Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 311: Codons are present in which type of RNA molecule?

A. Transfer RNA (t-RNA)
B. Ribosomal RNA (r-RNA)
C. Messenger RNA (m-RNA) (Correct Answer)
D. Small interfering RNA (si-RNA)

Explanation: **Explanation:** The correct answer is **Messenger RNA (m-RNA)**. In the process of translation, m-RNA serves as the template that carries genetic information from DNA to the ribosomes. A **codon** is a specific sequence of three consecutive nucleotides on the m-RNA molecule that codes for a specific amino acid or a stop signal during protein synthesis. **Analysis of Options:** * **Messenger RNA (m-RNA):** It contains the "genetic code" in the form of codons. There are 64 possible codons (61 sense codons and 3 stop codons). * **Transfer RNA (t-RNA):** t-RNA does not contain codons; instead, it contains the **anticodon**. The anticodon is a triplet sequence complementary to the m-RNA codon, allowing the t-RNA to deliver the correct amino acid to the ribosome. * **Ribosomal RNA (r-RNA):** This is a structural and catalytic component of ribosomes (e.g., the 28S r-RNA in eukaryotes acts as a peptidyl transferase ribozyme). It does not carry the triplet code for amino acids. * **Small interfering RNA (si-RNA):** These are short, double-stranded RNA molecules involved in the RNA interference (RNAi) pathway, primarily functioning in gene silencing and regulation rather than coding for proteins. **High-Yield NEET-PG Pearls:** * **Start Codon:** AUG (codes for Methionine in eukaryotes and N-formylmethionine in prokaryotes). * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), and UGA (Opal). * **Degeneracy/Redundancy:** A single amino acid can be coded by multiple codons (except Methionine and Tryptophan). * **Unambiguous:** One specific codon always codes for only one specific amino acid. * **Wobble Hypothesis:** Proposed by Francis Crick, it explains why the third base of a codon can undergo non-standard pairing with the anticodon.

Question 312: Which of the following enzymes acts as a free radical scavenger?

A. Glutathione peroxidase (Correct Answer)
B. NADH oxidase
C. Hydrogen peroxide
D. Hypochlorous acid

Explanation: **Explanation:** **Glutathione peroxidase (GPx)** is the correct answer because it is a key antioxidant enzyme that protects cells from oxidative damage. It functions by reducing lipid hydroperoxides to their corresponding alcohols and reducing free hydrogen peroxide ($H_2O_2$) into water. To perform this reaction, it utilizes **reduced glutathione (GSH)** as an electron donor, converting it into glutathione disulfide (GSSG). **Analysis of Incorrect Options:** * **NADH oxidase:** This enzyme is involved in the electron transport chain or the respiratory burst in phagocytes. Instead of scavenging radicals, it often contributes to the production of superoxide radicals to kill pathogens. * **Hydrogen peroxide ($H_2O_2$):** This is not an enzyme; it is a **Reactive Oxygen Species (ROS)** itself. While it is a substrate for GPx and Catalase, it acts as an oxidizing agent that can cause cellular damage. * **Hypochlorous acid (HOCl):** This is a potent oxidant produced by the enzyme Myeloperoxidase (MPO) in neutrophils. It is a "microbicidal agent" used to kill bacteria, not a scavenger. **High-Yield Clinical Pearls for NEET-PG:** * **Trace Element:** Glutathione peroxidase is a **selenoprotein**, meaning it requires **Selenium** as a co-factor (in the form of selenocysteine) for its catalytic activity. * **The GSH Cycle:** The enzyme **Glutathione Reductase** (which requires **NADPH** from the HMP shunt) is essential to regenerate GSH from GSSG so that GPx can continue scavenging radicals. * **Other Scavengers:** Remember the "Antioxidant Trio": **Superoxide Dismutase (SOD)**, **Catalase**, and **Glutathione Peroxidase**. Non-enzymatic scavengers include Vitamins A, C, and E.

Question 313: Which of the following statements regarding Nitric Oxide Synthase is true?

