Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 331: What is the predominant isoenzyme of Lactate Dehydrogenase (LDH) in cardiac muscle?

A. LDH-1 (Correct Answer)
B. LDH-2
C. LDH-3
D. LDH-5

Explanation: ### Explanation **Lactate Dehydrogenase (LDH)** is a tetrameric enzyme composed of two types of subunits: **H (Heart)** and **M (Muscle)**. These combine in five different ways to form isoenzymes (LDH-1 to LDH-5), which exhibit tissue-specific distribution. #### Why LDH-1 is Correct: **LDH-1 (H₄)** consists of four H subunits. It is the predominant isoenzyme found in **cardiac muscle** and **erythrocytes**. It has a high affinity for lactate, converting it into pyruvate for aerobic metabolism. In the setting of a Myocardial Infarction (MI), LDH-1 levels rise, often exceeding LDH-2 levels—a phenomenon known as the **"LDH Flipped Pattern."** #### Analysis of Incorrect Options: * **LDH-2 (H₃M₁):** Predominantly found in the **reticuloendothelial system** and serum. Under normal physiological conditions, LDH-2 is the most abundant isoenzyme in human serum (LDH-2 > LDH-1). * **LDH-3 (H₂M₂):** Primarily located in the **lungs**, spleen, and pancreas. Elevations are typically seen in pulmonary embolism or pneumonia. * **LDH-5 (M₄):** Predominant in **skeletal muscle** and the **liver**. It is a marker for muscular dystrophy or hepatic injury (e.g., hepatitis). #### High-Yield Clinical Pearls for NEET-PG: * **MI Marker Kinetics:** LDH starts rising 12–24 hours after an MI, peaks at 48–72 hours, and remains elevated for 10–14 days. While Troponins are now the gold standard, LDH is useful for **late diagnosis** of MI. * **LDH Flipped Pattern:** Normally LDH-2 > LDH-1. In MI or Hemolytic Anemia, this reverses to **LDH-1 > LDH-2**. * **LDH-4 (HM₃):** Found mainly in the kidneys and placenta. * **Total LDH:** Elevated in megaloblastic anemia (highest levels), malignancies, and hemolysis.

Question 332: Dehydrogenation of succinic acid to fumaric acid requires which of the following hydrogen carriers?

A. NAD+
B. NADP+
C. Flavoprotein (Correct Answer)
D. Glutathione

Explanation: **Explanation:** The conversion of succinate to fumarate is a key step in the **Tricarboxylic Acid (TCA) Cycle**, catalyzed by the enzyme **Succinate Dehydrogenase (SDH)**. **Why Flavoprotein is correct:** Succinate dehydrogenase is unique because it is the only enzyme in the TCA cycle that is membrane-bound (located in the inner mitochondrial membrane as **Complex II** of the Electron Transport Chain). The reaction involves the removal of two hydrogen atoms from succinate. The free energy change ($\Delta G$) of this specific reaction is insufficient to reduce $NAD^+$. Therefore, it requires a stronger oxidizing agent, **FAD (Flavin Adenine Dinucleotide)**. FAD is covalently bound to the enzyme, making SDH a **flavoprotein**. The FAD accepts electrons to become $FADH_2$, which then transfers them directly into the respiratory chain via Coenzyme Q. **Why other options are incorrect:** * **NAD+:** Used by other TCA cycle dehydrogenases (Isocitrate, $\alpha$-ketoglutarate, and Malate dehydrogenases) where the energy change is higher. * **NADP+:** Primarily functions as a reducing agent in anabolic pathways (like fatty acid synthesis) and the Hexose Monophosphate (HMP) shunt, not as an electron acceptor in the TCA cycle. * **Glutathione:** Acts as a major intracellular antioxidant and a cofactor for enzymes like glutathione peroxidase to neutralize free radicals; it does not participate in the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Competitive Inhibition:** This reaction is classically inhibited by **Malonate**, which is a structural analog of succinate. * **Dual Role:** Succinate Dehydrogenase is the only enzyme that participates in both the TCA cycle and the Electron Transport Chain (Complex II). * **Marker Enzyme:** It is often used as a marker enzyme for the inner mitochondrial membrane.

