Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 191: The uvrABC endonuclease is involved in which one of the following processes?

A. DNA replication
B. RNA splicing
C. DNA repair (Correct Answer)
D. DNA recombination

Explanation: **Explanation:** The **uvrABC endonuclease** is a multi-enzyme complex in *E. coli* that plays a critical role in **Nucleotide Excision Repair (NER)**. This pathway is the primary mechanism for identifying and removing bulky DNA lesions, most notably **pyrimidine dimers** (thymine dimers) caused by Ultraviolet (UV) radiation. * **Mechanism:** The complex consists of three proteins: **UvrA** and **UvrB** scan the DNA to identify the distortion; **UvrC** (the endonuclease) then performs two incisions on the damaged strand—one on each side of the lesion. The excised segment is removed by UvrD (helicase), and the gap is filled by DNA Polymerase I and sealed by Ligase. **Analysis of Incorrect Options:** * **A. DNA Replication:** This process involves DNA Polymerases, Helicase, and Primase to duplicate the genome. While it involves DNA synthesis, uvrABC is specific to damage correction, not replication. * **B. RNA Splicing:** This is a post-transcriptional modification where introns are removed from pre-mRNA, mediated by the **spliceosome** (snRNPs), not endonucleases like uvrABC. * **D. DNA Recombination:** This involves the exchange of genetic material (e.g., during meiosis or via the RecBCD pathway in bacteria), focusing on genetic diversity rather than repairing UV-induced bulky lesions. **Clinical Pearls for NEET-PG:** * **Xeroderma Pigmentosum (XP):** This is the human clinical correlate. It is an autosomal recessive disorder caused by a deficiency in human NER enzymes (orthologous to the uvrABC system). Patients present with extreme photosensitivity and a 2000-fold increased risk of skin cancer. * **Key Distinction:** Remember that **Nucleotide** Excision Repair (uvrABC) handles *bulky* lesions, while **Base** Excision Repair (DNA Glycosylases) handles *non-bulky* lesions like cytosine deamination.

Question 192: Which of the following enzymes is deficient in Crigler-Najjar syndrome?

A. UDP glucuronyl transferase 1 (Correct Answer)
B. UDP glucuronyl transferase 2
C. Bilirubin synthase
D. Heme synthase

Explanation: **Explanation:** **Crigler-Najjar Syndrome (Type I and II)** is a rare genetic disorder characterized by non-hemolytic unconjugated hyperbilirubinemia. The underlying defect is a deficiency in the conjugation of bilirubin. 1. **Why Option A is Correct:** Bilirubin, produced from heme breakdown, is lipid-soluble (unconjugated) and must be converted into water-soluble bilirubin diglucuronide to be excreted. This process occurs in the liver and is catalyzed by the enzyme **UDP-glucuronyltransferase 1A1 (UGT1A1)**. In Crigler-Najjar syndrome, mutations in the *UGT1A1* gene lead to either a total absence (Type I) or a severe deficiency (Type II) of this enzyme, resulting in dangerously high levels of unconjugated bilirubin. 2. **Why Other Options are Incorrect:** * **Option B:** While there are various UGT isoforms (like UGT2), **UGT1A1** is the specific isoform responsible for bilirubin conjugation. * **Option C:** "Bilirubin synthase" is not a recognized enzyme in the heme degradation pathway. * **Option D:** Heme synthase (also known as Ferrochelatase) is the final enzyme in the **heme synthesis** pathway (converting protoporphyrin IX to heme). Its deficiency leads to Erythropoietic Protoporphyria, not hyperbilirubinemia. **High-Yield Clinical Pearls for NEET-PG:** * **Crigler-Najjar Type I:** Autosomal recessive; total enzyme absence; serum bilirubin >20 mg/dL; high risk of **Kernicterus**; does not respond to Phenobarbital. * **Crigler-Najjar Type II (Arias Syndrome):** Autosomal dominant; partial enzyme deficiency; lower bilirubin levels; **responds to Phenobarbital** (which induces enzyme synthesis). * **Gilbert Syndrome:** Most common hereditary hyperbilirubinemia; mild decrease in UGT1A1 activity (~30% of normal); usually asymptomatic until triggered by stress or fasting.