A. It is activated by Calcium
B. It accepts electrons from NADH
C. It catalyzes a dioxygenase reaction
D. It requires NADPH, FAD, FMN and Heme iron (Correct Answer)

Explanation: **Explanation:** Nitric Oxide Synthase (NOS) is a complex enzyme responsible for the synthesis of **Nitric Oxide (NO)** from the amino acid **L-arginine**. **1. Why Option D is Correct:** NOS is a unique enzyme that functions as both a reductase and an oxygenase. To facilitate the multi-step transfer of electrons required to convert L-arginine to L-citrulline and NO, it requires five essential cofactors: **NADPH** (electron donor), **FAD**, **FMN**, **Heme iron**, and **Tetrahydrobiopterin (BH4)**. The presence of FAD and FMN makes it structurally similar to Cytochrome P450 reductase. **2. Why Other Options are Incorrect:** * **Option A:** While the constitutive isoforms (eNOS and nNOS) are regulated by Calcium-Calmodulin, the inducible isoform (**iNOS**) is **calcium-independent**. Therefore, "activated by calcium" is not a universal truth for all NOS types. * **Option B:** NOS specifically accepts electrons from **NADPH**, not NADH. * **Option C:** NOS is a **monooxygenase** (mixed-function oxidase), not a dioxygenase. It incorporates only one atom of molecular oxygen into the product (NO), while the other oxygen atom is reduced to water. **High-Yield Clinical Pearls for NEET-PG:** * **Isoforms:** * **nNOS (NOS-1):** Neuronal; involved in neurotransmission. * **iNOS (NOS-2):** Inducible (Macrophages); involved in immune response and septic shock. * **eNOS (NOS-3):** Endothelial; maintains vascular tone (vasodilation). * **Substrate:** L-Arginine is the precursor. * **Potent Inhibitor:** Asymmetric dimethylarginine (ADMA). * **Mechanism of NO:** It activates **Guanylyl Cyclase**, increasing **cGMP**, which leads to smooth muscle relaxation.

Question 314: Which of the following is the activator of the enzyme sulfite oxidase?

A. Copper
B. Zinc
C. Molybdenum (Correct Answer)
D. Iron

Explanation: **Explanation:** The correct answer is **Molybdenum (C)**. **Sulfite oxidase** is a mitochondrial enzyme responsible for the final step in the catabolism of sulfur-containing amino acids (methionine and cysteine). It catalyzes the oxidation of sulfite ($SO_3^{2-}$) to sulfate ($SO_4^{2-}$). This enzyme requires **Molybdenum** as an essential cofactor, specifically in the form of the **molybdopterin cofactor**. **Analysis of Options:** * **Copper (A):** Acts as a cofactor for enzymes like Cytochrome c oxidase, Superoxide dismutase (Cu-Zn SOD), and Tyrosinase. * **Zinc (B):** A versatile cofactor for over 300 enzymes, including Carbonic anhydrase, Alcohol dehydrogenase, and Carboxypeptidase. * **Iron (D):** Essential for heme-containing enzymes (Catalase, Peroxidase, Cytochromes) and non-heme enzymes like Aconitase. **Clinical Pearls for NEET-PG:** 1. **Molybdenum-dependent enzymes:** There are four key enzymes in humans: Sulfite oxidase, Xanthine oxidase (purine catabolism), Aldehyde oxidase, and Mitochondrial amidoxime reducing component (mARC). 2. **Sulfite Oxidase Deficiency:** A rare but severe genetic disorder presenting in neonates with intractable seizures, neurological deterioration, and **ectopia lentis** (dislocation of the lens). 3. **Diagnosis:** Characterized by elevated urinary sulfite levels and low serum sulfate. It mimics the clinical presentation of Molybdenum cofactor deficiency. 4. **High-Yield Link:** Remember "Molybdenum" for both Sulfite oxidase and Xanthine oxidase; the latter is the target of Allopurinol in gout management.

Question 315: Which of the following is a copper-containing enzyme?

