Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 231: Clinically, enzymes are classified as functional plasma enzymes and non-functional plasma enzymes. Which among the following is an example of a functional enzyme?

A. ALT
B. Lipoprotein lipase (Correct Answer)
C. Prothrombin
D. Amylase

Explanation: ### Explanation Enzymes in the plasma are broadly categorized based on their site of action and physiological role: **1. Functional Plasma Enzymes:** These enzymes are actively secreted into the blood by specific organs (like the liver) and perform their primary physiological function within the circulation. They are present in higher concentrations in plasma than in tissues. * **Lipoprotein Lipase (LPL):** This is a classic functional enzyme. It is synthesized in extrahepatic tissues and acts on the surface of capillary endothelium to hydrolyze triglycerides in chylomicrons and VLDLs into free fatty acids and glycerol. * **Other examples:** Procoagulants (Thrombin, Factor X), Pseudocholinesterase, and Plasmin. **2. Non-functional Plasma Enzymes:** These enzymes perform their primary functions **intracellularly**. They are present in the plasma in very low concentrations due to normal cell turnover. An elevation in their levels usually indicates tissue damage or necrosis. * **ALT (Alanine Aminotransferase):** An intracellular enzyme primarily found in the liver; its elevation signifies hepatocyte injury. * **Amylase:** Secreted by the pancreas and salivary glands into the gastrointestinal tract. Its presence in plasma is incidental; high levels indicate pancreatitis. * **Prothrombin:** Note that while Prothrombin is a precursor to a functional enzyme (Thrombin), it is technically a **zymogen** (pro-enzyme). In many classifications, the active form (Thrombin) is cited as the functional enzyme. However, between LPL and Prothrombin in standard biochemistry texts (like Vasudevan or Harper), LPL is the definitive example of a functional enzyme constantly active in lipid metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Value:** Non-functional enzymes are used as **biomarkers** for organ damage (e.g., CK-MB for MI, Lipase for Pancreatitis). * **Functional Enzyme Deficiency:** A decrease in the level of functional enzymes usually indicates liver dysfunction (reduced synthesis) or genetic deficiency (e.g., Type I Hyperlipoproteinemia due to LPL deficiency). * **Cofactor:** LPL requires **Apo C-II** as a mandatory cofactor for its activation.

Question 232: Aspartate aminotransferase is released:

A. by viable cells
B. after cell necrosis (Correct Answer)
C. either of the two
D. none of the above

Explanation: ### Explanation **Correct Option: B. after cell necrosis** **Mechanism:** Aspartate aminotransferase (AST), also known as SGOT, is an intracellular enzyme located primarily in the **mitochondria (80%)** and **cytoplasm (20%)** of hepatocytes, cardiac myocytes, and skeletal muscle cells. In healthy, viable cells, the plasma membrane is intact and acts as a barrier, keeping these large enzyme molecules within the cell. When a cell undergoes **necrosis** (due to ischemia, toxins, or inflammation), the integrity of the cell membrane is lost. This "leakiness" allows intracellular contents, including AST, to diffuse into the interstitial fluid and subsequently into the bloodstream. Therefore, an elevated serum AST level is a biochemical marker of **cellular death or significant injury**, not normal physiological release. **Analysis of Incorrect Options:** * **Option A (Viable cells):** Healthy cells do not actively secrete AST. While there is a negligible "turnover" of cells that maintains a low baseline level in the blood, active release only occurs upon structural damage. * **Option C & D:** These are incorrect because the release is specifically triggered by the loss of membrane integrity associated with cell death. **High-Yield Clinical Pearls for NEET-PG:** * **Tissue Specificity:** AST is less specific for the liver than ALT (Alanine Aminotransferase) because AST is also found in the heart, muscles, kidneys, and RBCs. * **De Ritis Ratio (AST/ALT):** * **Ratio > 2:1** is highly suggestive of **Alcoholic Liver Disease** (Alcohol depletes pyridoxal phosphate, which inhibits ALT more than AST). * **Ratio < 1** is typically seen in **Acute Viral Hepatitis**. * **Myocardial Infarction:** AST was historically used as a cardiac marker; it rises 6–8 hours after an MI, peaks at 24 hours, and returns to normal within 5 days.

