Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 61: Cyanides primarily inhibit which enzyme?

A. G-6-P dehydrogenase
B. Isomerase
C. Cytochrome oxidase (Correct Answer)
D. None of the above

Explanation: **Explanation:** **1. Why Cytochrome Oxidase is correct:** Cyanide is a potent inhibitor of the **Electron Transport Chain (ETC)**. It binds with high affinity to the ferric iron ($Fe^{3+}$) in the heme group of **Cytochrome oxidase (Complex IV)**. By inhibiting this final step of the ETC, cyanide prevents the transfer of electrons to oxygen, halting ATP production. This leads to cellular hypoxia despite adequate oxygen saturation in the blood, a condition known as **histotoxic hypoxia**. **2. Why the other options are incorrect:** * **G-6-P Dehydrogenase (G6PD):** This is the rate-limiting enzyme of the Hexose Monophosphate (HMP) Shunt. Its deficiency leads to hemolytic anemia, but it is not inhibited by cyanide. * **Isomerase:** These are a general class of enzymes (like Phosphohexose isomerase) that catalyze structural rearrangements. They are not the primary targets of cyanide toxicity. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Cyanide binds to the **$Fe^{3+}$ (ferric)** state of Cytochrome $a_3$. * **Antidote Rationale:** * **Amyl Nitrite/Sodium Nitrite:** These induce **Methemoglobinemia**. Methemoglobin contains $Fe^{3+}$, which acts as a "decoy" to pull cyanide away from Cytochrome oxidase. * **Sodium Thiosulfate:** Converts cyanide to non-toxic **Thiocyanate** via the enzyme **Rhodanese**. * **Hydroxocobalamin (Vitamin B12a):** Binds cyanide to form Cyanocobalamin, which is excreted by the kidneys. * **Classic Presentation:** Bitter almond odor on the breath and "cherry-red" skin/venous blood (due to failure of tissues to extract oxygen from the blood).

Question 62: All of the following cells contain telomerase enzyme except?

A. Cancer cells
B. Germ cells
C. Somatic cells (Correct Answer)
D. Hemopoietic cells

Explanation: **Explanation:** **Concept:** Telomerase is a specialized **ribonucleoprotein reverse transcriptase** enzyme that maintains chromosomal stability by adding repetitive DNA sequences (TTAGGG) to the 3' ends of chromosomes (telomeres). This prevents the "end-replication problem" and cellular senescence. **1. Why Somatic Cells are the Correct Answer:** Most mature **somatic cells** (differentiated body cells like skin or muscle cells) lack telomerase activity. Consequently, their telomeres shorten with every cell division. Once the telomeres reach a critical minimum length, the cell enters **replicative senescence** (the Hayflick limit) and eventually undergoes apoptosis. This acts as a biological clock and a protective mechanism against uncontrolled growth. **2. Analysis of Incorrect Options:** * **Germ Cells (B):** These cells (sperm and ova) must pass on a full-length genome to the next generation. They express high levels of telomerase to ensure telomere length is reset and maintained. * **Hemopoietic Cells (D):** Stem cells, including hematopoietic stem cells and basal cells of the epidermis, require high proliferative capacity. They express telomerase to allow for continuous self-renewal throughout life. * **Cancer Cells (A):** Approximately 85–90% of cancer cells abnormally reactivate telomerase. This grants them "replicative immortality," allowing them to divide indefinitely without telomere shortening. **Clinical Pearls for NEET-PG:** * **Composition:** Telomerase consists of **TERT** (catalytic protein subunit with reverse transcriptase activity) and **TERC** (RNA template). * **Progeria:** Mutations affecting telomere maintenance lead to premature aging syndromes like **Dyskeratosis Congenita**. * **Target:** Telomerase inhibitors (e.g., Imetelstat) are being researched as potential anti-cancer therapies to induce senescence in malignant cells.

Question 63: If Km remains the same and Vmax is reduced, what type of enzyme inhibition is this?

