Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 101: Ferrochelatase is inhibited by which of the following?

A. Arsenic
B. Lead (Correct Answer)
C. Chromium
D. Mercury

Explanation: ### Explanation **Correct Option: B (Lead)** **Mechanism of Action:** Ferrochelatase is the final enzyme in the heme synthesis pathway, located in the mitochondria. It catalyzes the insertion of ferrous iron ($Fe^{2+}$) into Protoporphyrin IX to form Heme. Lead ($Pb$) is a potent inhibitor of this enzyme because it competes with iron for the binding site. Additionally, lead inhibits **$\delta$-aminolevulinic acid dehydratase (ALAD)**. The inhibition of these two enzymes leads to the accumulation of Protoporphyrin IX and $\delta$-ALA, which are hallmark biochemical findings in lead poisoning. **Analysis of Incorrect Options:** * **A. Arsenic:** Arsenic primarily inhibits enzymes requiring **Lipoic acid** as a cofactor, such as the Pyruvate Dehydrogenase (PDH) complex and $\alpha$-ketoglutarate dehydrogenase, by binding to sulfhydryl groups. * **C. Chromium:** While hexavalent chromium is toxic and carcinogenic, it does not specifically target the heme synthesis pathway or ferrochelatase. * **D. Mercury:** Mercury binds to various sulfhydryl-containing enzymes and proteins, causing systemic toxicity (especially neurological and renal), but it is not the classic inhibitor associated with ferrochelatase in medical curricula. **High-Yield Clinical Pearls for NEET-PG:** * **Microcytic Hypochromic Anemia:** Lead poisoning causes anemia due to decreased heme synthesis. * **Basophilic Stippling:** A classic peripheral smear finding in lead poisoning due to the inhibition of pyrimidine 5'-nucleotidase, leading to RNA degradation products. * **Diagnostic Markers:** Elevated **Zinc Protoporphyrin (ZPP)** levels are seen because zinc is incorporated into the porphyrin ring when iron cannot be used. * **Clinical Presentation:** Look for "Burton’s lines" (bluish-purple gingival margin), colicky abdominal pain, and wrist/foot drop in adults.

Question 102: Activity of which of the following enzymes is NOT affected by insulin?

A. Pyruvate kinase
B. Glycogen synthase
C. Hexokinase (Correct Answer)
D. Glucokinase

Explanation: ### Explanation The correct answer is **Hexokinase (Option C)**. **1. Why Hexokinase is the correct answer:** Insulin is an anabolic hormone that regulates glucose metabolism primarily through **transcriptional regulation** (inducing/repressing enzyme synthesis) and **covalent modification** (dephosphorylation). **Hexokinase (specifically Type I, II, and III)** is a constitutive enzyme found in most extrahepatic tissues. It has a very low $K_m$ (high affinity) for glucose, allowing it to function at maximum velocity even during fasting. Its activity is primarily regulated by **allosteric inhibition** by its product, **Glucose-6-Phosphate**, and it is **not** induced or activated by insulin. **2. Why the other options are incorrect:** * **Glucokinase (Hexokinase IV):** Unlike other hexokinases, Glucokinase (found in liver and pancreatic $\beta$-cells) is **induced by insulin** at the transcriptional level. It has a high $K_m$ and is not inhibited by G6P. * **Pyruvate Kinase:** Insulin promotes the **dephosphorylation** (activation) of the L-type Pyruvate Kinase in the liver via protein phosphatase-1. It also induces its synthesis to promote glycolysis. * **Glycogen Synthase:** This is the rate-limiting enzyme of glycogenesis. Insulin triggers a signaling cascade that inactivates Glycogen Synthase Kinase-3 (GSK3) and activates protein phosphatase-1, keeping Glycogen Synthase in its **active (dephosphorylated) state**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Insulin **dephosphorylates** to activate (Exceptions: Glycogen phosphorylase and Hormone-sensitive lipase are *inactivated* by dephosphorylation). * **Glucokinase vs. Hexokinase:** Glucokinase acts as a "glucose sensor" in the pancreas; mutations in the Glucokinase gene lead to **MODY type 2** (Maturity-Onset Diabetes of the Young). * **Key Insulin-Induced Enzymes:** Glucokinase, PFK-1, Pyruvate Kinase, Acetyl-CoA Carboxylase, and HMG-CoA Reductase.

