Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 281: Which organ injury is associated with elevated levels of LDH isoenzyme-5?

A. Lungs
B. Brain
C. Heart
D. Liver and muscles (Correct Answer)

Explanation: **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme composed of two subunits: **H (Heart)** and **M (Muscle)**. These combine to form five isoenzymes (LDH1 to LDH5), which are tissue-specific. **Why Liver and Muscles are correct:** **LDH-5 (M4)** consists of four M subunits. It is primarily found in tissues that function under anaerobic conditions or have high glycolytic activity, specifically **skeletal muscle** and the **liver parenchyma**. An elevation in LDH-5 is a highly sensitive marker for hepatocellular injury (e.g., hepatitis) or skeletal muscle damage (e.g., muscular dystrophy or strenuous exercise). **Analysis of Incorrect Options:** * **Lungs:** Associated with **LDH-3 (H2M2)**. Elevations are seen in pulmonary embolism or pneumonia. * **Brain:** Primarily contains **LDH-1 and LDH-2**, though LDH-3 is also present. Brain injury or infarct typically shows a rise in these fractions. * **Heart:** Rich in **LDH-1 (H4)**. In myocardial infarction, LDH-1 levels rise and exceed LDH-2 levels (known as the **"Flipped Ratio"**). **High-Yield Clinical Pearls for NEET-PG:** * **LDH-1 (H4):** Heart and RBCs (Elevated in MI and Hemolytic anemia). * **LDH-2 (H3M1):** Reticuloendothelial system (Normal predominant serum fraction). * **LDH-4 (HM3):** Kidney and Pancreas. * **LDH-X (LDH-6):** Found in the mid-piece of spermatozoa; used as a marker for germ cell tumors. * **Total LDH:** A non-specific marker of cell turnover; significantly elevated in **Megaloblastic anemia** and **Pneumocystis jirovecii** pneumonia.

Question 282: Which of the following is NOT a principal digestive enzyme produced by the exocrine pancreas?

A. Trypsinogen
B. Lipase
C. Lactase (Correct Answer)
D. Amylase

Explanation: **Explanation:** The exocrine pancreas is responsible for secreting a potent juice containing enzymes that digest proteins, fats, and carbohydrates. These enzymes are secreted into the duodenum via the pancreatic duct. **Why Lactase is the correct answer:** Lactase is **not** a pancreatic enzyme. It is a **brush-border enzyme** produced by the enterocytes of the small intestinal mucosa. Its specific role is to hydrolyze lactose (milk sugar) into glucose and galactose. Since it is produced by the intestine and not the pancreas, it is the correct choice for this "NOT" question. **Analysis of incorrect options:** * **Trypsinogen (Option A):** This is a proteolytic proenzyme (zymogen) secreted by the pancreas. It is converted into its active form, **trypsin**, by the enzyme enteropeptidase (enterokinase) in the duodenum. * **Lipase (Option B):** Pancreatic lipase is the primary enzyme for fat digestion. It breaks down triglycerides into monoglycerides and free fatty acids in the presence of bile salts and colipase. * **Amylase (Option C):** Pancreatic amylase (α-amylase) is secreted in its active form to digest complex carbohydrates (starch and glycogen) into maltose and maltotriose. **High-Yield Clinical Pearls for NEET-PG:** * **Lactose Intolerance:** Caused by a deficiency of lactase, leading to osmotic diarrhea and abdominal bloating after dairy consumption. * **Zymogens:** To prevent autodigestion of the pancreas, proteases (Trypsin, Chymotrypsin, Elastase) are stored as inactive zymogens. * **Acute Pancreatitis:** Serum **Lipase** is more specific than Amylase for diagnosing acute pancreatitis due to its longer half-life and superior tissue specificity. * **Steatorrhea:** Occurs when pancreatic lipase secretion falls below 10% of normal levels, leading to foul-smelling, fatty stools.

Question 283: Which of the following is an autocatalytic enzyme?

