Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 291: Which enzyme is considered a marker for the plasma membrane?

A. 5'Nucleotidase (Correct Answer)
B. Catalase
C. Acid Phosphatase
D. GGT

Explanation: ### Explanation **Correct Option: A. 5'Nucleotidase** 5'Nucleotidase is a classic **marker enzyme for the plasma membrane**. It is an intrinsic membrane protein that catalyzes the hydrolysis of nucleoside 5'-monophosphates (like AMP) into nucleosides and inorganic phosphate. In clinical practice, its levels are elevated in hepatobiliary diseases, particularly those involving cholestasis, making it a specific marker for liver pathology alongside Alkaline Phosphatase (ALP). **Analysis of Incorrect Options:** * **B. Catalase:** This is the hallmark marker enzyme for **Peroxisomes**. It protects cells from oxidative damage by breaking down hydrogen peroxide ($H_2O_2$) into water and oxygen. * **C. Acid Phosphatase:** This is the characteristic marker enzyme for **Lysosomes**. It functions optimally at an acidic pH to degrade cellular debris. (Note: Prostatic acid phosphatase is a specific isoenzyme used in prostate cancer screening). * **D. GGT (Gamma-Glutamyl Transferase):** While GGT is found in the plasma membrane and endoplasmic reticulum of cells in the liver and bile ducts, it is primarily used as a clinical marker for **alcohol consumption** and hepatobiliary obstruction rather than a definitive structural marker for the plasma membrane in general cell biology. **High-Yield Marker Enzymes for NEET-PG:** * **Mitochondria:** ATP Synthase (Inner membrane), Monoamine Oxidase (Outer membrane), Citrate Synthase (Matrix). * **Cytosol:** Lactate Dehydrogenase (LDH). * **Endoplasmic Reticulum:** Glucose-6-Phosphatase. * **Golgi Complex:** Galactosyltransferase. * **Nucleus:** DNA Polymerase / RNA Polymerase.

Question 292: Selenium is a cofactor in which of the following enzymes?

A. Glutathione peroxidase (Correct Answer)
B. Cytochrome oxidase
C. Cytochrome reductase
D. Xanthine oxidase

Explanation: **Explanation:** **1. Why Glutathione Peroxidase is Correct:** Selenium is an essential trace element that is incorporated into proteins as the amino acid **Selenocysteine** (often called the 21st amino acid). **Glutathione peroxidase (GPx)** is the most well-known selenoenzyme. It plays a critical role in the cellular antioxidant system by reducing hydrogen peroxide ($H_2O_2$) and lipid hydroperoxides to water and alcohols, respectively, using reduced glutathione (GSH) as a donor. This protects cell membranes from oxidative damage. **2. Analysis of Incorrect Options:** * **B. Cytochrome oxidase:** This is Complex IV of the electron transport chain. It requires **Copper (Cu)** and **Iron (Fe)** for its catalytic activity, not selenium. * **C. Cytochrome reductase:** These enzymes (like NADPH-cytochrome P450 reductase) typically utilize **Flavin nucleotides (FAD/FMN)** as cofactors. * **D. Xanthine oxidase:** This enzyme, involved in purine catabolism (converting hypoxanthine to xanthine and then to uric acid), requires **Molybdenum (Mo)**, Iron, and FAD. **3. High-Yield Clinical Pearls for NEET-PG:** * **Other Selenoenzymes:** Apart from GPx, other important selenium-dependent enzymes include **Thioredoxin reductase** and **Deiodinase** (specifically Type 1 iodothyronine deiodinase, which converts $T_4$ to $T_3$). * **Deficiency:** Selenium deficiency is associated with **Keshan disease** (an endemic cardiomyopathy) and **Kashin-Beck disease** (an osteoarthropathy). * **Toxicity:** Excess selenium (Selenosis) leads to garlic breath, hair loss (alopecia), and nail changes. * **Codon:** Selenocysteine is encoded by the **UGA** codon, which normally acts as a stop codon but is recoded in the presence of a specific SECIS (Selenocysteine Insertion Sequence) element.

