Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 251: Glutamine synthetase is a type of enzyme that:

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

Explanation: **Explanation:** **1. Why Ligase is Correct:** Glutamine synthetase catalyzes the synthesis of **Glutamine** from **Glutamate** and **Ammonia**. This reaction involves the joining of two molecules coupled with the hydrolysis of a high-energy phosphate bond (ATP → ADP + Pi). By definition, **Ligases** (Class 6 enzymes) are enzymes that catalyze the synthetic joining of two molecules using energy derived from ATP or similar nucleoside triphosphates. The suffix "-synthetase" is a classic nomenclature hallmark for enzymes belonging to the Ligase class. **2. Why other options are incorrect:** * **Isomerase (Class 5):** These enzymes catalyze structural or geometric changes within a single molecule (e.g., Phosphohexose isomerase). They do not join two molecules together. * **Lyase (Class 4):** These enzymes catalyze the cleavage of C-C, C-O, or C-N bonds by means other than hydrolysis or oxidation, often forming double bonds. They do not require ATP for synthesis. * **Transferase (Class 2):** These enzymes transfer a functional group (e.g., methyl or phosphate) from one substrate to another (e.g., Hexokinase). While they move groups, they are not primarily involved in joining two large molecules using ATP hydrolysis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Ammonia Detoxification:** Glutamine synthetase is the primary mechanism for ammonia detoxification in the **Brain**. It converts toxic ammonia into non-toxic glutamine, which can then be transported to the liver. * **Synthetase vs. Synthase:** This is a frequent "trap" in exams. **Synthetases** (Ligases) require ATP; **Synthases** (Lyases/Transferases) do not require ATP directly for the reaction. * **Regulation:** Glutamine synthetase is a key regulatory enzyme in nitrogen metabolism and is subject to cumulative feedback inhibition.

Question 252: Which enzyme is used in the treatment of acute myocardial infarction?

A. Urokinase (Correct Answer)
B. Papain
C. Asparginase
D. Serratiopeptidase

Explanation: **Explanation:** **Urokinase** is the correct answer because it belongs to a class of drugs known as **thrombolytics** or "clot busters." In the context of an acute myocardial infarction (AMI), the primary pathology is the occlusion of a coronary artery by a thrombus. Urokinase acts as a serine protease that directly converts **plasminogen to plasmin**. Plasmin then degrades the fibrin meshwork of the thrombus, restoring blood flow to the ischemic myocardium (reperfusion). **Analysis of Incorrect Options:** * **Papain:** Derived from papaya, this proteolytic enzyme is primarily used for wound debridement (removing dead tissue) and as a digestive aid, not for systemic thrombolysis. * **Asparaginase:** This enzyme is used as a chemotherapeutic agent, specifically in **Acute Lymphoblastic Leukemia (ALL)**. It breaks down asparagine; since leukemic cells cannot synthesize asparagine, they undergo apoptosis. * **Serratiopeptidase:** A proteolytic enzyme used to reduce inflammation and edema in conditions like sinusitis or post-traumatic swelling. It has no role in treating acute coronary syndromes. **Clinical Pearls for NEET-PG:** * **Thrombolytic Generations:** * *1st Gen:* Streptokinase (non-specific, antigenic), Urokinase. * *2nd/3rd Gen:* Alteplase (tPA), Reteplase, Tenecteplase (fibrin-specific, preferred in modern practice). * **Mechanism:** All thrombolytics ultimately increase **plasmin** levels. * **Contraindications:** Always check for history of hemorrhagic stroke, active internal bleeding, or recent major surgery before administration.

Question 253: Which of the following is NOT an endopeptidase?

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

Explanation: **Explanation:** Proteolytic enzymes (proteases) are classified into two main categories based on their site of action on the polypeptide chain: **Endopeptidases** and **Exopeptidases**. 1. **Endopeptidases:** These enzymes hydrolyze internal peptide bonds within the protein molecule, breaking it into smaller peptides. 2. **Exopeptidases:** These enzymes act on the terminal ends of the polypeptide chain. They are further divided into **Aminopeptidases** (acting at the N-terminus) and **Carboxypeptidases** (acting at the C-terminus). **Why Aminopeptidase is the correct answer:** Aminopeptidase is an **exopeptidase** secreted by the intestinal mucosa (succus entericus). It cleaves the peptide bond nearest to the free amino-terminal (N-terminal) end of the protein, releasing a single amino acid. Therefore, it is NOT an endopeptidase. **Analysis of incorrect options:** * **Trypsin:** A pancreatic endopeptidase that specifically cleaves peptide bonds where the carboxyl group is contributed by basic amino acids (Lysine and Arginine). * **Pepsin:** A gastric endopeptidase that functions in an acidic pH, primarily cleaving bonds involving aromatic amino acids (Phenylalanine, Tyrosine, Tryptophan). * **Chymotrypsin:** A pancreatic endopeptidase that targets peptide bonds involving the carboxyl group of aromatic amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogens:** Most proteases are secreted as inactive proenzymes (e.g., Trypsinogen) to prevent autolysis of the secreting organ. * **Activation:** Enteropeptidase (Enterokinase) is the "master switch" that activates Trypsinogen to Trypsin, which then activates all other pancreatic proteases. * **Carboxypeptidase:** Note that Carboxypeptidase (A and B) is also an **exopeptidase**, but it is secreted by the pancreas, unlike Aminopeptidase which is intestinal.

