Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 351: Which of the following enzymes is classified as a Lyase?

A. Aldolase (Correct Answer)
B. Acetyl CoA Synthetase
C. Fatty Acyl CoA Dehydrogenase
D. Acetyl CoA Carboxylase

Explanation: **Explanation:** Enzymes are classified into six major classes based on the type of reaction they catalyze (IUBMB classification). **Lyases (Class 4)** are enzymes that catalyze the cleavage of C-C, C-O, C-N, and other bonds by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of groups to double bonds. **1. Why Aldolase is correct:** **Aldolase** (specifically Fructose-1,6-bisphosphate aldolase) is a classic example of a Lyase. In glycolysis, it cleaves the 6-carbon Fructose-1,6-bisphosphate into two 3-carbon molecules (DHAP and Glyceraldehyde-3-phosphate) without the use of water or redox cofactors. **2. Analysis of Incorrect Options:** * **Acetyl CoA Synthetase:** This is a **Ligase (Class 6)**. It joins Acetate and Coenzyme A using the energy from ATP hydrolysis. (Note: *Synthetases* require ATP, while *Synthases* do not). * **Fatty Acyl CoA Dehydrogenase:** This is an **Oxidoreductase (Class 1)**. It catalyzes the removal of hydrogen atoms during beta-oxidation, utilizing FAD as an electron acceptor. * **Acetyl CoA Carboxylase:** This is a **Ligase (Class 6)**. It catalyzes the ATP-dependent carboxylation of Acetyl CoA to Malonyl CoA (the rate-limiting step in fatty acid synthesis). **Clinical Pearls & High-Yield Facts:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**yases **I**somerize **L**igases (**O**xidoreductase, **T**ransferase, **H**ydrolase, **L**yase, **I**somerase, **L**igase). * **Aldolase B Deficiency:** Leads to **Hereditary Fructose Intolerance**, characterized by hypoglycemia and jaundice after fructose ingestion. * **Synthase vs. Synthetase:** For NEET-PG, remember that **Synthases** are usually Lyases (e.g., ATP Synthase), whereas **Synthetases** are always Ligases.

Question 352: All of the following are trypsin inhibitors, except:

A. Alpha-1 antitrypsin
B. Alpha-1 antiproteinase
C. Enterokinase (Correct Answer)
D. Egg white

Explanation: **Explanation:** The correct answer is **Enterokinase** because it is an **activator** of trypsin, not an inhibitor. **1. Why Enterokinase is the correct answer:** Enterokinase (also known as enteropeptidase) is an enzyme secreted by the duodenal mucosa. Its primary physiological role is to convert the inactive zymogen **trypsinogen** into active **trypsin** by cleaving a specific hexapeptide from the N-terminal end. Once a small amount of trypsin is formed, it auto-catalyzes the activation of more trypsinogen and other pancreatic zymogens (chymotrypsinogen, procarboxypeptidase). Therefore, it acts as the "molecular switch" for protein digestion. **2. Why the other options are incorrect:** * **Alpha-1 antitrypsin (A) & Alpha-1 antiproteinase (B):** These are two names for the same serine protease inhibitor (Serpin). Despite its name, it is a potent inhibitor of several proteases, including trypsin, though its most critical clinical role is inhibiting neutrophil elastase. * **Egg white (D):** Raw egg white contains **ovomucoid** and **ovoinhibitor**, which are potent natural trypsin inhibitors. This is why consuming large amounts of raw egg whites can interfere with protein digestion. **Clinical Pearls for NEET-PG:** * **Deficiency of Enterokinase:** Leads to severe protein malabsorption, presenting with failure to thrive, hypoproteinemia, and edema in infants. * **Alpha-1 Antitrypsin Deficiency:** Results in uninhibited neutrophil elastase activity, leading to **Panacinar Emphysema** (lungs) and **Liver Cirrhosis** (due to accumulation of misfolded proteins in hepatocytes). * **Pancreatic Secretory Trypsin Inhibitor (PSTI/SPINK1):** A specific protein in the pancreas that prevents premature activation of trypsinogen within the pancreatic ducts, protecting against **acute pancreatitis**.

