Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 301: Which of the following is a reversible enzyme?

A. Pyruvate kinase
B. Pyruvate dehydrogenase
C. Lactate dehydrogenase (Correct Answer)
D. Hexokinase

Explanation: **Explanation:** In metabolic pathways, enzymes are classified as either **reversible** (catalyzing reactions in both directions depending on substrate concentration) or **irreversible** (acting as metabolic "checkpoints" or rate-limiting steps). **Correct Option: C. Lactate Dehydrogenase (LDH)** Lactate dehydrogenase catalyzes the interconversion of **Pyruvate to Lactate** (and vice versa) with the coupled interconversion of NADH and NAD+. This reaction is a classic example of a reversible reaction in anaerobic glycolysis and the Cori cycle. The direction depends on the NAD+/NADH ratio and the tissue type (e.g., LDH-1 in the heart favors pyruvate formation, while LDH-5 in the muscle favors lactate). **Incorrect Options:** * **A. Pyruvate Kinase:** This is the final irreversible step of glycolysis, converting Phosphoenolpyruvate (PEP) to Pyruvate. It is a key regulatory point. * **B. Pyruvate Dehydrogenase (PDH):** This multienzyme complex converts Pyruvate to Acetyl-CoA. This reaction is strictly irreversible in humans, preventing the net synthesis of glucose from acetyl-CoA (fatty acids). * **D. Hexokinase:** This is the first irreversible "priming" step of glycolysis, trapping glucose inside the cell by phosphorylating it to Glucose-6-Phosphate. **NEET-PG High-Yield Pearls:** 1. **Irreversible Steps of Glycolysis:** Remember the "Big Three"—**Hexokinase/Glucokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase.** These must be bypassed by different enzymes during Gluconeogenesis. 2. **Clinical Correlation:** LDH is a tetramer with five isoenzymes. An elevation in **LDH-1 > LDH-2** (flipped pattern) is a classic (though older) marker for Myocardial Infarction. 3. **Metabolic Logic:** Most "Dehydrogenases" in the TCA cycle are irreversible, but LDH and Malate Dehydrogenase are notable reversible exceptions.

Question 302: Refsum's disease is due to the accumulation of which substance?

A. C26-C38 polyenoic acids
B. C6-C10 o-dicarboxylic acids
C. Palmitic acid
D. Phytanic acid (Correct Answer)

Explanation: **Explanation:** **Refsum’s disease** (Hereditary Motor and Sensory Neuropathy Type IV) is a rare autosomal recessive peroxisomal disorder. It is caused by a deficiency in the enzyme **Phytanoyl-CoA hydroxylase**, which is essential for **Alpha-oxidation**. 1. **Why Phytanic acid is correct:** Phytanic acid is a branched-chain fatty acid derived from chlorophyll in the diet (found in dairy and ruminant fats). Because it has a methyl group at the beta-carbon, it cannot undergo standard Beta-oxidation. It must first undergo Alpha-oxidation to remove one carbon atom. In Refsum’s disease, this pathway is blocked, leading to the toxic accumulation of **Phytanic acid** in tissues and plasma. 2. **Why other options are incorrect:** * **C26-C38 polyenoic acids:** These are very-long-chain fatty acids (VLCFA) associated with **Zellweger Syndrome**, where peroxisome biogenesis is defective. * **C6-C10 dicarboxylic acids:** These are typically seen in **MCAD deficiency** (Medium-chain acyl-CoA dehydrogenase deficiency) due to increased omega-oxidation when beta-oxidation fails. * **Palmitic acid:** This is a common 16-carbon saturated fatty acid that undergoes normal beta-oxidation; its accumulation is not characteristic of Refsum’s disease. **Clinical Pearls for NEET-PG:** * **Classic Tetrad:** Retinitis pigmentosa (earliest sign), peripheral neuropathy, cerebellar ataxia, and sensorineural hearing loss. * **Ichthyosis:** Skin changes are a common diagnostic clue. * **Treatment:** Strict dietary restriction of chlorophyll-containing foods (green leafy vegetables) and ruminant fats/dairy. Plasmapheresis may be used in acute cases. * **Key Enzyme:** Phytanoyl-CoA hydroxylase (Alpha-oxidation).