Question 293: Molybdenum is a component of which enzyme?

A. Cytochrome oxidase
B. Xanthine oxidase (Correct Answer)
C. Glutathione peroxidase
D. Urease

Explanation: **Explanation:** **Xanthine Oxidase** is a complex metalloenzyme that requires **Molybdenum** (as a molybdopterin cofactor), Iron, and FAD for its catalytic activity. It plays a critical role in purine catabolism, catalyzing the oxidation of hypoxanthine to xanthine and xanthine to uric acid. **Analysis of Options:** * **Cytochrome Oxidase (Option A):** This is the terminal enzyme of the electron transport chain (Complex IV). It contains **Copper (Cu)** and **Iron (Fe)** (in heme groups), not molybdenum. * **Glutathione Peroxidase (Option C):** This enzyme protects cells from oxidative damage by reducing lipid hydroperoxides. It is a well-known **Selenium-dependent** enzyme. * **Urease (Option D):** Found in bacteria (like *H. pylori*) and plants, this enzyme catalyzes the hydrolysis of urea. It requires **Nickel (Ni)** for its activity. **Clinical Pearls & High-Yield Facts:** 1. **Gout Connection:** Allopurinol, a drug used to treat chronic gout, acts as a suicide inhibitor of Xanthine Oxidase, thereby lowering serum uric acid levels. 2. **Molybdenum-dependent enzymes:** Besides Xanthine Oxidase, other human enzymes requiring Molybdenum include **Sulfite Oxidase** (deficiency leads to neurological symptoms) and **Aldehyde Oxidase**. 3. **Genetic Deficiency:** Hereditary Xanthinuria is caused by a deficiency of Xanthine Oxidase, leading to hypouricemia and potential xanthine stones in the urinary tract.

Question 294: Which enzyme is involved in the cleavage of glycine in liver mitochondria?

A. Pyruvate dehydrogenase
B. Dihydrolipoyl transacetylase
C. Dihydrolipoyl dehydrogenase (Correct Answer)
D. None of these

Explanation: ### Explanation The cleavage of glycine in liver mitochondria is catalyzed by the **Glycine Cleavage System (GCS)**, also known as the glycine synthase complex. This multienzyme complex is structurally and functionally similar to the Pyruvate Dehydrogenase (PDH) and α-Ketoglutarate Dehydrogenase complexes. **Why Dihydrolipoyl dehydrogenase is correct:** The GCS consists of four protein components: 1. **P-protein:** A pyridoxal phosphate-dependent decarboxylase. 2. **H-protein:** A lipoic acid-containing protein. 3. **T-protein:** A tetrahydrofolate-dependent aminomethyltransferase. 4. **L-protein (Dihydrolipoyl dehydrogenase):** This enzyme is responsible for the re-oxidation of the dihydrolipoyl group on the H-protein, using NAD+ as an electron acceptor. Since the L-protein is identical to the E3 subunit found in the PDH complex, **Dihydrolipoyl dehydrogenase** is the correct enzyme involved in this process. **Why other options are incorrect:** * **Pyruvate dehydrogenase (Option A):** This is the E1 subunit of the PDH complex specifically involved in the decarboxylation of pyruvate, not glycine. * **Dihydrolipoyl transacetylase (Option B):** This is the E2 subunit of the PDH complex. While the GCS has a similar lipoic acid-bearing protein (H-protein), it does not involve a transacetylase reaction. **Clinical Pearls for NEET-PG:** * **Non-ketotic Hyperglycinemia:** A deficiency in any of the GCS components (most commonly the P-protein) leads to high glycine levels in the blood and CSF, causing severe neurological distress and seizures in neonates. * **Common Subunit:** Remember that **Dihydrolipoyl dehydrogenase (E3)** is a shared component among three major complexes: PDH, α-Ketoglutarate Dehydrogenase, and Branched-chain α-keto acid Dehydrogenase. * **Cofactors:** The GCS requires PLP, Lipoic acid, THF, and NAD+.