Question 353: Which one of the following enzymes is used as an anti-cancer drug?

A. Alpha-1-antitrypsin
B. Streptokinase
C. Asparaginase (Correct Answer)
D. Papain

Explanation: **Explanation:** **L-Asparaginase** is used as a chemotherapeutic agent, primarily in the treatment of **Acute Lymphoblastic Leukemia (ALL)**. The underlying medical concept is based on a metabolic vulnerability: normal cells can synthesize the non-essential amino acid **asparagine** from aspartate using the enzyme *asparagine synthetase*. However, certain malignant lymphoid cells lack this enzyme and depend on the systemic circulation for their asparagine supply. L-Asparaginase catalyzes the hydrolysis of circulating asparagine into aspartic acid and ammonia, depriving the tumor cells of this vital nutrient, leading to inhibited protein synthesis and apoptosis. **Analysis of Incorrect Options:** * **Alpha-1-antitrypsin (A):** This is a protease inhibitor used as replacement therapy in patients with hereditary Alpha-1-antitrypsin deficiency to prevent panacinar emphysema. * **Streptokinase (B):** This is a fibrinolytic (thrombolytic) enzyme used to dissolve blood clots in conditions like acute myocardial infarction, pulmonary embolism, and stroke. * **Papain (D):** Derived from papaya, this proteolytic enzyme is used for wound debridement (removing dead tissue) and as a digestive aid, but it has no anti-cancer properties. **High-Yield Clinical Pearls for NEET-PG:** * **Side Effects:** The most common side effect of L-Asparaginase is **hypersensitivity/anaphylaxis** (as it is a bacterial product from *E. coli* or *Erwinia*). It can also cause **acute pancreatitis** and a decrease in clotting factors (leading to thrombosis or hemorrhage). * **Cell Cycle Specificity:** It is considered a **G1 phase-specific** drug. * **Other therapeutic enzymes:** Note that **Pegaspargase** is a pegylated form of asparaginase with a longer half-life and reduced immunogenicity.

Question 354: All of the following are secreted in proenzyme form except?

A. Trypsin
B. Chymotrypsin
C. Pepsin
D. Ribonuclease (Correct Answer)

Explanation: ### Explanation The core concept tested here is the physiological significance of **Zymogens (Proenzymes)**. Zymogens are inactive precursors of enzymes that require biochemical change (usually selective proteolysis) to become active. This is a protective mechanism to prevent the autodigestion of the organs that synthesize them. **Why Ribonuclease is the Correct Answer:** **Ribonuclease (RNase)** is an enzyme that catalyzes the degradation of RNA into smaller components. Unlike proteases, RNase does not pose a threat of digesting the cellular structure of the pancreas or stomach (which are primarily protein and lipid-based). Therefore, it is secreted in its **active form** directly. **Analysis of Incorrect Options:** * **Trypsin (as Trypsinogen):** Secreted by the pancreas. If active within the pancreas, it would cause acute pancreatitis. It is activated by enteropeptidase in the duodenum. * **Chymotrypsin (as Chymotrypsinogen):** Also a pancreatic protease. It is activated by trypsin in the small intestine. * **Pepsin (as Pepsinogen):** Secreted by the chief cells of the stomach. It requires the acidic pH of gastric HCl to undergo autocatalytic cleavage into active pepsin. This prevents the digestion of the gastric mucosa during synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Activation Cascade:** Trypsin is the "master activator." Once trypsinogen is converted to trypsin by **enteropeptidase (enterokinase)**, trypsin goes on to activate chymotrypsinogen, proelastase, and procarboxypeptidases. * **Acute Pancreatitis:** This condition occurs when zymogens (especially trypsin) are prematurely activated within the pancreatic parenchyma, leading to autodigestion. * **Pancreatic Secretions:** Remember that while proteases are secreted as proenzymes, **pancreatic amylase and pancreatic lipase** are secreted in their active forms.