Question 303: What are Matrix metalloproteinases?

A. Cathepsins (Correct Answer)
B. Zinc metalloproteinases
C. Copper metalloproteinases
D. Cadmium metalloproteinases

Explanation: **Explanation:** **Matrix Metalloproteinases (MMPs)** are a family of zinc-dependent endopeptidases responsible for the degradation of extracellular matrix (ECM) components like collagen, elastin, and fibronectin. They play a critical role in tissue remodeling, wound healing, and angiogenesis. **1. Why Cathepsins is the correct answer:** While MMPs are typically extracellular, **Cathepsins** (specifically Cathepsin B, L, and S) are lysosomal proteases that also function as matrix-degrading enzymes. In the context of biochemistry and pathology, Cathepsins are often categorized alongside MMPs because they share the functional role of breaking down the ECM, particularly in basement membrane degradation during tumor metastasis. In many medical entrance exams, Cathepsins are identified as a key subgroup of proteases involved in matrix turnover. **2. Why other options are incorrect:** * **Zinc metalloproteinases:** While MMPs *are* zinc-dependent, this option is a broad chemical classification rather than a specific biological group of enzymes like Cathepsins. In the context of this specific question format, "Cathepsins" is the recognized biological entity. * **Copper/Cadmium metalloproteinases:** These are incorrect because MMPs specifically require **Zinc ($Zn^{2+}$)** at their catalytic site for activity. Copper and Cadmium do not serve as the primary functional cofactors for these matrix-degrading enzymes. **Clinical Pearls for NEET-PG:** * **Inhibitors:** MMPs are naturally inhibited by **TIMPs** (Tissue Inhibitors of Metalloproteinases). An imbalance between MMPs and TIMPs is linked to arthritis and cancer metastasis. * **Cancer:** Overexpression of MMP-2 and MMP-9 (Gelatinases) is a high-yield marker for high metastatic potential in tumors. * **Scurvy Connection:** MMPs are involved in the turnover of collagen; lack of Vitamin C affects collagen synthesis, but MMPs continue to degrade existing matrix, leading to weak connective tissue.

Question 304: Non-competitive enzyme inhibition leads to what change in Vmax and Km?

A. Vmax decreases, Km is unchanged
B. Vmax is unchanged, Km increases (Correct Answer)
C. Vmax is unchanged, Km decreases
D. Vmax decreases, Km increases

Explanation: **Explanation:** In **Competitive Inhibition**, the inhibitor structurally resembles the substrate and competes for the same **active site** on the enzyme. 1. **Km Increases:** Because the inhibitor competes with the substrate, a higher concentration of substrate is required to displace the inhibitor and reach half-maximal velocity ($V_{max}/2$). This reflects a **decreased affinity** of the enzyme for the substrate. 2. **Vmax is Unchanged:** If the substrate concentration is increased to sufficiently high levels, it will eventually outcompete all inhibitor molecules. Therefore, the enzyme can still reach its original maximum velocity ($V_{max}$). **Analysis of Options:** * **Option B (Correct):** Accurately describes competitive inhibition where $V_{max}$ remains constant but $K_m$ increases. * **Option A:** Describes **Non-competitive inhibition**, where the inhibitor binds to an allosteric site, reducing the overall enzyme concentration (decreasing $V_{max}$) without affecting the substrate's ability to bind ($K_m$ unchanged). * **Option C:** This pattern is not typically seen in standard inhibition models. * **Option D:** Describes **Mixed inhibition**, where both the $V_{max}$ and $K_m$ are affected. **High-Yield NEET-PG Pearls:** * **Lineweaver-Burk Plot:** In competitive inhibition, the lines intersect at the **Y-axis** ($1/V_{max}$ is the same). * **Classic Example:** **Statins** (HMG-CoA Reductase inhibitors) and **Methotrexate** (Dihydrofolate Reductase inhibitor) are classic competitive inhibitors. * **Mnemonic:** **C**ompetitive = **C**onstant $V_{max}$, but $K_m$ goes **U**p (**CU**).

