Enzymes Practice Questions

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

Colipase. **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).

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

Glucose-6-phosphate dehydrogenase. **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.

Q: NAD acts as a cofactor for all except?

Succinyl thiokinase. **Explanation:** The correct answer is **Succinyl thiokinase** (also known as Succinyl-CoA synthetase). This enzyme catalyzes the conversion of Succinyl-CoA to Succinate in the TCA cycle. Unlike the other enzymes listed, it does not involve a redox reaction requiring NAD+; instead, it is responsible for **substrate-level phosphorylation**, where the energy from the thioester bond is used to synthesize GTP (or ATP). **Analysis of Options:** * **Isocitrate dehydrogenase:** This is the rate-limiting enzyme of the TCA cycle. It catalyzes the oxidative decarboxylation of isocitrate to $\alpha$-ketoglutarate, reducing **NAD+ to NADH**. * **Alpha-ketoglutarate dehydrogenase:** This multi-enzyme complex requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, and **NAD+**). It reduces NAD+ to NADH during the conversion of $\alpha$-ketoglutarate to Succinyl-CoA. * **Malate dehydrogenase:** This enzyme catalyzes the final step of the TCA cycle, converting malate to oxaloacetate. This is a dehydrogenation reaction that requires **NAD+** as an electron acceptor. **High-Yield Clinical Pearls for NEET-PG:** 1. **Substrate-Level Phosphorylation:** In the TCA cycle, this occurs only at the **Succinyl thiokinase** step. 2. **NADH Production:** Three steps in the TCA cycle produce NADH: Isocitrate DH, $\alpha$-ketoglutarate DH, and Malate DH. 3. **FADH2 Production:** Occurs only at the **Succinate dehydrogenase** step (which is also Complex II of the Electron Transport Chain). 4. **Mnemonic for TCA Cofactors:** $\alpha$-ketoglutarate DH and Pyruvate DH both require "**T**ender **L**oving **C**are **F**or **N**ancy" (Thiamine, Lipoate, CoA, FAD, NAD).

Q: Which of the following statements regarding enzyme inhibition is true?

Fluoroacetate competitively inhibits aconitase.. ### Explanation **Correct Option: D. Fluoroacetate competitively inhibits aconitase.** Fluoroacetate is a classic example of **"Lethal Synthesis."** It is not toxic itself but is converted in the body to **fluorocitrate**. Fluorocitrate then acts as a competitive inhibitor of the enzyme **aconitase** in the Krebs cycle. This blockade halts cellular respiration, leading to toxicity. **Analysis of Incorrect Options:** * **A. Sulfonamides:** These are structural analogs of PABA and **competitively** (not irreversibly) inhibit **Dihydropteroate synthase**. It is *Trimethoprim* and *Methotrexate* that inhibit Dihydrofolate reductase. * **B. Ethanol in Methanol Poisoning:** Ethanol acts as a **competitive inhibitor** of Alcohol Dehydrogenase (ADH). By having a higher affinity for ADH, ethanol prevents the conversion of methanol into its toxic metabolites (formaldehyde and formic acid). * **C. Malathion:** This is an organophosphate that inhibits Acetylcholinesterase **irreversibly** (covalent bonding). Unlike competitive inhibition, irreversible inhibition **cannot** be reversed by simply increasing the substrate (Acetylcholine) concentration. **NEET-PG High-Yield Pearls:** 1. **Competitive Inhibition:** $V_{max}$ remains unchanged, $K_m$ increases. It can be overcome by increasing substrate concentration (e.g., Neostigmine for Curare poisoning). 2. **Non-competitive Inhibition:** $V_{max}$ decreases, $K_m$ remains unchanged (e.g., Acetazolamide, Digoxin). 3. **Suicide Inhibition:** A form of irreversible inhibition where the enzyme converts an inactive substrate into a reactive inhibitor that binds permanently (e.g., Allopurinol to Alloxanthine, Aspirin, 5-Fluorouracil).

Q: All of the following are trypsin inhibitors, except:

Enterokinase. **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**.

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

Asparaginase. **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.

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

Both Iron and Copper. ### 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 $H_2O$.

