Receptor-Ligand Interactions — MCQs
Which of the following is a G protein coupled receptor?
Which of the following best demonstrates the variability in drug responsiveness among individuals?
Which of the following statements best describes the mechanism of action of insulin on target cells?
All these hormones primarily use cyclic adenosine monophosphate (cAMP) as their main second messenger pathway, except:
G-protein coupled receptor that does not act through opening of potassium channels is:
What is the primary mechanism of action of opioids in pain management?
All the following mediate their action using cAMP as second messenger except:
Thyroid hormone binds to which receptor ?
A research team is developing a gene therapy approach using CRISPR-Cas9 to correct a point mutation causing sickle cell disease. They must decide between two strategies: (A) correcting the mutation in hematopoietic stem cells ex vivo, or (B) in vivo correction in bone marrow. Considering molecular physiology principles, what is the most significant advantage of strategy A over strategy B?
A novel drug is designed to treat a genetic disorder caused by a nonsense mutation in the dystrophin gene. The drug works by allowing the ribosome to skip over the premature stop codon and continue translation. Evaluation of this therapeutic strategy reveals partial restoration of dystrophin protein with 60% of normal length but sufficient function. What is the most critical molecular consideration in determining if this approach will be clinically beneficial?
Receptor-Ligand Interactions Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is a G protein coupled receptor?
- A. M2 muscarinic receptor (Correct Answer)
- B. NMDA receptor
- C. Insulin receptors
- D. Steroid receptors
Explanation: ***M2 muscarinic receptor***- The **M2 muscarinic receptor** is a classic example of a **G protein-coupled receptor (GPCR)** [1]. When a ligand binds to a G-protein-coupled receptor, it triggers a mechanism where GDP is exchanged for GTP, causing the G-protein's alpha subunit to separate and initiate signaling pathways [1]. These heterotrimeric G-proteins couple cell surface receptors to catalytic units that form second messengers or directly to ion channels [1]. GPCRs are important regulators of nerve activity in the CNS and are receptors for neurotransmitters of the peripheral autonomic nervous system, with acetylcholine (ACh) being a ligand that regulates functions of glands and smooth muscle [2]. The **M2 muscarinic receptor** specifically activates an **inhibitory G protein (G_i)**, leading to a decrease in **cAMP** and opening of **potassium channels**. The effects of metabotropic receptors, like GPCRs, can last tens of seconds to minutes, contrasting with the brief effects of ionotropic receptors [4].*NMDA receptor*- The **NMDA receptor** is a **ligand-gated ion channel** that allows the influx of calcium and sodium ions [3]. It does not couple to G proteins, but directly mediates ion flow upon activation by **glutamate** and **glycine**. Ligand-gated ion channels open a central transmembrane ion channel when a neurotransmitter binds to sites on its extracellular domain [3].*Steroid*- **Steroid hormones** primarily act on **intracellular receptors** that, once activated, translocate to the nucleus to regulate gene expression. They are not cell surface receptors and do not utilize G protein signaling.*Insulin receptors*- **Insulin receptors** are **receptor tyrosine kinases** that, upon binding insulin, undergo autophosphorylation and activate intracellular signaling pathways. They signal through a cascade of protein phosphorylations, not through G proteins.
Question 2: Which of the following best demonstrates the variability in drug responsiveness among individuals?
- A. Potency
- B. Quantal Dose Response Curve (Correct Answer)
- C. Efficacy
- D. Graded Dose Response Curve
Explanation: ***Quantal Dose Response Curve*** - A **quantal dose-response curve** plots the percentage of individuals exhibiting a discrete, all-or-none effect against the log dose of a drug. - This curve directly illustrates the **variability in drug responsiveness** within a population by showing the range of doses required to produce a specific effect in different individuals. *Efficacy* - **Efficacy** refers to the maximum effect a drug can produce, regardless of the dose. - While efficacy is an important pharmacological parameter, it describes the drug's overall therapeutic potential, not the **individual variability** in response. *Potency* - **Potency** is a measure of the amount of drug needed to produce an effect of given intensity. - It relates to the absolute dose required for a particular effect but does not directly demonstrate the **inter-individual differences** in biological response. *Graded Dose Response Curve* - A **graded dose-response curve** depicts the relationship between the dose of a drug and the **magnitude of the effect** in a **single biological unit** (e.g., an individual, a tissue, or a cell). - This curve reflects the relationship between drug concentration and effect intensity, but not the **variability in response among different individuals** in a population.