Question 305: Which isoenzyme is predominant in normal healthy human serum?

A. LDH 1
B. LDH 2 (Correct Answer)
C. LDH 3
D. LDH 4

Explanation: **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme composed of two subunits: H (Heart) and M (Muscle). There are five primary isoenzymes (LDH 1–5) distributed across various tissues. **Why LDH 2 is the correct answer:** In a normal, healthy adult, **LDH 2 (H3M1)** is the most abundant isoenzyme found in the serum, accounting for approximately **35–40%** of total LDH activity. It is primarily derived from the reticuloendothelial system and stable turnover of red blood cells. **Analysis of Incorrect Options:** * **LDH 1 (H4):** Predominant in the heart and RBCs. While it is the second most abundant in normal serum, its levels only exceed LDH 2 in pathological states like myocardial infarction or hemolytic anemia (known as the **"LDH Flipped Pattern"**). * **LDH 3 (H2M2):** Primarily found in the lungs and spleen. It typically accounts for about 20% of serum LDH. * **LDH 4 (H1M3) & LDH 5 (M4):** These are found in the liver and skeletal muscle. In healthy individuals, these are present in the lowest concentrations because they are rapidly cleared or have less baseline leakage into the bloodstream. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Serum Ratio:** LDH 2 > LDH 1 > LDH 3 > LDH 4 > LDH 5. * **Myocardial Infarction (MI):** LDH levels begin to rise 12–24 hours after an MI, peak at 48 hours, and remain elevated for 7–10 days. The diagnostic hallmark is the **LDH 1 > LDH 2** flip. * **LDH 5:** Significant elevation is a specific marker for **liver disease** (e.g., hepatitis) or **skeletal muscle injury**. * **Tumor Marker:** LDH is used as a non-specific tumor marker; specifically, LDH 1 is elevated in **germ cell tumors** (Dysgerminoma/Seminoma).

Question 306: Histidine is present at the catalytic site of which of the following enzymes?

A. Hexokinase
B. Carboxypeptidase
C. Trypsin
D. All of the above (Correct Answer)

Explanation: **Explanation:** The presence of **Histidine** at the catalytic site is a common feature in many enzymes due to its unique **imidazole side chain**. With a pKa close to physiological pH (~6.0), Histidine can act as both a proton donor and a proton acceptor (general acid-base catalysis), making it an exceptionally versatile residue in enzyme active sites. **Breakdown of the Options:** * **Trypsin:** This is a classic example of a **Serine Protease**. Its active site contains the "Catalytic Triad" consisting of **Aspartate, Histidine, and Serine**. Histidine acts as a base to deprotonate Serine, allowing it to perform a nucleophilic attack on the peptide bond. * **Carboxypeptidase:** This is a **Zinc-containing metalloenzyme**. In Carboxypeptidase A, the Zinc ion is coordinated by two **Histidine** residues and one Glutamate residue. These Histidines are crucial for positioning the metal ion that activates the water molecule for hydrolysis. * **Hexokinase:** This glycolytic enzyme utilizes **Histidine** (specifically His-213 in some isoforms) at its active site to facilitate the transfer of a phosphate group from ATP to glucose by acting as a general base. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **The "Amphoteric" Nature:** Histidine is the only amino acid that functions as an effective buffer at physiological pH. 2. **Catalytic Triad:** Remember the triad **Asp-His-Ser** for Trypsin, Chymotrypsin, Elastase, and Thrombin. 3. **Bohr Effect:** Histidine residues in Hemoglobin (specifically His-146) are responsible for binding H+ ions, facilitating the release of oxygen in peripheral tissues. 4. **Carbonic Anhydrase:** Another high-yield enzyme where Histidine residues coordinate with a Zinc ion to catalyze the hydration of CO₂.

Question 307: Glycogen phosphorylase is classified as which type of enzyme?