Question 355: Which element is a component of cytochrome oxidase?

A. Copper (Correct Answer)
B. Iodine
C. Manganese
D. Molybdenum

Explanation: **Explanation:** **Cytochrome Oxidase (Complex IV)** is the terminal enzyme of the mitochondrial electron transport chain. It plays a critical role in aerobic respiration by transferring electrons to oxygen to form water. This enzyme contains two heme groups ($a$ and $a_3$) and **two copper centers ($Cu_A$ and $Cu_B$)**. Copper is essential for the catalytic reduction of oxygen; specifically, the $a_3-Cu_B$ binuclear center is where the actual reduction of $O_2$ occurs. **Analysis of Incorrect Options:** * **B. Iodine:** Primarily required for the synthesis of thyroid hormones ($T_3$ and $T_4$) in the thyroid gland. It has no role in the electron transport chain. * **C. Manganese:** Acts as a cofactor for enzymes like Pyruvate Carboxylase, Arginase, and Mitochondrial Superoxide Dismutase (Mn-SOD). * **D. Molybdenum:** A vital cofactor for "molybdopterin" dependent enzymes, including Xanthine Oxidase (purine catabolism), Sulfite Oxidase, and Aldehyde Oxidase. **High-Yield Clinical Pearls for NEET-PG:** * **Cyanide and Carbon Monoxide Poisoning:** Both inhibit Cytochrome Oxidase (Complex IV) by binding to the iron/copper centers, halting ATP production and causing cellular hypoxia. * **Menkes Disease:** A defect in copper absorption (ATP7A) leads to a deficiency of copper-dependent enzymes, including cytochrome oxidase, resulting in neurological symptoms and "kinky" hair. * **Other Copper-containing enzymes:** Superoxide dismutase (cytosolic), Tyrosinase (melanin synthesis), Lysyl oxidase (collagen cross-linking), and Ferroxidase (Ceruloplasmin).

Question 356: Which serum enzyme is primarily used for the diagnosis of Myocardial Infarction?

A. Creatine Kinase (CK) (Correct Answer)
B. Alkaline Phosphatase
C. Acid Phosphatase
D. Lipase

Explanation: ### Explanation **1. Why Creatine Kinase (CK) is Correct:** Creatine Kinase (specifically the **CK-MB isoenzyme**) is a classic biochemical marker for Myocardial Infarction (MI). CK-MB is found predominantly in cardiac muscle. Following myocardial injury, it rises within 4–6 hours, peaks at 24 hours, and returns to baseline within 48–72 hours. While Cardiac Troponins (I and T) are now the "gold standard" due to higher sensitivity and specificity, CK-MB remains clinically significant for detecting **re-infarction** because of its rapid clearance from the blood. **2. Why the Other Options are Incorrect:** * **Alkaline Phosphatase (ALP):** This enzyme is primarily used to diagnose hepatobiliary diseases (especially obstructive jaundice) and bone disorders (like Rickets or Paget’s disease). It has no diagnostic value in MI. * **Acid Phosphatase (ACP):** Historically used as a marker for prostate cancer (specifically Prostatic Acid Phosphatase), it is also found in lysosomes and RBCs. It is not associated with cardiac injury. * **Lipase:** This is a highly specific marker for **Acute Pancreatitis**. It rises within hours of pancreatic inflammation and remains elevated longer than amylase. **3. NEET-PG High-Yield Clinical Pearls:** * **Sequence of markers in MI:** Myoglobin (Earliest, 1-3h) → CK-MB/Troponins (4-6h) → LDH (Late marker). * **LDH Flip:** In MI, LDH-1 becomes higher than LDH-2 (normally LDH-2 > LDH-1). * **AST (Aspartate Aminotransferase):** Was the first enzyme used for MI diagnosis but is now obsolete for this purpose due to lack of specificity. * **Key takeaway:** If a patient has a second chest pain 3 days after an initial MI, **CK-MB** is the investigation of choice to diagnose re-infarction.