A. Competitive Inhibition
B. Noncompetitive Inhibition (Correct Answer)
C. Uncompetitive inhibition
D. Suicide Inhibition

Explanation: ### Explanation In enzyme kinetics, the relationship between substrate concentration and reaction velocity is defined by the Michaelis-Menten equation. Understanding how inhibitors affect **$K_m$** (affinity) and **$V_{max}$** (maximum velocity) is a high-yield topic for NEET-PG. **1. Why Noncompetitive Inhibition is Correct:** In **Noncompetitive Inhibition**, the inhibitor binds to an allosteric site (a site other than the active site) on both the free enzyme (E) and the enzyme-substrate complex (ES). * **$V_{max}$ decreases:** Because the inhibitor reduces the overall concentration of functional enzymes, the maximum reaction rate is lowered regardless of how much substrate is added. * **$K_m$ remains the same:** Since the inhibitor does not compete for the active site, the affinity of the remaining functional enzymes for the substrate remains unchanged. **2. Analysis of Incorrect Options:** * **Competitive Inhibition:** The inhibitor competes for the active site. Adding more substrate can overcome this, so **$V_{max}$ remains unchanged**, but **$K_m$ increases** (lower affinity). * **Uncompetitive Inhibition:** The inhibitor binds only to the ES complex. This results in a **decrease in both $V_{max}$ and $K_m$**. On a Lineweaver-Burk plot, this produces parallel lines. * **Suicide Inhibition:** This is a form of irreversible inhibition where the enzyme converts the inhibitor into a reactive form that covalently binds to the active site (e.g., Aspirin, Allopurinol). While it reduces $V_{max}$, it is characterized by its irreversible nature rather than specific $K_m$ shifts. **High-Yield Clinical Pearls for NEET-PG:** * **Lineweaver-Burk Plot:** In noncompetitive inhibition, the lines intersect on the **negative x-axis** ($-1/K_m$ is constant). * **Classic Example:** Heavy metal poisoning (e.g., Lead, Mercury) binding to functional groups of enzymes. * **Mnemonic:** **C**ompetitive = **C**hanges $K_m$; **N**on-competitive = **N**o change in $K_m$.

Question 64: Which enzyme is studied in the Triphenyl tetrazolium chloride test (TTC) for detecting myocardial infarction?

A. Succinate dehydrogenase (SDH)
B. Lactate dehydrogenase (LDH) (Correct Answer)
C. MDM
D. Lecithinase

Explanation: ### Explanation **Correct Answer: B. Lactate dehydrogenase (LDH)** **Mechanism and Clinical Significance:** The Triphenyl tetrazolium chloride (TTC) test is a macroscopic staining technique used during autopsy to identify early myocardial infarction (MI). The test relies on the presence of **Lactate Dehydrogenase (LDH)**. * **In healthy myocardium:** LDH is present and active. It reacts with the colorless TTC substrate, reducing it to a bright red, insoluble pigment called **formazan**. * **In infarcted myocardium:** Due to cell membrane damage (necrosis), LDH leaks out of the cells into the systemic circulation. Consequently, the depleted tissue cannot reduce the TTC. * **Result:** The viable tissue stains **red**, while the infarcted (dead) area remains **pale/white**. This allows for the detection of MI as early as 2–3 hours post-insult. **Analysis of Incorrect Options:** * **A. Succinate dehydrogenase (SDH):** While SDH is a mitochondrial enzyme used in other histochemical stains (like the "SDH stain" for muscle biopsies), it is not the primary enzyme utilized in the standard TTC macroscopic test for MI. * **C. MDM:** This is not a standard enzyme related to cardiac necrosis or the TTC test. * **D. Lecithinase:** Also known as Phospholipase C, this is a toxin produced by *Clostridium perfringens* (causing gas gangrene) and is not used in cardiac pathology staining. **High-Yield Pearls for NEET-PG:** * **TTC Test Timing:** Most effective for detecting MI between **2 to 12 hours** after the event (before gross changes are visible to the naked eye). * **LDH Isoenzymes:** In clinical biochemistry, **LDH-1** (found in the heart) rises 24–48 hours after MI. The "flipped ratio" (LDH-1 > LDH-2) is a classic (though now historical) diagnostic marker. * **Gold Standard:** While TTC is used in pathology, **Troponin I and T** are the gold standard biochemical markers for diagnosing MI in living patients.