Question 295: Fatty acid-linked dehydrogenase is:

A. Enoyl reductase
B. Glyceraldehyde 3-phosphate dehydrogenase
C. Succinate dehydrogenase (Correct Answer)
D. Isocitrate dehydrogenase

Explanation: **Explanation:** **Succinate Dehydrogenase (SDH)** is the correct answer because it is a unique enzyme that utilizes **FAD (Flavin Adenine Dinucleotide)** as its coenzyme. In biochemistry, dehydrogenases that utilize FAD are often referred to as "Flavoproteins" or "Fatty acid-linked" (though the term more accurately refers to the **Flavin** prosthetic group, FAD). SDH catalyzes the oxidation of Succinate to Fumarate in the TCA cycle. Unlike other TCA cycle enzymes, SDH is **integral to the inner mitochondrial membrane** and functions as **Complex II** of the Electron Transport Chain (ETC). It transfers electrons directly from succinate to the ubiquinone pool via FADH₂. **Analysis of Incorrect Options:** * **Enoyl reductase:** This is an enzyme involved in Fatty Acid Synthesis (FAS complex). It typically utilizes **NADPH** as a reducing equivalent, not FAD. * **Glyceraldehyde 3-phosphate dehydrogenase (GAPDH):** A key glycolytic enzyme that converts GAP to 1,3-bisphosphoglycerate. It is strictly **NAD⁺-dependent**. * **Isocitrate dehydrogenase:** A rate-limiting enzyme of the TCA cycle. The mitochondrial isoform primarily uses **NAD⁺** to produce NADH, while the cytosolic isoform uses **NADP⁺**. **High-Yield Clinical Pearls for NEET-PG:** * **Marker Enzyme:** SDH is a specific marker enzyme for **Mitochondria**. * **Competitive Inhibition:** Malonate is a classic competitive inhibitor of SDH (structural analog of succinate), a frequent exam topic. * **Dual Role:** It is the only enzyme that participates in both the **TCA Cycle** and the **Electron Transport Chain**. * **Prosthetic Group:** FAD is covalently bound to SDH, unlike NAD⁺ which is a loosely bound coenzyme.

Question 296: All of the following are true about Xanthine oxidase, EXCEPT:

A. Contains iron
B. Contains molybdenum
C. Flavoprotein
D. Produces H2O (Correct Answer)

Explanation: **Explanation:** Xanthine oxidase (XO) is a critical enzyme in the catabolism of purines, responsible for converting hypoxanthine to xanthine and xanthine to **uric acid**. The correct answer is **D** because Xanthine oxidase does not produce $H_2O$; instead, it reduces molecular oxygen ($O_2$) to produce **Hydrogen peroxide ($H_2O_2$)** and superoxide radicals. * **Why Option D is the exception:** During the oxidation of xanthine, electrons are transferred to oxygen. This reaction typically generates $H_2O_2$ (a reactive oxygen species), not water ($H_2O$). * **Why Option A is incorrect:** XO is a complex metalloenzyme that contains **Iron-Sulfur (Fe-S) clusters**, which are essential for electron transfer within the enzyme. * **Why Option B is incorrect:** XO is one of the few human enzymes that requires **Molybdenum** (as a molybdopterin cofactor) for its catalytic activity. * **Why Option C is incorrect:** It is a **Flavoprotein**, containing **FAD** (Flavin Adenine Dinucleotide) as a prosthetic group to facilitate the redox reaction. **High-Yield Clinical Pearls for NEET-PG:** 1. **Allopurinol:** A suicide inhibitor of Xanthine oxidase used to treat **Gout** by lowering serum uric acid levels. 2. **Xanthine Stones:** Rare renal stones formed in patients with hereditary xanthine oxidase deficiency or those on high-dose Allopurinol. 3. **Reperfusion Injury:** XO is a major source of free radicals during ischemia-reperfusion injury, as it generates superoxide anions. 4. **Molybdenum Deficiency:** Can lead to secondary xanthine oxidase deficiency, resulting in hypouricemia.

