Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 31: Which of the following methods most accurately estimates blood creatinine level?

A. Jaffe method
B. Kinetic Jaffe method
C. Technicon method
D. Enzyme assay (Correct Answer)

Explanation: **Explanation:** The **Enzymatic Assay** (using Creatininase and Creatinase) is considered the most accurate method for estimating blood creatinine because of its high **specificity**. 1. **Why Enzyme Assay is Correct:** Unlike chemical methods, enzymes are highly specific to their substrate. The enzymatic method (often involving a peroxidase-coupled reaction) eliminates interference from "non-creatinine chromogens." It is currently the method of choice in clinical laboratories seeking to minimize errors in Calculated Glomerular Filtration Rate (eGFR). 2. **Analysis of Incorrect Options:** * **Jaffe Method (Option A):** This is the traditional method based on the reaction of creatinine with alkaline picrate to form a red-orange complex. It is notorious for **positive interference** from substances like glucose, ketones, protein, and cephalosporins, leading to overestimation. * **Kinetic Jaffe Method (Option B):** An improvement over the manual Jaffe method, it measures the rate of color formation to reduce interference from slow-reacting chromogens (like glucose). While faster and more common, it still lacks the absolute specificity of enzymatic assays. * **Technicon Method (Option C):** This refers to older automated analyzers (like the AutoAnalyzer) which typically utilized a modified Jaffe reaction. It is not a distinct biochemical principle for accuracy. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** The analytical "Gold Standard" for creatinine measurement is **IDMS** (Isotope Dilution Mass Spectrometry), but among routine laboratory methods, **Enzymatic Assay** is the most accurate. * **Interference:** Bilirubin causes **negative interference** in the Jaffe reaction (falsely low results), while ketones and cephalosporins cause **positive interference**. * **Creatinine Source:** It is an anhydride of creatine, produced at a constant rate proportional to **muscle mass**.

Question 32: Which of the following is NOT a serine protease?

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

Explanation: **Explanation:** The classification of proteases is based on the specific amino acid residue at their active site that facilitates the catalytic mechanism. **Why Pepsin is the correct answer:** Pepsin is an **Aspartic protease** (Acid protease). It utilizes two highly conserved aspartic acid residues in its active site to activate a water molecule, which then attacks the peptide bond of the substrate. Pepsin functions optimally in the highly acidic environment of the stomach (pH 1.5–2.5). **Why the other options are incorrect:** * **Trypsin, Chymotrypsin, and Elastase** are all classic examples of **Serine proteases**. * They share a common catalytic mechanism involving a **"Catalytic Triad"** consisting of **Serine, Histidine, and Aspartate**. * In these enzymes, the Serine residue acts as a nucleophile to attack the carbonyl carbon of the substrate's peptide bond. These enzymes are primarily secreted by the pancreas as zymogens and function in the neutral-to-alkaline pH of the small intestine. **High-Yield NEET-PG Pearls:** 1. **Catalytic Triad:** Remember the mnemonic **"SHA"** (Serine, Histidine, Aspartate) for serine proteases. 2. **Blood Clotting Factors:** Most coagulation factors (II, VII, IX, X, XI, XII) and Thrombin are also Serine proteases. 3. **Substrate Specificity:** * **Trypsin:** Cleaves at Basic AAs (Arg, Lys). * **Chymotrypsin:** Cleaves at Aromatic AAs (Phe, Tyr, Trp). * **Elastase:** Cleaves at Small Neutral AAs (Gly, Ala, Ser). 4. **Other Protease Classes:** * **Cysteine Proteases:** Caspases (apoptosis), Cathepsins. * **Metalloproteases:** Carboxypeptidases (require Zinc), Matrix Metalloproteinases (MMPs).

Question 33: Papain is an enzyme used for what purpose?

A. To decrease intestinal gas produced during the digestive process (Correct Answer)
B. As an antihelminthic
C. To treat herpes zoster
D. To treat infected wounds