Question 65: Which of the following enzymes does not utilize copper?

A. Prolyl hydroxylase (Correct Answer)
B. Tyrosinase
C. Ceruloplasmin
D. Superoxide dismutase

Explanation: **Explanation:** The correct answer is **Prolyl hydroxylase** because it is a **Vitamin C and Iron (Fe²⁺)** dependent enzyme, not a copper-dependent one. **1. Why Prolyl hydroxylase is correct:** Prolyl hydroxylase is essential for the post-translational modification of collagen. It hydroxylates proline residues to hydroxyproline, which stabilizes the collagen triple helix. This reaction requires **Ferrous iron (Fe²⁺)**, molecular oxygen, alpha-ketoglutarate, and **Ascorbic acid (Vitamin C)**. Vitamin C maintains iron in its reduced ferrous state; its deficiency leads to Scurvy. **2. Why the other options are incorrect:** * **Tyrosinase:** This is a copper-containing enzyme responsible for converting Tyrosine to DOPA and then to Dopachrome in the synthesis of **melanin**. Deficiency leads to Albinism. * **Ceruloplasmin:** This is the primary copper-carrying protein in the blood. It also functions as a **ferroxidase** (converting Fe²⁺ to Fe³⁺ for binding to transferrin), utilizing its internal copper atoms for the redox reaction. * **Superoxide dismutase (SOD):** The cytosolic form of SOD (SOD1) requires both **Copper and Zinc** (Cu-Zn SOD) to scavenge free radicals. (Note: The mitochondrial form, SOD2, requires Manganese). **High-Yield Clinical Pearls for NEET-PG:** * **Copper-containing enzymes (Mnemonic: "C-Cyto-T-S-L"):** **C**eruloplasmin, **Cyto**chrome c oxidase (Complex IV), **T**yrosinase, **S**uperoxide dismutase, **L**ysyl oxidase (essential for collagen cross-linking). * **Menkes Disease:** ATP7A mutation leading to copper deficiency; symptoms include "kinky" hair due to defective lysyl oxidase. * **Wilson Disease:** ATP7B mutation leading to copper toxicity and low serum ceruloplasmin.

Question 66: The enzyme phosphofructo kinase-1 is strongly activated by which of the following?

A. Cyclic AMP
B. Adenosine triphosphate
C. Citrate
D. Fructose 2,6 bisphosphate (Correct Answer)

Explanation: **Explanation:** Phosphofructokinase-1 (PFK-1) is the **rate-limiting and key committed step** of Glycolysis, converting Fructose 6-phosphate to Fructose 1,6-bisphosphate. Its regulation is a high-yield topic for NEET-PG. **Why Fructose 2,6-bisphosphate (F2,6-BP) is correct:** F2,6-BP is the **most potent allosteric activator** of PFK-1. It is produced by the bifunctional enzyme PFK-2. F2,6-BP functions by increasing the affinity of PFK-1 for its substrate (Fructose 6-phosphate) and decreasing the inhibitory effect of ATP. This ensures that glycolysis proceeds rapidly even when energy levels are relatively high. **Analysis of Incorrect Options:** * **B. Adenosine Triphosphate (ATP):** ATP acts as an **allosteric inhibitor**. High ATP levels signal that the cell has sufficient energy, thus "turning off" glycolysis to conserve glucose. * **C. Citrate:** Citrate is an intermediate of the TCA cycle. High levels indicate that energy production is meeting demand, acting as an **allosteric inhibitor** of PFK-1. * **A. Cyclic AMP (cAMP):** In the liver, high cAMP (triggered by Glucagon) leads to the phosphorylation and inactivation of PFK-2, which **decreases** F2,6-BP levels, thereby inhibiting PFK-1 and glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **PFK-1 Inhibitors:** ATP, Citrate, and H+ ions (low pH/acidosis). * **PFK-1 Activators:** Fructose 2,6-BP and AMP. * **Insulin vs. Glucagon:** Insulin increases F2,6-BP (stimulating glycolysis), while Glucagon decreases it (stimulating gluconeogenesis). * **Tauri Disease (GSD Type VII):** Caused by a deficiency of the PFK-1 enzyme in muscles, leading to exercise intolerance and hemolysis.