A. Proelastase
B. Procarboxylase
C. Pepsinogen (Correct Answer)
D. Chymotrypsinogen

Explanation: ### Explanation **Correct Answer: C. Pepsinogen** **Mechanism of Autocatalysis:** Autocatalysis refers to a process where the product of a reaction acts as a catalyst for its own formation. **Pepsinogen**, the inactive zymogen secreted by gastric chief cells, is initially activated into **pepsin** by the low pH (HCl) of the stomach. Once a small amount of pepsin is formed, it acts proteolytically on remaining pepsinogen molecules to rapidly convert them into more pepsin. This positive feedback loop ensures rapid protein digestion in the stomach. **Analysis of Incorrect Options:** * **A, B, and D (Proelastase, Procarboxypeptidase, Chymotrypsinogen):** These are all pancreatic zymogens. Unlike pepsinogen, they are not autocatalytic. Their activation follows a specific cascade: 1. The duodenal enzyme **Enteropeptidase (Enterokinase)** first converts Trypsinogen to **Trypsin**. 2. Trypsin then acts as the common activator for all other pancreatic zymogens (Proelastase → Elastase; Procarboxypeptidase → Carboxypeptidase; Chymotrypsinogen → Chymotrypsin). *Note: Trypsinogen itself is also considered autocatalytic because once trypsin is formed, it can activate more trypsinogen.* **High-Yield Clinical Pearls for NEET-PG:** * **Zymogens:** Inactive enzyme precursors that require covalent modification (proteolysis) for activation. This prevents autodigestion of the secreting organs (e.g., the pancreas). * **Trypsin:** Often called the "master switch" of pancreatic digestion. * **Enteropeptidase Deficiency:** Leads to severe protein malabsorption because no pancreatic enzymes can be activated. * **Acute Pancreatitis:** Occurs when trypsin is prematurely activated within the pancreas, leading to autodigestion.

Question 284: Which of the following is an anchoring protein?

A. Myosin
B. Actinin (Correct Answer)
C. Troponin
D. Tropomyosin

Explanation: **Explanation:** The correct answer is **Actinin**. **1. Why Actinin is the Correct Answer:** Anchoring proteins (also known as linking or scaffolding proteins) are responsible for tethering the cytoskeleton to specific structures within the cell. **$\alpha$-Actinin** is a major cross-linking protein found in the **Z-disk** of skeletal muscle. Its primary function is to anchor the ends of the thin (actin) filaments to the Z-disk, ensuring the structural stability of the sarcomere during muscle contraction. **2. Analysis of Incorrect Options:** * **Myosin (Option A):** This is a **contractile protein**. It forms the thick filaments and possesses ATPase activity to generate force via the sliding filament mechanism. * **Troponin (Option C):** This is a **regulatory protein** complex (consisting of subunits T, I, and C). It binds calcium and moves tropomyosin to uncover myosin-binding sites on actin. * **Tropomyosin (Option D):** This is also a **regulatory protein**. It winds around the actin helix and blocks the active sites in a resting muscle to prevent contraction. **3. NEET-PG High-Yield Clinical Pearls:** * **Dystrophin:** Another critical anchoring protein. It links the actin cytoskeleton to the extracellular matrix. Mutations in the *DMD* gene lead to **Duchenne Muscular Dystrophy**. * **Titin:** The largest known protein; it acts as a molecular spring, anchoring the thick (myosin) filaments to the Z-disk and providing passive elasticity. * **Desmin:** An intermediate filament that anchors Z-disks of adjacent myofibrils to each other and to the plasma membrane (sarcolemma). * **Z-Disk Composition:** Remember that the Z-disk marks the boundary of a sarcomere and contains $\alpha$-actinin, desmin, and vimentin.

Question 285: Which of the following is NOT a feature of competitive inhibition?

A. Inhibitor and substrate can bind simultaneously to the same enzyme molecule. (Correct Answer)
B. Inhibitor is a structural analogue of the substrate.
C. Km increases.
D. Vmax is unaffected.

