Molecular Basis of Enzyme Function — MCQs
Assertion: RMP depends on proteins and phosphate ions. Reason: Diffusion potential can be calculated using nernst equation. Choose the best statement regarding the assertion and reason.
Which of the following is true about non-competitive inhibition?
Which of the following is an example of allosteric inhibition?
Which of the following statements about isozymes is true?
Identify the false statement regarding suicide inhibition
What is the specific activity of an enzyme?
Enzymes that move a molecular group from one molecule to another are known as -
The primary hormone secreted by duodenal cells in response to dietary fats and proteins is:
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?
Molecular Basis of Enzyme Function Indian Medical PG Practice Questions and MCQs
Question 1: Assertion: RMP depends on proteins and phosphate ions. Reason: Diffusion potential can be calculated using nernst equation. Choose the best statement regarding the assertion and reason.
- A. Assertion false, Reason true
- B. Both true, Reason is the explanation of assertion
- C. Assertion true, Reason false
- D. Both true, Reason is not the explanation of assertion (Correct Answer)
Explanation: ***Both true, Reason is not the explanation of assertion*** - The **Assertion is TRUE**: The resting membrane potential (RMP) does depend on intracellular **proteins and phosphate ions**, which are large, non-diffusible anions that remain trapped inside the cell. These molecules contribute significantly to the **net negative charge** of the intracellular compartment and create the **Gibbs-Donnan effect**. At physiological pH, most intracellular proteins are negatively charged, and phosphate ions (HPO₄²⁻, H₂PO₄⁻) are major intracellular anions. While the primary determinants of RMP are the concentration gradients and membrane permeabilities of K⁺, Na⁺, and Cl⁻ ions, the presence of non-diffusible anions (proteins and phosphates) is essential for establishing the baseline negative intracellular environment. - The **Reason is TRUE**: The **Nernst equation** (E = RT/zF × ln[ion]out/[ion]in) is indeed used to calculate the **equilibrium potential** (also called diffusion potential) for a single permeable ion. This equation determines the membrane potential at which the electrical gradient exactly balances the concentration gradient for that specific ion, resulting in no net ion movement. - **However, the Reason does NOT explain the Assertion**: The Nernst equation calculates equilibrium potentials for diffusible ions like K⁺, Na⁺, and Cl⁻. It does NOT explain the contribution of **non-diffusible** anions (proteins and phosphates) to the RMP. The actual RMP, which involves multiple ions with different permeabilities, is calculated using the **Goldman-Hodgkin-Katz (GHK) equation**, not the Nernst equation. The two statements are independently true but address different aspects of membrane potential physiology. *Assertion false, Reason true* - This is **incorrect** because the assertion is actually TRUE. Intracellular proteins and phosphate ions do contribute to the RMP by providing fixed negative charges that influence the distribution of diffusible ions and create the electrochemical environment necessary for RMP establishment. *Both true, Reason is the explanation of assertion* - This is **incorrect** because while both statements are true, the Nernst equation (Reason) does not explain how proteins and phosphate ions contribute to RMP (Assertion). The Nernst equation applies only to permeable ions, whereas proteins and phosphates are impermeant molecules whose role is explained by the Gibbs-Donnan equilibrium and their contribution to fixed negative charges. *Assertion true, Reason false* - This is **incorrect** because the reason is TRUE. The Nernst equation is a fundamental and valid equation in membrane physiology that accurately calculates the equilibrium potential for any permeable ion based on its concentration gradient.
Question 2: Which of the following is true about non-competitive inhibition?
- A. Km increases, Vmax remains same
- B. Km decreases, Vmax increases
- C. Km increases, Vmax increases
- D. Km remains same, Vmax decreases (Correct Answer)
Explanation: ***Km remains same, Vmax decreases*** - In **non-competitive inhibition**, the inhibitor binds to an allosteric site on the enzyme, altering its conformation, thereby **reducing its catalytic efficiency**. - This binding does not affect the **enzyme's affinity for the substrate (Km remains the same)**, but it **reduces the maximum reaction rate (Vmax decreases)** because fewer enzyme molecules are able to perform catalysis per unit time. *Km increases, Vmax remains same* - This describes **competitive inhibition**, where the inhibitor competes with the substrate for the enzyme's active site. - While it **increases the apparent Km** (more substrate needed to reach half Vmax), **Vmax remains unchanged** as high substrate concentrations can overcome the inhibition. *Km decreases, Vmax increases* - This scenario would imply an activation rather than inhibition, where both enzyme affinity and catalytic efficiency are enhanced. - This is not characteristic of any standard **enzyme inhibition mechanism**. *Km increases, Vmax increases* - This combination is not observed in any typical **enzyme inhibition pattern**. - An increase in **Vmax** implies enhanced catalytic activity, while an increase in **Km** suggests reduced substrate affinity, which are contradictory effects for a single inhibitor.