Question 103: Which of the following is an allosteric stimulator of glycogen phosphorylase?

A. ATP
B. AMP (Correct Answer)
C. Insulin
D. Glucose-6-phosphate

Explanation: **Explanation:** The regulation of **Glycogen Phosphorylase**, the rate-limiting enzyme of glycogenolysis, is a high-yield topic for NEET-PG. This enzyme exists in two forms: *Phosphorylase a* (active, phosphorylated) and *Phosphorylase b* (inactive, dephosphorylated). **Why AMP is the Correct Answer:** In muscle tissue, **AMP** acts as a potent **allosteric activator**. A high concentration of AMP signifies a low-energy state in the cell. AMP binds to glycogen phosphorylase *b*, inducing a conformational change that activates the enzyme even without covalent phosphorylation. This ensures that glycogen is broken down rapidly to provide glucose for ATP production during vigorous exercise. **Analysis of Incorrect Options:** * **ATP (Option A):** High levels of ATP indicate an energy-rich state. ATP competes with AMP for the same allosteric site, acting as an **inhibitor** to prevent unnecessary glycogen breakdown. * **Insulin (Option C):** Insulin is a hormone, not an allosteric modulator. It regulates the enzyme **hormonally** by promoting dephosphorylation (via Protein Phosphatase-1), which converts the active form to the inactive *phosphorylase b*. * **Glucose-6-phosphate (Option D):** This is a downstream product of glycogenolysis. High levels signal that the cell's energy needs are met; thus, it acts as an **allosteric inhibitor**. **High-Yield Clinical Pearls for NEET-PG:** * **Muscle vs. Liver:** In the **liver**, glycogen phosphorylase is primarily inhibited by **Free Glucose**, which acts as a sensor for blood sugar levels. AMP does *not* significantly stimulate the liver isoform. * **Calcium Regulation:** During muscle contraction, **Ca²⁺** binds to the calmodulin subunit of Phosphorylase Kinase, activating it and further stimulating glycogenolysis. * **McArdle Disease (GSD Type V):** Caused by a deficiency of skeletal muscle glycogen phosphorylase, leading to exercise intolerance and "second wind" phenomenon.

Question 104: Which of the following acts as a free radical scavenger?

A. Peroxidase
B. Catalase (Correct Answer)
C. Dehydrogenase
D. All of the above

Explanation: **Explanation:** Free radicals and Reactive Oxygen Species (ROS) are highly reactive molecules that can cause cellular damage. To counter this, the body utilizes **antioxidant enzymes** (scavengers) to neutralize these species. **Why Catalase is Correct:** Catalase is a crucial heme-containing antioxidant enzyme primarily located in **peroxisomes**. It specifically targets **hydrogen peroxide ($H_2O_2$)**, a potent ROS, and converts it into water and molecular oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$). By rapidly decomposing $H_2O_2$, it prevents the formation of the highly toxic hydroxyl radical ($\cdot OH$) via the Fenton reaction. **Analysis of Incorrect Options:** * **Peroxidase:** While some specific peroxidases (like Glutathione Peroxidase) act as scavengers, "Peroxidase" as a general class is often involved in synthetic pathways or using $H_2O_2$ to oxidize other substrates rather than primarily acting as a universal scavenger in the same capacity as Catalase or SOD. In the context of standard medical exams, Catalase is the definitive "scavenger" enzyme for $H_2O_2$. * **Dehydrogenase:** These enzymes catalyze oxidation-reduction reactions by transferring hydrogen atoms from a substrate to an electron acceptor (like $NAD^+$ or $FAD$). They are central to metabolic pathways (e.g., Glycolysis, TCA cycle) but do not function as free radical scavengers. **High-Yield Clinical Pearls for NEET-PG:** * **Superoxide Dismutase (SOD):** Converts Superoxide ($\cdot O_2^-$) to $H_2O_2$. It is the "first line" of enzymatic defense. * **Glutathione Peroxidase:** Requires **Selenium** as a cofactor; it neutralizes $H_2O_2$ while converting reduced glutathione (GSH) to oxidized glutathione (GSSG). * **Acatalasia:** A rare genetic deficiency of catalase leading to oral ulcers and gangrene. * **Vitamin Scavengers:** Remember **ACE** (Vitamin A, C, and E) as the primary non-enzymatic antioxidants.