Explanation: In competitive inhibition, the inhibitor competes directly with the substrate for the **active site** of the enzyme. Because they both target the same specific site, their binding is **mutually exclusive**. If the inhibitor is bound, the substrate cannot bind, and vice versa. Therefore, the statement that they can bind simultaneously is false, making Option A the correct answer. **Explanation of Options:** * **Option B (Structural Analogue):** This is a hallmark of competitive inhibition. The inhibitor mimics the substrate's shape to "trick" the enzyme into binding it (e.g., Malonate is a structural analogue of Succinate for Succinate Dehydrogenase). * **Option C (Km increases):** Since the inhibitor competes for the active site, a higher concentration of substrate is required to reach half-maximal velocity ($V_{max}/2$). This results in an increased Michaelis constant ($K_m$), indicating decreased affinity. * **Option D (Vmax is unaffected):** Competitive inhibition can be overcome by increasing the substrate concentration. At infinitely high substrate levels, the substrate outcompetes the inhibitor, allowing the reaction to reach its original maximum velocity ($V_{max}$). **NEET-PG High-Yield Pearls:** * **Lineweaver-Burk Plot:** In competitive inhibition, the lines intersect on the **Y-axis** ($1/V_{max}$ remains constant). * **Clinical Examples:** * **Statins** (HMG-CoA Reductase inhibitors). * **Methanol poisoning treatment:** Ethanol competes with methanol for Alcohol Dehydrogenase. * **Sulfonamides:** Compete with PABA for dihydropteroate synthase in bacteria. * **Non-competitive inhibition:** In contrast, the inhibitor binds to an allosteric site, $V_{max}$ decreases, and $K_m$ remains unchanged.

Question 286: Which of the following is an allosteric inhibitor of pyruvate dehydrogenase?

A. AMP
B. Acetyl CoA (Correct Answer)
C. ADP
D. Citrate

Explanation: **Explanation:** The **Pyruvate Dehydrogenase (PDH) complex** is a multi-enzyme system that catalyzes the irreversible oxidative decarboxylation of pyruvate into Acetyl CoA, serving as the critical bridge between glycolysis and the TCA cycle. **1. Why Acetyl CoA is correct:** PDH is regulated primarily by **product inhibition**. The immediate products of the reaction are **Acetyl CoA** and **NADH**. When these accumulate, they act as potent allosteric inhibitors of the enzyme complex. High levels of Acetyl CoA signal that the cell’s energy needs are met or that fatty acid oxidation is providing sufficient fuel, thus slowing down the conversion of pyruvate. **2. Why the other options are incorrect:** * **AMP and ADP (Options A & C):** These are indicators of a "low energy state." They act as **allosteric activators** of the PDH complex (and inhibitors of PDH kinase) to promote ATP production. * **Citrate (Option D):** While citrate is an important allosteric inhibitor of **Phosphofructokinase-1 (PFK-1)** in glycolysis, it does not directly inhibit the PDH complex. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Covalent Modification:** PDH is also regulated by phosphorylation. **PDH Kinase** inactivates it (stimulated by Acetyl CoA/NADH), while **PDH Phosphatase** activates it (stimulated by $Ca^{2+}$ and Insulin). * **Co-factors:** PDH requires five co-enzymes: **T**hiamine pyrophosphate ($B_1$), **L**ipoic acid, **C**oenzyme A ($B_5$), **F**AD ($B_2$), and **N**AD ($B_3$). (Mnemonic: **T**ender **L**oving **C**are **F**or **N**o-one). * **Arsenic Poisoning:** Arsenite inhibits PDH by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and neurological symptoms.

Question 287: What is the principal serum enzyme used in the clinical diagnosis of Wilson disease?