Question 193: Which of the following statements regarding Nitric Oxide Synthase is correct?

A. It is inhibited by Ca+
B. It catalyzes a dioxygenase reaction
C. It accepts electrons from NADH
D. It requires NADPH, FAD, FMN, and heme iron (Correct Answer)

Explanation: **Explanation:** **Nitric Oxide Synthase (NOS)** is a complex enzyme responsible for synthesizing Nitric Oxide (NO) from the amino acid **L-arginine**. **1. Why Option D is Correct:** NOS is a unique enzyme that functions as both a reductase and an oxygenase. To catalyze the conversion of L-arginine to L-citrulline and NO, it requires five essential cofactors: **NADPH, FAD, FMN, Heme iron (protoporphyrin IX), and Tetrahydrobiopterin (BH4)**. The electrons flow from NADPH through the flavins (FAD/FMN) to the heme center, where oxygen is reduced and incorporated into the substrate. **2. Why Other Options are Incorrect:** * **Option A:** NOS (specifically the eNOS and nNOS isoforms) is actually **activated by Calcium-Calmodulin** binding, not inhibited by it. * **Option B:** It catalyzes a **monooxygenase** reaction. In the process, one atom of oxygen is incorporated into the hydroxyl group of the intermediate (L-hydroxyarginine), and the other is reduced to water. * **Option C:** The primary electron donor for NOS is **NADPH**, not NADH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isoforms:** There are three isoforms: **nNOS** (neuronal/Type I), **iNOS** (inducible/Type II), and **eNOS** (endothelial/Type III). * **iNOS:** Unlike the others, iNOS is **calcium-independent** and is expressed in macrophages during inflammation to produce large amounts of NO for bactericidal activity. * **Substrate:** L-arginine is the precursor; L-citrulline is the byproduct. * **Inhibitor:** Asymmetric dimethylarginine (ADMA) acts as an endogenous inhibitor of NOS.

Question 194: In cytochrome P450, P stands for what?

A. Pigment (Correct Answer)
B. Polymer
C. Protein
D. Plasma

Explanation: **Explanation:** The "P" in Cytochrome P450 stands for **Pigment**. This nomenclature is derived from the enzyme's unique spectral property: when the heme iron in the enzyme is in a reduced state and bound to carbon monoxide (CO), it exhibits a characteristic absorption maximum at a wavelength of **450 nm**. Because it absorbs light in the visible spectrum and functions as a colored cellular component, it is classified as a pigment. **Analysis of Options:** * **B. Polymer:** Cytochrome P450 is a monomeric hemeprotein, not a polymer (a large molecule composed of repeating subunits). * **C. Protein:** While Cytochrome P450 *is* a protein, the specific "P" designation in its name historically and scientifically refers to its properties as a pigment. * **D. Plasma:** These enzymes are primarily located in the **Smooth Endoplasmic Reticulum** (microsomes) and mitochondria of hepatocytes, not in the plasma. **High-Yield Clinical Pearls for NEET-PG:** * **Function:** They are **Monooxygenases** (Mixed Function Oxidases) involved in Phase I detoxification reactions (hydroxylation). * **Location:** Highest concentration is found in the **Liver**. * **Components:** The system requires NADPH and the enzyme NADPH-cytochrome P450 reductase. * **Inducers vs. Inhibitors:** * *Inducers:* Phenytoin, Rifampicin, Griseofulvin, Carbamazepine (increase drug metabolism). * *Inhibitors:* Ketoconazole, Erythromycin, Cimetidine, Grapefruit juice (decrease drug metabolism, leading to toxicity). * **Key Isoenzyme:** **CYP3A4** is the most abundant isoform responsible for metabolizing nearly 50% of clinical drugs.

