Signal Transduction — MCQs

Signal Transduction Indian Medical PG Practice Questions and MCQs

Question 101: All are true regarding G protein Except?

A. It has a trimeric structure
B. Alpha subunit has GTPase activity
C. It is activated by GTP binding
D. Active conformation has all three subunits (Correct Answer)

Explanation: ***Active conformation has all three subunits*** - The **active conformation** of a G protein typically involves the dissociation of the **alpha subunit** from the **beta-gamma dimer** upon GTP binding. - The alpha subunit, now bound to GTP, and the beta-gamma dimer then activate downstream effector proteins independently; thus, the fully trimeric structure is not the active conformation. *It has a trimeric structure* - In its **inactive state**, a G protein is indeed a **heterotrimer** composed of alpha (α), beta (β), and gamma (γ) subunits, with GDP bound to the alpha subunit. - This **trimeric structure** is a defining characteristic of G proteins before activation by a G protein-coupled receptor. *Alpha subunit has GTPase activity* - The alpha subunit of a G protein possesses intrinsic **GTPase activity**, which is crucial for terminating signaling. - This activity allows it to **hydrolyze bound GTP back to GDP**, returning the G protein to its inactive state. *It is activated by GTP binding* - G proteins are activated when a **guanosine diphosphate (GDP)** molecule bound to the alpha subunit is exchanged for a **guanosine triphosphate (GTP)** molecule. - This **GTP binding** is triggered by the activation of a G protein-coupled receptor (GPCR) by its ligand, leading to conformational changes that facilitate nucleotide exchange.

Question 102: If there is a Gs alpha subunit gain-of-function mutation, this results in

A. Increased cAMP (Correct Answer)
B. Decreased cAMP
C. Increased GTPase activity
D. Decreased IP₃

Explanation: ***Increased cAMP*** - A **gain-of-function mutation** in the **Gs alpha subunit** means it remains in its active, GTP-bound state for longer. - The activated Gs alpha subunit stimulates **adenylyl cyclase**, leading to persistently high levels of **cAMP**. *Decreased cAMP* - This would result from a **loss-of-function** mutation in the Gs alpha subunit or a gain-of-function in an inhibitory G protein (Gi), not a Gs gain-of-function. - A decrease in cAMP would inhibit downstream signaling pathways, which is the opposite of what occurs with Gs activation. *Increased GTPase activity* - **GTPase activity** is responsible for hydrolyzing GTP to GDP, which inactivates the G alpha subunit. - A gain-of-function mutation often implies **reduced GTPase activity**, causing the G protein to stay active longer, not increased activity. *Decreased IP* - **IP3 (inositol trisphosphate)** is a secondary messenger produced via the activation of **phospholipase C**, typically by Gq proteins. - Gs alpha subunit mutations primarily affect the **adenylyl cyclase/cAMP pathway**, not the inositol phosphate pathway.

Question 103: JAK-STAT kinase receptors are associated with?

A. Ion channels
B. Nuclear transcription (Correct Answer)
C. IP3 - DAG
D. cAMP

Explanation: ***Nuclear transcription*** - JAK-STAT signaling is a **ligand-activated pathway** that directly regulates gene transcription in response to cytokines and growth factors. - Upon activation, **STAT proteins** translocate to the nucleus to act as transcription factors, modulating gene expression. *Ion channels* - **Ion channels** are transmembrane proteins that allow the passage of ions across cell membranes, typically involved in electrical signaling. - Their activity is usually regulated by **voltage changes** or direct ligand binding, not primarily by JAK-STAT signaling. *IP3 - DAG* - The **inositol triphosphate (IP3)** and **diacylglycerol (DAG)** pathway is a second messenger system primarily activated by G protein-coupled receptors (GPCRs). - This pathway leads to intracellular **calcium release** and protein kinase C activation, distinct from JAK-STAT. *cAMP* - **cAMP** (cyclic adenosine monophosphate) is a common second messenger activated by GPCRs, leading to the activation of **protein kinase A (PKA)**. - This signaling cascade is distinct from the direct transcriptional role of the JAK-STAT pathway.

Question 104: Which of the following requires calcium for ligand binding?

