Enzymes Practice Questions

Q: An increase in alkaline phosphatase may be seen in all of the following conditions except one. Which one is the exception?

Osteoporosis.. Alkaline Phosphatase (ALP) is a marker of **osteoblastic activity**. Its serum levels rise whenever there is active bone formation or high bone turnover. **Explanation of the Correct Answer:** **B. Osteoporosis:** In osteoporosis, there is a decrease in total bone mass, but the bone that remains is chemically normal. Crucially, the rate of bone resorption exceeds bone formation, but there is **no significant increase in osteoblastic activity**. Therefore, serum calcium, phosphate, and **ALP levels remain characteristically normal** in patients with primary osteoporosis. This is a classic "trap" in NEET-PG questions. **Explanation of Incorrect Options:** * **A. Hyperparathyroidism:** High levels of Parathyroid Hormone (PTH) stimulate osteoclastic bone resorption. This is followed by a compensatory increase in osteoblastic activity to repair the bone, leading to elevated ALP. * **C. Osteitis deformans (Paget’s Disease):** This condition is characterized by disordered, excessive bone remodeling. It features the **highest levels of serum ALP** seen in any bone disease due to massive osteoblastic compensation. * **D. Adenocarcinoma of the prostate:** Prostate cancer frequently metastasizes to the bone, typically causing **osteoblastic (sclerotic) lesions**. These lesions trigger intense osteoblast activity, resulting in significantly raised ALP. **High-Yield Clinical Pearls for NEET-PG:** 1. **ALP vs. ACP:** While ALP is a marker for osteoblastic activity (and prostate mets), **Acid Phosphatase (ACP)** and PSA are specific markers for the prostate gland itself. 2. **Heat Stability:** To differentiate the source of ALP, remember: **"Regan is Heat Stable"** (Placental isoenzyme) and **"Bone is Bone-y"** (Bone isoenzyme is heat-labile). 3. **Normal Labs in Osteoporosis:** Always remember: Calcium, Phosphorus, and ALP are **Normal** in Osteoporosis, but ALP is **High** in Osteomalacia and Paget’s.

Q: Which of the following inhibitors in the TCA cycle acts by blocking citrate?

Fluoroacetate. **Explanation:** **1. Why Fluoroacetate is Correct:** Fluoroacetate is a potent inhibitor of the TCA cycle, often referred to as a "suicide inhibitor." It does not inhibit the cycle directly in its original form. Instead, it is converted into **Fluorocitrate** by the enzyme Citrate Synthase. Fluorocitrate then competitively inhibits the enzyme **Aconitase**. This blockage prevents the conversion of Citrate to Isocitrate, leading to a toxic accumulation of **Citrate** in the mitochondria and a complete halt of the cycle. **2. Why Other Options are Incorrect:** * **Arsenite (B):** This inhibits the **Pyruvate Dehydrogenase (PDH) complex** and the **$\alpha$-Ketoglutarate Dehydrogenase** complex. It acts by binding to the -SH groups of Lipoic acid (a required co-factor), not by blocking citrate. * **Malonate (C):** This is a classic example of a competitive inhibitor that acts on **Succinate Dehydrogenase**. It competes with the substrate Succinate due to structural similarity. **3. NEET-PG High-Yield Pearls:** * **Suicide Inhibition:** Fluoroacetate is the most common example of suicide inhibition (where the enzyme converts a non-toxic substrate into a toxic inhibitor) asked in exams. * **Aconitase:** This enzyme contains an **Iron-Sulfur (Fe-S) cluster**, making it sensitive to oxidative stress. * **Arsenic Poisoning:** Clinically presents with "garlic breath" and rice-water stools; it inhibits enzymes requiring Lipoic acid. * **Inhibitor Summary:** * Fluoroacetate $\rightarrow$ Aconitase * Arsenite $\rightarrow$ $\alpha$-Ketoglutarate Dehydrogenase * Malonate $\rightarrow$ Succinate Dehydrogenase

Q: "Brain Natriuretic Peptide" is degraded by:

Neutral endopeptidase. **Explanation:** **1. Why Neutral Endopeptidase (NEP) is correct:** Brain Natriuretic Peptide (BNP), along with Atrial Natriuretic Peptide (ANP), is a hormone involved in cardiovascular homeostasis by promoting natriuresis and vasodilation. These peptides are primarily degraded by **Neutral Endopeptidase (NEP)**, also known as **Neprilysin**. NEP is a zinc-dependent metalloendopeptidase found in various tissues, particularly the kidneys. By cleaving the peptide bonds of natriuretic peptides, NEP terminates their biological action. **2. Why the other options are incorrect:** * **Elastase:** This is a protease that breaks down elastin in connective tissue. While clinically relevant in conditions like emphysema (Alpha-1 antitrypsin deficiency), it does not play a role in BNP metabolism. * **Collagenase:** This enzyme breaks the peptide bonds in collagen. It is involved in wound healing and tissue remodeling, not hormone degradation. * **Omeprazole:** This is a pharmacological agent (Proton Pump Inhibitor) used to inhibit gastric acid secretion. It is not an enzyme and has no degradative effect on peptides. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sacubitril:** This is a potent **Neprilysin inhibitor**. When combined with Valsartan (an ARB), it forms the drug class **ARNI** (Angiotensin Receptor-Neprilysin Inhibitor), which is now a cornerstone in the management of Heart Failure with reduced Ejection Fraction (HFrEF). * **Diagnostic Marker:** While BNP is degraded by NEP, **NT-proBNP** (the N-terminal fragment) is not a substrate for Neprilysin. Therefore, in patients taking Sacubitril, BNP levels will be falsely elevated, making **NT-proBNP** the preferred marker for monitoring heart failure. * **Second Pathway:** Apart from NEP, natriuretic peptides are also cleared via **Natriuretic Peptide Receptor-C (NPR-C)** through receptor-mediated endocytosis.

Q: Which of the following is responsible for respiratory burst and production of superoxide ions?

NADPH oxidase. Explanation: 1. Why NADPH Oxidase is Correct: Respiratory burst (or oxidative burst) is a critical process in phagocytes (neutrophils and macrophages) used to kill ingested pathogens. The enzyme NADPH oxidase, located in the phagosomal membrane, catalyzes the transfer of an electron from NADPH to molecular oxygen ($O_2$). This reaction produces the superoxide anion ($O_2^•-$), which is the "starting gun" for the production of other reactive oxygen species (ROS) like hydrogen peroxide and hypochlorite [1]. 2. Why the Other Options are Incorrect: * Hydrolases (A): These are enzymes that catalyze the cleavage of bonds (like peptide or glycosidic bonds) by adding water. They are involved in digestion and lysosomal degradation, not the production of free radicals. * Catalase (B): This is an antioxidant enzyme that breaks down hydrogen peroxide ($H_2O_2$) into water and oxygen [1]. It protects cells from oxidative damage rather than initiating the respiratory burst [2]. * Peroxidase (C): While Myeloperoxidase (MPO) is involved in the respiratory burst pathway, its role is to convert $H_2O_2$ into hypochlorous acid (HOCl/bleach) [1]. It does not produce the initial superoxide ion. 3. Clinical Pearls & High-Yield Facts: * Chronic Granulomatous Disease (CGD): A high-yield deficiency of NADPH oxidase. Patients suffer from recurrent infections with catalase-positive organisms (e.g., *S. aureus, Aspergillus*) because they cannot produce their own ROS. * Nitroblue Tetrazolium (NBT) Test: Used to diagnose CGD. Normal cells turn blue (positive), while CGD cells remain colorless (negative). * Reaction Formula: $NADPH + 2O_2 \xrightarrow{\text{NADPH Oxidase}} NADP^+ + 2O_2^•- + H^+$

Q: Creatine kinase is elevated in Myocardial Infarction after:

2 - 4 hours. **Explanation:** In the context of Myocardial Infarction (MI), **Creatine Kinase (specifically the CK-MB isoenzyme)** is one of the earliest cardiac biomarkers to rise in the bloodstream following myocardial necrosis. 1. **Why A is correct:** After an acute MI, cell membrane integrity is lost, causing intracellular enzymes to leak into the circulation. CK-MB levels typically begin to rise within **2 to 4 hours** of the onset of chest pain. It reaches its peak concentration at 12–24 hours and usually returns to baseline within 48–72 hours. This rapid rise makes it a sensitive early marker. 2. **Why the other options are incorrect:** * **B (4 - 8 hours):** While CK-MB is certainly elevated by this time, the *initial* rise is detectable as early as 2 hours. 4–8 hours is more characteristic of the rise in **Aspartate Aminotransferase (AST)**. * **C (12 - 24 hours):** This is the timeframe for the **peak concentration** of CK-MB and Troponins, not the initial elevation. * **D (> 24 hours):** By this time, CK-MB levels are often starting to decline. **Lactate Dehydrogenase (LDH)** typically begins its rise after 24 hours. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** Cardiac **Troponins (I and T)** are the most sensitive and specific markers for MI (rising within 3–4 hours and staying elevated for 7–14 days). * **Re-infarction:** CK-MB is the **marker of choice for diagnosing re-infarction** because it returns to baseline quickly (within 3 days), whereas Troponins remain elevated for up to two weeks. * **Earliest Marker:** **Myoglobin** is the earliest marker to rise (1–2 hours) but lacks cardiac specificity.

Q: Peroxisomes contain what class of enzymes?

Oxidative. **Explanation:** Peroxisomes (also known as microbodies) are membrane-bound organelles found in almost all eukaryotic cells. They are primarily characterized by the presence of **oxidative enzymes**, such as **catalase, urate oxidase, and D-amino acid oxidase**. The correct answer is **A (Oxidative)** because these enzymes function by removing hydrogen atoms from specific organic substrates in an oxidative reaction that produces hydrogen peroxide ($H_2O_2$). Catalase, the most abundant peroxisomal enzyme, then utilizes this $H_2O_2$ to oxidize other substrates (like phenols and alcohols) or decomposes excess $H_2O_2$ into water and oxygen, protecting the cell from oxidative damage. **Analysis of Incorrect Options:** * **B, C, and D:** These are all examples of **Reactive Oxygen Species (ROS)** or free radicals. While peroxisomes are involved in the metabolism of these substances (specifically neutralizing them), they are the *products* or *byproducts* of metabolic pathways, not the class of enzymes contained within the organelle. Hypochlorous acid (D) is specifically associated with the "respiratory burst" in neutrophils via the enzyme myeloperoxidase. **High-Yield Facts for NEET-PG:** * **Major Functions:** $\beta$-oxidation of Very Long Chain Fatty Acids (VLCFA), bile acid synthesis, and plasmalogen synthesis (essential for myelin). * **Zellweger Syndrome:** A high-yield clinical correlation where a genetic defect in protein import into peroxisomes leads to "empty" peroxisomes. It presents with neurological deficits, hepatomegaly, and early death. * **Adrenoleukodystrophy (X-linked):** Caused by a defect in transporting VLCFAs into peroxisomes, leading to their accumulation and damage to the adrenal glands and white matter of the brain.

Q: Aspartate aminotransferase is released:

after cell necrosis. ### 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.

Q: Which of the following is NOT a coenzyme required for the pyruvate dehydrogenase complex?

Folic acid. **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme cluster that catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. This complex requires **five specific coenzymes** to function, often remembered by the mnemonic **"Tender Loving Care For Nancy."** 1. **Thiamine pyrophosphate (TPP):** Derived from Vitamin B1. 2. **Lipoate (Lipoic acid):** Acts as an acyl carrier. 3. **Coenzyme A (CoA):** Derived from Vitamin B5 (Pantothenic acid). 4. **FAD:** Derived from Vitamin B2 (Riboflavin). 5. **NAD+:** Derived from Vitamin B3 (Niacin). **Why Folic Acid is the correct answer:** **Folic acid (Vitamin B9)** is primarily involved in **one-carbon metabolism** (e.g., DNA synthesis and amino acid metabolism). It plays no role in the PDH complex. **Analysis of Incorrect Options:** * **FAD (Option A):** Serves as the prosthetic group for E3 (Dihydrolipoyl dehydrogenase), accepting electrons from reduced lipoamide. * **NAD (Option B):** Acts as the final electron acceptor in the complex, forming NADH. * **Lipoate (Option D):** Attached to E2 (Dihydrolipoyl transacetylase), it undergoes oxidation/reduction to swing the hydroxyethyl group from TPP to CoA. **High-Yield Clinical Pearls for NEET-PG:** * **Arsenic Poisoning:** Arsenic inhibits the PDH complex by binding to the **SH (sulfhydryl) groups of Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Leads to Beriberi and Wernicke-Korsakoff syndrome because PDH and Alpha-ketoglutarate dehydrogenase cannot function without TPP. * The same five cofactors are also required by the **Alpha-ketoglutarate dehydrogenase** and **Branched-chain alpha-keto acid dehydrogenase** complexes.