A. Cytochrome c oxidase (Correct Answer)
B. Catalase
C. Lactate dehydrogenase
D. None of the above

Explanation: **Explanation:** **Cytochrome c oxidase (Complex IV)** is the correct answer because it is a critical multi-subunit enzyme in the mitochondrial electron transport chain that contains **two copper centers (CuA and CuB)** in addition to two heme groups (a and a3). These copper ions are essential for transferring electrons to oxygen, the final electron acceptor, to form water. **Analysis of Incorrect Options:** * **Catalase:** This is a **heme-containing (iron)** enzyme found in peroxisomes. It protects cells from oxidative damage by catalyzing the decomposition of hydrogen peroxide into water and oxygen. * **Lactate Dehydrogenase (LDH):** This is a glycolytic enzyme that converts pyruvate to lactate. It does not require a metal cofactor like copper; instead, it utilizes **NAD+/NADH** as a coenzyme. * **None of the above:** Incorrect, as Cytochrome c oxidase is a well-known cuproenzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Other Copper-containing enzymes:** Superoxide dismutase (cytosolic), Tyrosinase (melanin synthesis), Lysyl oxidase (collagen cross-linking), Ceruloplasmin (ferroxidase), and Dopamine $\beta$-hydroxylase. * **Menkes Disease:** An X-linked recessive disorder of copper absorption leading to a deficiency in these enzymes (characterised by "kinky" hair). * **Wilson Disease:** A disorder of copper excretion leading to toxic accumulation in the liver and brain (Kayser-Fleischer rings). * **Inhibitors:** Cytochrome c oxidase is inhibited by **Cyanide, Carbon Monoxide (CO), and Azide**, which halt ATP production.

Question 316: What is true about Glutathione reductase?

A. It is a sulfur-containing enzyme.
B. It is important in methemoglobinemia.
C. It is a free radical scavenger. (Correct Answer)
D. All of the above.

Explanation: **Explanation:** **Glutathione Reductase (GR)** is a critical antioxidant enzyme that maintains the pool of reduced glutathione (GSH) in the cell. Its primary role is to catalyze the reduction of glutathione disulfide (GSSG) back to reduced glutathione (GSH) using **NADPH** as a reducing equivalent. **Why Option C is Correct:** Reduced glutathione (GSH) acts as a potent **free radical scavenger**. It neutralizes reactive oxygen species (ROS) like hydrogen peroxide ($H_2O_2$) via the enzyme Glutathione Peroxidase. Since Glutathione Reductase is essential for regenerating GSH from its oxidized form, it is fundamentally classified as a key component of the cellular antioxidant defense system and a free radical scavenger. **Analysis of Incorrect Options:** * **Option A:** While glutathione itself is a tripeptide containing sulfur (cysteine), Glutathione Reductase is a **flavoprotein** containing **FAD** (Flavin Adenine Dinucleotide) as a cofactor, not primarily characterized as a "sulfur-containing enzyme." * **Option B:** Methemoglobinemia is primarily managed by **NADH-cytochrome b5 reductase** (Diaphorase I). While the HMP shunt provides NADPH for RBC integrity, the specific enzyme linked to preventing methemoglobin is not Glutathione Reductase. **High-Yield Clinical Pearls for NEET-PG:** * **Source of NADPH:** The NADPH required by Glutathione Reductase is supplied by the **HMP Shunt** (Pentose Phosphate Pathway) via the enzyme **G6PD**. * **RBC Hemolysis:** In G6PD deficiency, a lack of NADPH leads to a failure of Glutathione Reductase. This results in accumulated $H_2O_2$, causing hemoglobin denaturation and the formation of **Heinz bodies** and **Bite cells**. * **Riboflavin Connection:** Since GR requires FAD, its activity is used as a functional assay to bridge a diagnosis of **Vitamin B2 (Riboflavin) deficiency**.

Question 317: NADPH+ and H+ are generated in the reaction catalyzed by which enzyme?

