Enzymes Practice Questions

Q: What is the number of isoenzymes of Lactate Dehydrogenase (LDH)?

5. **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme (composed of four subunits) that catalyzes the reversible conversion of lactate to pyruvate. The correct answer is **5** because LDH is formed by the combination of two distinct polypeptide chains: **H (Heart)** and **M (Muscle)**. Since the enzyme is a tetramer, these two subunits can combine in five different permutations, resulting in five distinct isoenzymes: * **LDH-1 (H4):** Predominantly in the heart and RBCs. * **LDH-2 (H3M1):** Predominantly in the Reticuloendothelial system. * **LDH-3 (H2M2):** Predominantly in the lungs. * **LDH-4 (H1M3):** Predominantly in the kidneys and pancreas. * **LDH-5 (M4):** Predominantly in the liver and skeletal muscle. **Analysis of Incorrect Options:** * **Option A (1):** Incorrect. Enzymes with multiple subunits and tissue-specific expressions usually exist in multiple isoforms. * **Option B (3):** Incorrect. This might be confused with Creatine Kinase (CK), which has 3 isoenzymes (CK-MM, CK-MB, CK-BB). * **Option C (4):** Incorrect. While LDH is a tetramer (4 subunits), the mathematical combinations of two different subunits (H and M) result in five possible arrangements. **High-Yield Clinical Pearls for NEET-PG:** * **Flipped Pattern:** Normally, LDH-2 > LDH-1. In **Myocardial Infarction (MI)** or hemolytic anemia, LDH-1 levels exceed LDH-2 (LDH-1/LDH-2 ratio >1), known as the "flipped pattern." * **LDH-5:** Is the most heat-labile isoenzyme and serves as a marker for liver cell damage or skeletal muscle injury. * **Total LDH:** Is a non-specific marker of cellular turnover; significantly elevated levels are seen in megaloblastic anemia and certain malignancies (e.g., Seminoma, Lymphoma).

Q: Raised serum alkaline phosphatase is seen in all of the following conditions EXCEPT:

Multiple myeloma. Serum Alkaline Phosphatase (ALP) is a marker of **osteoblastic activity** (bone formation). In conditions where bone remodeling or bone formation is increased, ALP levels rise. **Why Multiple Myeloma is the correct answer:** In Multiple Myeloma, the bone lesions are characteristically **purely lytic**. The malignant plasma cells produce factors (like DKK-1 and sclerostin) that inhibit osteoblasts while activating osteoclasts. Because there is a lack of compensatory osteoblastic activity, the **ALP remains normal** despite extensive bone destruction. This is a classic diagnostic "negative" finding used to differentiate myeloma from other bone pathologies. **Analysis of incorrect options:** * **Paget’s Disease:** Characterized by focal areas of increased and disorganized bone remodeling [1]. It features the highest levels of serum ALP seen in clinical practice due to intense osteoblastic compensation. * **Osteomalacia:** Defective mineralization leads to an accumulation of unmineralized osteoid. This triggers a compensatory increase in osteoblastic activity, resulting in elevated ALP. * **Hyperthyroidism:** Thyroid hormones have a direct stimulatory effect on bone turnover. Increased bone resorption is coupled with increased bone formation, leading to mildly to moderately raised ALP. **NEET-PG High-Yield Pearls:** * **The "Cold" Scan:** Because Bone Scans (Technetium-99m) depend on osteoblastic activity, they are often negative in Multiple Myeloma. Skeletal surveys (X-rays) or MRI are preferred. * **Isolated ALP Elevation:** If ALP is raised with normal GGT, the source is bone; if GGT is also raised, the source is hepatobiliary. * **Hypercalcemia + Normal ALP:** Always consider Multiple Myeloma as a primary differential in an elderly patient [1].

Q: All of the following are serine proteases, except?