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

Explanation: **Explanation:** **Why Transferase is correct:** Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis. It catalyzes the breakdown of glycogen by adding an inorganic phosphate ($P_i$) to the terminal $\alpha$-1,4-glycosidic bond. This process is called **phosphorolysis**, resulting in the release of Glucose-1-Phosphate. According to the IUBMB classification, enzymes that transfer a functional group (in this case, a phosphate group) from one substrate to another are classified as **Transferases (EC 2)**. Specifically, glycogen phosphorylase is a glycosyltransferase. **Why other options are incorrect:** * **Oxidoreductases (EC 1):** These catalyze oxidation-reduction reactions (e.g., LDH). Glycogen phosphorylase does not involve the transfer of electrons or hydrogen. * **Hydrolases (EC 3):** These break bonds by adding water (hydrolysis). While glycogen phosphorylase breaks a bond, it uses phosphate instead of water. If it were a hydrolase, it would release free glucose (like the debranching enzyme's $\alpha$-1,6-glucosidase activity). * **Lyases (EC 4):** These catalyze the cleavage of bonds by means other than hydrolysis or oxidation, often forming a double bond or adding groups to double bonds (e.g., Aldolase). **High-Yield Clinical Pearls for NEET-PG:** 1. **Co-enzyme:** Glycogen phosphorylase requires **Pyridoxal Phosphate (PLP/Vitamin B6)** as an essential cofactor. 2. **Regulation:** It is activated by phosphorylation (via Phosphorylase Kinase) and allosterically activated by **AMP** in the muscle. 3. **Clinical Correlation:** A deficiency of liver glycogen phosphorylase leads to **Hers Disease (GSD Type VI)**, characterized by hepatomegaly and mild hypoglycemia. 4. **Key Distinction:** Do not confuse "Phosphorylase" (Transferase) with "Phosphatase" (Hydrolase).

Question 308: Metastasis of cancer has its roots in structural abnormalities of which of the following?

A. Glycolipids in nervous tissue
B. Glycoproteins on cell surface (Correct Answer)
C. Lipoproteins in blood
D. None of the above

Explanation: **Explanation:** The correct answer is **B. Glycoproteins on cell surface.** **Why it is correct:** Metastasis is a complex process involving the detachment of cancer cells from the primary tumor, their migration through the extracellular matrix (ECM), and colonization of distant sites. This process is heavily dependent on **cell-surface glycoproteins**, which act as cell adhesion molecules (CAMs). 1. **Cadherins:** A loss of E-cadherin (a glycoprotein) reduces cell-to-cell adhesion, allowing cells to detach. 2. **Integrins:** These glycoproteins mediate the attachment of cells to the ECM; alterations in integrin expression facilitate migration and "homing" to distant organs. 3. **Selectins:** These glycoproteins are involved in the "docking" of circulating tumor cells to the vascular endothelium during extravasation. **Why incorrect options are wrong:** * **A. Glycolipids in nervous tissue:** While glycolipids (like gangliosides) are vital for signal transduction and myelin stability in the nervous system, they are not the primary structural drivers of systemic cancer metastasis. * **C. Lipoproteins in blood:** Lipoproteins (HDL, LDL, VLDL) function as transport vehicles for lipids and cholesterol. While they may play a minor role in providing energy to cancer cells, they do not form the structural basis for metastatic spread. **High-Yield Clinical Pearls for NEET-PG:** * **E-Cadherin:** Often referred to as a "metastasis suppressor." Its downregulation is a hallmark of **Epithelial-Mesenchymal Transition (EMT)**. * **MMPs (Matrix Metalloproteinases):** These are zinc-dependent enzymes that degrade the basement membrane (collagen type IV), working alongside glycoprotein changes to facilitate invasion. * **CEA (Carcinoembryonic Antigen):** A well-known clinical tumor marker that is itself a cell-surface glycoprotein.

Question 309: Enzymes increase reaction rates by?