Question 195: Which of the following enzymes does not contain Cu2+?

A. Ceruloplasmin
B. Cytochrome Oxidase
C. Xanthine oxidase (Correct Answer)
D. Dopamine beta-hydroxylase

Explanation: **Explanation:** The correct answer is **Xanthine oxidase** because it is a metalloenzyme that requires **Molybdenum (Mo)**, Iron (Fe), and FAD as cofactors, rather than Copper (Cu²⁺). It plays a critical role in purine catabolism by converting hypoxanthine to xanthine and xanthine to uric acid. **Analysis of Options:** * **Ceruloplasmin (Option A):** This is the primary copper-carrying protein in the blood. It contains 6 to 8 copper atoms per molecule and functions as a ferroxidase, converting Fe²⁺ to Fe³⁺ for binding to transferrin. * **Cytochrome Oxidase (Option B):** Also known as Complex IV of the Electron Transport Chain, it contains two copper centers ($Cu_A$ and $Cu_B$) along with heme groups ($a$ and $a_3$). It is essential for transferring electrons to oxygen. * **Dopamine beta-hydroxylase (Option C):** This enzyme converts dopamine to norepinephrine in the catecholamine synthesis pathway. It requires **Copper** and Vitamin C (ascorbic acid) as essential cofactors. **High-Yield Clinical Pearls for NEET-PG:** * **Menkes Disease:** A defect in copper absorption (ATP7A) leading to "kinky hair" and neurological issues due to the failure of copper-dependent enzymes like Lysyl oxidase and Dopamine $\beta$-hydroxylase. * **Wilson Disease:** A defect in copper biliary excretion (ATP7B) leading to low serum ceruloplasmin and copper deposition in the liver and basal ganglia (Kayser-Fleischer rings). * **Other Copper Enzymes:** Lysyl oxidase (collagen cross-linking), Tyrosinase (melanin synthesis), and Superoxide Dismutase (cytosolic antioxidant). * **Xanthine Oxidase Inhibitor:** Allopurinol and Febuxostat are used to treat Gout by inhibiting this molybdenum-containing enzyme.

Question 196: Lysosomal enzymes are maximally active at which pH?

A. Acidic pH (Correct Answer)
B. Alkaline pH
C. Neutral pH
D. Has no relation with pH

Explanation: **Explanation:** Lysosomes are membrane-bound organelles often referred to as the "suicide bags" of the cell. They contain approximately 50 different degradative enzymes known as **acid hydrolases** (e.g., phosphatases, glycosidases, proteases, and lipases). **1. Why Acidic pH is Correct:** The enzymes within lysosomes are specifically designed to function at an **optimum pH of approximately 4.5 to 5.0**. This acidity is maintained by a **V-type ATPase (proton pump)** located in the lysosomal membrane, which actively pumps H+ ions from the cytosol into the lysosome. This acidic environment is crucial for the denaturation of macromolecules, making them easier for hydrolases to degrade. **2. Why Other Options are Incorrect:** * **Alkaline & Neutral pH:** Most cytosolic and extracellular enzymes function best at a physiological pH (~7.2–7.4). If lysosomal enzymes were active at a neutral or alkaline pH, any accidental leakage or rupture of a lysosome would lead to the immediate autodigestion of the cell's vital components. * **No relation with pH:** Enzyme activity is strictly dependent on the ionization state of the amino acids at the active site, which is directly influenced by pH. **High-Yield Clinical Pearls for NEET-PG:** * **Protective Mechanism:** The requirement for an acidic pH serves as a protective mechanism for the cell; if a lysosome leaks, the hydrolases become inactive in the neutral cytosol, preventing cellular damage. * **I-Cell Disease:** A high-yield pathology where lysosomal enzymes fail to be phosphorylated (Man-6-P tag) in the Golgi, leading to their secretion outside the cell rather than being targeted to the lysosome. * **Lysosomal Storage Disorders (LSDs):** Deficiencies in specific acid hydrolases lead to the accumulation of undigested substrates (e.g., Gaucher’s, Tay-Sachs, and Niemann-Pick disease).