Question 233: Which amino acid is present in Glutathione peroxidase?

A. Alanine
B. Selenocysteine (Correct Answer)
C. Cysteine
D. Serine

Explanation: **Explanation:** The correct answer is **Selenocysteine**. **1. Why Selenocysteine is correct:** Glutathione peroxidase (GPx) is a critical antioxidant enzyme that protects cells from oxidative damage by reducing lipid hydroperoxides and free hydrogen peroxide. It belongs to a unique class of proteins called **selenoproteins**. The active site of this enzyme contains **Selenocysteine**, often referred to as the "21st amino acid." Unlike other post-translational modifications, selenocysteine is incorporated into the polypeptide chain during translation, encoded by the **UGA stop codon** (with the help of a specific SECIS element). The selenium atom in selenocysteine is essential for the enzyme's catalytic activity, providing higher nucleophilic strength than sulfur. **2. Why other options are incorrect:** * **Alanine:** A simple non-polar amino acid that does not possess the redox-active properties required for the catalytic center of antioxidant enzymes. * **Cysteine:** While structurally similar to selenocysteine (containing sulfur instead of selenium), it is not the primary amino acid in Glutathione peroxidase. However, it is found in the related enzyme *Glutathione Reductase*. * **Serine:** Although selenocysteine is biosynthetically derived from serine (attached to tRNA), serine itself lacks the selenium atom necessary for GPx function. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Selenium Deficiency:** Leads to **Keshan Disease** (endemic cardiomyopathy), primarily due to the decreased activity of Glutathione peroxidase. * **Cofactor:** Glutathione peroxidase requires **Reduced Glutathione (GSH)** as a hydrogen donor. * **Other Selenoproteins:** **Thioredoxin reductase** and **Deiodinase** (which converts T4 to T3) also contain Selenocysteine. * **Key Enzyme Pair:** While GPx reduces $H_2O_2$, **Glutathione Reductase** regenerates GSH using **NADPH** (derived from the HMP Shunt).

Question 234: Which divalent cation is essential for the function of most kinases?

A. Manganese (Mn2+)
B. Copper (Cu2+)
C. Magnesium (Mg2+) (Correct Answer)
D. Inorganic phosphate

Explanation: **Explanation:** The correct answer is **Magnesium (Mg²⁺)**. **1. Why Magnesium is Correct:** Kinases are enzymes that catalyze the transfer of a phosphate group from a high-energy molecule (usually ATP) to a substrate. The true substrate for most kinases is not just ATP, but a **Mg²⁺-ATP complex**. Magnesium ions coordinate with the negatively charged oxygen atoms of the phosphate groups on ATP. This interaction neutralizes the charge, stabilizes the molecule, and facilitates a nucleophilic attack on the gamma-phosphate, making the transfer energetically favorable. Without Mg²⁺, the highly negative charge of ATP would repel the nucleophilic functional groups of the substrate. **2. Why Other Options are Incorrect:** * **Manganese (Mn²⁺):** While Mn²⁺ can act as a cofactor for some enzymes (like Pyruvate carboxylase or Arginase), it is not the primary physiological activator for the majority of kinases. * **Copper (Cu²⁺):** Copper is typically a cofactor for redox enzymes (oxidoreductases) such as Cytochrome c oxidase and Superoxide dismutase, not phosphate transfer. * **Inorganic Phosphate:** This is often a product or a substrate in phosphorylase reactions, but it is an anion, not a divalent cation cofactor. **3. High-Yield Clinical Pearls for NEET-PG:** * **Hypomagnesemia Link:** Clinically, low magnesium levels can lead to "refractory hypokalemia" and "hypocalcemia" because Mg²⁺ is required for the function of the Na⁺/K⁺ ATPase pump and the secretion/action of Parathyroid Hormone (PTH). * **Hexokinase/Glucokinase:** These are the classic examples of Mg²⁺-dependent kinases in glycolysis. * **Rule of Thumb:** Whenever ATP is used by an enzyme, Mg²⁺ is almost always required as a cofactor.