A. Aspartate aminotransferase
B. Ceruloplasmin (Correct Answer)
C. beta-Glucocerebrosidase
D. Lactate dehydrogenase isozyme 5

Explanation: **Explanation:** **Wilson Disease (Hepatolenticular Degeneration)** is an autosomal recessive disorder caused by a mutation in the **ATP7B gene** on chromosome 13. This defect impairs biliary copper excretion and prevents the incorporation of copper into apo-ceruloplasmin. **Why Ceruloplasmin is the correct answer:** Ceruloplasmin is the primary copper-carrying protein in the blood. In Wilson disease, the failure to form holoceruloplasmin leads to the release of unstable apo-ceruloplasmin, which is rapidly degraded in the circulation. Consequently, **low serum ceruloplasmin levels (<20 mg/dL)** are a hallmark diagnostic finding and the most commonly used serum biochemical marker for the disease. **Analysis of Incorrect Options:** * **A. Aspartate aminotransferase (AST):** While AST levels may rise due to liver damage in Wilson disease, it is a non-specific marker of hepatocellular injury found in many conditions (e.g., viral hepatitis, alcohol use). * **C. beta-Glucocerebrosidase:** This enzyme is deficient in **Gaucher disease**, a lysosomal storage disorder. It has no diagnostic role in copper metabolism. * **D. Lactate dehydrogenase isozyme 5 (LDH-5):** This isozyme is found primarily in the liver and skeletal muscle. While elevated in liver injury, it lacks the specificity required to diagnose Wilson disease. **NEET-PG High-Yield Pearls:** * **Gold Standard Diagnosis:** Liver biopsy showing increased copper content (>250 μg/g dry weight). * **Classic Triad:** Liver cirrhosis, Basal ganglia degeneration (Parkinsonian symptoms), and **Kayser-Fleischer (KF) rings** in the cornea (Descemet's membrane). * **Urinary Findings:** Increased 24-hour urinary copper excretion (>100 μg/day). * **Treatment:** Copper chelators like **D-Penicillamine** (first-line) or Trientine, and Zinc (to inhibit intestinal absorption).

Question 288: Lactate dehydrogenase is:

A. Isoenzyme (Correct Answer)
B. Coenzyme
C. Antienzyme
D. Zymogen

Explanation: **Explanation:** **Lactate Dehydrogenase (LDH)** is a classic example of an **Isoenzyme** (or Isozyme) [3]. Isoenzymes are physically distinct forms of the same enzyme that catalyze the same chemical reaction but differ in their amino acid sequence, physical properties (like electrophoretic mobility), and kinetic parameters ($K_m$ and $V_{max}$) [3]. LDH is a tetramer composed of two types of subunits: **H (Heart)** and **M (Muscle)** [3]. These combine in five different ways to form the five isoenzymes (LDH1 to LDH5), which are distributed tissue-specifically [3]. **Why other options are incorrect:** * **Coenzyme:** These are non-protein organic molecules (like $NAD^+$ or $FAD$) that assist enzymes in catalysis [1]. LDH *uses* $NAD^+$ as a coenzyme but is not one itself [1]. * **Antienzyme:** These are substances (often antibodies or inhibitors) that inhibit enzymatic activity (e.g., Trypsin inhibitors). * **Zymogen:** Also known as proenzymes, these are inactive precursors that require cleavage to become active (e.g., Pepsinogen, Trypsinogen). LDH is synthesized in its active form. **High-Yield Clinical Pearls for NEET-PG:** 1. **LDH Composition:** * **LDH1 ($H_4$):** Found in Heart and RBCs [3]. * **LDH2 ($H_3M_1$):** Predominant form in normal serum [2]. * **LDH4 & LDH5 ($M_4$):** Found in Liver and Skeletal muscle [3]. 2. **Diagnostic Significance:** In Myocardial Infarction (MI), the **"LDH Flip"** occurs where LDH1 levels exceed LDH2 (normally LDH2 > LDH1) [2]. 3. **Cancer Marker:** LDH is a non-specific marker of high cell turnover (e.g., Lymphoma, Germ cell tumors).