Question 197: Cyanide inhibits which of the following enzymes?

A. Pyruvate kinase
B. Cytochrome C oxidase (Correct Answer)
C. Enolase
D. None of the above

Explanation: **Explanation:** **Correct Answer: B. Cytochrome C oxidase** Cyanide is a potent inhibitor of the **Electron Transport Chain (ETC)**. It binds with high affinity to the ferric ($Fe^{3+}$) iron in the heme group of **Cytochrome oxidase (Complex IV)**. By blocking this final step, cyanide prevents the transfer of electrons to oxygen, halting ATP production and leading to cellular hypoxia despite adequate oxygen saturation in the blood. **Analysis of Incorrect Options:** * **A. Pyruvate kinase:** This is a key regulatory enzyme in **Glycolysis** (converting phosphoenolpyruvate to pyruvate). It is inhibited by ATP and alanine, but not by cyanide. * **C. Enolase:** This glycolytic enzyme converts 2-phosphoglycerate to phosphoenolpyruvate. It is classically inhibited by **Fluoride** (the basis for using grey-top vacutainers for blood glucose estimation). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Cyanide inhibits Complex IV ($a-a_3$). Other inhibitors of Complex IV include **Carbon Monoxide (CO)** and **Hydrogen Sulfide ($H_2S$)**. * **Clinical Presentation:** Patients present with "cherry-red" skin (due to high venous oxygen saturation as tissues cannot utilize $O_2$) and severe lactic acidosis. * **Antidote:** The treatment involves **Amyl nitrite/Sodium nitrite** (to induce methemoglobinemia, which sequesters cyanide) and **Sodium thiosulfate** (to convert cyanide to non-toxic thiocyanate via the enzyme rhodanese). **Hydroxocobalamin** is also a first-line treatment. * **Differentiation:** While Cyanide binds $Fe^{3+}$, Carbon Monoxide binds $Fe^{2+}$.

Question 198: In transaminases, Pyridoxal Phosphate (PLP) is covalently attached to which amino acid?

A. Glutamate
B. Lysine (Correct Answer)
C. Alanine
D. Threonine

Explanation: **Explanation:** In transamination reactions, **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6, serves as an essential coenzyme. The correct answer is **Lysine** because PLP is covalently bound to the enzyme (transaminase/aminotransferase) at its active site via a specific linkage called a **Schiff base** (aldimine linkage). This bond forms between the aldehyde group of PLP and the **$\epsilon$-amino group** of a specific Lysine residue within the enzyme. During the reaction, the substrate amino acid displaces this lysine to form a new Schiff base with PLP. **Analysis of Incorrect Options:** * **A. Glutamate:** While glutamate is a frequent product or reactant in transamination (acting as the universal amino group collector), it does not provide the covalent attachment site for the coenzyme. * **C. Alanine:** Alanine is a substrate for ALT (Alanine Transaminase) but does not participate in the structural binding of PLP to the enzyme. * **D. Threonine:** Threonine is one of the few amino acids (along with Lysine) that **does not** undergo transamination; it is instead deaminated by dehydratases. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin B6 Deficiency:** Leads to decreased transaminase activity, manifesting as convulsions (due to decreased GABA synthesis) and sideroblastic anemia. * **Non-transaminating Amino Acids:** Remember the mnemonic **"KLT"** (Lysine, Leucine, Threonine) — these do not participate in transamination. * **Diagnostic Markers:** AST (SGOT) and ALT (SGPT) are key biomarkers for liver and cardiac injury; both require PLP as a cofactor. * **Other PLP-dependent reactions:** Decarboxylation (e.g., Histidine to Histamine), Heme synthesis (ALA synthase), and Cystathionine synthesis.

Question 199: Zinc is an essential component of which of the following enzymes?