Question 357: Which of the following elements is contained in cytochrome oxidase?

A. Iron (Fe)
B. Copper (Cu)
C. Both Iron and Copper (Correct Answer)
D. Neither Iron nor Copper

Explanation: ### Explanation **Core Concept:** Cytochrome oxidase, also known as **Complex IV** of the Electron Transport Chain (ETC), is the terminal enzyme that catalyzes the transfer of electrons from cytochrome *c* to molecular oxygen, reducing it to water. To perform this redox reaction, the enzyme requires specific metallic prosthetic groups. **Why Option C is Correct:** Cytochrome oxidase is a large transmembrane protein complex that contains: 1. **Two Heme groups:** Heme $a$ and Heme $a_3$. These contain **Iron (Fe)**, which cycles between the ferrous ($Fe^{2+}$) and ferric ($Fe^{3+}$) states. 2. **Two Copper centers:** $Cu_A$ and $Cu_B$. Specifically, electrons flow from Cytochrome *c* $\rightarrow$ $Cu_A$ $\rightarrow$ Heme $a$ $\rightarrow$ Heme $a_3$–$Cu_B$ binuclear center $\rightarrow$ $O_2$. Because both metals are essential for the structural integrity and electron transfer capability of the enzyme, "Both Iron and Copper" is the correct answer. **Why Other Options are Incorrect:** * **Option A & B:** While both iron and copper are present, selecting only one is incomplete. Iron is found in many cytochromes, but the presence of copper is a unique defining feature of Complex IV (Cytochrome oxidase). * **Option D:** This is incorrect as the enzyme cannot function without these metallic cofactors. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitors:** Cyanide, Carbon Monoxide (CO), and Azide inhibit Complex IV by binding to the iron in heme $a_3$, halting cellular respiration. * **Copper Deficiency:** Can lead to decreased activity of cytochrome oxidase, contributing to the neurological symptoms seen in Menkes disease. * **Final Electron Acceptor:** Oxygen is the final electron acceptor in the ETC, and it is specifically at the $a_3$–$Cu_B$ site where $O_2$ is reduced to $H_2O$.

Question 358: Which enzyme contains copper?

A. Dopamine hydroxylase (Correct Answer)
B. Dopamine decarboxylase
C. Dopamine carboxylase
D. Tyrosine hydroxylase

Explanation: **Explanation:** **1. Why Dopamine Hydroxylase is correct:** Dopamine $\beta$-hydroxylase (DBH) is the enzyme responsible for converting dopamine into norepinephrine within the catecholamine synthesis pathway. It is a **copper-containing enzyme** that requires **Vitamin C (Ascorbic acid)** as a co-factor to maintain the copper in its reduced state ($Cu^+$). This is a high-yield biochemical fact often tested in the context of trace elements and vitamin functions. **2. Why the other options are incorrect:** * **Dopamine decarboxylase:** This enzyme converts DOPA to dopamine. It requires **Pyridoxal Phosphate (Vitamin B6)** as a co-factor, not copper. * **Dopamine carboxylase:** This is not a standard functional enzyme in the catecholamine biosynthetic pathway. * **Tyrosine hydroxylase:** This is the rate-limiting enzyme of catecholamine synthesis (converting Tyrosine to DOPA). It requires **Tetrahydrobiopterin ($BH_4$)** and **Iron ($Fe^{2+}$)** as co-factors. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Other Copper-containing enzymes:** Cytochrome c oxidase (Complex IV), Superoxide dismutase (cytosolic), Tyrosinase (deficiency leads to Albinism), Lysyl oxidase (collagen cross-linking), and Ceruloplasmin (Ferroxidase). * **Menkes Disease:** A defect in copper absorption (ATP7A gene) leading to "kinky hair" and connective tissue defects due to the failure of copper-dependent enzymes like Lysyl oxidase. * **Wilson Disease:** A defect in copper excretion (ATP7B gene) leading to toxic accumulation in the liver and basal ganglia. * **Vitamin C connection:** Scurvy presents with poor wound healing partly because Vitamin C is a co-factor for prolyl hydroxylase (iron-dependent) and dopamine hydroxylase (copper-dependent).

