Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 431: The Michaelis Menten hypothesis states that:

A. The rate of an enzymatic reaction is independent of substrate concentration.
B. The rate of a non-enzymatic reaction is proportional to the substrate concentration.
C. Km represents the enzyme-substrate complex dissociation constant.
D. The formation of an enzyme-substrate complex is essential in enzymatic reactions. (Correct Answer)

Explanation: ### Explanation The **Michaelis-Menten hypothesis** is a fundamental model of enzyme kinetics. Its core postulate is that an enzyme (E) must physically combine with its substrate (S) to form a transient **Enzyme-Substrate (ES) complex** before the product (P) can be formed and the enzyme released. This interaction occurs at the active site and is the rate-limiting step that dictates the velocity of the reaction. **Analysis of Options:** * **Option D (Correct):** The formation of the **ES complex** is the mandatory first step in the Michaelis-Menten model ($E + S \rightleftharpoons ES \rightarrow E + P$). Without this complex, catalysis cannot occur. * **Option A:** Incorrect. The rate is highly dependent on substrate concentration ([S]). At low [S], the rate is first-order; at high [S], the enzyme becomes saturated, and the rate becomes zero-order (independent of [S]). * **Option B:** Incorrect. Michaelis-Menten kinetics specifically describe **enzymatic** reactions, which show "saturation kinetics," unlike simple non-enzymatic chemical reactions. * **Option C:** Incorrect. $K_m$ (Michaelis constant) is the substrate concentration at which the reaction velocity is half of $V_{max}$. While it reflects affinity, it is technically $(k_{-1} + k_2) / k_1$, not just the dissociation constant ($K_d$). **High-Yield Clinical Pearls for NEET-PG:** * **$K_m$ and Affinity:** $K_m$ is inversely proportional to enzyme-substrate affinity. A **low $K_m$** means high affinity (e.g., Hexokinase), while a **high $K_m$** means low affinity (e.g., Glucokinase). * **Lineweaver-Burk Plot:** A double-reciprocal plot ($1/v$ vs $1/[S]$) used to determine $K_m$ and $V_{max}$. * **X-intercept** = $-1/K_m$ * **Y-intercept** = $1/V_{max}$ * **Competitive Inhibition:** $V_{max}$ remains unchanged, but $K_m$ increases (overcome by increasing [S]).

Question 432: Which of the following groups of enzymes act by mediating the transfer of one molecule to another?

A. Lyases
B. Oxidases
C. Peptidases
D. Transferases (Correct Answer)

Explanation: **Explanation:** Enzymes are classified into six major classes based on the type of reaction they catalyze (EC classification). **1. Why Transferases is correct:** **Transferases (Class 2)** are enzymes that catalyze the transfer of a specific functional group (e.g., methyl, phosphate, amino, or acyl groups) from one substrate (the donor) to another (the acceptor). A classic example is **Hexokinase**, which transfers a phosphate group from ATP to glucose. **2. Why the other options are incorrect:** * **Lyases (Class 4):** These enzymes catalyze the cleavage of C-C, C-O, or C-N bonds by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of a group to a double bond (e.g., Fumarase). * **Oxidases (Subclass of Class 1 - Oxidoreductases):** These catalyze oxidation-reduction reactions where electrons are transferred, typically involving NAD+/FADH2 or oxygen as an electron acceptor. They do not transfer functional groups between molecules. * **Peptidases (Subclass of Class 3 - Hydrolases):** These enzymes catalyze the cleavage of peptide bonds through the **addition of water** (hydrolysis). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**I**L** (**O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases). * **Kinases** are a vital subgroup of Transferases that specifically transfer phosphate groups from high-energy compounds (like ATP) to substrates. * **Aminotransferases (ALT/AST)** are diagnostic markers for liver injury and belong to the Transferase class. * **Ligases (Class 6)** require ATP to join two molecules together, unlike Lyases.