Question 297: Selenium is a cofactor for which of the following enzymes?

A. Glutathione peroxidase (Correct Answer)
B. Glutathione reductase
C. Glutathione synthetase
D. Glutathione dehydrogenase

Explanation: **Explanation:** **1. Why Glutathione Peroxidase is Correct:** Glutathione peroxidase (GPx) is a vital antioxidant enzyme that protects cells from oxidative damage by reducing lipid hydroperoxides and free hydrogen peroxide ($H_2O_2$) into water. It requires **Selenium** in the form of the 21st amino acid, **Selenocysteine**, at its active site to function. This makes Selenium an essential trace element for maintaining the integrity of RBC membranes and preventing hemolysis. **2. Why the Other Options are Incorrect:** * **Glutathione Reductase:** This enzyme regenerates reduced glutathione (GSH) from its oxidized form (GSSG). Its essential cofactor is **Riboflavin (Vitamin $B_2$)** in the form of FAD, and it requires NADPH (from the HMP shunt) as a reducing equivalent. * **Glutathione Synthetase:** This is an ATP-dependent enzyme involved in the de novo synthesis of glutathione from $\gamma$-glutamylcysteine and glycine. It does not require selenium. * **Glutathione Dehydrogenase:** This is not a primary enzyme in the glutathione redox cycle. The term is sometimes used synonymously with enzymes involved in glutathione metabolism, but none are selenium-dependent. **3. High-Yield Clinical Pearls for NEET-PG:** * **Selenium Deficiency:** Can lead to **Keshan Disease** (an endemic cardiomyopathy) and **Kashin-Beck Disease** (an osteoarthropathy). * **Other Selenoenzymes:** Besides GPx, Selenium is a cofactor for **Thioredoxin reductase** and **Deiodinase** (which converts $T_4$ to $T_3$). * **The Redox Cycle:** Remember the "GR-GPx" duo: **G**lutathione **R**eductase uses **B2**, while **G**lutathione **P**eroxidase uses **Selenium**.

Question 298: ATP synthetase is a marker of which cellular organelle?

A. Golgi apparatus
B. Mitochondria (Correct Answer)
C. Cytosol
D. Endoplasmic reticulum

Explanation: **Explanation:** **Correct Answer: B. Mitochondria** ATP synthetase (also known as Complex V or $F_oF_1$-ATPase) is the enzyme responsible for synthesizing ATP from ADP and inorganic phosphate. This process occurs during **oxidative phosphorylation** on the **inner mitochondrial membrane**. The enzyme utilizes the proton gradient generated by the Electron Transport Chain (ETC) to drive the phosphorylation of ADP. Because of its exclusive and vital role in mitochondrial energy production, it serves as a specific biochemical marker for this organelle. **Why other options are incorrect:** * **Golgi Apparatus:** Markers for the Golgi include **Galactosyltransferase** and Thiamine pyrophosphatase. Its primary role is protein packaging and modification, not ATP synthesis. * **Cytosol:** Common markers include **Lactate Dehydrogenase (LDH)** and Glucose-6-phosphate dehydrogenase. While glycolysis (which produces some ATP) occurs here, the specific ATP synthetase complex is absent. * **Endoplasmic Reticulum (ER):** The classic marker for the ER is **Glucose-6-phosphatase** (smooth ER) or Cytochrome P450 enzymes. The ER is involved in protein synthesis and lipid metabolism. **High-Yield NEET-PG Pearls:** * **Structure:** ATP synthetase consists of two subunits: $F_o$ (proton channel, inhibited by **Oligomycin**) and $F_1$ (catalytic unit that projects into the matrix). * **Mechanism:** It operates via the "Binding Change Mechanism" (Rotary catalysis) proposed by Paul Boyer. * **Other Mitochondrial Markers:** Succinate dehydrogenase (Inner membrane), Citrate synthase (Matrix), and Monoamine oxidase (Outer membrane).

Question 299: What does G6PD stand for?

