Enzymes — MCQs
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Which of the following genetic disorders is treated with enzyme replacement therapy?
Which statement is false about allosteric regulation?
Non-competitive inhibition is:
Which of the following enzymes is a constituent of the HMP shunt?
Which of the following is an example of a reverse transcriptase?
Enzymes Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following genetic disorders is treated with enzyme replacement therapy?
- A. Gaucher's disease (Correct Answer)
- B. Krabbe's disease
- C. Metachromatic leukodystrophy
- D. Tay-Sachs disease
Explanation: **Explanation:** **Gaucher’s Disease (Option A)** is the correct answer because it was the first lysosomal storage disorder (LSD) for which **Enzyme Replacement Therapy (ERT)** was developed. It is caused by a deficiency of the enzyme **Glucocerebrosidase** (Acid $\beta$-glucosidase), leading to the accumulation of glucosylceramide in macrophages (Gaucher cells). Recombinant enzymes like **Imiglucerase** are administered intravenously to clear these deposits, particularly improving hepatosplenomegaly and hematological parameters in Type 1 Gaucher’s. **Why the other options are incorrect:** * **Krabbe’s disease (Option B):** Caused by **Galactocerebrosidase** deficiency. ERT is not the standard of care because the enzyme cannot cross the blood-brain barrier (BBB) to treat the severe central nervous system (CNS) demyelination. Hematopoietic stem cell transplantation (HSCT) is the preferred intervention. * **Metachromatic leukodystrophy (Option C):** Caused by **Arylsulfatase A** deficiency. Similar to Krabbe’s, the primary pathology is in the CNS, making standard ERT ineffective. Gene therapy and HSCT are the focus of current management. * **Tay-Sachs disease (Option D):** Caused by **Hexosaminidase A** deficiency. It involves rapid neurodegeneration. ERT cannot reach the brain tissues effectively, and currently, treatment remains supportive. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Described as having a **"wrinkled paper"** or "crumpled silk" appearance of the cytoplasm. * **ERT Success:** ERT is highly effective for LSDs with significant **systemic/visceral** involvement (e.g., Gaucher Type 1, Fabry, Pompe, and MPS I/Hurler) but is generally ineffective for purely **neurodegenerative** conditions due to the BBB. * **Alternative Treatment:** Substrate Reduction Therapy (SRT) using **Miglustat** is also used in Gaucher’s to decrease the synthesis of the accumulating substrate.
Question 2: Which statement is false about allosteric regulation?
- A. It is usually the mode of regulation for the first committed step in reaction pathways. (Correct Answer)
- B. Cellular response is faster with allosteric control than by controlling enzyme concentration in the cell.
- C. The regulation is important to the conservation of energy and materials in cells.
- D. Allosteric modulators bind non-covalently at sites other than the active site and induce conformational changes in the enzyme.
Explanation: ### Explanation **Why Option A is the Correct Answer (The False Statement):** In the context of this specific question, Option A is technically a **true** statement regarding biochemistry. However, in many NEET-PG style assessments, if this is marked as the "false" option, it is often due to a technicality in phrasing or a specific textbook context where allosteric regulation is contrasted with other forms of control. *Correction/Refinement:* Allosteric regulation **is** indeed the most common mode of regulation for the **first committed step** (rate-limiting step) of a metabolic pathway (e.g., PFK-1 in glycolysis). If the question identifies this as the "false" statement, it may be implying that not *all* committed steps are regulated *exclusively* by allosteric means (some use covalent modification or induction). **Analysis of Other Options:** * **Option B (True):** Allosteric control involves simple binding/unbinding of a ligand, causing an immediate conformational change. This is significantly faster than **enzyme induction/repression**, which requires transcription and translation (taking hours to days). * **Option C (True):** By inhibiting the first committed step via feedback inhibition, the cell prevents the unnecessary accumulation of intermediates and the wasteful expenditure of ATP and substrates. * **Option D (True):** By definition, allosteric ("other site") modulators bind **non-covalently** to a regulatory site. This induces a conformational change (T-state to R-state or vice versa) that alters the affinity of the active site for the substrate. **High-Yield Clinical Pearls for NEET-PG:** * **Kinetics:** Allosteric enzymes show a **Sigmoidal (S-shaped)** curve on a velocity-substrate plot, unlike the hyperbolic curve of Michaelis-Menten enzymes. * **Feedback Inhibition:** The end-product of a pathway often acts as a negative allosteric effector of the rate-limiting enzyme. * **Key Example:** **Phosphofructokinase-1 (PFK-1)** is the rate-limiting enzyme of glycolysis; it is allosterically inhibited by ATP and Citrate, and activated by AMP and Fructose 2,6-bisphosphate. * **Aspartate Transcarbamoylase (ATCase):** A classic example of allosteric regulation in pyrimidine synthesis, inhibited by CTP.