A. Sialyl-Lewis X
B. LFA-1
C. PSGL-1
D. P-selectin (Correct Answer)

Explanation: ***P-selectin*** - **P-selectin** is a **C-type (calcium-dependent) lectin** that requires **Ca²⁺ ions** for its carbohydrate recognition domain to function. - The calcium is essential for maintaining the proper **conformational structure** of the lectin domain, enabling it to bind carbohydrate ligands like **sialyl-Lewis X** on leukocytes. - Expressed on activated endothelial cells and platelets during inflammation. *Sialyl-Lewis X* - This is a **carbohydrate ligand** (not a receptor) found on leukocytes that binds to selectins. - It does not itself require calcium for binding; rather, it is recognized by calcium-dependent selectins. *LFA-1* - **LFA-1** is an **integrin** that binds to ICAMs. - While integrins require **Mg²⁺** (not Ca²⁺) for optimal binding, LFA-1 is not primarily characterized as calcium-dependent for ligand binding. *PSGL-1* - **PSGL-1** is a **glycoprotein ligand** on leukocytes, not a receptor. - It does not require calcium for its own function; the calcium dependency is on the selectin side of the interaction.

Question 105: In the mitogen activated protein kinase pathway, the activation of RAS is counteracted by

A. Inositol triphosphate
B. GTPase activating protein (Correct Answer)
C. Phosphatidyl inositol
D. Protein kinase C

Explanation: ***GTPase activating protein*** - **GTPase Activating Proteins (GAPs)** facilitate the hydrolysis of **GTP bound to RAS** to GDP, converting active RAS back to its inactive state. - This inactivation is crucial for turning off the downstream signaling of the **MAPK pathway** and preventing uncontrolled cell proliferation. *Inositol triphosphate* - **Inositol triphosphate (IP3)** is a secondary messenger that triggers the release of **intracellular calcium** from the endoplasmic reticulum. - It is involved in various signaling pathways, but its primary role is not to directly counteract RAS activation. *Phosphatidyl inositol* - **Phosphatidylinositol (PI)** is a component of cell membranes and can be phosphorylated to produce various **phosphatidylinositol phosphates (PIPs)**, like **PIP2** and **PIP3**. - These molecules act as docking sites for signaling proteins but do not directly inactivate RAS. *Protein kinase C* - **Protein kinase C (PKC)** is a family of enzymes involved in signal transduction, typically activated by **diacylglycerol (DAG)** and calcium. - It phosphorylates various proteins, mediating diverse cellular responses, but it does not directly counteract the activation of RAS.

Question 106: A small Ca+2 binding protein that modifies the activity of many enzymes and other proteins in response to changes of Ca+2 concentration is known as:

A. Cyclin
B. Kinesin
C. Collagen
D. Calmodulin (Correct Answer)

Explanation: ***Calmodulin*** - **Calmodulin** is a highly conserved, small **calcium-binding protein** that acts as a central mediator of calcium signaling pathways. - It undergoes a conformational change upon binding **Ca2+**, enabling it to interact with and regulate the activity of numerous target enzymes and proteins, thereby orchestrating various cellular processes. *Cyclin* - **Cyclins** are a family of proteins that activate **cyclin-dependent kinases (CDKs)**, playing a crucial role in regulating the progression through the cell cycle. - While essential for cell division, cyclins do not primarily function as **Ca2+-binding regulatory proteins**. *Kinesin* - **Kinesin** is a motor protein that moves along **microtubules**, transporting vesicles, organelles, and other cellular components within the cell. - Its primary function is in intracellular transport and cell division, not in **Ca2+-dependent enzymatic regulation**. *Collagen* - **Collagen** is the most abundant protein in mammals, forming the main structural component of connective tissues like skin, bone, and cartilage. - It provides structural integrity and tensile strength to tissues and is not involved in **calcium-dependent enzyme modulation**.