Question 3: Which of the following is an example of allosteric inhibition?
- A. Decreased synthesis of glucokinase by glucagon
- B. Inactivation of glycogen synthase by phosphorylation
- C. Inhibition of PFK-1 by citrate (Correct Answer)
- D. None of the options
Explanation: ***Inhibition of PFK-1 by citrate*** - **Citrate** acts as an **allosteric inhibitor** of **phosphofructokinase-1 (PFK-1)**, a key enzyme in glycolysis. - Citrate binds to a site distinct from the active site, inducing a conformational change that reduces PFK-1's affinity for **fructose-6-phosphate**, thus slowing glycolysis. *Inactivation of glycogen synthase by phosphorylation* - This is an example of **covalent modification** (phosphorylation), not allosteric regulation. - Phosphorylation alters the enzyme's activity by adding a phosphate group, changing its structure and function. *Decreased synthesis of glucokinase by glucagon* - This describes **transcriptional regulation** or **gene expression control**, where glucagon affects the amount of enzyme produced. - It is not an example of allosteric regulation, which involves direct binding of a molecule to an enzyme to alter its activity. *None of the options* - This option is incorrect because the inhibition of PFK-1 by citrate is a classic example of allosteric inhibition.
Question 4: Which of the following statements about isozymes is true?
- A. They catalyze the same reaction but may differ in structure. (Correct Answer)
- B. They have the same quaternary structure.
- C. They have the same enzyme classification but differ in number and name.
- D. They are distributed uniformly across different organs.
Explanation: ***They catalyze the same reaction but may differ in structure.*** - Isozymes are **different forms of an enzyme** that catalyze the **same biochemical reaction** but have distinct amino acid sequences. - Due to their different amino acid sequences, isozymes can exhibit variations in their **molecular structure**, kinetic properties, and regulatory mechanisms. *They have the same quaternary structure.* - While some isozymes might have a similar quaternary structure (e.g., both being tetramers), it is not a defining characteristic; they often have **different subunit compositions** or arrangements. - Their structural differences, including quaternary structure, contribute to their distinct properties and often reflect their expression in **different tissues or developmental stages**. *They have the same enzyme classification but differ in number and name.* - Isozymes belong to the **same enzyme classification** (e.g., EC number) because they catalyze the identical reaction, but they are **not necessarily numbered differently** as distinct enzymes. - Their differing names typically reflect the tissue they are found in or their specific subunits (e.g., lactate dehydrogenase isozymes **LDH-1 to LDH-5**). *They are distributed uniformly across different organs.* - Isozymes typically exhibit a **tissue-specific distribution**, meaning their presence and relative abundance vary significantly between different organs and tissues. - This differential distribution allows for **fine-tuning metabolic pathways** to meet the specific physiological demands of each tissue.
Question 5: Identify the false statement regarding suicide inhibition
- A. The binding of the enzyme to the substrate analogue is irreversible
- B. The inhibitor forms a product with the enzyme and the product inhibits it
- C. The inhibitor can bind with any site resulting in suicidal inhibition (Correct Answer)
- D. They are enzyme specific and used in rational drug design
Explanation: ***The inhibitor can bind with any site resulting in suicidal inhibition*** - Suicide inhibition, also known as **mechanism-based inhibition**, is highly specific and requires the inhibitor to bind to the **active site** of the enzyme. - The enzyme then catalyzes a transformation of the inhibitor into a **reactive intermediate** that irreversibly binds to the active site. *The binding of the enzyme to the substrate analogue is irreversible* - This statement is true; once the suicide inhibitor is metabolically activated by the enzyme, it forms a **covalent bond** with a residue in the active site. - This irreversible binding permanently inactivates the enzyme. *The inhibitor forms a product with the enzyme and the product inhibits it* - This statement is true; the enzyme's catalytic action converts the inhibitor (a substrate analogue) into a **highly reactive compound**. - This reactive product then binds covalently and irreversibly to the enzyme's **active site**, leading to its inactivation. *They are enzyme specific and used in rational drug design* - This statement is true; suicide inhibitors are designed to be highly specific for a particular enzyme, as they rely on that enzyme's catalytic mechanism for their activation. - Their specificity and irreversible action make them valuable tools in **drug discovery** and **rational drug design**, allowing for targeted inactivation of disease-related enzymes.