Q: Alkaline phosphatase is elevated in all the following conditions except?

Hypoparathyroidism. Serum **Alkaline Phosphatase (ALP)** is a marker of osteoblastic activity. Its levels rise whenever there is increased bone turnover or remodeling [3]. **Why Hypoparathyroidism is the correct answer:** In **Hypoparathyroidism**, there is a deficiency of Parathyroid Hormone (PTH). PTH is the primary stimulator of bone remodeling; without it, bone turnover becomes abnormally low (adynamic bone). Consequently, osteoblastic activity is minimal, leading to **low or normal ALP levels**. This distinguishes it from most other metabolic bone diseases where ALP is typically elevated [1]. **Analysis of Incorrect Options:** * **Rickets & Osteomalacia:** These conditions involve defective mineralization of the osteoid [4]. To compensate for the weak bone matrix, osteoblasts increase their activity, leading to a significant **elevation in ALP** [2]. This is a hallmark diagnostic feature. * **Hypophosphatemia:** Low serum phosphate (as seen in Vitamin D resistant rickets or Fanconi syndrome) impairs mineralization, similarly triggering a compensatory increase in osteoblastic activity and **elevated ALP**. **NEET-PG High-Yield Pearls:** 1. **ALP is NOT elevated in:** Multiple Myeloma (punched-out lesions have no osteoblastic reaction), Scurvy, and Hypoparathyroidism. 2. **Highest ALP levels:** Seen in **Paget’s Disease of bone** and Hyperparathyroidism (Osteitis fibrosa cystica). 3. **Hypophosphatasia:** A rare genetic disorder where ALP is pathologically **low**, leading to rickets-like symptoms. 4. **Isoenzymes:** Remember that ALP also originates from the liver (biliary tract), placenta (Regan isoenzyme), and intestine. To confirm bone origin, check Heat stability (Bone ALP is heat-labile) or GGT levels.

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

Hydrolases. **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 1

An increase in alkaline phosphatase may be seen in all of the following conditions except one. Which one is the exception?

Accepted Answer

Osteoporosis.

Answer

Hyperparathyroidism.

Answer

Osteitis deformans.

Answer

Adenocarcinoma of the prostate.

Question 2

Which of the following inhibitors in the TCA cycle acts by blocking citrate?

Accepted Answer

Fluoroacetate

Answer

Arsenite

Answer

Malonate

Answer

None of the above

Question 3

"Brain Natriuretic Peptide" is degraded by:

Accepted Answer

Neutral endopeptidase

Answer

Elastase

Answer

Collagenase

Answer

Omeprazole

Question 4

Which of the following is responsible for respiratory burst and production of superoxide ions?

Accepted Answer

NADPH oxidase

Answer

Hydrolase

Answer

Catalase

Answer

Peroxidase

Question 5

Creatine kinase is elevated in Myocardial Infarction after:

Accepted Answer

2 - 4 hours

Answer

4 - 8 hours

Answer

12 - 24 hours

Answer

> 24 hours

Question 6

Peroxisomes contain what class of enzymes?

Accepted Answer

Oxidative

Answer

Superoxide anion

Answer

Hydroxyl radical

Answer

Hypochlorous acid

Question 7

Aspartate aminotransferase is released:

Accepted Answer

after cell necrosis

Answer

by viable cells

Answer

either of the two

Answer

none of the above

Question 8

Which of the following is NOT a coenzyme required for the pyruvate dehydrogenase complex?

Accepted Answer

Folic acid

Answer

FAD

Answer

NAD

Answer

Lipoate

Question 9

Alkaline phosphatase is elevated in all the following conditions except?

Accepted Answer

Hypoparathyroidism

Answer

Rickets

Answer

Hypophosphatemia

Answer

Osteomalacia

Question 10

Digestive enzymes are primarily classified as which type of enzyme?

Accepted Answer

Hydrolases

Answer

Oxidoreductases

Answer

Dehydrogenases

Answer

Ligases

Enzymes — MCQs

Enzymes — MCQs

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