Question 105: Which enzyme contains zinc as a prosthetic group?

A. Carbonic anhydrase
B. Carboxypeptidase
C. Lactate dehydrogenase
D. All of these (Correct Answer)

Explanation: **Explanation:** Zinc is one of the most common trace elements acting as a **metalloenzyme cofactor**. It functions as a Lewis acid, stabilizing negative charges or activating water molecules during catalysis. 1. **Carbonic Anhydrase:** This is the classic example of a zinc-containing enzyme. The $Zn^{2+}$ ion is coordinated to three histidine residues and a water molecule, facilitating the rapid interconversion of $CO_2$ and water into bicarbonate and $H^+$. 2. **Carboxypeptidase:** This digestive protease (secreted by the pancreas) utilizes zinc to coordinate the carbonyl group of the peptide bond, making it more susceptible to nucleophilic attack during hydrolysis. 3. **Lactate Dehydrogenase (LDH):** While LDH is primarily known for its NAD+ dependency, it is a metalloenzyme that requires zinc for structural stability and catalytic activity during the conversion of pyruvate to lactate. **Why "All of these" is correct:** All three enzymes listed are metalloenzymes that require Zinc ($Zn^{2+}$) for their functional or structural integrity. Other notable zinc-containing enzymes include **Alcohol Dehydrogenase**, **Alkaline Phosphatase**, and **RNA Polymerase**. **High-Yield Clinical Pearls for NEET-PG:** * **Zinc Finger Motifs:** Zinc is essential for the structure of "zinc fingers," which are the most common DNA-binding motifs in transcription factors (e.g., Steroid hormone receptors). * **Acrodermatitis Enteropathica:** An autosomal recessive disorder caused by impaired intestinal zinc absorption, characterized by periorificial dermatitis, alopecia, and diarrhea. * **Wound Healing:** Zinc is a cofactor for **Collagenase** (Matrix Metalloproteinases), making it vital for tissue repair and remodeling.

Question 106: When Km changes and Vmax remains the same, what is the type of enzyme inhibition?

A. Competitive inhibition (Correct Answer)
B. Non-competitive inhibition
C. Uncompetitive inhibition
D. Suicide inhibition

Explanation: In enzyme kinetics, the relationship between substrate concentration and reaction velocity is defined by the Michaelis-Menten equation. ### **Why Competitive Inhibition is Correct** In **competitive inhibition**, the inhibitor structurally resembles the substrate and competes for the same **active site** on the enzyme. * **Vmax remains the same:** If substrate concentration is increased to a very high level, it eventually outcompetes the inhibitor, allowing the enzyme to reach its maximum velocity. * **Km increases:** Because the inhibitor interferes with substrate binding, a higher concentration of substrate is required to reach half of the Vmax ($1/2 Vmax$). On a Lineweaver-Burk plot, this is seen as the lines intersecting at the Y-axis. ### **Why Other Options are Incorrect** * **Non-competitive inhibition:** The inhibitor binds to an allosteric site. This decreases the functional enzyme concentration, leading to a **decreased Vmax**, while the **Km remains unchanged** (affinity is unaffected). * **Uncompetitive inhibition:** The inhibitor binds only to the enzyme-substrate (ES) complex. This results in a **decrease in both Vmax and Km**, typically showing parallel lines on a Lineweaver-Burk plot. * **Suicide inhibition:** This is a form of irreversible inhibition where the enzyme converts the inhibitor into a reactive form that covalently binds to the active site (e.g., Aspirin, Allopurinol). ### **High-Yield NEET-PG Pearls** * **Mnemonic:** **C**ompetitive = **C**ommon Vmax (Vmax stays same). * **Statins** (HMG-CoA reductase inhibitors) and **Methanol poisoning treatment** (Ethanol/Fomepizole) are classic clinical examples of competitive inhibition. * **Lineweaver-Burk Plot:** In competitive inhibition, the X-intercept moves closer to zero (Km increases), but the Y-intercept remains constant ($1/Vmax$).