Question 254: Which of the following does not cause an increase in serum amylase?

A. Pancreatitis
B. Renal failure
C. Cardiac failure (Correct Answer)
D. Carcinoma lung

Explanation: **Explanation:** Serum amylase is a key biochemical marker primarily used to diagnose pancreatic disorders, but its elevation can occur due to non-pancreatic causes. **Why Cardiac Failure is the Correct Answer:** Cardiac failure does not typically cause an elevation in serum amylase. While severe congestive heart failure can lead to "shock liver" (elevated transaminases), it does not involve the salivary glands or the pancreas, nor does it impair the renal clearance of amylase significantly enough to cause hyperamylasemia. **Analysis of Incorrect Options:** * **Pancreatitis:** This is the most common cause. Inflammation leads to the leakage of amylase from pancreatic acinar cells into the systemic circulation (usually >3x the upper limit of normal). * **Renal Failure:** Amylase is a small molecule filtered by the kidneys. In renal insufficiency, the decreased glomerular filtration rate (GFR) leads to reduced clearance, causing a persistent, moderate rise in serum amylase levels. * **Carcinoma Lung:** Certain tumors, particularly small cell lung cancer and non-small cell lung cancer, can produce amylase ectopically (ectopic hyperamylasemia). **NEET-PG High-Yield Pearls:** 1. **Macroamylasemia:** A condition where amylase binds to Immunoglobulins (IgA/IgG), forming a complex too large to be filtered by the kidney. Result: **High serum amylase but low urinary amylase.** 2. **Lipase vs. Amylase:** Lipase is more specific for acute pancreatitis and remains elevated longer (7–14 days) than amylase (2–5 days). 3. **Other causes of high amylase:** Mumps (parotitis), ectopic pregnancy, and perforated peptic ulcer.

Question 255: Which enzyme catalyzes the addition of water to a C-C bond?

A. Hydroxylase
B. Dehydrogenase
C. Hydrolase (Correct Answer)
D. Hydratase

Explanation: ### Explanation The correct answer is **C. Hydrolase**. **1. Why Hydrolase is Correct:** Hydrolases are a major class of enzymes (EC 3) that catalyze the cleavage of various chemical bonds (such as C-O, C-N, or C-C) by the **addition of water**. This process is known as hydrolysis. In the context of a C-C bond, a hydrolase breaks the bond by incorporating a water molecule, typically resulting in two smaller molecules. Common examples include digestive enzymes like peptidases and lipases. **2. Why the Other Options are Incorrect:** * **A. Hydroxylase:** These belong to the Oxidoreductase class. They catalyze the addition of a hydroxyl group (-OH) to a substrate, usually requiring molecular oxygen ($O_2$) rather than water ($H_2O$). * **B. Dehydrogenase:** These are Oxidoreductases that catalyze the removal of hydrogen atoms from a substrate, transferring them to electron carriers like $NAD^+$ or $FAD$. They are involved in redox reactions, not bond cleavage via water. * **D. Hydratase:** While the name sounds similar, hydratases belong to the **Lyase** class. They add water to a double bond (e.g., Fumarase adding water to Fumarate to form Malate) **without** breaking the bond to split the molecule. Hydrolases split the molecule; Hydratases do not. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **EC Classification:** Remember the mnemonic **OTH LIL** (Oxidoreductase, Transferase, Hydrolase, Lyase, Isomerase, Ligase). * **Hydrolase vs. Lyase:** This is a common trap. **Hydrolases** use water to *break* a bond (Hydrolysis). **Lyases** (like Hydratases) add water to a *double bond* or remove groups to form double bonds without hydrolysis. * **Key Example:** Acetylcholinesterase is a clinically significant hydrolase; its inhibition is the basis of Organophosphate poisoning.

Question 256: Which of the following enzymes is NOT typically elevated in liver disorders?