Question 3: Which of the following statements best describes the mechanism of action of insulin on target cells?
- A. Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.
- B. Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.
- C. Insulin enters the cell and causes the release of calcium ions from intracellular stores.
- D. Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor. (Correct Answer)
Explanation: ***Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor.*** - **Insulin** is a **peptide hormone** and cannot freely pass through the lipid bilayer, thus it binds to a **transmembrane receptor** on the cell surface. - This binding leads to the activation of the receptor's intrinsic **tyrosine kinase activity** in the intracellular domain, initiating a signaling cascade. *Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.* - This mechanism describes the action of **steroid hormones**, which are lipid-soluble and can cross the cell membrane, binding to **intracellular receptors**. - **Insulin** acts via a **cell surface receptor** and its downstream effects are mediated through signal transduction pathways, not direct nuclear translocation. *Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.* - This mechanism is characteristic of **G-protein coupled receptors (GPCRs)**, which activate or inhibit enzymes like adenylate cyclase via G-proteins to produce second messengers like cyclic AMP. - The **insulin receptor** is a **receptor tyrosine kinase**, not a GPCR, and does not directly activate adenylate cyclase via Gs protein. *Insulin enters the cell and causes the release of calcium ions from intracellular stores.* - While some hormones and neurotransmitters can trigger the release of intracellular **calcium ions**, this is typically mediated by specific pathways (e.g., GPCRs linked to phospholipase C). - **Insulin** does not directly enter target cells to cause calcium release; its actions are primarily mediated through receptor tyrosine kinase signaling pathways.
Question 4: All these hormones primarily use cyclic adenosine monophosphate (cAMP) as their main second messenger pathway, except:
- A. Dopamine (Correct Answer)
- B. Glucagon
- C. vasopressin
- D. Corticotropin
Explanation: ***Dopamine*** - **Dopamine** has dual signaling mechanisms depending on receptor subtype, making it unique among the listed hormones. - **D1-like receptors** (D1, D5) couple to Gs proteins and **increase cAMP** levels. - **D2-like receptors** (D2, D3, D4) couple to Gi proteins and **decrease/inhibit cAMP** production. - Since dopamine's effects are mediated through both cAMP-increasing and cAMP-decreasing pathways with significant physiological roles for both, it does **not primarily use cAMP** as a straightforward second messenger like the other hormones listed. - Therefore, dopamine is the exception as it has mixed cAMP signaling rather than primarily activating the cAMP pathway. *Corticotropin (ACTH)* - **Corticotropin** (ACTH) binds to melanocortin-2 receptors (MC2R) on the adrenal cortex and **primarily utilizes the cAMP pathway**. - Activation of adenylyl cyclase leads to increased intracellular cAMP, which activates protein kinase A (PKA). - This stimulates the synthesis and release of glucocorticoids (primarily cortisol). *Glucagon* - **Glucagon** binds to its G-protein coupled receptors on hepatocytes, leading to activation of adenylyl cyclase and increased intracellular **cAMP**. - The cAMP then activates protein kinase A, mediating glucagon's metabolic effects including **glycogenolysis and gluconeogenesis**. - This is a classic example of cAMP-mediated hormone action. *Vasopressin* - **Vasopressin** (ADH) primarily acts through **V2 receptors** in the renal collecting ducts, which use the **cAMP pathway** to increase water reabsorption (its primary physiological function). - V1 receptors (vasoconstriction) use the IP3/DAG pathway, but this is a secondary effect. - Since vasopressin's main clinical action is via cAMP-mediated V2 receptors, it primarily uses cAMP as its second messenger.
Question 5: G-protein coupled receptor that does not act through opening of potassium channels is:
- A. Dopamine D2 receptor
- B. Muscarinic M2 receptor
- C. Serotonin 5 HT 1 receptor
- D. Angiotensin 1 receptor (Correct Answer)
Explanation: ***Angiotensin 1 receptor*** - The **angiotensin 1 receptor (AT1R)** is a **Gq-coupled receptor** that primarily activates the **phospholipase C (PLC)** pathway, leading to increased intracellular **calcium** and **IP3/DAG** signaling. - Its activation mediates vasoconstriction, aldosterone release, and cardiac hypertrophy, none of which involve direct opening of potassium channels. *Dopamine D2 receptor* - **Dopamine D2 receptors** are **Gi/o-coupled receptors** that inhibit adenylyl cyclase and **open potassium channels**, leading to **hyperpolarization** and reduced neuronal excitability. - This action contributes to its **antipsychotic** and **motor control** effects. *Muscarinic M2 receptor* - **Muscarinic M2 receptors** are **Gi/o-coupled receptors** found in the heart that cause **bradycardia** by activating **acetylcholine-gated inwardly rectifying potassium (GIRK) channels**, leading to hyperpolarization. - They also inhibit adenylyl cyclase, reducing cAMP levels and decreasing heart rate and contractility. *Serotonin 5 HT 1 receptor* - **Serotonin 5-HT1 receptors** (e.g., 5-HT1A) are **Gi/o-coupled receptors** that, upon activation, **increase potassium conductance** (hyperpolarization) and inhibit adenylyl cyclase. - This leads to a reduction in neuronal firing and is implicated in the anxiolytic and antidepressant effects of these receptors.
