMP

Answer

cGMP

Answer

NO

Question 10

G-proteins function primarily as:

Accepted Answer

Signal transducers

Answer

Hormone carriers

Answer

Hormone receptors

Answer

Second messengers

Signal Transduction — MCQs

Signal Transduction — MCQs

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