Carboxypeptidase. **Explanation:** The classification of proteases is based on the functional group present at their active site that initiates the peptide bond cleavage. **1. Why Carboxypeptidase is the correct answer:** Carboxypeptidase is a **Zinc-containing Metalloenzyme** (specifically a metalloprotease). It requires a metal ion (Zn²⁺) to activate a water molecule, which then attacks the peptide bond. Unlike serine proteases, it does not utilize a serine residue for its catalytic mechanism. It is an exopeptidase that cleaves amino acids from the C-terminal end of proteins. **2. Why the other options are incorrect:** * **Chymotrypsin & Trypsin:** These are classic examples of pancreatic serine proteases. They utilize a "Catalytic Triad" consisting of **Serine, Histidine, and Aspartate**. The serine residue acts as a nucleophile to attack the carbonyl carbon of the substrate. * **Thrombin:** This is a critical serine protease in the coagulation cascade. It converts fibrinogen to fibrin. Like trypsin, it cleaves peptide bonds following specific basic amino acid residues. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Catalytic Triad:** Always remember the three amino acids involved in serine proteases: **Ser-His-Asp**. * **Serine Protease Inhibitors (SERPINs):** Deficiencies in these lead to clinical conditions (e.g., **α1-Antitrypsin deficiency** leading to emphysema and liver cirrhosis). * **Other Protease Classes:** * **Cysteine Proteases:** Caspases (apoptosis), Cathepsins. * **Aspartyl Proteases:** Pepsin, Renin, HIV Protease. * **Metalloproteases:** Carboxypeptidase, Matrix Metalloproteinases (MMPs), ACE. * **Zymogens:** Most serine proteases are secreted as inactive precursors (e.g., Trypsinogen) to prevent autodigestion of the pancreas.

Q: Raised serum amylase levels are used to diagnose which of the following conditions?

Acute pancreatitis. **Explanation:** **Correct Answer: D. Acute pancreatitis** **Why it is correct:** Serum amylase is a digestive enzyme primarily produced by the acinar cells of the pancreas and the salivary glands. In **acute pancreatitis**, inflammation and damage to the pancreatic acinar cells lead to the leakage of this enzyme into the interstitial space and subsequently into the bloodstream. Serum amylase levels typically rise within 2–12 hours of the onset of symptoms and remain elevated for 3–5 days. A level **three times the upper limit of normal** is highly suggestive of acute pancreatitis. **Why the other options are incorrect:** * **A & B (Autoimmune and Degenerative diseases):** These are chronic, systemic, or localized processes that do not typically cause the acute cellular rupture or ductal leakage required to significantly elevate serum amylase. * **C (Acute cholecystitis):** While biliary tract diseases can sometimes cause a mild rise in amylase (especially if a gallstone obstructs the common bile duct), it is not the primary diagnostic marker for cholecystitis. Cholecystitis is better diagnosed via ultrasound and clinical signs like Murphy’s sign. **High-Yield Clinical Pearls for NEET-PG:** * **Amylase vs. Lipase:** Serum **Lipase** is more specific and sensitive than amylase for acute pancreatitis because it stays elevated longer (7–14 days) and has fewer non-pancreatic sources. * **Urinary Amylase:** Remains elevated longer than serum amylase and can be useful in late presentations. * **Macroamylasemia:** A benign condition where amylase binds to immunoglobulins, causing high serum levels but low urinary excretion (due to the large size of the complex). * **Other causes of raised amylase:** Mumps (parotitis), ruptured ectopic pregnancy, and perforated peptic ulcer.

Q: Which amino acid is responsible for the activation of Thioredoxin reductase?

Selenocysteine. **Explanation:** **Thioredoxin reductase** is a critical flavoprotein enzyme that reduces thioredoxin, a protein essential for DNA synthesis (via ribonucleotide reductase) and the defense against oxidative stress. **Why Selenocysteine is correct:** The catalytic activity of Thioredoxin reductase is dependent on **Selenocysteine (Sec)**, often referred to as the **21st amino acid**. Selenocysteine contains a selenium atom in place of the sulfur found in cysteine. Because selenium has a lower pKa and higher nucleophilicity than sulfur, it allows the enzyme to carry out redox reactions more efficiently at physiological pH. The Selenocysteine residue is located at the C-terminal active site of the enzyme; without it, the enzyme loses its catalytic power. **Why incorrect options are wrong:** * **Cysteine:** While cysteine is structurally similar and involved in disulfide bond formation within the enzyme, it cannot replace the specific redox efficiency provided by the selenium atom in the active site. * **Serine:** Serine contains a hydroxyl group (-OH) which is not redox-active and cannot facilitate the electron transfer required by this enzyme. * **Alanine:** Alanine is a non-polar amino acid with a simple methyl side chain; it lacks the functional groups necessary for catalysis. **Clinical Pearls for NEET-PG:** * **Genetic Coding:** Selenocysteine is encoded by the **UGA stop codon**, requiring a specific mRNA structure called the **SECIS element** (Selenocysteine Insertion Sequence). * **Other Selenoenzymes:** Glutathione peroxidase, Deiodinases (converting T4 to T3), and Selenoprotein P. * **Trace Element:** Selenium deficiency can lead to **Keshan disease** (cardiomyopathy) or **Kashin-Beck disease** (osteoarthropathy).