A. Lactate dehydrogenase (LDH)
B. Glucose-6-phosphate dehydrogenase (G-6-PD) (Correct Answer)
C. Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD)
D. Alcohol dehydrogenase

Explanation: **Explanation:** The correct answer is **Glucose-6-phosphate dehydrogenase (G-6-PD)**. This enzyme catalyzes the first and rate-limiting step of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). In this reaction, Glucose-6-phosphate is oxidized to 6-phosphogluconolactone, and **NADP+ is reduced to NADPH + H+**. This is the primary source of NADPH in the body, which is essential for reductive biosynthesis (e.g., fatty acid and steroid synthesis) and maintaining the pool of reduced glutathione to protect cells against oxidative stress. **Analysis of Incorrect Options:** * **Lactate dehydrogenase (LDH):** Involved in anaerobic glycolysis, it interconverts pyruvate and lactate using **NADH/NAD+** as the coenzyme, not NADPH. * **Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD):** A key enzyme in glycolysis that converts G-3-P to 1,3-bisphosphoglycerate, generating **NADH**. * **Alcohol dehydrogenase:** Responsible for ethanol metabolism in the cytosol, it converts ethanol to acetaldehyde using **NAD+** as an electron acceptor to produce **NADH**. **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** under oxidative stress (e.g., fava beans, primaquine, infections) because the RBCs cannot generate enough NADPH to maintain reduced glutathione. * **Bite Cells & Heinz Bodies:** Classic peripheral smear findings in G6PD deficiency. * **Tissue Distribution:** The HMP shunt is highly active in the adrenal cortex, liver, mammary glands, and RBCs. * **Rule of Thumb:** Enzymes using **NAD+** are generally involved in **catabolic** pathways (energy production), while those using **NADP+** are involved in **anabolic** pathways (biosynthesis) or detoxification.

Question 318: What is true about competitive inhibition?

A. Increases Vmax
B. Increases Km (Correct Answer)
C. Decreases Vmax
D. Decreases Km

Explanation: ### Explanation In **Competitive Inhibition**, the inhibitor structurally resembles the substrate and competes for the same **active site** on the enzyme [1]. **1. Why the Correct Answer is Right (Increases Km):** * **Concept:** $K_m$ (Michaelis constant) represents the substrate concentration required to reach half of the maximum velocity ($V_{max}$). It is an inverse measure of the enzyme's affinity for its substrate [1]. * **Mechanism:** Because the inhibitor competes for the active site, more substrate is required to displace the inhibitor and achieve the same reaction rate. This effectively **lowers the affinity** of the enzyme for the substrate, leading to an **increase in $K_m$** [1]. **2. Why the Incorrect Options are Wrong:** * **Options A & C (Vmax):** In competitive inhibition, **$V_{max}$ remains unchanged**. If the substrate concentration is increased to a sufficiently high level, it will eventually outcompete the inhibitor, allowing the enzyme to reach its maximum potential velocity [1]. * **Option D (Decreases Km):** A decrease in $K_m$ would imply an increase in affinity, which does not occur in any standard form of inhibition. **3. High-Yield Clinical Pearls for NEET-PG:** * **Lineweaver-Burk Plot:** On a double-reciprocal plot, competitive inhibition shows lines that **intersect on the Y-axis** (same $V_{max}$) but have different X-intercepts (increased $K_m$) [1]. * **Classic Clinical Examples:** * **Statins** (e.g., Atorvastatin) compete with HMG-CoA for HMG-CoA reductase. * **Methanol poisoning** is treated with **Ethanol** (competitive inhibition of Alcohol Dehydrogenase). * **Methotrexate** competes with dihydrofolate for Dihydrofolate Reductase. * **Sulfonamides** compete with PABA in bacterial folic acid synthesis.

Question 319: Pyruvate dehydrogenase complex requires all the following coenzymes, except?