Explanation: **Explanation:** **Papain** is a proteolytic enzyme (cysteine protease) derived from the latex of the raw fruit of the papaya plant (*Carica papaya*). 1. **Why Option A is Correct:** Papain functions similarly to human pepsin. It aids in the breakdown of complex proteins into smaller peptides and amino acids. In clinical practice, it is used as a **digestive aid** to supplement pancreatic enzymes. By improving protein digestion, it prevents the accumulation of undigested matter in the gut, thereby **decreasing intestinal gas (flatulence)** and bloating associated with dyspepsia or pancreatic insufficiency. 2. **Analysis of Incorrect Options:** * **Option B (Antihelminthic):** While some traditional medicines use papaya seeds for parasites, papain itself is not a standard clinical antihelminthic. * **Option C (Herpes Zoster):** There is no established clinical role for papain in treating viral infections like shingles. * **Option D (Infected Wounds):** While papain was historically used in enzymatic debridement (to remove dead tissue), it is not used to treat the *infection* itself. Furthermore, the FDA has restricted several topical papain products due to hypersensitivity risks. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** It is a **Cysteine Protease** (contains a thiol group at the active site). * **Industrial Use:** It is widely used as a **meat tenderizer** because it breaks down tough muscle fibers (collagen). * **Biochemical Application:** In immunology, papain is used to cleave **Immunoglobulin G (IgG)** into three fragments: **two Fab fragments** (antigen-binding) and **one Fc fragment** (crystallizable). This is a frequent high-yield question in both Biochemistry and Microbiology.

Question 34: Which enzyme is not released as a proenzyme?

A. Elastase
B. Trypsin
C. Amylase (Correct Answer)
D. Chymotrypsin

Explanation: **Explanation:** The core concept tested here is the distinction between **proteolytic enzymes** and **non-proteolytic enzymes**. **Why Amylase is the Correct Answer:** Amylase is a carbohydrate-digesting enzyme (hydrolase) that breaks down starch into maltose. Unlike proteases, amylase does not pose a threat to the structural integrity of the pancreatic cells or ducts because it does not digest cellular proteins. Therefore, it is synthesized and secreted in its **active form**. It only requires calcium ions ($Ca^{2+}$) and chloride ions ($Cl^-$) for optimal activity, rather than proteolytic cleavage for activation. **Why the Other Options are Incorrect:** * **Trypsin, Chymotrypsin, and Elastase** are all **proteases** (enzymes that digest proteins). * If these were secreted in their active forms, they would cause **autodigestion** of the pancreas, leading to acute pancreatitis. * To prevent this, they are secreted as inactive **proenzymes (zymogens)**: Trypsinogen, Chymotrypsinogen, and Proelastase. * **Trypsinogen** is activated to **Trypsin** by the enzyme **Enteropeptidase** (Enterokinase) in the duodenum. Once formed, Trypsin acts as a common activator for Chymotrypsinogen and Proelastase. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogens:** Inactive precursors of enzymes. Most gastrointestinal proteases (including Pepsinogen in the stomach) are zymogens. * **Pancreatitis:** In acute pancreatitis, premature activation of trypsinogen within the pancreas leads to a cascade of zymogen activation and hemorrhagic necrosis. * **Diagnostic Marker:** Serum amylase and lipase are elevated in acute pancreatitis, but **Lipase** is considered more specific. * **Inhibitor:** Pancreatic Secretory Trypsin Inhibitor (PSTI/SPINK1) is a protective protein that inhibits any small amounts of trypsin prematurely formed within the pancreas.

Question 35: Cytochrome oxidase can be poisoned by all of the following, EXCEPT:

A. Cyanide
B. Hydrogen sulphide
C. Carbon monoxide
D. Carbon dioxide (Correct Answer)

Explanation: **Explanation:** Cytochrome oxidase (also known as **Complex IV** of the Electron Transport Chain) is the terminal enzyme that transfers electrons to oxygen. It contains iron (heme) and copper centers. The correct answer is **Carbon dioxide**, as it does not bind to or inhibit this enzyme; instead, it is a byproduct of the TCA cycle and is primarily transported in the blood as bicarbonate. **Why the other options are incorrect (Inhibitors of Complex IV):** * **Cyanide (CN⁻):** Binds to the ferric iron ($Fe^{3+}$) in the heme $a_3$ component of Cytochrome oxidase, halting the ETC and causing rapid cellular hypoxia. * **Hydrogen Sulphide ($H_2S$):** Acts similarly to cyanide by binding to the heme iron in Complex IV. It is a potent occupational hazard (e.g., in sewage workers). * **Carbon Monoxide (CO):** Binds to the ferrous iron ($Fe^{2+}$) in Cytochrome oxidase. While its primary toxicity is due to binding hemoglobin (forming carboxyhemoglobin), its inhibition of the mitochondrial ETC contributes to cellular toxicity. * **Azide ($N_3^-$):** Another classic inhibitor of Complex IV (often tested alongside these options). **High-Yield Clinical Pearls for NEET-PG:** 1. **Antidote for Cyanide:** Amyl nitrite/Sodium nitrite (induces methemoglobinemia to sequester cyanide) and **Hydroxocobalamin** (forms cyanocobalamin). 2. **Complex IV** is unique because it is the only complex that reduces $O_2$ to $H_2O$. 3. **Inhibitors vs. Uncouplers:** Remember that inhibitors (like Cyanide) stop both the ETC and ATP synthesis, whereas uncouplers (like 2,4-DNP) stop ATP synthesis but actually *increase* the rate of the ETC and heat production.