A. Glucose 6 phosphatase dehydratase
B. Glucose 6 phosphate dehydrogenase (Correct Answer)
C. Glucose 6 phosphodiesterase
D. Glucose 6 phosphate decarboxylase

Explanation: **Explanation:** **1. Why Option B is Correct:** **Glucose 6-phosphate dehydrogenase (G6PD)** is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). It catalyzes the conversion of Glucose 6-phosphate to 6-phosphogluconolactone. This reaction is critical because it reduces NADP⁺ to **NADPH**. In RBCs, NADPH is the only source of reducing equivalents used by Glutathione Reductase to maintain **reduced glutathione**, which protects the cell against oxidative damage from free radicals and H₂O₂. **2. Why Other Options are Incorrect:** * **Option A (Dehydratase):** Dehydratases remove water molecules to form double bonds; they are not involved in the initial step of the HMP shunt. * **Option C (Phosphodiesterase):** These enzymes break phosphodiester bonds (e.g., in cAMP or DNA/RNA) and have no role in glucose metabolism. * **Option D (Decarboxylase):** While decarboxylation does occur later in the HMP shunt (converting 6-phosphogluconate to Ribulose 5-phosphate via 6-phosphogluconate dehydrogenase), G6PD itself does not remove CO₂. **3. Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It is an **X-linked recessive** disorder. * **Pathophysiology:** Deficiency leads to decreased NADPH, causing hemoglobin to denature and precipitate as **Heinz Bodies**. * **Morphology:** Splenic macrophages pluck out these bodies, resulting in **"Bite Cells"** (Degmacytes) seen on peripheral smears. * **Triggers:** Hemolysis is typically triggered by oxidative stress, such as **Fava beans**, infections, or drugs (e.g., Primaquine, Sulfa drugs, Dapsone). * **Protective Effect:** G6PD deficiency offers a selective advantage against *Plasmodium falciparum* malaria.

Question 300: Which enzyme is part of the free radical scavenging system?

A. NADPH oxidase
B. Glutathione peroxidase (Correct Answer)
C. Endonuclease
D. Phospholipase

Explanation: **Explanation:** **Glutathione peroxidase (GPx)** is a critical antioxidant enzyme that protects cells from oxidative damage. It functions by reducing lipid hydroperoxides to their corresponding alcohols and reducing free hydrogen peroxide ($H_2O_2$) to water. This reaction requires **reduced glutathione (GSH)** as a hydrogen donor. During this process, GSH is oxidized to glutathione disulfide (GSSG), which is subsequently regenerated by Glutathione Reductase using NADPH. **Analysis of Incorrect Options:** * **A. NADPH Oxidase:** This enzyme is actually a **pro-oxidant**. It is found in the membranes of phagocytes and is responsible for the "Respiratory Burst," producing superoxide radicals ($O_2^{\bullet-}$) to kill invading microorganisms. * **C. Endonuclease:** These are enzymes that cleave the phosphodiester bonds within a polynucleotide chain (DNA/RNA). They are involved in DNA repair and replication, not free radical scavenging. * **D. Phospholipase:** These enzymes hydrolyze phospholipids into fatty acids and other lipophilic substances. While phospholipase $A_2$ can be activated by oxidative stress, its primary role is in lipid metabolism and signaling (e.g., arachidonic acid release), not scavenging. **High-Yield Clinical Pearls for NEET-PG:** * **Selenium Dependency:** Glutathione peroxidase is a **selenoprotein**; it contains **selenocysteine** at its active site. Selenium deficiency can lead to reduced GPx activity (e.g., Keshan disease). * **The NADPH Link:** The HMP Shunt (Pentose Phosphate Pathway) is vital for the antioxidant system because it provides the **NADPH** required to keep glutathione in its reduced state. * **Other Scavengers:** Remember the "Antioxidant Trio": **Superoxide Dismutase (SOD)** (converts $O_2^{\bullet-}$ to $H_2O_2$), **Catalase** (decomposes $H_2O_2$), and **Glutathione Peroxidase**.

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Enzymes — MCQs

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