Question 107: Deficiency of glucose-6-phosphate dehydrogenase may cause which of the following?

A. Diabetes mellitus
B. Haemolytic anemia (Correct Answer)
C. Wernicke-Korsakoff syndrome
D. Porphyria

Explanation: **Explanation:** **Glucose-6-Phosphate Dehydrogenase (G6PD)** is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary role is the production of **NADPH**. 1. **Why Haemolytic Anemia is correct:** In red blood cells (RBCs), NADPH is essential to maintain a pool of **reduced glutathione**. Reduced glutathione acts as an antioxidant that neutralizes Reactive Oxygen Species (ROS) like hydrogen peroxide. In G6PD deficiency, NADPH levels drop, leading to the accumulation of oxidized glutathione. This causes oxidative damage to hemoglobin, which denatures and precipitates as **Heinz bodies**. These damaged RBCs are destroyed in the spleen (hemolysis), especially after exposure to oxidative stress (e.g., Fava beans, Primaquine, or infections). 2. **Why other options are incorrect:** * **Diabetes Mellitus:** Caused by insulin deficiency or resistance, not HMP shunt defects. * **Wernicke-Korsakoff Syndrome:** Caused by **Thiamine (Vitamin B1)** deficiency, affecting enzymes like Transketolase and Pyruvate Dehydrogenase. * **Porphyria:** A group of disorders caused by defects in the **Heme synthesis pathway** (e.g., deficiency of Ferrochelatase or PBG deaminase). **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is an **X-linked recessive** disorder. * **Morphology:** Look for **Heinz bodies** (supravital stain) and **Bite cells** (degmacytes) on a peripheral smear. * **Protective Effect:** G6PD deficiency provides a selective advantage against *Plasmodium falciparum* malaria. * **Contraindicated Drugs:** Sulfa drugs, Antimalarials (Primaquine), and Nitrofurantoin.

Question 108: Fischer's lock and key theory postulates that:

A. The active site of an enzyme exists in a proper conformation to the substrate molecule even in the absence of the substrate. (Correct Answer)
B. The active site of an enzyme undergoes conformational changes to fit the substrate during binding.
C. There is absolute specificity in the reaction between an enzyme and a substrate.
D. Enzymes possess catalytic power and accelerate reactions by reducing the energy of activation.

Explanation: ### Explanation **1. Why Option A is Correct:** Emil Fischer’s **Lock and Key Theory (1894)** proposes that the enzyme’s active site is a **rigid, pre-formed structure**. Much like a specific key fits into a specific lock, the substrate’s geometry is perfectly complementary to the active site **even before binding occurs**. This model emphasizes that the enzyme does not change its shape to accommodate the substrate; the conformation is static and predetermined. **2. Analysis of Incorrect Options:** * **Option B:** This describes the **Induced Fit Theory** proposed by Daniel Koshland. It suggests that the active site is flexible and undergoes conformational changes *only after* the substrate binds to achieve a perfect fit. This is the more modern and widely accepted model. * **Option C:** While the Lock and Key model implies high specificity, "absolute specificity" is a property of certain enzymes (like urease), but it is not the *postulate* or defining mechanism of Fischer’s theory itself. * **Option D:** This is a general characteristic of all enzymes (lowering activation energy), but it does not describe the specific structural interaction defined by the Lock and Key model. **3. High-Yield Clinical Pearls for NEET-PG:** * **Key Distinction:** Lock and Key = **Rigid/Static** active site; Induced Fit = **Flexible/Dynamic** active site. * **Transition State:** Modern biochemistry (and Koshland's model) suggests enzymes are actually most complementary to the **transition state** of the substrate, rather than the ground-state substrate itself. * **Enzyme Specificity Types:** * *Stereospecificity:* Enzyme acts only on one isomer (e.g., L-amino acid oxidase). * *Group Specificity:* Enzyme acts on a specific chemical group (e.g., Hexokinase acts on various hexoses). * **Thermodynamics:** Remember that enzymes **do not** alter the equilibrium constant ($K_{eq}$) or the $\Delta G$ of a reaction; they only increase the *rate* by lowering activation energy.

