Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 401: Which of the following is a functional plasma enzyme?

A. Lipoprotein lipase (Correct Answer)
B. Lactate dehydrogenase (LDH)
C. Creatine phosphokinase (CPK)
D. Lipase

Explanation: ### Explanation Enzymes in the plasma are broadly classified into two categories: **Functional** and **Non-functional** plasma enzymes. **1. Why Lipoprotein Lipase (LPL) is the Correct Answer:** Functional plasma enzymes are those that are actively secreted into the blood by specific organs (usually the liver) and perform their primary physiological function within the circulation. **Lipoprotein lipase** is a classic example; it is synthesized by extrahepatic tissues and attached to the capillary endothelium. It acts on circulating chylomicrons and VLDL to hydrolyze triglycerides into free fatty acids and glycerol. Other examples include pseudocholinesterase and enzymes involved in blood coagulation. **2. Why the Other Options are Incorrect:** * **Lactate Dehydrogenase (LDH) & Creatine Phosphokinase (CPK):** These are **non-functional plasma enzymes**. They perform their metabolic roles exclusively *inside* cells. Their presence in the plasma in high concentrations is abnormal and indicates cell damage or necrosis (e.g., myocardial infarction or hepatitis). * **Lipase:** While secreted, pancreatic lipase is an **exocrine enzyme** intended for the digestive tract. Its presence in the blood is a diagnostic marker for pancreatic injury (e.g., acute pancreatitis) rather than a functional component of plasma. **3. High-Yield Clinical Pearls for NEET-PG:** * **Functional Enzymes:** Substrate is always present in the blood; concentration is higher in plasma than in tissues. * **Non-functional Enzymes:** Substrate is absent in the blood; concentration is much higher in tissues than in plasma. * **Heparin Connection:** Intravenous heparin releases LPL from the endothelial wall into the plasma, a phenomenon known as "post-heparin lipolytic activity." * **Diagnostic Value:** Non-functional enzymes are used as **biomarkers** for organ-specific damage (e.g., ALT for liver, CK-MB for heart).

Question 402: What is the cofactor of carbonic anhydrase?

A. Molybdenum
B. Zinc (Correct Answer)
C. Copper
D. Selenium

Explanation: **Explanation:** **Carbonic anhydrase** is a vital metalloenzyme that catalyzes the reversible hydration of carbon dioxide ($CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$). The correct answer is **Zinc ($Zn^{2+}$)** because it is an absolute requirement for the enzyme's catalytic activity. The Zinc ion is coordinated to three histidine residues at the active site, where it polarizes a bound water molecule to generate a nucleophilic hydroxide ion, facilitating the reaction with $CO_2$. **Analysis of Incorrect Options:** * **Molybdenum (A):** This is a cofactor for enzymes involved in redox reactions, such as **Xanthine oxidase** (purine metabolism), Sulfite oxidase, and Aldehyde oxidase. * **Copper (C):** Copper is a cofactor for enzymes like **Cytochrome c oxidase** (ETC), Superoxide dismutase (cytosolic), Tyrosinase, and Lysyl oxidase. * **Selenium (D):** This is a key component of **Glutathione peroxidase**, which protects cells from oxidative damage, and Deiodinase (thyroid hormone metabolism). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Location:** Carbonic anhydrase is found in high concentrations in RBCs (for $CO_2$ transport), gastric mucosa (HCL secretion), and renal tubules (acid-base balance). * **Inhibitors:** **Acetazolamide** is a potent inhibitor used clinically to treat glaucoma, altitude sickness, and as a weak diuretic. * **Speed:** It is one of the fastest known enzymes, with a turnover number ($K_{cat}$) of $10^6$ reactions per second. * **Other Zinc-containing enzymes:** Alcohol dehydrogenase, Carboxypeptidase, and DNA/RNA polymerases.

Question 403: Which of the following is NOT an allosteric inhibitor of the TCA cycle?