Q: What is a marker of the lysosome?

Acid Phosphatase. **Explanation:** **Correct Answer: A. Acid Phosphatase** Lysosomes are membrane-bound organelles containing hydrolytic enzymes responsible for intracellular digestion. The hallmark of a lysosome is its acidic internal environment (pH ~4.5–5.0), maintained by a proton pump (V-type ATPase). **Acid phosphatase** is the primary marker enzyme for lysosomes because it is specifically localized within these organelles and requires an acidic pH for its catalytic activity. It is used histochemically to identify lysosomes in various tissues. **Analysis of Incorrect Options:** * **B. Lactate dehydrogenase (LDH):** This is a key enzyme in anaerobic glycolysis and is a classic marker for the **Cytosol**. Its presence in extracellular fluid often indicates cell membrane damage or necrosis. * **C. Oxidase:** Enzymes like Urate oxidase and Catalase are characteristic markers for **Peroxisomes** (microbodies), which are involved in long-chain fatty acid oxidation and hydrogen peroxide metabolism. * **D. Na-K ATPase:** This is the primary active transport pump located on the **Plasma Membrane**. It is the standard marker used to identify cell membrane fractions during cell fractionation. **High-Yield Clinical Pearls for NEET-PG:** * **Other Lysosomal Markers:** Cathepsins, Glucuronidase, and LAMP-1 (Lysosome-associated membrane protein 1). * **I-Cell Disease:** A clinical condition where lysosomal enzymes fail to be phosphorylated (Man-6-P tag) in the Golgi, leading to their secretion outside the cell rather than being transported to the lysosome. * **Mitochondrial Markers:** ATP synthase (Inner membrane) and Monoamine oxidase (Outer membrane). * **Golgi Marker:** Galactosyltransferase.

Q: Which type of enzyme is a transaminase?

Transferase. **Explanation:** **1. Why Transferase is Correct:** Transaminases (also known as aminotransferases) belong to **Class 2: Transferases**. These enzymes catalyze the transfer of a functional group from one molecule (the donor) to another (the acceptor). Specifically, transaminases transfer an **amino group (-NH₂)** from an amino acid to a keto acid (usually α-ketoglutarate), converting the keto acid into a new amino acid. This process requires **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6, as an essential coenzyme. **2. Why Other Options are Incorrect:** * **Oxidoreductases (Class 1):** Catalyze oxidation-reduction reactions (e.g., Dehydrogenases). While transamination is linked to the urea cycle, it does not involve the direct transfer of electrons or hydrogen in this specific step. * **Isomerases (Class 5):** Catalyze structural rearrangements within a single molecule (e.g., Phosphohexose isomerase). They do not transfer groups between different molecules. * **Ligases (Class 6):** Catalyze the joining of two molecules using ATP energy (e.g., DNA ligase, Pyruvate carboxylase). Transaminases do not require ATP for the transfer. **3. Clinical Pearls for NEET-PG:** * **Diagnostic Markers:** AST (Aspartate Transaminase) and ALT (Alanine Transaminase) are critical biomarkers for liver injury. ALT is more specific for liver pathology, while AST is also found in cardiac and skeletal muscle. * **Coenzyme Dependency:** A common high-yield question involves **Vitamin B6 (Pyridoxine)** deficiency, which impairs transamination, leading to neurological symptoms and sideroblastic anemia. * **Mechanism:** Transaminases utilize a "Ping-Pong" (Double Displacement) kinetic mechanism.

Q: All of the following are examples of competitive enzyme inhibition except?