Question 67: An increase in LDH-5 enzyme is seen in the following conditions, except:

A. Malignancies of the central nervous system
B. Muscular dystrophies
C. Breast carcinoma
D. Pulmonary embolism (Correct Answer)

Explanation: **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme with five isoenzymes (LDH 1-5). **LDH-5 ($M_4$)** is primarily found in the liver and skeletal muscle. **Why Pulmonary Embolism is the Correct Answer:** In **Pulmonary Embolism (PE)**, the characteristic finding is an increase in **LDH-3**. LDH-3 is the predominant isoenzyme in lung tissue. While a "triad" of increased LDH, increased bilirubin, and normal AST is classically associated with PE, the specific isoenzyme elevation involves LDH-3, not LDH-5. **Analysis of Other Options:** * **Malignancies of the CNS:** While LDH-1 and 2 are usually associated with the brain, certain CNS tumors and metastatic deposits can cause a shift toward anaerobic metabolism, leading to a significant rise in **LDH-5**. * **Muscular Dystrophies:** Since LDH-5 is the primary isoenzyme in skeletal muscle, any condition causing muscle necrosis or dystrophy (like Duchenne Muscular Dystrophy) will result in a marked elevation of LDH-5. * **Breast Carcinoma:** Many solid tumors, including breast and hepatic carcinomas, show an increase in LDH-5. This is often used as a marker for tumor burden and metabolic shift (Warburg effect) in malignant tissues. **High-Yield NEET-PG Pearls:** * **LDH-1 ($H_4$):** Heart and RBCs. Elevated in Myocardial Infarction (shows "flipped pattern" where LDH-1 > LDH-2). * **LDH-2 ($H_3M_1$):** Reticuloendothelial system. Normally the most abundant fraction in serum. * **LDH-3 ($H_2M_2$):** Lungs, Spleen, and Pancreas. * **LDH-4 ($H_1M_3$):** Kidney and Placenta. * **LDH-5 ($M_4$):** Liver and Skeletal Muscle. * **LDH-X (LDH-6):** Found in post-pubertal testes.

Question 68: Malonate is a competitive inhibitor of which of the following?

A. Complex I
B. Complex II (Correct Answer)
C. Complex III
D. Complex IV

Explanation: **Explanation:** **1. Why Complex II is the Correct Answer:** Malonate is a classic example of a **competitive inhibitor**. It is a structural analog of **succinate**, the natural substrate for the enzyme **Succinate Dehydrogenase (SDH)**. In the Citric Acid Cycle (TCA) and the Electron Transport Chain (ETC), SDH is known as **Complex II**. Because malonate mimics the structure of succinate, it competes for the same active site on the enzyme. By binding to Complex II, malonate prevents the oxidation of succinate to fumarate, thereby halting the flow of electrons from FADH₂ into the ETC. **2. Why Other Options are Incorrect:** * **Complex I (NADH Dehydrogenase):** Inhibited by substances like **Rotenone**, Amobarbital (Amytal), and Piericidin A. It transfers electrons from NADH to Coenzyme Q. * **Complex III (Cytochrome bc1 complex):** Inhibited by **Antimycin A** and British Anti-Lewisite (BAL). It transfers electrons from Coenzyme Q to Cytochrome c. * **Complex IV (Cytochrome c Oxidase):** Inhibited by **Cyanide (CN⁻)**, **Carbon Monoxide (CO)**, Hydrogen Sulfide (H₂S), and Azide (N₃⁻). These bind to the iron in heme, preventing the final transfer of electrons to oxygen. **3. High-Yield Facts for NEET-PG:** * **Complex II Unique Feature:** It is the only enzyme that participates in both the TCA cycle and the ETC. It is also the only complex that does **not** pump protons across the inner mitochondrial membrane. * **Competitive Inhibition Rule:** The inhibitory effect of malonate can be overcome by increasing the concentration of the substrate (succinate). * **Kinetics:** In competitive inhibition, the **Km increases** (affinity decreases) while the **Vmax remains unchanged**.