A. Lipase (Correct Answer)
B. Alkaline phosphatase (ALP)
C. Aspartate aminotransferase (AST)
D. Alanine aminotransferase (ALT)

Explanation: **Explanation:** The correct answer is **Lipase**. **Why Lipase is the correct answer:** Lipase is a digestive enzyme primarily synthesized and secreted by the **pancreas** into the duodenum to facilitate the hydrolysis of dietary fats. While trace amounts are found in other tissues, its clinical utility is almost exclusively as a highly specific marker for **acute pancreatitis**. It is not produced by hepatocytes or biliary epithelial cells; therefore, its levels remain normal in primary liver disorders like hepatitis or cirrhosis. **Analysis of Incorrect Options:** * **ALT (Alanine Aminotransferase):** This is the most specific marker for hepatocellular injury. It is found primarily in the cytosol of hepatocytes. * **AST (Aspartate Aminotransferase):** While also found in cardiac and skeletal muscle, AST is significantly elevated in liver cell necrosis. A characteristic high-yield fact is the **AST:ALT ratio > 2:1**, which is highly suggestive of alcoholic liver disease. * **ALP (Alkaline Phosphatase):** This enzyme is located on the canalicular membranes of hepatocytes. It is the hallmark marker for **cholestasis** (biliary obstruction) and infiltrative liver diseases. **NEET-PG High-Yield Pearls:** 1. **Specificity:** ALT is more specific for the liver than AST. Lipase is more specific for the pancreas than Amylase. 2. **De Ritis Ratio:** An AST/ALT ratio < 1 is typical for viral hepatitis, while > 2 indicates alcohol-induced damage. 3. **GGT (Gamma-Glutamyl Transferase):** Often tested alongside ALP; if both are elevated, the source of ALP is confirmed to be hepatic rather than bone. 4. **Half-life:** ALT has a longer half-life (approx. 47 hours) compared to AST (approx. 17 hours).

Question 257: Competitive inhibition is characterized by all of the following, except?

A. Inhibitor is a structural analogue of the substrate
B. Inhibitor binds to the active site
C. Increases Km value
D. Irreversible (Correct Answer)

Explanation: ### Explanation **Competitive inhibition** is a reversible form of enzyme inhibition where the inhibitor competes directly with the substrate for the same binding site. **1. Why "Irreversible" is the Correct Answer (The Exception):** Competitive inhibition is inherently **reversible**. Because the inhibitor and substrate compete for the active site, the inhibition can be completely overcome by increasing the substrate concentration ($[S]$). In contrast, irreversible inhibition involves covalent bonding or permanent denaturation of the enzyme, which cannot be reversed by adding more substrate. **2. Analysis of Incorrect Options:** * **Option A (Structural Analogue):** Competitive inhibitors typically mimic the chemical structure of the substrate (e.g., Malonate mimics Succinate), allowing them to "fit" into the active site. * **Option B (Binds to Active Site):** This is the hallmark of competitive inhibition. The inhibitor occupies the active site, physically blocking the substrate from binding. * **Option C (Increases $K_m$):** Since the inhibitor interferes with substrate binding, the apparent affinity of the enzyme for the substrate decreases. Therefore, a higher concentration of substrate is required to reach half-maximal velocity ($1/2 V_{max}$), leading to an **increased $K_m$**. Note that $V_{max}$ remains unchanged. --- ### High-Yield Clinical Pearls for NEET-PG * **Lineweaver-Burk Plot:** In competitive inhibition, the lines intersect on the **Y-axis** (same $V_{max}$), but the X-intercept ($-1/K_m$) moves closer to zero. * **Classic Example:** **Statins** (e.g., Atorvastatin) are competitive inhibitors of **HMG-CoA reductase**. * **Methanol Poisoning:** Ethanol is used as a competitive inhibitor of **Alcohol Dehydrogenase** to prevent the formation of toxic formaldehyde. * **Succinate Dehydrogenase:** Inhibited competitively by **Malonate, Oxaloacetate, and Glutarate**. * **Mnemonic:** **C**ompetitive = **C**ommon $V_{max}$ (stays the same), but $K_m$ **C**limbs (increases).

Question 258: What is the major monooxygenase in the endoplasmic reticulum?