A. FAD
B. NAD+
C. THF (Correct Answer)
D. TPP

Explanation: **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme assembly that catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. This process requires **five specific cofactors**, often remembered by the mnemonic **"Tender Loving Care For Nancy."** 1. **T**hiamine Pyrophosphate (**TPP**) – Derived from Vitamin B1. 2. **L**ipoic Acid (Lipoamide). 3. **C**oenzyme A (**CoA**) – Derived from Vitamin B5 (Pantothenic acid). 4. **F**lavin Adenine Dinucleotide (**FAD**) – Derived from Vitamin B2 (Riboflavin). 5. **N**icotinamide Adenine Dinucleotide (**NAD+**) – Derived from Vitamin B3 (Niacin). **Why THF is the correct answer:** **Tetrahydrofolate (THF)** is the active form of Folic Acid (Vitamin B9). Its primary role in biochemistry is **one-carbon metabolism** (transferring methyl, formyl, or methylene groups), which is essential for purine and pyrimidine synthesis. It plays no role in the oxidative decarboxylation of alpha-keto acids like pyruvate. **Analysis of Incorrect Options:** * **TPP (Option D):** Acts as a prosthetic group for the E1 subunit (Pyruvate decarboxylase), facilitating the cleavage of the C-C bond. * **FAD (Option A):** Required by the E3 subunit (Dihydrolipoyl dehydrogenase) to re-oxidize the lipoamide arm. * **NAD+ (Option B):** Serves as the final electron acceptor, forming NADH, which then enters the electron transport chain. **High-Yield Clinical Pearls for NEET-PG:** * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the SH-groups of **Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Leads to Beriberi and Wernicke-Korsakoff syndrome because PDH and Alpha-ketoglutarate dehydrogenase cannot function without TPP. * The same five cofactors are also required by **Alpha-ketoglutarate dehydrogenase** and **Branched-chain alpha-keto acid dehydrogenase**.

Question 320: Which condition is characterized by a flipped pattern of LDH isoenzymes?

A. Myositis
B. Myocardial infarction (Correct Answer)
C. Grave's disease
D. Myasthenia gravis

Explanation: **Explanation:** **1. Why Myocardial Infarction is correct:** Lactate Dehydrogenase (LDH) exists in five isoenzyme forms. In a healthy individual, **LDH-2** (found primarily in the reticuloendothelial system) is the most abundant fraction, meaning **LDH-2 > LDH-1**. However, **LDH-1** is highly concentrated in cardiac muscle. Following a **Myocardial Infarction (MI)**, damaged cardiac cells release large amounts of LDH-1 into the bloodstream. This causes the serum levels of LDH-1 to exceed LDH-2, a phenomenon known as the **"LDH Flipped Pattern" (LDH-1 > LDH-2)**. While Troponins are now the preferred biomarkers, the flipped LDH pattern remains a classic biochemical hallmark of MI. **2. Why other options are incorrect:** * **Myositis:** This involves skeletal muscle inflammation. Skeletal muscle is rich in **LDH-5**. Therefore, myositis would show an elevation in LDH-5, not a flip in the LDH-1/LDH-2 ratio. * **Grave’s Disease:** This is an autoimmune hyperthyroidism. While it may occasionally cause non-specific enzyme elevations, it does not typically present with a flipped LDH pattern. * **Myasthenia Gravis:** This is a neuromuscular junction disorder (acetylcholine receptor antibodies) and does not involve significant muscle cell necrosis or LDH release. **3. High-Yield Clinical Pearls for NEET-PG:** * **LDH Isoenzymes:** LDH-1 (Heart/RBCs), LDH-2 (RES), LDH-3 (Lungs), LDH-4 (Kidney/Pancreas), LDH-5 (Liver/Skeletal Muscle). * **Diagnostic Window:** LDH begins to rise 12–24 hours after MI, peaks at 48 hours, and remains elevated for 7–10 days (useful for late diagnosis). * **Hemolysis:** Since LDH-1 is also high in RBCs, **hemolytic anemia** can also cause a flipped LDH pattern. * **Total LDH:** A non-specific marker of cell turnover/damage; highly elevated in Megaloblastic Anemia and Lymphomas.

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Enzyme Classification and Nomenclature

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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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Enzymes — MCQs

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