Question 359: Which of the following is a selenium-dependent enzyme?

A. Glutathione peroxidase (Correct Answer)
B. Xanthine oxidase
C. Cytochrome oxidase
D. Carbonic anhydrase

Explanation: **Explanation:** **Glutathione Peroxidase (GPx)** is the correct answer because it contains **Selenocysteine** at its active site. This enzyme plays a critical role in the cellular antioxidant system by reducing hydrogen peroxide ($H_2O_2$) and lipid hydroperoxides to water and alcohols, respectively, using reduced glutathione (GSH) as a donor. This prevents oxidative damage to cell membranes. **Analysis of Incorrect Options:** * **Xanthine Oxidase:** This enzyme, involved in purine catabolism (converting hypoxanthine to xanthine and xanthine to uric acid), requires **Molybdenum (Mo)**, Iron, and FAD as cofactors. * **Cytochrome Oxidase:** A key component of the Electron Transport Chain (Complex IV), it requires **Copper (Cu)** and **Iron (Fe)** for its function. * **Carbonic Anhydrase:** Found in RBCs and renal tubules for $CO_2$ transport and acid-base balance, it is a classic example of a **Zinc (Zn)** dependent metalloenzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Selenocysteine** is often referred to as the **21st amino acid**, encoded by the stop codon **UGA** when a specific insertion sequence (SECIS) is present. * Other Selenium-dependent enzymes include **Thioredoxin reductase** and **Deiodinase** (which converts $T_4$ to $T_3$). * **Keshan Disease:** A cardiomyopathy resulting from Selenium deficiency. * **Glutathione Reductase**, often confused with GPx, requires **Riboflavin (Vitamin $B_2$)** as a cofactor (FAD), not Selenium.

Question 360: Zymogen activation by partial proteolysis is an example of?

A. Allosteric modification
B. Enzyme induction
C. Enzyme repression
D. Covalent modification (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Covalent Modification** **Mechanism:** Zymogens (proenzymes) are inactive precursors of enzymes. Activation occurs through **partial proteolysis**, where specific peptide bonds are hydrolyzed to remove an inhibitory peptide fragment. This cleavage causes a conformational change that exposes the active site. Because this process involves the breaking and forming of chemical bonds (covalent bonds), it is classified as an **irreversible covalent modification**. **Why other options are incorrect:** * **A. Allosteric modification:** This involves the non-covalent, reversible binding of an effector molecule at a site other than the active site (e.g., ATP inhibiting PFK-1). It does not involve peptide bond cleavage. * **B & C. Enzyme Induction and Repression:** These refer to the regulation of enzyme **synthesis** at the genetic level (transcription/translation). Induction increases the quantity of enzyme (e.g., Phenobarbital inducing CYP450), while repression decreases it. Zymogen activation regulates the **activity** of pre-existing protein molecules. **High-Yield Clinical Pearls for NEET-PG:** * **Examples of Zymogens:** Digestive enzymes (Pepsinogen → Pepsin; Trypsinogen → Trypsin) and Blood Clotting Factors (Prothrombin → Thrombin). * **Clinical Correlation:** Premature activation of pancreatic zymogens (like trypsinogen) within the pancreas leads to **Acute Pancreatitis**. * **Key Distinction:** While phosphorylation/dephosphorylation is the most common *reversible* covalent modification, zymogen activation is the classic example of *irreversible* covalent modification. * **Master Regulator:** Trypsin acts as a common activator for other pancreatic zymogens (Chymotrypsinogen, Proelastase, Procarboxypeptidase).

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Enzyme Classification and Nomenclature

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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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Enzymes — MCQs

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