Question 289: Which of the following covalent modifications does NOT regulate enzyme kinetics?

A. Phosphorylation
B. Acetylation
C. ADP-Ribosylation
D. Glycosylation (Correct Answer)

Explanation: **Explanation:** Enzyme activity is regulated through various mechanisms, including **covalent modification**, where the addition or removal of a chemical group alters the enzyme's conformation and kinetics. **Why Glycosylation is the Correct Answer:** While glycosylation (the addition of carbohydrate chains) is a vital post-translational modification, its primary roles are **protein folding, stability, cell-cell recognition, and trafficking** (e.g., targeting enzymes to lysosomes via Mannose-6-Phosphate). Unlike phosphorylation, it is generally not used as a "molecular switch" to acutely turn enzyme catalytic activity on or off in response to metabolic signals. **Analysis of Incorrect Options:** * **Phosphorylation (Option A):** The most common covalent modification. It occurs on Serine, Threonine, or Tyrosine residues. Example: **Glycogen phosphorylase** is activated by phosphorylation, while **Glycogen synthase** is inactivated. * **Acetylation (Option B):** Common in histones and metabolic enzymes. Acetylation of lysine residues can alter the charge and affinity of enzymes for DNA or substrates. * **ADP-Ribosylation (Option C):** Involves the transfer of ADP-ribose from NAD+. This is a key mechanism for several bacterial toxins (e.g., **Cholera toxin** ADP-ribosylates Gs proteins, and **Diphtheria toxin** inhibits Elongation Factor-2). **High-Yield Clinical Pearls for NEET-PG:** * **Zymogen Activation:** A form of irreversible covalent modification (proteolysis), e.g., Pepsinogen to Pepsin. * **Master Regulator:** Protein Kinase A (PKA) mediates most phosphorylation events triggered by cAMP. * **Lysosomal Targeting:** Deficiency in the glycosylation step that adds Mannose-6-Phosphate leads to **I-Cell Disease**, where enzymes are secreted extracellularly instead of being sent to lysosomes.

Question 290: In muscle, phosphorylase b is inactivated by which of the following?

A. cAMP
B. Ca ions
C. Glucose
D. ATP (Correct Answer)

Explanation: **Explanation:** Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis. In muscle, it exists in two forms: **Phosphorylase *a*** (phosphorylated, active) and **Phosphorylase *b*** (dephosphorylated, usually inactive). **Why ATP is the correct answer:** Muscle phosphorylase *b* is regulated **allosterically**. It serves as an energy sensor for the cell. When the cell has high energy levels (high **ATP** and **Glucose-6-Phosphate**), these molecules bind to the enzyme and stabilize its inactive state (T-state). This prevents unnecessary glycogen breakdown when energy is abundant. **Analysis of Incorrect Options:** * **A. cAMP:** cAMP does not bind directly to phosphorylase *b*. Instead, it activates Protein Kinase A (PKA), which leads to the phosphorylation (activation) of the enzyme into phosphorylase *a*. * **B. Ca²⁺ ions:** Calcium is a potent **activator**. During muscle contraction, Ca²⁺ binds to the calmodulin subunit of phosphorylase kinase, which then activates phosphorylase *b* to *a*. * **C. Glucose:** While glucose is an allosteric inhibitor of **liver** phosphorylase, it does not play a significant role in inhibiting the **muscle** isoform. **NEET-PG High-Yield Pearls:** * **AMP** is the most potent allosteric **activator** of muscle phosphorylase *b*, signaling low energy status. * **McArdle Disease (GSD Type V):** Caused by a deficiency of skeletal muscle glycogen phosphorylase, leading to exercise intolerance and "second wind" phenomenon. * **Covalent Modification:** Phosphorylation (via Phosphorylase Kinase) activates the enzyme; Dephosphorylation (via Protein Phosphatase-1) inactivates it.

Practice by Chapter

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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