Question 6: What is the primary mechanism of action of opioids in pain management?
- A. Inhibition of cyclooxygenase (COX) enzymes
- B. Activation of opioid receptors in the spinal cord only
- C. Activation of opioid receptors in the brain only
- D. Activation of opioid receptors at both spinal and supraspinal levels (Correct Answer)
Explanation: ***Activation of opioid receptors at both spinal and supraspinal levels*** - Opioids primarily exert their analgesic effects by binding to and activating **mu (μ), delta (δ), and kappa (κ) opioid receptors** located throughout the central nervous system, including the brain and spinal cord. - Activation of these receptors modulates **pain perception**, emotional responses to pain, and descending pain inhibitory pathways. *Inhibition of cyclooxygenase (COX) enzymes* - This is the primary mechanism of action for **Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)**, not opioids. - NSAIDs reduce pain, inflammation, and fever by blocking the synthesis of **prostaglandins**. *Activation of opioid receptors in the spinal cord only* - While opioids do activate receptors in the spinal cord to inhibit **pain transmission**, their action is not confined to this area. - Significant **supraspinal analgesic effects** contribute to their overall pain-relieving properties, affecting higher brain centers involved in pain processing. *Activation of opioid receptors in the brain only* - Opioids act on opioid receptors in the brain to modulate pain perception and emotional aspects of pain, but they also have crucial effects at the **spinal cord level**. - Their action at the spinal cord level helps to prevent pain signals from reaching the brain, making **both levels crucial** for their comprehensive pain management.
Question 7: All the following mediate their action using cAMP as second messenger except:
- A. Glucagon
- B. Dopamine
- C. Corticotropin
- D. Vasopressin (Correct Answer)
Explanation: ***Vasopressin (ADH)*** - Vasopressin has **dual signaling mechanisms** depending on receptor type: - **V2 receptors** (kidney collecting duct): Use **Gs-protein → cAMP pathway** for water reabsorption via aquaporin-2 insertion - **V1 receptors** (vascular smooth muscle): Use **Gq-protein → IP3/DAG pathway** for vasoconstriction - In the context of this question, vasopressin is considered the exception because it has **significant non-cAMP mediated actions** through V1 receptors, unlike the other hormones listed which **predominantly or exclusively** use cAMP - **Note**: This is a teaching point about receptor subtypes; vasopressin DOES use cAMP at V2 receptors *Glucagon* - **Exclusively uses cAMP pathway** in hepatocytes and adipocytes - Binds to **glucagon receptor** (GPCR) → **Gs-protein** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Promotes glycogenolysis, gluconeogenesis, and lipolysis *Dopamine* - **D1 and D5 receptors** are **Gs-coupled** → **stimulate adenylyl cyclase** → **increase cAMP** - Important for neurotransmission (motor control, reward) and renal vasodilation - D2-family receptors (D2, D3, D4) inhibit cAMP but D1-family predominates in many physiological contexts *Corticotropin (ACTH)* - Binds to **melanocortin-2 receptor (MC2R)** on adrenal cortex - **Gs-protein coupled** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Stimulates steroidogenesis and cortisol secretion - **Exclusively cAMP-dependent mechanism**
Question 8: Thyroid hormone binds to which receptor ?
- A. Membrane
- B. Cytoplasmic
- C. Nuclear (Correct Answer)
- D. None of the options
Explanation: ***Nuclear*** - Thyroid hormones, being **lipid-soluble**, readily diffuse across the **cell membrane** to bind to receptors located in the nucleus. - This binding directly influences **gene expression** and protein synthesis, mediating the hormone's effects. *Membrane* - Membrane receptors typically bind **water-soluble hormones** (e.g., peptide hormones, catecholamines) that cannot freely cross the cell membrane. - These interactions usually trigger a **second messenger cascade** within the cell. *Cytoplasmic* - While some **steroid hormones** bind to cytoplasmic receptors which then translocate to the nucleus, thyroid hormones bind directly to nuclear receptors. - Cytoplasmic receptors are located in the **cytosol** before their ligand-induced translocation. *None of the options* - This option is incorrect, as thyroid hormones have a specific and well-defined receptor location. - The direct action on **gene regulation** necessitates a nuclear receptor.