Question 235: Digestive enzymes are primarily classified as which type of enzyme?

A. Hydrolases (Correct Answer)
B. Oxidoreductases
C. Dehydrogenases
D. Ligases

Explanation: **Explanation:** **Correct Answer: A. Hydrolases** Digestive enzymes (such as amylase, pepsin, trypsin, and lipase) function by catalyzing the cleavage of chemical bonds (ester, ether, peptide, or glycosidic bonds) through the **addition of a water molecule ($H_2O$)**. This process is known as hydrolysis. Since digestion involves breaking down complex macromolecules (proteins, carbohydrates, and fats) into simpler units using water, these enzymes are classified under Class 3 of the IUBMB enzyme classification: **Hydrolases**. **Analysis of Incorrect Options:** * **B. Oxidoreductases:** These enzymes catalyze oxidation-reduction reactions involving the transfer of electrons or hydrogen (e.g., Lactate Dehydrogenase). Digestion does not primarily involve redox reactions. * **C. Dehydrogenases:** This is a sub-class of Oxidoreductases. They remove hydrogen atoms from a substrate and transfer them to an acceptor like $NAD^+$ or $FAD$. * **D. Ligases:** These enzymes catalyze the joining of two molecules, usually coupled with the hydrolysis of ATP (e.g., DNA Ligase, Pyruvate Carboxylase). They are involved in synthesis, not breakdown. **High-Yield NEET-PG Pearls:** * **IUBMB Classification Mnemonic:** **O**ver **T**he **H**ill **L**yases **I**somereases **L**igases (**O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases). * **Lyases vs. Hydrolases:** Both break bonds, but **Lyases** do so without water or oxidation, often forming a double bond (e.g., Carbonic Anhydrase, Aldolase). * **Clinical Note:** Most lysosomal enzymes are also hydrolases (acid hydrolases), which is why lysosomal storage diseases result from their deficiency.

Question 236: Which of the following causes hydrolysis of peptidoglycans?

A. Lysozyme (Correct Answer)
B. Lactoferrin
C. Protease
D. Aflatoxin

Explanation: **Explanation:** **Lysozyme (Option A)** is the correct answer. It is a glycoside hydrolase enzyme found in secretions like tears, saliva, and mucus. Its primary mechanism of action is the **hydrolysis of the β(1→4) glycosidic bond** between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in the bacterial **peptidoglycan** cell wall. This disruption leads to osmotic lysis of the bacteria, making lysozyme a critical component of the innate immune system. **Analysis of Incorrect Options:** * **Lactoferrin (Option B):** This is an iron-binding protein found in secretions. It acts as a bacteriostatic agent by sequestering free iron, which is essential for bacterial growth, rather than degrading the cell wall. * **Protease (Option C):** These enzymes catalyze the proteolysis of peptide bonds in proteins. While peptidoglycan contains peptide cross-links, the primary backbone is a polysaccharide chain targeted by lysozymes, not general proteases. * **Aflatoxin (Option D):** This is a potent hepatocarcinogenic toxin produced by *Aspergillus flavus*. It is not an enzyme and does not cause hydrolysis; instead, it causes DNA damage and is linked to hepatocellular carcinoma. **NEET-PG High-Yield Pearls:** * **Target Site:** Lysozyme specifically cleaves the **β-1,4 linkage** between NAM and NAG. * **Gram-Positive vs. Negative:** Lysozyme is more effective against Gram-positive bacteria because their peptidoglycan layer is exposed, whereas Gram-negative bacteria have an outer membrane protecting the cell wall. * **Clinical Relevance:** Deficiencies in lysozyme are associated with increased susceptibility to conjunctival and pulmonary infections. It is often used as a marker in the diagnosis of sarcoidosis and certain leukemias (monocytic leukemia).