A. Altering the free energy of the reaction
B. Inhibiting the backward reaction
C. Enhancing the forward reaction
D. Decreasing the energy of activation (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right (Decreasing the Energy of Activation)** Enzymes are biological catalysts that accelerate chemical reactions without being consumed. Every chemical reaction requires an initial input of energy, known as the **Activation Energy ($E_a$)**, to reach the unstable **transition state** where bonds can be broken or formed. Enzymes function by stabilizing this transition state and providing an alternative reaction pathway. By lowering the $E_a$ barrier, a larger fraction of substrate molecules can achieve the required energy to react at body temperature, thereby significantly increasing the reaction rate. **2. Why the Incorrect Options are Wrong** * **Option A:** Enzymes **do not alter the free energy ($\Delta G$)** of the reaction. The net energy difference between reactants and products remains constant; enzymes only change the speed at which equilibrium is reached, not the equilibrium position itself. * **Option B:** Enzymes do not selectively inhibit the backward reaction. They catalyze **both** the forward and backward reactions equally to reach equilibrium faster. * **Option C:** While enzymes do enhance the forward reaction rate, "enhancing" is a descriptive result rather than the *mechanism*. The fundamental mechanism by which they achieve this enhancement is the lowering of activation energy. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Transition State Analogs:** Drugs that mimic the transition state of a substrate (e.g., **Statins** mimicking the HMG-CoA transition state) are potent competitive inhibitors because enzymes bind the transition state more tightly than the substrate itself. * **Active Site:** The specific region where the substrate binds; it is often a hydrophobic cleft. * **Models:** The **Induced Fit Model** (Koshland) is more accurate than the Lock and Key model, as it accounts for the conformational changes in the enzyme upon substrate binding. * **Thermodynamics:** Enzymes affect **kinetics** (rate), not **thermodynamics** (spontaneity/$\Delta G$).

Question 310: The type of enzyme inhibition observed when xanthine oxidase is inhibited by allopurinol is an example of:

A. Noncompetitive
B. Uncompetitive
C. Allosteric
D. Suicidal (Correct Answer)

Explanation: **Explanation:** **Suicide Inhibition (Mechanism-Based Inhibition)** occurs when an enzyme converts a substrate analogue into a highly reactive intermediate that binds irreversibly to the enzyme's active site, permanently inactivating it. In the case of **Allopurinol**, the enzyme **Xanthine Oxidase** initially treats it as a substrate and converts it into **Alloxanthine (Oxypurinol)**. Alloxanthine then binds tightly to the molybdenum-sulfide complex at the active site of xanthine oxidase, effectively "killing" the enzyme it helped create. This is a classic example of "suicide" because the enzyme participates in its own destruction. **Why other options are incorrect:** * **Noncompetitive:** The inhibitor binds to a site other than the active site (E or ES complex). It decreases $V_{max}$ but $K_m$ remains unchanged. * **Uncompetitive:** The inhibitor binds only to the Enzyme-Substrate (ES) complex. Both $V_{max}$ and $K_m$ decrease. * **Allosteric:** These enzymes have a "regulatory site" distinct from the active site. Binding of an effector causes a conformational change, often following sigmoidal kinetics rather than Michaelis-Menten kinetics. **NEET-PG High-Yield Pearls:** * **Other Suicide Inhibitors:** Aspirin (Cyclooxygenase), 5-Fluorouracil (Thymidylate synthase), Penicillin (Transpeptidase), and Disulfiram (Aldehyde dehydrogenase). * **Clinical Use:** Allopurinol is the drug of choice for **Chronic Gout** as it lowers serum uric acid levels. * **Key Distinction:** Unlike standard competitive inhibition, suicide inhibition is **irreversible**.

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

Enzyme Kinetics and Michaelis-Menten Equation

Practice Questions

Enzyme Inhibition: Competitive and Non-competitive

Practice Questions

Allosteric Regulation

Practice Questions

Coenzymes and Cofactors

Practice Questions

Isoenzymes and Clinical Significance

Practice Questions

Enzyme Regulation: Covalent Modification

Practice Questions

Enzyme Regulation: Zymogen Activation

Practice Questions

Enzyme Induction and Repression

Practice Questions

Ribozymes and Catalytic RNA

Practice Questions

Enzyme Diagnostic Applications

Practice Questions

Enzyme Therapy and Inhibitors as Drugs

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?