A. Cytochrome P450 (Correct Answer)
B. Cytochromes
C. Epoxide reductase
D. Glutathione reductase

Explanation: **Explanation:** **1. Why Cytochrome P450 is correct:** Cytochrome P450 (CYP450) enzymes are the primary **monooxygenases** located in the smooth endoplasmic reticulum (microsomes) of hepatocytes and other tissues. They are also known as **mixed-function oxidases**. Their primary role is to catalyze the hydroxylation of a wide variety of hydrophobic substrates, including endogenous compounds (steroids, bile acids) and exogenous substances (drugs, toxins). The reaction typically involves the incorporation of one atom of molecular oxygen into the substrate (forming a hydroxyl group) and the reduction of the other oxygen atom to water, utilizing **NADPH** as a reducing equivalent. **2. Why the other options are incorrect:** * **B. Cytochromes:** This is a broad category of heme-proteins. While CYP450 is a cytochrome, "Cytochromes" as a general term includes Cytochrome c and Cytochrome a/a3, which are part of the mitochondrial Electron Transport Chain (ETC) involved in ATP production, not monooxygenase activity. * **C. Epoxide reductase:** Specifically, Vitamin K epoxide reductase (VKORC1) is involved in the recycling of Vitamin K. It is the target of Warfarin but does not function as a major monooxygenase. * **D. Glutathione reductase:** This enzyme reduces oxidized glutathione (GSSG) back to reduced glutathione (GSH) using NADPH. It is crucial for protecting cells against oxidative stress but is not a monooxygenase. **Clinical Pearls for NEET-PG:** * **Inducers vs. Inhibitors:** Drugs like Phenobarbital and Rifampicin **induce** CYP450 (increasing metabolism), while Ketoconazole and Grapefruit juice **inhibit** it (increasing drug toxicity). * **Most Common Isoform:** **CYP3A4** is the most abundant isoform in the liver and is responsible for metabolizing nearly 50% of all clinical drugs. * **Requirement:** CYP450 requires **NADPH-Cytochrome P450 reductase** and molecular oxygen to function.

Question 259: Coenzyme A requires which of the following?

A. Pantothenic acid (Correct Answer)
B. Biotin
C. Folic acid
D. Cobalamin

Explanation: **Explanation:** **Coenzyme A (CoA-SH)** is a vital cofactor involved in the metabolism of fatty acids, carbohydrates, and amino acids. It is derived from **Pantothenic acid (Vitamin B5)**. 1. **Why Pantothenic Acid is Correct:** The structure of Coenzyme A consists of three main components: **Adenosine 3', 5'-bisphosphate**, **Pantothenic acid**, and **β-mercaptoethylamine**. The functional part of the molecule is the terminal thiol group (-SH), which forms high-energy thioester bonds with acyl groups (e.g., Acetyl-CoA). Pantothenic acid is an essential precursor that must be obtained from the diet to synthesize CoA. 2. **Why Other Options are Incorrect:** * **Biotin (Vitamin B7):** Acts as a coenzyme for **carboxylation reactions** (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It carries activated CO₂. * **Folic Acid (Vitamin B9):** Functions as Tetrahydrofolate (THF), which is involved in **one-carbon metabolism** (transfer of methyl, formyl, or methylene groups), crucial for DNA synthesis. * **Cobalamin (Vitamin B12):** Acts as a coenzyme for only two human enzymes: **Methionine synthase** and **Methylmalonyl-CoA mutase**. **High-Yield Clinical Pearls for NEET-PG:** * **Acyl Carrier Protein (ACP):** Pantothenic acid is also a component of ACP, which is part of the Fatty Acid Synthase complex. * **Key Reactions:** CoA is essential for the **PDH complex** (Pyruvate to Acetyl-CoA), the **α-ketoglutarate dehydrogenase complex**, and **Fatty acid β-oxidation**. * **Mnemonic:** "Panto" means "everywhere" in Greek; Vitamin B5 is found in almost all foods, making deficiency extremely rare (presents as "Burning Feet Syndrome").

Question 260: Aldolase is classified as which type of enzyme?

A. Transferase
B. Isomerase
C. Lyase (Correct Answer)
D. Reductase

Explanation: **Explanation:** **1. Why Lyase is the Correct Answer:** Enzymes are classified into six major classes by the IUBMB (International Union of Biochemistry and Molecular Biology). **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. **Aldolase** (specifically Aldolase A in glycolysis) catalyzes the reversible cleavage of **Fructose 1,6-bisphosphate** (a 6-carbon sugar) into two 3-carbon molecules: **Dihydroxyacetone phosphate (DHAP)** and **Glyceraldehyde 3-phosphate (GAP)**. Since this involves breaking a C-C bond without the use of water (hydrolysis) or redox changes, it is a classic example of a Lyase. **2. Why Other Options are Incorrect:** * **Transferases (Class 2):** These transfer functional groups (e.g., methyl, phosphate) from one substrate to another. Example: Hexokinase. * **Isomerases (Class 5):** These catalyze structural rearrangements within a single molecule. Example: Phosphofructoisomerase. * **Reductases (Class 1 - Oxidoreductases):** These catalyze oxidation-reduction reactions involving the transfer of electrons/hydrogen. Example: Lactate dehydrogenase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isoenzymes:** Aldolase A (Muscle/RBCs), Aldolase B (Liver/Kidney), Aldolase C (Brain). * **Clinical Correlation:** **Aldolase B deficiency** leads to **Hereditary Fructose Intolerance (HFI)**. In HFI, Fructose-1-phosphate accumulates, depleting intracellular phosphate and causing severe hypoglycemia and liver damage. * **Diagnostic Marker:** Serum Aldolase A levels are elevated in skeletal muscle disorders like Duchenne Muscular Dystrophy (DMD) and inflammatory myopathies.

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Enzymes — MCQs

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