Question 433: Insulin increases the activities of all of the following enzymes, EXCEPT:

A. Glucokinase
B. Pyruvate carboxylase (Correct Answer)
C. Glycogen synthase
D. Acetyl-CoA carboxylase

Explanation: **Explanation:** The regulation of metabolic enzymes by insulin follows a simple physiological logic: **Insulin promotes energy storage (Anabolism) and glucose utilization, while inhibiting glucose production (Gluconeogenesis).** **Why Pyruvate Carboxylase is the correct answer:** Pyruvate carboxylase is the first regulatory enzyme of **gluconeogenesis**, converting pyruvate to oxaloacetate. Since insulin aims to lower blood glucose levels, it suppresses the expression and activity of gluconeogenic enzymes. Instead, pyruvate carboxylase is stimulated by **Acetyl-CoA** and hormones like **glucagon and cortisol**, which act in opposition to insulin. **Analysis of Incorrect Options:** * **Glucokinase (Option A):** Insulin induces the synthesis of Glucokinase in the liver. This facilitates the uptake and phosphorylation of glucose, trapping it in the cell for glycolysis or glycogenesis. * **Glycogen Synthase (Option B):** Insulin promotes glycogenesis (storage of glucose as glycogen). It activates glycogen synthase by stimulating a phosphatase that dephosphorylates the enzyme into its active form. * **Acetyl-CoA Carboxylase (Option D):** This is the rate-limiting enzyme for **fatty acid synthesis**. Insulin promotes the conversion of excess glucose into fat for storage, thereby activating this enzyme. **NEET-PG High-Yield Pearls:** * **The "Dephosphorylation Rule":** Insulin generally activates enzymes by **dephosphorylating** them (via Protein Phosphatase-1). * **Key Insulin-Induced Enzymes:** Glucokinase, PFK-1, Pyruvate Kinase (Glycolysis); Glycogen Synthase (Glycogenesis); Acetyl-CoA Carboxylase (Lipogenesis); HMG-CoA Reductase (Cholesterol synthesis). * **Key Insulin-Repressed Enzymes:** Pyruvate carboxylase, PEP carboxykinase, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase (all involved in Gluconeogenesis).

Question 434: Which of the following is NOT a non-competitive inhibitor?

A. Arsenate (Correct Answer)
B. Fluoroacetate
C. Arsenate
D. Disulfiram

Explanation: In biochemistry, inhibitors are classified based on their mechanism of action. This question tests the distinction between **Competitive** and **Non-competitive** inhibition. ### **Explanation of the Correct Answer** **Arsenate (Option A/C)** is a **Competitive Inhibitor**. It is a structural analog of inorganic phosphate ($P_i$). In the glycolysis pathway, specifically the reaction catalyzed by *Glyceraldehyde-3-phosphate dehydrogenase*, arsenate competes with phosphate to bind at the active site. This results in the formation of 1-arseno-3-phosphoglycerate, which spontaneously hydrolyzes, bypassing ATP production (substrate-level phosphorylation). Because it competes for the same binding site as the substrate (phosphate), it is classified as competitive. ### **Analysis of Incorrect Options** * **Fluoroacetate (Option B):** This is a classic example of **Suicide Inhibition** (a form of irreversible non-competitive inhibition). It is converted to fluorocitrate, which inhibits the enzyme *Aconitase* in the TCA cycle. * **Disulfiram (Option D):** This is an **Irreversible Non-competitive Inhibitor** of the enzyme *Aldehyde Dehydrogenase*. It binds to the enzyme (not at the active site) and prevents the metabolism of acetaldehyde, leading to "Antabuse" reactions. ### **High-Yield Clinical Pearls for NEET-PG** * **Competitive Inhibition:** $V_{max}$ remains unchanged; $K_m$ increases. (Mnemonic: **C**ompetitive = **C**onstant $V_{max}$). * **Non-competitive Inhibition:** $V_{max}$ decreases; $K_m$ remains unchanged. * **Suicide Inhibition Examples:** Allopurinol (Xanthine Oxidase), Aspirin (COX), Penicillin (Transpeptidase), and 5-Fluorouracil (Thymidylate Synthase). * **Arsenite vs. Arsenate:** While Arsenate competes with phosphate, **Arsenite** inhibits enzymes requiring Lipoic acid (e.g., Pyruvate Dehydrogenase) by binding to -SH groups.