Cyclooxygenase by Aspirin. **Explanation** The correct answer is **A. Cyclooxygenase by Aspirin**. In **competitive inhibition**, the inhibitor binds reversibly to the active site of the enzyme, competing with the substrate. However, **Aspirin** is a classic example of **Irreversible Inhibition**. It works by covalently acetylating a specific serine residue (Serine 529 in COX-1) at the active site of the Cyclooxygenase enzyme. This permanent modification prevents the substrate (arachidonic acid) from binding for the entire lifespan of the enzyme/cell (e.g., the 7–10 day lifespan of a platelet). **Analysis of other options:** * **B. Thymidylate synthase by 5-Fluorouracil:** This is a "Suicide Inhibition" (a specialized form of irreversible inhibition). However, in many standard textbooks and competitive exams, it is categorized under the broad umbrella of antimetabolites that compete for the active site. *Note: If both "Irreversible" and "Competitive" are options, 5-FU is suicide/irreversible.* * **C. Coagulation cascade by Dicumarol:** Dicumarol (and Warfarin) acts as a competitive inhibitor of **Vitamin K Epoxide Reductase**. It structurally resembles Vitamin K and competes for the same binding site. * **D. Dihydrofolate reductase (DHFR) by Methotrexate:** Methotrexate is a structural analog of folic acid. It binds to the active site of DHFR with a much higher affinity than the natural substrate, acting as a classic competitive inhibitor. **High-Yield Clinical Pearls for NEET-PG:** * **Competitive Inhibition:** $V_{max}$ remains unchanged; $K_m$ increases. Can be overcome by increasing substrate concentration. * **Non-competitive Inhibition:** $V_{max}$ decreases; $K_m$ remains unchanged. * **Suicide Inhibition Examples:** Allopurinol (on Xanthine Oxidase), Aspirin (on COX), Penicillin (on Transpeptidase), and 5-Fluorouracil. * **Statin drugs** (e.g., Atorvastatin) are competitive inhibitors of HMG-CoA Reductase.

Question 1

What is the number of isoenzymes of Lactate Dehydrogenase (LDH)?

Accepted Answer

5

Answer

1

Answer

3

Answer

4

Question 2

Raised serum alkaline phosphatase is seen in all of the following conditions EXCEPT:

Accepted Answer

Multiple myeloma

Answer

Paget's disease

Answer

Osteomalacia

Answer

Hyperthyroidism

Question 3

All of the following are serine proteases, except?

Accepted Answer

Carboxypeptidase

Answer

Chymotrypsin

Answer

Trypsin

Answer

Thrombin

Question 4

Raised serum amylase levels are used to diagnose which of the following conditions?

Accepted Answer

Acute pancreatitis

Answer

Autoimmune disease

Answer

Degenerative diseases

Answer

Acute cholecystitis

Question 5

Which amino acid is responsible for the activation of Thioredoxin reductase?

Accepted Answer

Selenocysteine

Answer

Serine

Answer

Cysteine

Answer

Alanine

Question 6

What is the definition of enzyme specificity?

Accepted Answer

The amount of enzyme causing the transformation of 1 micromole of substrate per minute under standard conditions.

Answer

The amount of enzyme required per second for the formation of one mole of product.

Answer

The number of binding sites on an enzyme for a substrate.

Answer

The ability of an enzyme to bind with various substrates.

Question 7

What is a marker of the lysosome?

Accepted Answer

Acid Phosphatase

Answer

Lactate dehydrogenase

Answer

Oxidase

Answer

Na K ATPase

Question 8

Which type of enzyme is a transaminase?

Accepted Answer

Transferase

Answer

Oxidoreductase

Answer

Isomerase

Answer

Ligase

Question 9

Hormone substrate concentration affects the velocity of enzymatic action. This is based upon which of the following?

Accepted Answer

Michaelis-Menten equation

Answer

Zimmermann reaction

Answer

Salkowski reaction

Answer

Liebermann-Burchard reaction

Question 10

All of the following are examples of competitive enzyme inhibition except?

Accepted Answer

Cyclooxygenase by Aspirin

Answer

Thymidylate synthase by 5-Fluorouracil

Answer

Coagulation cascade by Dicumarol

Answer

Dihydrofolate reductase by Methotrexate

Enzymes — MCQs

Enzymes — MCQs

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