Question 3: Non-competitive inhibition is:
- A. Reversible
- B. Irreversible
- C. Any of the above (Correct Answer)
- D. None of the above
Explanation: ### Explanation In biochemistry, **Non-competitive inhibition** occurs when an inhibitor binds to a site other than the active site (the allosteric site). This binding induces a conformational change in the enzyme, reducing its catalytic activity regardless of whether the substrate is bound. **1. Why "Any of the above" is correct:** Non-competitive inhibition is traditionally categorized based on the nature of the bond formed between the inhibitor and the enzyme: * **Reversible Non-competitive Inhibition:** The inhibitor binds via weak, non-covalent interactions (e.g., hydrogen bonds). The inhibitor can dissociate, and the enzyme's function can be restored. * **Irreversible Non-competitive Inhibition:** The inhibitor binds via strong covalent bonds or destroys a functional group necessary for catalysis. This is often referred to as "irreversible inhibition" or "enzyme poisoning." Because the term "non-competitive" describes the **site and mechanism** of binding (not competing for the active site), it can technically be either reversible or irreversible. **2. Analysis of Incorrect Options:** * **Option A (Reversible):** While many classic examples (like Ferrochelatase inhibition by Lead) are reversible, this is too restrictive as it excludes irreversible inhibitors. * **Option B (Irreversible):** Similarly, many non-competitive inhibitors (like Cyanide) act irreversibly, but this option ignores the reversible class. **3. NEET-PG High-Yield Pearls:** * **Kinetics:** In non-competitive inhibition, **$V_{max}$ decreases** (the engine is broken), but **$K_m$ remains unchanged** (affinity for the substrate is the same). * **Classic Example:** Heavy metal poisoning (Lead, Mercury) and Cyanide (inhibiting Cytochrome Oxidase). * **Comparison:** Unlike Competitive inhibition, non-competitive inhibition **cannot** be overcome by increasing the substrate concentration. * **Graph:** On a Lineweaver-Burk plot, the lines intersect on the negative x-axis ($-1/K_m$).
Question 4: Which of the following enzymes is a constituent of the HMP shunt?
- A. Glucose 6 Phosphatase
- B. Hexokinase
- C. G6P dehydrogenase (Correct Answer)
- D. Phosphorylase
Explanation: **Explanation:** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol and is essential for generating **NADPH** (for reductive biosynthesis) and **Ribose-5-phosphate** (for nucleotide synthesis). **Why G6P Dehydrogenase (G6PD) is correct:** G6PD is the **rate-limiting and key regulatory enzyme** of the HMP shunt. It catalyzes the first step of the oxidative phase, converting Glucose-6-Phosphate into 6-Phosphogluconolactone. This reaction reduces $NADP^+$ to $NADPH$. **Analysis of Incorrect Options:** * **A. Glucose-6-Phosphatase:** This enzyme is involved in **Gluconeogenesis** and **Glycogenolysis** (found in liver/kidneys). It converts G6P to free glucose to maintain blood sugar levels. * **B. Hexokinase:** This is the first enzyme of **Glycolysis**, responsible for phosphorylating glucose to Glucose-6-Phosphate in extrahepatic tissues. * **D. Phosphorylase:** This is the key enzyme of **Glycogenolysis**, responsible for breaking down glycogen into Glucose-1-Phosphate. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** under oxidative stress (e.g., Fava beans, Primaquine, infections) because RBCs cannot generate enough NADPH to maintain reduced glutathione. 2. **Heinz Bodies & Bite Cells:** Classic peripheral smear findings in G6PD deficiency. 3. **Tissue Distribution:** The HMP shunt is highly active in tissues requiring NADPH for lipid/steroid synthesis (Adrenal cortex, Liver, Lactating mammary glands) and in RBCs to combat oxidative stress. 4. **Transketolase:** Another HMP shunt enzyme; it requires **Thiamine (Vitamin B1)** as a cofactor. Measuring its activity is used to diagnose Thiamine deficiency.
Question 5: Which of the following is an example of a reverse transcriptase?
- A. Gyrase
- B. Helicase
- C. Telomerase (Correct Answer)
- D. RNA polymerase
Explanation: **Explanation:** The correct answer is **Telomerase**. **Why Telomerase is a Reverse Transcriptase:** Reverse transcriptase is an enzyme that synthesizes DNA using an RNA template (RNA-dependent DNA polymerase). Telomerase is a specialized ribonucleoprotein complex responsible for maintaining the length of telomeres (the repetitive TTAGGG sequences at the ends of chromosomes). It contains an intrinsic RNA molecule that serves as a template to synthesize telomeric DNA, thereby preventing the "end-replication problem" and chromosomal shortening during cell division. **Analysis of Incorrect Options:** * **Gyrase (Topoisomerase II):** This enzyme relieves torsional strain (supercoiling) ahead of the replication fork by creating double-stranded breaks in DNA. * **Helicase:** This enzyme uses ATP to unwind the DNA double helix into single strands by breaking hydrogen bonds between nitrogenous bases. * **RNA Polymerase:** This enzyme performs transcription, synthesizing RNA from a DNA template (DNA-dependent RNA polymerase). **High-Yield Clinical Pearls for NEET-PG:** * **Cancer & Aging:** Telomerase activity is high in germ cells, stem cells, and **cancer cells** (conferring immortality), but low or absent in most somatic cells, leading to cellular senescence. * **Other Reverse Transcriptases:** Retroviruses like **HIV** utilize reverse transcriptase to integrate their viral genome into the host DNA. * **Zidovudine (AZT):** A common NEET-PG topic; it is a drug that inhibits reverse transcriptase, used in the treatment of HIV. * **Telomere Sequence:** In humans, the repeating hexanucleotide sequence is **5'-TTAGGG-3'**.
Practice by Chapter
Enzyme Classification and Nomenclature
Practice Questions
Enzyme Kinetics and Michaelis-Menten Equation
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Enzyme Inhibition: Competitive and Non-competitive
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Allosteric Regulation
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Coenzymes and Cofactors
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Isoenzymes and Clinical Significance
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Enzyme Regulation: Covalent Modification
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Enzyme Regulation: Zymogen Activation
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Enzyme Induction and Repression
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Ribozymes and Catalytic RNA
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Enzyme Diagnostic Applications
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Enzyme Therapy and Inhibitors as Drugs
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