Question 36: Regan isoenzyme is increased in which of the following conditions?

A. Seminoma (Correct Answer)
B. Paget's disease
C. Osteoporosis
D. Cholestasis

Explanation: **Explanation:** **Regan isoenzyme** is a heat-stable alkaline phosphatase (ALP) isoenzyme that is biochemically identical to the placental ALP (PALP) but is produced ectopically by certain tumors. 1. **Why Seminoma is correct:** Regan isoenzyme is a classic **oncofetal protein**. It is most characteristically associated with **Seminoma** (germ cell tumor of the testis) and dysgerminoma of the ovary. It serves as a useful tumor marker for monitoring treatment response and recurrence in these patients. 2. **Why the other options are incorrect:** * **Paget’s disease & Osteoporosis:** These conditions involve increased bone turnover. The ALP elevated here is the **Bone isoenzyme** (heat-labile), not the placental-like Regan isoenzyme. * **Cholestasis:** This leads to an elevation of the **Liver isoenzyme** of ALP due to increased synthesis and "leaking" from the canalicular membrane into the blood. **High-Yield Clinical Pearls for NEET-PG:** * **Heat Stability Rule:** "Regan is Resistant." Unlike the bone isoenzyme (which is heat-labile), the Regan isoenzyme is highly heat-stable (resists denaturation at 65°C). * **Nagao Isoenzyme:** A variant of the Regan isoenzyme, also found in germ cell tumors and metastatic carcinomas, but inhibited by L-leucine. * **ALP Isoenzyme Sources (Mnemonic: BLP-R):** * **B**one (Heat-labile) * **L**iver (Most common) * **P**lacental (Normal in 3rd trimester) * **R**egan (Pathological/Ectopic)

Question 37: Which element is found in the active site of glutathione peroxidase for its function?

A. Chromium
B. Manganese
C. Zinc
D. Selenium (Correct Answer)

Explanation: **Explanation:** **Glutathione Peroxidase (GPx)** is a critical antioxidant enzyme that protects cells from oxidative damage by reducing lipid hydroperoxides and free hydrogen peroxide ($H_2O_2$) into water. The correct answer is **Selenium** because this enzyme contains the unique amino acid **Selenocysteine** at its active site. Selenocysteine is often referred to as the "21st amino acid" and is essential for the enzyme's catalytic activity; without Selenium, the enzyme cannot neutralize reactive oxygen species (ROS). **Analysis of Incorrect Options:** * **Chromium (A):** Primarily functions as a component of the "Glucose Tolerance Factor," enhancing the action of insulin. It is not involved in the glutathione system. * **Manganese (B):** Acts as a cofactor for mitochondrial Superoxide Dismutase (Mn-SOD), Arginase, and Pyruvate Carboxylase. * **Zinc (C):** A structural or catalytic component for over 300 enzymes, including Carbonic Anhydrase, Alcohol Dehydrogenase, and cytoplasmic SOD (Cu-Zn SOD), but not GPx. **Clinical Pearls for NEET-PG:** * **Keshan Disease:** A cardiomyopathy resulting from Selenium deficiency, leading to decreased GPx activity. * **Antioxidant Synergy:** GPx works in tandem with **Vitamin E**; while Vitamin E prevents lipid peroxidation in membranes, GPx removes peroxides from the cytosol. * **The Reaction:** $2GSH + H_2O_2 \xrightarrow{GPx} GSSG + 2H_2O$. (Note: Glutathione Reductase then regenerates GSH using NADPH). * **Codon:** Selenocysteine is encoded by the **UGA** codon (normally a stop codon) through a specialized recoding mechanism involving the SECIS element.

Question 38: Which enzyme deficiency causes hemolytic anemia?