Question 333: Which of the following is NOT a true statement about glucokinase?

A. Its Km value is higher than normal blood glucose levels.
B. It is found in the liver.
C. Glucose enters cells via GLUT2 transporters when glucokinase is involved.
D. It is inhibited by Glucose-6-phosphate. (Correct Answer)

Explanation: **Explanation** The key to this question lies in distinguishing between the two major hexokinase isoenzymes: **Hexokinase (Types I-III)** and **Glucokinase (Hexokinase Type IV)**. **Why Option D is the correct answer:** Unlike Hexokinase, **Glucokinase is NOT inhibited by its product, Glucose-6-phosphate (G6P).** Instead, it is regulated by the **Glucokinase Regulatory Protein (GKRP)** in the liver. This lack of feedback inhibition allows the liver to continue phosphorylating glucose even when G6P levels are high, facilitating the storage of excess glucose as glycogen after a meal. **Analysis of Incorrect Options:** * **Option A:** Glucokinase has a **high Km** (approx. 10 mmol/L), which is higher than normal fasting blood glucose levels. This means it has a low affinity for glucose and only becomes highly active when blood glucose levels rise significantly (post-prandial). * **Option B:** Glucokinase is primarily located in the **liver parenchymal cells** and the **beta cells of the pancreas**. * **Option C:** In the liver and pancreas, glucose entry is mediated by **GLUT2**, a high-capacity, high-Km transporter. This ensures that the intracellular glucose concentration equilibrates rapidly with the blood glucose concentration, allowing glucokinase to act as a "glucose sensor." **High-Yield Clinical Pearls for NEET-PG:** * **Glucose Sensor:** Glucokinase acts as the glucose sensor for insulin release in pancreatic beta cells. * **MODY Type 2:** Mutations in the glucokinase gene lead to Maturity-Onset Diabetes of the Young (MODY) Type 2. * **Sigmoidal Kinetics:** Unlike the hyperbolic curve of Hexokinase, Glucokinase exhibits sigmoidal (positive cooperativity) kinetics. * **Induction:** Glucokinase is induced by **Insulin**, whereas Hexokinase is constitutive.

Question 334: Which of the following is not an antioxidant?

A. Superoxide dismutase
B. Catalase
C. Coagulase (Correct Answer)
D. Glutathione peroxidase

Explanation: ### Explanation The correct answer is **Coagulase**. **1. Why Coagulase is the correct answer:** Coagulase is not an antioxidant; it is an **enzyme produced by certain bacteria** (most notably *Staphylococcus aureus*). Its primary function is to convert fibrinogen to fibrin, causing blood plasma to clot. In a clinical context, this serves as a virulence factor by coating the bacteria in fibrin to evade the host's immune system. It plays no role in neutralizing reactive oxygen species (ROS). **2. Why the other options are incorrect (Antioxidant Enzymes):** The other three options are the primary enzymatic defenses against oxidative stress: * **Superoxide Dismutase (SOD):** Converts the highly reactive superoxide radical ($O_2^{\bullet-}$) into hydrogen peroxide ($H_2O_2$). * **Catalase:** A heme-containing enzyme (found in peroxisomes) that decomposes $H_2O_2$ into water and oxygen. * **Glutathione Peroxidase:** A **selenium-dependent** enzyme that reduces $H_2O_2$ to water while oxidizing glutathione (GSH to GSSG). **3. Clinical Pearls for NEET-PG:** * **Selenium Connection:** Glutathione peroxidase is a high-yield fact; remember it requires Selenium as a cofactor. * **Cellular Localization:** SOD is found in both mitochondria (Mn-SOD) and cytosol (Cu-Zn SOD). Catalase is primarily localized in **peroxisomes**. * **Non-Enzymatic Antioxidants:** For the exam, also remember Vitamin E (lipid-soluble, prevents lipid peroxidation), Vitamin C (water-soluble, regenerates Vit E), and Vitamin A/Carotenoids. * **Glutathione Reductase:** This enzyme requires **NADPH** (from the HMP shunt) to regenerate reduced glutathione, which is essential for RBC membrane integrity.