A. ADP (Correct Answer)
B. ATP
C. NADH
D. Succinyl CoA

Explanation: The TCA cycle is the central metabolic pathway for energy production. Its regulation is governed by the **energy status** of the cell. ### **Why ADP is the Correct Answer** ADP (Adenosine Diphosphate) signifies a **low-energy state**. When ADP levels are high, the cell needs more ATP; therefore, ADP acts as an **allosteric activator**, not an inhibitor. It specifically stimulates **Isocitrate Dehydrogenase**, the rate-limiting enzyme of the TCA cycle, to increase the flow of substrates through the pathway. ### **Why the Other Options are Incorrect** These molecules signify a **high-energy state** or feedback inhibition, thus they act as inhibitors: * **ATP:** High levels signal that the cell has sufficient energy. ATP allosterically inhibits Citrate Synthase and Isocitrate Dehydrogenase. * **NADH:** A high NADH/NAD+ ratio indicates an abundance of reducing equivalents. NADH inhibits Isocitrate Dehydrogenase and α-Ketoglutarate Dehydrogenase. * **Succinyl CoA:** This is an example of **product inhibition**. It competes with Acetyl CoA at Citrate Synthase and inhibits α-Ketoglutarate Dehydrogenase. ### **High-Yield NEET-PG Pearls** * **Rate-limiting enzyme:** Isocitrate Dehydrogenase. * **Key Regulatory Steps:** 1. Citrate Synthase 2. Isocitrate Dehydrogenase 3. α-Ketoglutarate Dehydrogenase * **Calcium (Ca²⁺):** In muscle cells, Ca²⁺ acts as an important **activator** of the TCA cycle (specifically Isocitrate DH and α-Ketoglutarate DH) to link muscle contraction with energy production. * **Fluoroacetate:** A potent inhibitor of the TCA cycle that inhibits the enzyme **Aconitase**.

Question 404: Which enzyme is a NAD+ linked dehydrogenase?

A. Pyruvate dehydrogenase (PDH) (Correct Answer)
B. Glucose-6-phosphate dehydrogenase (G6PD)
C. Flavin adenine dinucleotide (FAD)
D. Flavin mononucleotide (FMN)

Explanation: **Explanation:** **1. Why Pyruvate Dehydrogenase (PDH) is Correct:** The Pyruvate Dehydrogenase Complex (PDH) is a multi-enzyme system that catalyzes the oxidative decarboxylation of pyruvate to Acetyl-CoA. This reaction is a critical link between glycolysis and the TCA cycle. The PDH complex requires five cofactors: **Thiamine pyrophosphate (TPP), Lipoic acid, CoA, FAD, and NAD+**. Specifically, the E3 component (Dihydrolipoyl dehydrogenase) utilizes NAD+ as the final electron acceptor, reducing it to **NADH + H+**. Thus, it is a classic NAD+-linked dehydrogenase. **2. Analysis of Incorrect Options:** * **Glucose-6-phosphate dehydrogenase (G6PD):** This is the rate-limiting enzyme of the Hexose Monophosphate (HMP) Shunt. It is **NADP+-linked**, not NAD+-linked. It reduces NADP+ to **NADPH**, which is essential for reductive biosynthesis and maintaining reduced glutathione. * **Flavin adenine dinucleotide (FAD) & Flavin mononucleotide (FMN):** These are not enzymes; they are **coenzymes** (prosthetic groups) derived from Vitamin B2 (Riboflavin). While they are involved in redox reactions, they are the "tools" used by enzymes, not the dehydrogenases themselves. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for PDH Cofactors:** "**T**ender **L**oving **C**are **F**or **N**ancy" (TPP, Lipoic acid, CoA, FAD, NAD). * **Clinical Correlation:** PDH deficiency is the most common cause of congenital lactic acidosis. * **Inhibitors:** PDH is inhibited by **Arsenite**, which binds to the -SH groups of Lipoic acid. * **Key Distinction:** Generally, NAD+ is used in **catabolic** pathways (energy generation), while NADP+ is used in **anabolic** pathways (synthesis).

Question 405: Malonate is a competitive inhibitor of succinate dehydrogenase, a key enzyme in the Krebs tricarboxylic acid cycle. How does the presence of malonate affect the kinetic parameters of succinate dehydrogenase?