Question 109: Which of the following is NOT a component enzyme of the Pyruvate Dehydrogenase complex?

A. Pyruvate Dehydrogenase
B. Dihydrolipoyl Transacetylase
C. Dihydrolipoyl Dehydrogenase
D. Pyruvate Decarboxylase (Correct Answer)

Explanation: **Explanation:** The **Pyruvate Dehydrogenase Complex (PDH)** is a multi-enzyme mitochondrial complex that catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. **Why Option D is correct:** **Pyruvate Decarboxylase** is an enzyme involved in **ethanol fermentation** (found in yeast and some bacteria), which converts pyruvate into acetaldehyde. It is **not** part of the human PDH complex. While the first subunit of PDH (E1) does perform decarboxylation, its formal name is Pyruvate Dehydrogenase or Pyruvate Decarboxylase *component*, but "Pyruvate Decarboxylase" as a standalone term refers to the fermentation enzyme. **Why the other options are incorrect:** The PDH complex consists of three distinct catalytic subunits: * **Option A (E1): Pyruvate Dehydrogenase.** It uses Thiamine Pyrophosphate (TPP) as a cofactor to decarboxylate pyruvate. * **Option B (E2): Dihydrolipoyl Transacetylase.** It transfers the acetyl group to Coenzyme A and utilizes Lipoic acid. * **Option C (E3): Dihydrolipoyl Dehydrogenase.** It regenerates the oxidized form of lipoic acid using FAD and NAD+. **High-Yield Clinical Pearls for NEET-PG:** * **Cofactors:** PDH requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2/FAD), **N**iacin (B3/NAD), **P**antothenic acid (B5/CoA), and **L**ipoic acid (Mnemonic: **T**ender **R**eversed **N**eck **P**ads **L**oose). * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and garlic breath. * **Deficiency:** PDH deficiency is a common cause of congenital lactic acidosis; management involves a **ketogenic diet** (high fat, low carb).

Question 110: What are enzymes that catalyze the same reaction but differ in physical properties called?

A. Proenzyme
B. Isoenzyme (Correct Answer)
C. Coenzyme
D. Holoenzyme

Explanation: **Explanation:** **Correct Answer: B. Isoenzyme** Isoenzymes (or isozymes) are multiple forms of an enzyme that catalyze the **same chemical reaction** but differ in their **physical and chemical properties**, such as amino acid sequence, electrophoretic mobility, kinetic parameters ($K_m$ and $V_{max}$), and regulatory properties. They are often synthesized from different genes or through alternative splicing and are frequently tissue-specific. **Analysis of Incorrect Options:** * **A. Proenzyme (Zymogen):** This is an inactive precursor of an enzyme that requires biochemical change (like proteolytic cleavage) to become active (e.g., Pepsinogen to Pepsin). * **C. Coenzyme:** These are non-protein, low-molecular-weight organic molecules (often derived from vitamins) that bind to enzymes and are essential for their catalytic activity (e.g., NAD+, FAD). * **D. Holoenzyme:** This refers to the complete, catalytically active enzyme system consisting of the protein portion (Apoenzyme) and its required non-protein component (Cofactor/Coenzyme). **High-Yield Clinical Pearls for NEET-PG:** * **Lactate Dehydrogenase (LDH):** A tetramer with 5 isoenzymes. **LDH-1** (HHHH) is predominant in the heart, while **LDH-5** (MMMM) is found in the liver and skeletal muscle. * **Creatine Kinase (CK):** A dimer with 3 isoenzymes. **CK-MB** is a specific marker for Myocardial Infarction; **CK-MM** is found in skeletal muscle; **CK-BB** is found in the brain. * **Alkaline Phosphatase (ALP):** Isoenzymes help differentiate the source of pathology (e.g., **Regan isoenzyme** is a marker for certain cancers; Bone vs. Liver ALP).

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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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