Question 6: What is the specific activity of an enzyme?
- A. Enzyme units per mg of protein (Correct Answer)
- B. Concentration of substrate transformed per minute
- C. Enzyme units per mg of substrate
- D. Limit of enzyme per gram of substrate
Explanation: ***Enzyme units per mg of protein*** - **Specific activity** is defined as the number of **enzyme units** (representing catalytic activity) per milligram of total protein in the sample. - It is a measure of **purity**, indicating the amount of active enzyme relative to other proteins in a preparation. - Formula: Specific activity = Units of enzyme activity / mg of total protein - Used to track enzyme purification progress during isolation procedures. *Concentration of substrate transformed per minute* - This describes the **reaction velocity** or rate of catalysis, but not the specific activity of the enzyme. - While related to enzyme activity, it does not normalize the activity to the amount of **total protein**. - This would be expressed as reaction rate or velocity (V), not specific activity. *Enzyme units per mg of substrate* - This is an incorrect formulation that confuses substrate with protein. - **Specific activity** is normalized to the amount of **protein** in the enzyme preparation, not the substrate. - This option represents a common misconception in enzyme kinetics terminology. *Limit of enzyme per gram of substrate* - This phrase does not correspond to any standard biochemical measure of enzyme activity or concentration. - It does not provide information about the **catalytic efficiency** or **purity** of the enzyme preparation. - The term "limit" is not used in the context of specific activity measurements.
Question 7: Enzymes that move a molecular group from one molecule to another are known as -
- A. Transferases (Correct Answer)
- B. Ligases
- C. Dipeptidases
- D. Oxido-reductases
Explanation: ***Transferases*** - **Transferases** are a class of enzymes that catalyze the transfer of a specific functional group (e.g., methyl, acetyl, phosphate) from one molecule (the donor) to another (the acceptor). - This broad category includes enzymes vital for many metabolic pathways, such as **kinases** (transferring phosphate groups) and **transaminases** (transferring amino groups). *Ligases* - **Ligases** are enzymes responsible for joining two large molecules together, typically by forming a new chemical bond. - This process usually involves the concomitant hydrolysis of a small, energy-rich molecule such as **ATP**, to provide the necessary energy for bond formation. *Dipeptidases* - **Dipeptidases** are a type of hydrolase enzyme that specifically cleaves the peptide bond within a **dipeptide**, releasing two free amino acids. - They are crucial for the final stages of protein digestion, breaking down small peptides into absorbable **amino acid units**. *Oxido-reductases* - **Oxido-reductases** are enzymes that catalyze **oxidation-reduction reactions** (redox reactions), where electrons are transferred from one molecule to another. - This class includes enzymes like **dehydrogenases** and **oxidases**, which play critical roles in cellular respiration and energy production.
Question 8: The primary hormone secreted by duodenal cells in response to dietary fats and proteins is:
- A. Secretin
- B. Gastrin
- C. CCK (Correct Answer)
- D. Motilin
Explanation: ***Correct Answer: CCK (Cholecystokinin)*** - **Cholecystokinin (CCK)** is primarily released from the **duodenal I-cells** in response to the presence of **fats and proteins** in the chyme entering the duodenum. - Its main functions include stimulating **gallbladder contraction** (releasing bile for fat emulsification) and **pancreatic enzyme secretion** (for nutrient digestion). *Incorrect: Secretin* - **Secretin** is primarily released in response to **acidic chyme** entering the duodenum, not directly by fats and proteins. - Its main roles are to stimulate the pancreas to release **bicarbonate-rich fluid** to neutralize gastric acid and to inhibit gastric acid secretion. *Incorrect: Gastrin* - **Gastrin** is secreted by **G-cells** in the stomach and duodenum, primarily in response to food (especially proteins) and vagal stimulation. - Its main function is to stimulate **gastric acid secretion** by parietal cells, not directly to dietary fats and proteins as a primary duodenal response. *Incorrect: Motilin* - **Motilin** is released from the small intestine during the **interdigestive period** (when fasting). - It plays a key role in initiating the **migrating motor complex (MMC)**, which sweeps undigested food and bacteria from the stomach and small intestine into the colon.
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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