A. Increases the apparent Km but does not affect Vmax. (Correct Answer)
B. Decreases the apparent Km but does not affect Vmax.
C. Decreases Vmax but does not affect the apparent Km.
D. Increases Vmax but does not affect the apparent Km.

Explanation: ### Explanation **1. Why Option A is Correct (The Mechanism)** Malonate acts as a **competitive inhibitor** because its chemical structure closely resembles succinate (the natural substrate). In competitive inhibition, the inhibitor and substrate compete for the same **active site** on the enzyme. * **Effect on $K_m$:** Because malonate occupies the active site, a higher concentration of succinate is required to achieve half-maximal velocity ($1/2 V_{max}$). This results in an **increase in the apparent $K_m$** (decreased affinity). * **Effect on $V_{max}$:** Competitive inhibition can be overcome by increasing the substrate concentration. At infinitely high succinate levels, the inhibitor is displaced, allowing the enzyme to reach its original maximum velocity. Thus, **$V_{max}$ remains unchanged**. **2. Why Other Options are Wrong** * **Option B:** Decreased $K_m$ implies increased affinity, which never occurs with inhibitors. * **Option C:** This describes **Non-competitive inhibition**. In this case, the inhibitor binds to an allosteric site, reducing the total catalytic activity ($V_{max}$ decreases) regardless of substrate concentration, while $K_m$ remains constant. * **Option D:** No known inhibitor increases $V_{max}$; this would describe an enzyme activator. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Lineweaver-Burk Plot:** In competitive inhibition, the plots intersect at the **Y-axis** ($1/V_{max}$ is constant). * **Classic Example:** The use of **Statins** (HMG-CoA reductase inhibitors) to treat hypercholesterolemia is a clinically vital example of competitive inhibition. * **Ethylene Glycol Poisoning:** Ethanol acts as a competitive inhibitor of Alcohol Dehydrogenase, preventing the formation of toxic metabolites. * **Methanol Poisoning:** Fomepizole is the competitive inhibitor used as an antidote.

Question 406: Which enzymes are responsible for the synthesis of extracellular glucans and fructans?

A. Glucosyltransferase, fructosyltransferase (Correct Answer)
B. Glucosylconvertase, fructosylconvertase
C. Convertase
D. Both of the above

Explanation: ### Explanation **1. Why Option A is Correct:** The synthesis of extracellular polysaccharides, specifically **glucans** (like dextran) and **fructans** (like levan), is catalyzed by the enzymes **glucosyltransferase** and **fructosyltransferase**, respectively. These enzymes are primarily produced by oral bacteria, most notably *Streptococcus mutans*. The underlying mechanism involves the cleavage of **sucrose** (a disaccharide). These enzymes use the energy released from breaking the glycosidic bond of sucrose to transfer a monosaccharide unit to a growing polymer chain: * **Glucosyltransferase:** Transfers glucose from sucrose to form glucans. * **Fructosyltransferase:** Transfers fructose from sucrose to form fructans. **2. Why Other Options are Incorrect:** * **Options B & C:** The terms "Glucosylconvertase" and "Fructosylconvertase" are not standard biochemical nomenclature for these biosynthetic pathways. While "convertase" is a general term for enzymes that convert one substance to another (e.g., Proprotein convertase), it does not describe the specific transferase activity required for polysaccharide synthesis from sucrose. * **Option D:** Since B and C are incorrect, "Both of the above" is invalid. **3. Clinical Pearls for NEET-PG:** * **Dental Caries:** This is the most high-yield clinical correlation. *Streptococcus mutans* uses these extracellular glucans to form a sticky **biofilm (dental plaque)**, allowing bacteria to adhere to tooth enamel. * **Substrate Specificity:** Sucrose is the *only* sugar that can be used as a substrate for these specific extracellular enzymes, which is why high sucrose intake is directly linked to tooth decay. * **Lactic Acid:** Once the biofilm is formed, bacteria ferment other carbohydrates into lactic acid, which demineralizes the enamel.

Question 407: Which of the following enzymes is stable at acidic pH?