Question 335: Substrate-level phosphorylation is seen in the reaction catalyzed by which enzyme?

A. Succinate dehydrogenase
B. Alpha-ketoglutarate dehydrogenase
C. Succinyl CoA thiokinase (Correct Answer)
D. Malate dehydrogenase

Explanation: **Explanation:** **Substrate-level phosphorylation (SLP)** is the direct synthesis of ATP or GTP from ADP or GDP by the transfer of a high-energy phosphate group from a metabolic intermediate, without the involvement of the Electron Transport Chain (ETC) or molecular oxygen. **Why Succinyl CoA thiokinase is correct:** In the Citric Acid Cycle (TCA cycle), **Succinyl CoA thiokinase** (also known as Succinyl CoA synthetase) catalyzes the conversion of Succinyl CoA to Succinate. This reaction involves the cleavage of a high-energy thioester bond, which releases enough energy to drive the phosphorylation of GDP to GTP (or ADP to ATP in some tissues). This is the **only** step in the TCA cycle where SLP occurs. **Analysis of Incorrect Options:** * **A. Succinate dehydrogenase:** Catalyzes the oxidation of Succinate to Fumarate. It is part of Complex II of the ETC and generates FADH₂, leading to oxidative phosphorylation, not SLP. * **B. Alpha-ketoglutarate dehydrogenase:** Catalyzes the oxidative decarboxylation of α-ketoglutarate to Succinyl CoA. It produces NADH but does not directly generate a high-energy phosphate bond. * **C. Malate dehydrogenase:** Catalyzes the conversion of Malate to Oxaloacetate, producing NADH. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP sites in Glucose Metabolism:** There are 3 main sites: 1. **Phosphoglycerate kinase** (Glycolysis) 2. **Pyruvate kinase** (Glycolysis) 3. **Succinyl CoA thiokinase** (TCA Cycle) * **Tissue Specificity:** In the liver and kidneys, Succinyl CoA thiokinase typically produces **GTP** (used for gluconeogenesis), while in heart and skeletal muscle, it produces **ATP**. * **Arsenite Poisoning:** Inhibits α-ketoglutarate dehydrogenase, but SLP via Succinyl CoA thiokinase is bypassed if the cycle stops here.

Question 336: Which of the following enzymes catalyzes the formation of AMP from two molecules of ADP?

A. Adenylate kinase (Correct Answer)
B. Adenylyl cyclase
C. Adenosine kinase
D. Adenosine deaminase

Explanation: ### Explanation **Correct Option: A. Adenylate kinase** Adenylate kinase (also known as **myokinase**) is a phosphotransferase enzyme that maintains adenine nucleotide equilibrium within the cell. It catalyzes the reversible reaction: **2 ADP ⇌ ATP + AMP** This reaction is crucial for cellular energy homeostasis. When a cell rapidly consumes ATP (e.g., during vigorous muscle contraction), ADP levels rise. Adenylate kinase "recycles" two ADP molecules to generate one ATP for immediate energy and one AMP, which acts as a potent metabolic signal to activate pathways like glycolysis and fatty acid oxidation (via AMPK). **Why the other options are incorrect:** * **B. Adenylyl cyclase:** This enzyme converts **ATP into cyclic AMP (cAMP)** and pyrophosphate. It is a key component of G-protein signaling pathways, not nucleotide interconversion. * **C. Adenosine kinase:** This enzyme catalyzes the phosphorylation of **adenosine to AMP** using one molecule of ATP. It does not utilize ADP as a substrate. * **D. Adenosine deaminase (ADA):** This is a hydrolase involved in purine catabolism that converts **adenosine to inosine**. Deficiency of ADA leads to Severe Combined Immunodeficiency (SCID). **High-Yield Clinical Pearls for NEET-PG:** * **AMP as a Metabolic Sensor:** An increase in the [AMP]/[ATP] ratio (driven by adenylate kinase) activates **AMP-activated protein kinase (AMPK)**, the "master metabolic switch" that shifts the cell from anabolic to catabolic states. * **Myokinase in Muscle:** In skeletal muscle, this enzyme allows for the rapid regeneration of ATP during the initial seconds of exercise. * **Directionality:** The reaction is freely reversible; however, in a metabolically active cell, it typically proceeds toward AMP formation to signal energy depletion.