Question 69: What is the enzyme involved in the following conversion?

A. Sorbitol dehydrogenase
B. Glucose reductase
C. Aldose reductase (Correct Answer)
D. Glucose oxidase

Explanation: ***Aldose reductase*** - **Aldose reductase** catalyzes the first step of the **polyol pathway**, converting glucose to sorbitol using **NADPH** as a cofactor. - This enzyme is clinically significant in **diabetic complications** like neuropathy, retinopathy, nephropathy, and cataracts due to sorbitol accumulation. *Sorbitol dehydrogenase* - This enzyme catalyzes the **second step** of the polyol pathway, converting sorbitol to fructose using **NAD+**. - It does **not convert glucose** directly but acts on the product of aldose reductase. *Glucose reductase* - This is **not a recognized enzyme** in standard biochemical pathways involved in glucose metabolism. - The term is often confused with aldose reductase, but glucose reductase does not exist as a distinct enzyme. *Glucose oxidase* - This enzyme **oxidizes glucose** to gluconic acid and hydrogen peroxide, primarily used in **laboratory glucose assays**. - It does **not reduce glucose** to sorbitol and is not part of the polyol pathway.

Question 70: The enzyme trypsin is specific for peptide bonds of which type of amino acids?

A. Basic amino acids (Correct Answer)
B. Acidic amino acids
C. Aromatic amino acids
D. Amino acids adjacent to proline residues

Explanation: **Explanation:** Trypsin is a serine protease found in the digestive system that catalyzes the hydrolysis of peptide bonds. Its specificity is determined by its **substrate-binding pocket (S1 pocket)**, which contains a negatively charged **aspartate residue (Asp 189)** at its base. This negative charge attracts and stabilizes the positively charged side chains of **basic amino acids**, specifically **Lysine (K)** and **Arginine (R)**. Consequently, trypsin cleaves peptide bonds on the carboxyl side of these residues. **Analysis of Options:** * **A. Basic amino acids (Correct):** As explained, the electrostatic attraction between the enzyme's aspartate and the basic side chains of Lysine/Arginine dictates this specificity. * **B. Acidic amino acids:** Enzymes like **Pepsin** show some preference for acidic residues, but specific cleavage at Aspartate/Glutamate is more characteristic of **Endoproteinase Glu-C**. * **C. Aromatic amino acids:** This is the specificity of **Chymotrypsin**. Its binding pocket is large and hydrophobic, accommodating bulky rings like Phenylalanine, Tyrosine, and Tryptophan. * **D. Adjacent to proline:** Trypsin generally **cannot** cleave a bond if the following residue is Proline, as the rigid ring structure of proline creates steric hindrance that prevents the peptide bond from entering the active site. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogen Activation:** Trypsin is secreted as inactive **trypsinogen**. It is activated by **Enteropeptidase (Enterokinase)**, a brush-border enzyme. Once formed, trypsin autocatalytically activates more trypsinogen and other proenzymes (Chymotrypsinogen, Procarboxypeptidase). * **Pancreatitis:** Premature intra-pancreatic activation of trypsin leads to autodigestion of the gland, the hallmark of acute pancreatitis. * **SPINK1:** This is a specific trypsin inhibitor in the pancreas that prevents accidental activation. Mutations in *SPINK1* are associated with hereditary pancreatitis.

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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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