A. Pepsin (Correct Answer)
B. Trypsin
C. Chymotrypsin
D. Carboxypeptidase

Explanation: **Explanation:** The correct answer is **Pepsin**. **1. Why Pepsin is Correct:** Pepsin is the primary proteolytic enzyme of the stomach. It is secreted by the **Chief cells** as an inactive zymogen, pepsinogen. It requires a highly acidic environment (optimal pH **1.5 to 2.5**) for two reasons: first, to undergo autocatalytic cleavage from pepsinogen to active pepsin, and second, to maintain its tertiary structure for catalytic activity. At a pH above 5.0, pepsin becomes denatured and irreversibly inactivated. **2. Why the Other Options are Incorrect:** * **Trypsin, Chymotrypsin, and Carboxypeptidase:** These are all **pancreatic enzymes** secreted into the duodenum. The pancreas secretes bicarbonate to neutralize gastric acid, creating an alkaline environment (optimal pH **7.5 to 8.5**). These enzymes are stable and active only at this alkaline pH; they would be rapidly denatured in the acidic environment of the stomach. **3. NEET-PG High-Yield Clinical Pearls:** * **Zymogen Activation:** Pepsinogen is activated by **HCl** (secreted by Parietal cells). Once some pepsin is formed, it further activates pepsinogen via **positive feedback** (autocatalysis). * **Specificity:** Pepsin is an endopeptidase that prefers cleaving peptide bonds involving aromatic amino acids (Phenylalanine, Tyrosine, Tryptophan). * **Achlorhydria:** In conditions like Pernicious Anemia (where parietal cells are destroyed), the lack of HCl leads to a failure in pepsin activation, severely impairing protein digestion in the stomach. * **pH Memory:** Remember: **Stomach = Acidic (Pepsin)**; **Small Intestine = Alkaline (Trypsin/Chymotrypsin).**

Question 408: Which of the following is a multi-enzyme complex?

A. HMG-CoA synthase
B. Malic enzyme
C. Fatty acid synthase (Correct Answer)
D. Fatty acid oxidase

Explanation: ### Explanation **Correct Answer: C. Fatty acid synthase** **Why it is correct:** A **multi-enzyme complex** is a stable assembly of multiple enzymes that catalyze sequential steps in a metabolic pathway. In eukaryotes (including humans), **Fatty Acid Synthase (FAS)** is a classic example. It is a **homodimer**, where each monomer is a single large polypeptide chain containing **seven different catalytic activities** and an **Acyl Carrier Protein (ACP)** domain. This structural arrangement allows for "substrate channeling," where the growing fatty acid chain remains covalently attached to the complex, increasing catalytic efficiency and preventing the loss of intermediates. **Why the other options are incorrect:** * **A. HMG-CoA synthase:** This is a single enzyme involved in ketogenesis (mitochondria) and cholesterol synthesis (cytoplasm). It is not a multi-enzyme complex. * **B. Malic enzyme:** This is a single oxidative decarboxylase that converts malate to pyruvate, generating NADPH. * **C. Fatty acid oxidase:** This refers to the enzymes of **β-oxidation**. Unlike fatty acid synthesis, the enzymes for β-oxidation are **independent, soluble proteins** located in the mitochondrial matrix (except for the trifunctional protein in the inner membrane, but the system as a whole is not classified as a single multi-enzyme complex like FAS). **High-Yield Clinical Pearls for NEET-PG:** * **Other Multi-enzyme Complexes:** Pyruvate Dehydrogenase (PDH), α-Ketoglutarate Dehydrogenase, and Branched-chain α-keto acid Dehydrogenase. * **FAS End-product:** The primary product of the FAS complex is **Palmitate (C16)**. * **Requirement:** FAS requires **NADPH** as a reducing equivalent, primarily sourced from the Pentose Phosphate Pathway (HMP Shunt). * **Prokaryotic Difference:** In *E. coli* (Type II FAS), the enzymes are separate individual proteins, unlike the integrated Type I system in humans.

Question 409: Which of the following enzymes does not participate in a ping-pong reaction?