Question 337: Trypsin is a:

A. Serine protease (Correct Answer)
B. Lecithinase
C. Phospholipase
D. Elastase

Explanation: **Explanation:** **1. Why Serine Protease is Correct:** Trypsin is a proteolytic enzyme (protease) secreted by the pancreas as the inactive zymogen, trypsinogen. It belongs to the **Serine Protease** family because it contains a highly conserved **"Catalytic Triad"** consisting of three amino acids: **Serine (Ser 195), Histidine (His 57), and Aspartate (Asp 102)**. The serine residue acts as a nucleophile that attacks the peptide bond of the substrate. Trypsin specifically cleaves peptide bonds on the carboxyl side of basic amino acids, namely **Lysine and Arginine**. **2. Why the other options are incorrect:** * **Lecithinase:** Also known as Phospholipase C, this enzyme breaks down lecithin (phosphatidylcholine). It is a classic virulence factor for *Clostridium perfringens* (alpha-toxin), not a protease. * **Phospholipase:** These are enzymes that hydrolyze phospholipids into fatty acids and other lipophilic substances. While the pancreas secretes Phospholipase A2, it is distinct from trypsin. * **Elastase:** While Elastase is *also* a serine protease, it is a **distinct enzyme** from trypsin. Elastase specifically targets elastin and is characterized by its ability to cleave bonds next to small neutral amino acids like Alanine, Valine, or Serine. **3. NEET-PG High-Yield Pearls:** * **The Master Activator:** Trypsin is known as the "master activator" because once activated by **Enteropeptidase (Enterokinase)** in the duodenum, it autocatalytically activates more trypsinogen and other zymogens (Chymotrypsinogen, Procarboxypeptidase, and Proelastase). * **Inhibitor:** Pancreatic secretory trypsin inhibitor (PSTI/SPINK1) prevents premature activation of trypsin within the pancreas to prevent **Acute Pancreatitis**. * **Diagnostic Marker:** Serum Immunoreactive Trypsin (IRT) is used as a screening test for **Cystic Fibrosis** in newborns.

Question 338: Isocitrate dehydrogenase is linked to which cofactor?

A. NAD (Correct Answer)
B. FAD
C. NADP
D. FMN

Explanation: **Explanation:** **Isocitrate Dehydrogenase (ICDH)** is a critical rate-limiting enzyme of the **TCA cycle (Krebs cycle)**. It catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate. 1. **Why NAD is correct:** In the mitochondrial matrix, the primary isoform of ICDH involved in the TCA cycle utilizes **NAD+** as an electron acceptor, reducing it to **NADH**. This NADH then enters the Electron Transport Chain (Complex I) to generate ATP. 2. **Why other options are incorrect:** * **FAD/FMN:** These are flavin nucleotides. FAD is specifically used by **Succinate Dehydrogenase** (Complex II) in the TCA cycle. ICDH does not utilize flavin cofactors. * **NADP:** While an NADP-dependent isoform of ICDH exists (found in the cytosol and mitochondria), it is primarily involved in providing NADPH for reductive biosynthesis and antioxidant defense, not the primary energy-yielding steps of the TCA cycle. In the context of standard metabolic questions, the NAD-linked mitochondrial enzyme is the intended answer. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Step:** Isocitrate dehydrogenase is the most important rate-limiting enzyme of the TCA cycle. * **Regulation:** It is allosterically **activated by ADP and Ca²⁺** and **inhibited by ATP and NADH**. * **First Decarboxylation:** This reaction marks the first of two CO₂ molecules released in the TCA cycle. * **IDH Mutations:** Mutations in IDH1 and IDH2 are significant markers in neuro-oncology (e.g., Gliomas) and AML, leading to the production of the oncometabolite 2-hydroxyglutarate.