Question 107: One of the ways that cells communicate with each other is by secretion of various molecules. The secreted molecule is known as

A. A receptor molecule
B. An integrin
C. A spectrin tetramer
D. A signaling molecule (Correct Answer)

Explanation: ***A signaling molecule*** - Cells communicate by releasing **signaling molecules** that travel to other cells and bind to specific receptors. - These molecules transmit information, initiating a **response** in the target cell. *A receptor molecule* - A **receptor molecule** is located on the target cell's surface or inside the cell and binds to the signaling molecule, rather than being the secreted molecule itself. - Its role is to **receive** the signal, not to transmit it from the secreting cell. *An integrin* - **Integrins** are transmembrane proteins that link the cell's cytoskeleton to the extracellular matrix, facilitating cell adhesion and migration. - They are primarily involved in **cell-matrix interactions** and less directly in general cell-to-cell signaling via secretion. *A spectrin tetramer* - A **spectrin tetramer** is a component of the cytoskeleton, particularly important in maintaining the shape and structural integrity of red blood cells. - It functions as an **intracellular structural protein** and is not a secreted molecule involved in cell-to-cell communication.

Question 108: How many subunits are there in the insulin receptor?

A. 3
B. 2
C. 4 (Correct Answer)
D. 1

Explanation: ***4*** - The **insulin receptor** is a **tetramer** composed of two alpha (α) subunits and two beta (β) subunits. - The two α subunits are located extracellularly and are responsible for **insulin binding**, while the two β subunits span the cell membrane and possess **tyrosine kinase activity** upon insulin binding. *3* - The insulin receptor is not a trimeric protein. It contains four subunits. - There are no known isoforms of the insulin receptor that are trimeric. *2* - While it has two types of subunits (alpha and beta), the functional receptor is a tetramer. - The receptor requires both alpha and beta subunits to be present in pairs for proper function. *1* - The insulin receptor is a complex protein, not a monomer. - A single subunit would not be able to perform both insulin binding and intracellular signaling.

Question 109: Steroid hormone receptors have attachment site for all except:

A. Hormone responsive element
B. Steroid hormone
C. Transcription activators (Correct Answer)
D. Transcription repressors

Explanation: ***Transcription activators*** - **Steroid hormone receptors** bind directly to **steroid hormones** and **hormone response elements (HREs)** on DNA, as well as to **transcription repressors** in their inactive state. - They do not have a direct attachment site for **transcription activators**; rather, activated steroid receptors can *act* as transcription activators or co-activators through protein-protein interactions. *Hormone responsive element* - This is a specific **DNA sequence** in the promoter region of target genes where the **steroid hormone-receptor complex** binds to regulate gene transcription. - It defines the genomic target for the activated steroid receptor, ensuring **gene-specific responses**. *Steroid hormone* - The **steroid hormone** itself binds to its specific receptor, inducing a conformational change that allows the receptor to translocate to the nucleus and bind to DNA. - This binding is essential for the **receptor's activation** and subsequent gene regulation. *Transcription repressors* - In the absence of a steroid hormone, **transcription repressors** (e.g., heat shock proteins) are often bound to the **steroid hormone receptor**, preventing its activation and binding to DNA. - Upon hormone binding, these repressors dissociate, allowing the receptor to become active and modulate **gene expression**.

Question 110: cGMP is a second messenger for which of the following hormones?

A. Nitric oxide
B. Atrial natriuretic factor
C. Both Nitric oxide and Atrial natriuretic factor (Correct Answer)
D. None of the options

Explanation: ***Both Nitric oxide and Atrial natriuretic factor*** - Both **nitric oxide (NO)** and **atrial natriuretic factor (ANF/ANP)** utilize **cGMP as a second messenger**. - **ANF** binds to membrane-bound guanylyl cyclase receptors (GC-A/natriuretic peptide receptor A), which directly catalyze the conversion of GTP to cGMP, leading to **natriuresis, vasodilation, and decreased blood pressure**. - **Nitric oxide** activates soluble guanylyl cyclase in target cells, producing cGMP, which causes **smooth muscle relaxation and vasodilation**. - Both are classic examples of the cGMP signaling pathway in medical biochemistry. *Atrial natriuretic factor (alone)* - While ANF does use cGMP as a second messenger, this option is incomplete. - Selecting only ANF ignores the fact that **nitric oxide also uses cGMP**, making "both" the more accurate answer. *Nitric oxide (alone)* - While NO does use cGMP as a second messenger, this option is incomplete. - Selecting only NO ignores the fact that **ANF also uses cGMP**, making "both" the more accurate answer. *None of the options* - This is incorrect because both **ANF** and **NO** clearly utilize cGMP as their second messenger. - The cGMP pathway is well-established for both molecules in cardiovascular and renal physiology.

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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