A. Aminotransferases
B. Serine proteases
C. Pyruvate carboxylase
D. None of the above (Correct Answer)

Explanation: **Explanation:** The **Ping-Pong (Double-Displacement) mechanism** is a characteristic of multi-substrate enzyme reactions where the first substrate binds and releases a product before the second substrate binds. This process typically involves the formation of a covalently modified **enzyme intermediate** ($E^*$). 1. **Why "None of the above" is correct:** All three enzymes listed (Aminotransferases, Serine proteases, and Pyruvate carboxylase) utilize a Ping-Pong mechanism. Therefore, there is no enzyme in the list that *does not* participate in such a reaction. 2. **Analysis of Options:** * **Aminotransferases (e.g., ALT, AST):** These are the classic examples. The first substrate (amino acid) transfers its amino group to the prosthetic group **Pyridoxal Phosphate (PLP)**, releasing a keto acid and leaving the enzyme as Pyridoxamine Phosphate. The second substrate (keto acid) then binds to pick up the amino group. * **Serine Proteases (e.g., Chymotrypsin, Trypsin):** These involve a "catalytic triad." A peptide bond is cleaved, the first product is released, and an **acyl-enzyme intermediate** is formed. Water then enters as the second substrate to hydrolyze this intermediate. * **Pyruvate Carboxylase:** This ABC enzyme (ATP, Biotin, $CO_2$) uses **Biotin** as a carrier. Biotin is carboxylated (Phase 1), and then the carboxyl group is transferred to pyruvate to form oxaloacetate (Phase 2). **High-Yield Clinical Pearls for NEET-PG:** * **Key Feature:** Ping-pong reactions never form a **ternary complex** (where both substrates are bound to the enzyme simultaneously). * **Lineweaver-Burk Plot:** These reactions produce **parallel lines** when the concentration of the second substrate is varied. * **Common Co-factors:** Enzymes using **PLP, Biotin, or Thiamine Pyrophosphate (TPP)** often follow Ping-Pong kinetics.

Question 410: Zinc is essential for which of the following enzymes?

A. Pyruvate kinase
B. Cytochrome oxidase
C. Xanthine oxidase
D. Carbonic anhydrase (Correct Answer)

Explanation: **Explanation:** **1. Why Carbonic Anhydrase is Correct:** Carbonic anhydrase is a classic example of a **metalloenzyme** where Zinc ($Zn^{2+}$) is an essential structural and functional component. The zinc ion is coordinated to three histidine residues and a water molecule at the enzyme's active site. It facilitates the rapid interconversion of carbon dioxide and water into bicarbonate and protons ($CO_2 + H_2O \rightleftharpoons HCO_3^- + H^+$), a process vital for acid-base balance, respiration, and renal function. **2. Analysis of Incorrect Options:** * **Pyruvate Kinase:** This glycolytic enzyme requires **Potassium ($K^+$)** as a monovalent activator and **Magnesium ($Mg^{2+}$)** or Manganese ($Mn^{2+}$) as divalent activators. * **Cytochrome Oxidase:** This is the terminal enzyme of the Electron Transport Chain (Complex IV). It contains **Copper ($Cu^{2+}$)** and **Iron ($Fe^{2+}/Fe^{3+}$)** in its heme groups. * **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 Zinc-containing enzymes:** Alcohol dehydrogenase, Carboxypeptidase, DNA/RNA Polymerase, Alkaline Phosphatase (ALP), and Superoxide Dismutase (cytosolic form). * **Zinc Finger Motifs:** Zinc is crucial for the structural stability of many transcription factors. * **Clinical Correlation:** Zinc deficiency leads to **Acrodermatitis Enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. It also causes impaired wound healing and hypogeusia (decreased taste). * **Mnemonic for Zinc Enzymes:** "**Z**inc **C**an **A**lways **A**id **P**olymerase" (**Z**inc: **C**arbonic anhydrase, **A**LP/Alcohol dehydrogenase, **A**CE, **P**olymerase).

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Enzyme Classification and Nomenclature

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Enzyme Kinetics and Michaelis-Menten Equation

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Enzyme Inhibition: Competitive and Non-competitive

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Allosteric Regulation

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Coenzymes and Cofactors

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Isoenzymes and Clinical Significance

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Enzyme Regulation: Covalent Modification

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Enzyme Regulation: Zymogen Activation

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Enzyme Induction and Repression

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Ribozymes and Catalytic RNA

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Enzyme Diagnostic Applications

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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