Question 9: A research team is developing a gene therapy approach using CRISPR-Cas9 to correct a point mutation causing sickle cell disease. They must decide between two strategies: (A) correcting the mutation in hematopoietic stem cells ex vivo, or (B) in vivo correction in bone marrow. Considering molecular physiology principles, what is the most significant advantage of strategy A over strategy B?
- A. Strategy A allows for screening and selection of successfully edited cells before transplantation, minimizing off-target effects (Correct Answer)
- B. Strategy A requires lower doses of viral vectors
- C. Strategy A produces faster clinical improvement
- D. Strategy A is less expensive to implement
Explanation: ***Strategy A allows for screening and selection of successfully edited cells before transplantation, minimizing off-target effects*** - **Ex vivo** correction allows scientists to perform **quality control** by screening the patient's cells for the desired **on-target** modification and ensuring no harmful **off-target** mutations exist. - This selection process ensures that only **genetically verified** hematopoietic stem cells are re-infused, providing a significant safety and efficacy profile compared to blind **in vivo** delivery. *Strategy A requires lower doses of viral vectors* - While the total volume might be smaller, the primary advantage is the **precision** and **safety** of editing, not merely the quantity of the vector used. - **In vivo** methods actually face greater challenges with **vector distribution** and immune clearance, but this is less critical than the ability to screen cells. *Strategy A produces faster clinical improvement* - The **ex vivo** process is time-consuming, involving **cell harvesting**, laboratory editing, and **myeloablative conditioning** before re-infusion. - Clinical improvement depends on the **engraftment** of edited cells and the turnover of red blood cells, which is not necessarily faster than **in vivo** methods. *Strategy A is less expensive to implement* - **Ex vivo** gene therapy is highly expensive due to the need for **specialized laboratory facilities**, intensive cell culture protocols, and prolonged patient **hospitalization**. - **In vivo** strategies are conceptually cheaper and easier to scale, but currently lack the **safety oversight** provided by laboratory screening.
Question 10: A novel drug is designed to treat a genetic disorder caused by a nonsense mutation in the dystrophin gene. The drug works by allowing the ribosome to skip over the premature stop codon and continue translation. Evaluation of this therapeutic strategy reveals partial restoration of dystrophin protein with 60% of normal length but sufficient function. What is the most critical molecular consideration in determining if this approach will be clinically beneficial?
- A. Whether the drug prevents degradation of dystrophin mRNA
- B. Whether the drug enhances ribosomal binding to the start codon
- C. Whether the drug increases transcription of the dystrophin gene
- D. Whether the truncated protein retains the actin-binding domain and maintains membrane stability (Correct Answer)
Explanation: ***Whether the truncated protein retains the actin-binding domain and maintains membrane stability*** - For a truncated **dystrophin** protein to be clinically effective, it must preserve the functional linkage between the **actin cytoskeleton** and the **extracellular matrix**. - This is the fundamental mechanism behind converting a severe **Duchenne** phenotype into a milder **Becker** muscular dystrophy phenotype through **read-through** or exon-skipping therapies. *Whether the drug prevents degradation of dystrophin mRNA* - While **nonsense-mediated decay (NMD)** can reduce mRNA levels in nonsense mutations, preventing degradation is useless if the resulting translation still produces a non-functional protein. - The primary goal of read-through therapy is the quality and **functional domains** of the protein produced, rather than just the quantity of mRNA present. *Whether the drug enhances ribosomal binding to the start codon* - Enhancing **ribosomal binding** to the **start codon** (AUG) might increase the initiation of translation but does not address the premature stop codon issue. - Clinical benefit depends on the ribosome's ability to bypass the **premature termination codon (PTC)**, not the efficiency of initial binding. *Whether the drug increases transcription of the dystrophin gene* - Increasing **transcription** would only result in more mutated mRNA transcripts, which would still terminate at the **premature stop codon**. - Without a mechanism to ensure a functional protein product, simply increasing **gene expression** does not mitigate the mechanical instability of the muscle cell membrane.
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