Question 237: Who proposed the induced fit hypothesis?

A. Koshland (Correct Answer)
B. Niemann
C. Krisch
D. Marfan

Explanation: **Explanation:** The **Induced Fit Hypothesis** was proposed by **Daniel Koshland** in 1958. This model is a refinement of the older "Lock and Key" model. It suggests that the enzyme's active site is not a rigid structure but is flexible. When a substrate approaches, it induces a conformational change in the enzyme, allowing the active site to mold itself around the substrate to achieve a precise fit. This interaction stabilizes the transition state and enhances the catalytic rate. **Analysis of Options:** * **A. Koshland (Correct):** He challenged the rigid model by proving that enzymes undergo structural rearrangements upon substrate binding. * **B. Niemann:** Known for the Niemann-Pick disease (a lysosomal storage disorder involving sphingomyelinase deficiency), not enzyme kinetics theories. * **C. Krisch:** Associated with studies on esterases, but did not propose a fundamental model of enzyme-substrate binding. * **D. Marfan:** Antoine Marfan described Marfan Syndrome, a genetic connective tissue disorder caused by mutations in the FBN1 gene (Fibrillin-1). **High-Yield Clinical Pearls for NEET-PG:** * **Lock and Key Model:** Proposed by **Emil Fischer** (1894); it assumes the active site is rigid and pre-shaped. * **Michaelis-Menten Equation:** Describes the rate of enzymatic reactions ($V = V_{max}[S] / K_m + [S]$). * **Koshland’s Model** explains why some enzymes can act on multiple similar substrates (broad specificity) and how allosteric modulation occurs through shape changes. * **Transition State:** The induced fit model emphasizes that enzymes are most complementary to the **transition state** of the reaction, rather than the ground-state substrate.

Question 238: Which enzyme is deficient in RBCs?

A. Hexokinase
B. Phosphofructokinase
C. Pyruvate kinase (Correct Answer)
D. Glycerol kinase

Explanation: **Explanation:** **Pyruvate Kinase (PK)** is the correct answer because it is the most common enzyme deficiency in the glycolytic pathway involving Red Blood Cells (RBCs). Mature RBCs lack mitochondria and depend entirely on **anaerobic glycolysis** for ATP production. A deficiency in PK leads to decreased ATP, causing failure of the Na+/K+ ATPase pumps. This results in loss of intracellular potassium and water, leading to cell dehydration, "echinocyte" formation, and premature destruction in the spleen (**Hereditary Non-spherocytic Hemolytic Anemia**). **Analysis of Incorrect Options:** * **Hexokinase (A) & Phosphofructokinase (B):** While deficiencies in these enzymes can occur, they are extremely rare compared to Pyruvate Kinase deficiency. Both are essential "rate-limiting" steps of glycolysis present in RBCs. * **Glycerol Kinase (D):** This enzyme is **absent** in RBCs (and adipose tissue). It is primarily found in the liver and kidneys. Its absence in RBCs means they cannot utilize glycerol for energy; however, the question context refers to a clinically significant pathological deficiency in the existing glycolytic machinery. **Clinical Pearls for NEET-PG:** * **Inheritance:** PK deficiency is typically **Autosomal Recessive**. * **Biochemical marker:** It leads to an accumulation of **2,3-BPG**, which shifts the oxygen-dissociation curve to the **right**, helping in oxygen unloading to tissues (compensatory mechanism). * **Blood Smear:** Characterized by **Echinocytes** (Burr cells/spiculated cells). * **Second most common cause** of enzyme-deficient hemolytic anemia (after G6PD deficiency).