A. Carbonic anhydrase
B. Alkaline phosphatase
C. Xanthine oxidase (Correct Answer)
D. Alcohol dehydrogenase

Explanation: **Explanation:** The question focuses on the essential metal cofactors required for enzyme activity, a high-yield topic in Biochemistry. **Correct Answer: C. Xanthine Oxidase** Xanthine oxidase is a complex metalloenzyme that requires **Molybdenum (Mo)**, **Iron (Fe)**, and **Flavin (FAD)** for its catalytic activity. It plays a critical role in purine catabolism by converting hypoxanthine to xanthine and xanthine to uric acid. *(Note: There appears to be a discrepancy in the question stem provided; while Zinc is a common cofactor for many enzymes, Xanthine Oxidase specifically requires Molybdenum. In the context of standard NEET-PG patterns, if the question asks for Zinc, options A, B, and D are actually the correct associations, whereas C is the outlier).* **Analysis of Other Options (Zinc-containing Enzymes):** Zinc is a constituent of over 300 enzymes. The following are classic examples: * **A. Carbonic Anhydrase:** Contains Zinc at its active site; it is essential for the transport of $CO_2$ and acid-base balance. * **B. Alkaline Phosphatase (ALP):** A Zinc-metalloenzyme used clinically as a marker for cholestasis and bone turnover. * **D. Alcohol Dehydrogenase:** Requires Zinc to oxidize ethanol into acetaldehyde in the liver. **High-Yield Clinical Pearls for NEET-PG:** * **Molybdenum:** Remember the "Moly" connection—**Xanthine Oxidase** and **Sulfite Oxidase**. Deficiency leads to hyperoxanthinemia and urinary stones. * **Zinc Deficiency:** Presents as **Acrodermatitis enteropathica**, poor wound healing, hypogeusia (loss of taste), and growth retardation. * **Other Metal Cofactors:** * **Selenium:** Glutathione peroxidase. * **Copper:** Cytochrome c oxidase, Superoxide dismutase (cytosolic), and Tyrosinase. * **Manganese:** Pyruvate carboxylase.

Question 200: A person unable to digest carbohydrates will be deficient in which of the following enzymes?

A. Lipase
B. Amylase (Correct Answer)
C. Pepsin
D. Trypsin

Explanation: **Explanation:** **Correct Answer: B. Amylase** The digestion of carbohydrates involves the breakdown of complex polysaccharides (like starch and glycogen) into simpler disaccharides and monosaccharides. **Amylase** is the primary enzyme responsible for this process. It acts on $\alpha$-1,4-glycosidic bonds. There are two main types: **Salivary amylase (Ptyalin)**, which initiates digestion in the mouth, and **Pancreatic amylase**, which completes the process in the small intestine. A deficiency in amylase directly leads to carbohydrate malabsorption. **Analysis of Incorrect Options:** * **A. Lipase:** This enzyme is responsible for the hydrolysis of lipids (fats) into fatty acids and glycerol. It requires bile salts for effective emulsification. * **C. Pepsin:** Secreted as pepsinogen by the gastric chief cells, pepsin is a protease that initiates **protein digestion** in the acidic environment of the stomach. * **D. Trypsin:** A pancreatic protease secreted as trypsinogen. It plays a central role in **protein digestion** in the small intestine and also activates other pancreatic zymogens. **High-Yield Clinical Pearls for NEET-PG:** * **Digestion Site:** Carbohydrate digestion begins in the **mouth** (salivary amylase) but halts in the stomach due to the low pH inactivating amylase. It resumes in the duodenum. * **Final Products:** The final products of carbohydrate digestion are monosaccharides: **Glucose, Galactose, and Fructose**. * **Diagnostic Marker:** Serum amylase levels are a classic (though non-specific) marker for **Acute Pancreatitis**. * **Brush Border Enzymes:** Remember that disaccharides (Maltose, Lactose, Sucrose) are broken down by brush border enzymes (Maltase, Lactase, Sucrase) on the intestinal microvilli.

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Enzymes — MCQs

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