A. Glucose-6-phosphate dehydrogenase (G6PD) (Correct Answer)
B. Aldolase
C. Isomerase
D. Enolase

Explanation: **Explanation:** **1. Why G6PD is the Correct Answer:** Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary role in red blood cells (RBCs) is to produce **NADPH**. This NADPH is essential for maintaining a pool of **reduced glutathione**, which acts as a major antioxidant. Since RBCs lack mitochondria, the HMP shunt is their only source of NADPH. In G6PD deficiency, the cell cannot neutralize reactive oxygen species (like $H_2O_2$), leading to oxidative damage to hemoglobin. This results in the formation of **Heinz bodies**, which are removed by splenic macrophages (forming **Bite cells**), ultimately causing episodic **hemolytic anemia**. **2. Why the Other Options are Incorrect:** * **B, C, and D (Aldolase, Isomerase, Enolase):** These are all enzymes involved in **Glycolysis**. While a deficiency in Pyruvate Kinase (another glycolytic enzyme) can cause hemolytic anemia, deficiencies in Aldolase, Isomerase, or Enolase are extremely rare and are not the classic, high-yield causes of hemolysis tested in medical exams. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** G6PD deficiency is an **X-linked recessive** disorder (more common in males). * **Triggers:** Hemolysis is typically triggered by oxidative stress: **Fava beans**, Infections, or Drugs (e.g., **Primaquine**, Sulfa drugs, Nitrofurantoin). * **Morphology:** Look for **Heinz bodies** (denatured hemoglobin) on Supravital stain and **Bite cells** on peripheral smear. * **Protective Effect:** G6PD deficiency provides a survival advantage against *Plasmodium falciparum* malaria.

Question 39: Enzymes that catalyze synthetic reactions where two molecules are joined together and ATP is used belong to which class?

A. Hydrolase
B. Lyase
C. Ligase (Correct Answer)
D. Transferase

Explanation: **Explanation:** The International Union of Biochemistry (IUB) classifies enzymes into seven major classes based on the type of reaction they catalyze. **1. Why Ligase is Correct:** **Ligases (Class 6)** are enzymes that catalyze the joining (ligation) of two large molecules. This process involves the formation of new chemical bonds (C-O, C-S, C-N, or C-C). Crucially, these reactions are endergonic and require energy, which is provided by the **hydrolysis of ATP** or other high-energy nucleoside triphosphates. A classic example is *DNA Ligase*, which joins DNA strands, or *Pyruvate Carboxylase*. **2. Why Other Options are Incorrect:** * **Hydrolases (Class 3):** These catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. They do not join molecules using ATP. * **Lyases (Class 4):** These catalyze the cleavage of 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. They do not require ATP for synthetic joining. * **Transferases (Class 2):** These catalyze the transfer of a specific functional group (e.g., methyl, phosphate, or amino groups) from one substrate to another. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Classes:** **O T H L I L H** (Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases, and the newly added Translocases). * **Synthetase vs. Synthase:** In medical biochemistry, the term **Synthetase** is synonymous with **Ligase** (requires ATP), whereas **Synthase** belongs to other classes (like Lyases) and does **not** require ATP directly. * **Key Example:** *Pyruvate Carboxylase* (a ligase) is the rate-limiting enzyme for gluconeogenesis, requiring Biotin and ATP.

Question 40: Which of the following is NOT a rate-limiting enzyme?

A. Phosphofructokinase (PFK)
B. HMG CoA reductase
C. HMG CoA synthase
D. Aldolase (Correct Answer)

Explanation: **Explanation:** In metabolic pathways, **rate-limiting enzymes** catalyze the slowest, usually irreversible step that determines the overall flux of the pathway. These enzymes are typically under tight allosteric or hormonal regulation. **Why Aldolase is the Correct Answer:** **Aldolase** (specifically Aldolase A in glycolysis and Aldolase B in gluconeogenesis/fructose metabolism) catalyzes a **reversible, equilibrium reaction**. In glycolysis, it cleaves Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate. Because it operates near equilibrium and its activity is governed by substrate availability rather than complex regulatory signals, it is not a rate-limiting step. **Analysis of Incorrect Options:** * **Phosphofructokinase-1 (PFK-1):** This is the **key rate-limiting and committed step of Glycolysis**. It is allosterically inhibited by ATP and citrate, and activated by AMP and Fructose-2,6-bisphosphate. * **HMG CoA Reductase:** This is the **rate-limiting enzyme for Cholesterol synthesis**. It is the target of **Statins** and is regulated by phosphorylation and feedback inhibition by cholesterol. * **HMG CoA Synthase:** The mitochondrial isoform of this enzyme is the **rate-limiting step for Ketogenesis** (synthesis of ketone bodies). **NEET-PG High-Yield Pearls:** 1. **Aldolase B Deficiency:** Causes **Hereditary Fructose Intolerance**, leading to severe hypoglycemia and liver damage upon fructose ingestion. 2. **Rate-Limiting Enzymes to Remember:** * **Gluconeogenesis:** Pyruvate carboxylase / Fructose-1,6-bisphosphatase. * **Glycogenolysis:** Glycogen phosphorylase. * **Glycogenesis:** Glycogen synthase. * **PPP Pathway:** Glucose-6-phosphate dehydrogenase (G6PD). * **Urea Cycle:** Carbamoyl phosphate synthetase I (CPS-I).

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Enzymes — MCQs

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