Question 239: What is true about competitive inhibition?

A. The structure of the inhibitor molecule is similar to that of the substrate.
B. The inhibitor gets attached to the active site of the enzyme.
C. It does not alter the structure of the enzyme.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** Competitive inhibition is a reversible process where an inhibitor competes directly with the substrate for the same binding site on an enzyme. 1. **Structural Similarity (Option A):** The inhibitor is a **structural analogue** of the substrate. Because they look alike, the enzyme’s active site cannot distinguish between the two, allowing the inhibitor to bind instead of the substrate. 2. **Binding Site (Option B):** The inhibitor binds specifically to the **active site** (catalytic site). This physically blocks the substrate from entering, preventing the formation of the Enzyme-Substrate (ES) complex. 3. **Enzyme Structure (Option C):** Unlike non-competitive or allosteric inhibitors, competitive inhibitors do not induce a conformational change or alter the enzyme's primary/tertiary structure; they simply occupy the space. **Kinetics & NEET-PG High-Yield Facts:** * **$V_{max}$ remains unchanged:** Since the inhibition can be overcome by increasing the substrate concentration ($[S]$), the maximum velocity is eventually reached. * **$K_m$ increases:** Because the inhibitor interferes with substrate binding, the apparent affinity of the enzyme for the substrate decreases (higher $K_m$). * **Lineweaver-Burk Plot:** The lines for inhibited and uninhibited reactions intersect on the **y-axis** ($1/V_{max}$). **Clinical Pearls:** * **Statins** (e.g., Atorvastatin) are competitive inhibitors of HMG-CoA reductase. * **Methanol poisoning** is treated with **Ethanol**, which competitively inhibits Alcohol Dehydrogenase. * **Sulfonamides** are structural analogues of PABA, competitively inhibiting dihydropteroate synthase in bacteria.

Question 240: What is the term for the combination of a main supporting enzyme and its cofactor?

A. Apoenzyme
B. Coenzyme
C. Holoenzyme (Correct Answer)
D. Constitutive enzyme

Explanation: ### Explanation **1. Why Holoenzyme is Correct:** In biochemistry, many enzymes are not active on their own and require a non-protein component to function. A **Holoenzyme** represents the complete, catalytically active unit. It is formed by the combination of the protein part (**Apoenzyme**) and its required non-protein component (**Cofactor**). * **Formula:** Apoenzyme + Cofactor = Holoenzyme. * The cofactor can be a metal ion (e.g., $Zn^{2+}$, $Mg^{2+}$) or an organic molecule (Coenzyme). **2. Analysis of Incorrect Options:** * **Apoenzyme:** This is the protein portion of the enzyme alone. It is catalytically inactive because it lacks the necessary cofactor. * **Coenzyme:** This is a specific type of cofactor—a non-protein, organic molecule (often derived from vitamins like B-complex) that binds to the apoenzyme. It is only one part of the holoenzyme, not the whole complex. * **Constitutive enzyme:** These are "housekeeping" enzymes that are produced at a constant rate by the cell regardless of the physiological demand or substrate concentration (e.g., enzymes of glycolysis), unlike inducible enzymes. **3. NEET-PG High-Yield Pearls:** * **Prosthetic Group:** If a coenzyme is covalently or very tightly bound to the apoenzyme (e.g., Heme in Cytochrome P450 or Biotin in Carboxylases), it is called a prosthetic group. * **Metalloenzymes:** Enzymes that require a specific metal ion for their activity (e.g., Carbonic Anhydrase requires Zinc). * **Clinical Correlation:** Many vitamin deficiencies manifest as metabolic blocks because the body cannot form the necessary **Coenzymes** to complete the **Holoenzyme** complex (e.g., Thiamine deficiency leads to decreased activity of Pyruvate Dehydrogenase).

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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Enzymes — MCQs

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