Q: Which enzyme is typically elevated in myopathies?

Creatine phosphokinase (CPK). **Explanation:** **Creatine Phosphokinase (CPK)**, also known as Creatine Kinase (CK), is the most sensitive and specific enzyme marker for muscle damage. It catalyzes the reversible conversion of creatine and ATP to phosphocreatine and ADP. In myopathies (diseases of the muscle tissue), the integrity of the muscle cell membrane (sarcolemma) is compromised, leading to the leakage of intracellular enzymes into the bloodstream. Specifically, the **CK-MM isoenzyme** is found predominantly in skeletal muscle, making total CPK levels significantly elevated in conditions like Duchenne Muscular Dystrophy, polymyositis, and rhabdomyolysis. **Analysis of Incorrect Options:** * **SGOT (Serum Glutamic-Oxaloacetic Transaminase) / AST (Aspartate Aminotransferase):** These are the same enzyme (AST is the modern name). While AST is found in muscle and can be elevated in myopathies, it is much less specific than CPK as it is also highly concentrated in the liver and heart. * **SGPT (Serum Glutamic-Pyruvic Transaminase) / ALT:** This enzyme is primarily a marker for **hepatocellular injury** (liver damage). While trace amounts exist in muscle, it is not a diagnostic marker for myopathies. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** CPK is the first enzyme to rise in muscle injury. * **Isoenzymes:** Remember the "Rule of Three": **CK-MM** (Skeletal muscle), **CK-MB** (Cardiac muscle), and **CK-BB** (Brain/Smooth muscle). * **Aldolase:** Another enzyme elevated in myopathies, often used if CPK results are inconclusive. * **DMD:** In Duchenne Muscular Dystrophy, CPK levels can be 50–100 times the upper limit of normal even before clinical symptoms appear.

Question 1

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

Accepted Answer

Colipase

Answer

Vitamin B12

Answer

Pyridoxine

Answer

Tocopherol

Question 2

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

Accepted Answer

Glucose-6-phosphate dehydrogenase

Answer

Glyceraldehyde-3-phosphate dehydrogenase

Answer

Lactate dehydrogenase

Question 3

NAD acts as a cofactor for all except?

Accepted Answer

Succinyl thiokinase

Answer

Isocitrate dehydrogenase

Answer

Alpha ketoglutarate dehydrogenase

Answer

Malate dehydrogenase

Question 4

Which of the following statements regarding enzyme inhibition is true?

Accepted Answer

Fluoroacetate competitively inhibits aconitase.

Answer

Sulfonamides inhibit dihydrofolate reductase irreversibly.

Answer

Ethanol inhibits alcohol dehydrogenase when used in methanol poisoning.

Answer

Acetylcholinesterase inhibition by malathion can be reversed by increasing the levels of acetylcholine.

Question 5

Glucokinase is inhibited by which of the following?

Accepted Answer

Glucose 6 phosphate

Answer

ATP

Answer

ADP

Answer

All of the above

Question 6

Glyceraldehyde-3-phosphate dehydrogenase is inhibited by iodoacetate. This is an example of which type of enzyme inhibition?

Accepted Answer

Noncompetitive

Answer

Uncompetitive

Answer

Competitive

Answer

Allosteric

Question 7

All of the following are trypsin inhibitors, except:

Accepted Answer

Enterokinase

Answer

Alpha-1 antitrypsin

Answer

Alpha-1 antiproteinase

Answer

Egg white

Question 8

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

Accepted Answer

Asparaginase

Answer

Alpha-1-antitrypsin

Answer

Streptokinase

Answer

Papain

Question 9

Which of the following elements is contained in cytochrome oxidase?

Accepted Answer

Both Iron and Copper

Answer

Iron (Fe)

Answer

Copper (Cu)

Answer

Neither Iron nor Copper

Question 10

Which enzyme is typically elevated in myopathies?

Accepted Answer

Creatine phosphokinase (CPK)

Answer

Serum glutamic-oxaloacetic transaminase (SGOT)

Answer

Serum glutamic-pyruvic transaminase (SGPT)

Answer

Aspartate aminotransferase (AST)

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?