Question 339: Which factor is required by pancreatic lipase for the digestion of lipids?

A. Vitamin B12
B. Pyridoxine
C. Tocopherol
D. Colipase (Correct Answer)

Explanation: **Explanation:** **Pancreatic lipase** is the primary enzyme responsible for the hydrolysis of dietary triacylglycerols (TAGs) into 2-monoacylglycerols and free fatty acids. However, its activity is inhibited by bile salts, which displace the lipase from the lipid-water interface of fat droplets. **Colipase** is the essential protein cofactor required to overcome this inhibition. Secreted by the pancreas as an inactive zymogen (**procolipase**) and activated by trypsin, colipase binds to both the bile-salt-coated lipid droplet and the pancreatic lipase. This anchors the lipase to its substrate, allowing digestion to proceed efficiently. **Analysis of Incorrect Options:** * **A. Vitamin B12 (Cobalamin):** Acts as a cofactor for methionine synthase and methylmalonyl-CoA mutase; it has no role in lipid emulsification or lipase activation. * **B. Pyridoxine (Vitamin B6):** Primarily functions as Pyridoxal Phosphate (PLP), the essential cofactor for transamination and decarboxylation reactions in amino acid metabolism. * **C. Tocopherol (Vitamin E):** Functions as a lipid-soluble antioxidant that protects cell membranes from lipid peroxidation; it is not an enzymatic cofactor for digestion. **High-Yield Clinical Pearls for NEET-PG:** * **Activation:** Procolipase is converted to active colipase by **Trypsin** in the intestinal lumen. * **Orlistat:** An anti-obesity drug that works by inhibiting gastric and pancreatic lipases, thereby reducing fat absorption. * **Steatorrhea:** Deficiency of pancreatic lipase or colipase (as seen in chronic pancreatitis or cystic fibrosis) leads to malabsorption of fats and fat-soluble vitamins (A, D, E, K).

Question 340: Which one of the following enzymes uses NADP as a coenzyme?

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

Explanation: **Explanation:** The correct answer is **Glucose-6-phosphate dehydrogenase (G6PD)**. **1. Why G6PD is correct:** G6PD is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). This pathway does not generate ATP; instead, its primary purpose is to produce **NADPH**. G6PD catalyzes the oxidation of Glucose-6-phosphate to 6-phosphogluconolactone, specifically using **NADP+** as the electron acceptor. NADPH is crucial for reductive biosynthesis (e.g., fatty acids, steroids) and maintaining reduced glutathione to protect cells from oxidative stress. **2. Why the other options are incorrect:** * **Glyceraldehyde-3-phosphate dehydrogenase:** This is an enzyme of Glycolysis. It uses **NAD+** (not NADP+) to convert Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. * **Lactate dehydrogenase (LDH):** This enzyme is involved in anaerobic glycolysis. It uses **NAD+/NADH** as a coenzyme to interconvert pyruvate and lactate. * *Note: Most enzymes in catabolic pathways (Glycolysis, TCA cycle) use NAD+, while enzymes in anabolic pathways or antioxidant defense use NADP+.* **3. Clinical Pearls & High-Yield Facts 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. * **Heinz Bodies & Bite Cells:** Classic peripheral smear findings in G6PD deficiency. * **NADP+ vs. NAD+:** A simple mnemonic—**P** is for **P**hosphate, **P**hotosynthesis, and **P**roduction (Anabolism). * **Other NADPH-producing enzymes:** Malic enzyme and 6-phosphogluconate dehydrogenase.

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Enzymes — MCQs

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