Question 435: Fluoride ions act by inhibiting which enzyme?

A. Enolase (Correct Answer)
B. Hexokinase
C. Cytochrome oxidase
D. Carbonic anhydrase

Explanation: **Explanation:** **1. Why Enolase is the Correct Answer:** Fluoride is a potent inhibitor of **Enolase**, the ninth enzyme in the glycolytic pathway. Enolase catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). The mechanism of inhibition involves fluoride forming a complex with magnesium ions ($Mg^{2+}$) and phosphate, creating a **fluorophosphate-magnesium complex**. Since Enolase requires $Mg^{2+}$ as a cofactor for its activity, this complex effectively displaces the magnesium, leading to **competitive inhibition**. **2. Analysis of Incorrect Options:** * **Hexokinase:** This is the first enzyme of glycolysis. It is inhibited by its product, Glucose-6-Phosphate, but not by fluoride. * **Cytochrome oxidase:** This is a key enzyme in the Electron Transport Chain (Complex IV). It is classically inhibited by **Cyanide, Carbon Monoxide (CO), and Azide**, which bind to the iron or copper centers of the enzyme. * **Carbonic anhydrase:** This enzyme regulates acid-base balance and $CO_2$ transport. It is inhibited by **Acetazolamide**, a sulfonamide derivative. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood Glucose Estimation:** In clinical practice, blood samples for glucose testing are collected in **grey-topped vials** containing **Sodium Fluoride (NaF)**. This prevents "in vitro" glycolysis by RBCs, ensuring the measured glucose level reflects the patient's actual blood sugar at the time of draw. * **Anticoagulant Pairing:** NaF is usually paired with **Potassium Oxalate**, which acts as the anticoagulant by chelating calcium. * **Fluoride & Teeth:** While high doses inhibit enzymes, low doses of fluoride are used to prevent dental caries by converting hydroxyapatite in tooth enamel to the more acid-resistant **fluoroapatite**.

Question 436: Which of the following enzymes does NOT contain Zinc?

A. Alcohol Dehydrogenase
B. Arginase (Correct Answer)
C. Alkaline Phosphatase
D. Carbonic Anhydrase

Explanation: **Explanation:** The correct answer is **Arginase**. This question tests your knowledge of **metalloenzymes**, which are enzymes that require specific metal ions as integral components for their catalytic activity. **1. Why Arginase is the correct answer:** Arginase is a key enzyme in the Urea Cycle that converts Arginine to Urea and Ornithine. Unlike the other options, Arginase requires **Manganese ($Mn^{2+}$)** for its activity, not Zinc. In some contexts, Cobalt may also activate it, but Manganese is its primary physiological cofactor. **2. Analysis of Incorrect Options (Zinc-containing enzymes):** * **Alcohol Dehydrogenase:** A classic zinc-containing metalloenzyme responsible for the oxidation of ethanol to acetaldehyde. Zinc plays a structural and catalytic role in the active site. * **Alkaline Phosphatase (ALP):** This enzyme contains **Zinc** (essential for catalysis) and **Magnesium** (essential for stability). It is a high-yield clinical marker for obstructive jaundice and bone diseases. * **Carbonic Anhydrase:** One of the most efficient enzymes known, it requires **Zinc** to coordinate with water molecules to facilitate the rapid interconversion of $CO_2$ and $H_2O$ to bicarbonate. **3. NEET-PG High-Yield Clinical Pearls:** * **Zinc-containing enzymes:** Remember the mnemonic **"C-A-L-A"** (Carbonic anhydrase, Alcohol dehydrogenase, Lactate dehydrogenase, Alkaline phosphatase). Other examples include Carboxypeptidase and RNA/DNA Polymerases. * **Zinc Deficiency:** Classically presents as **Acrodermatitis Enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. * **Manganese ($Mn^{2+}$) Enzymes:** Besides Arginase, **Pyruvate Carboxylase** and **Superoxide Dismutase (mitochondrial)** are important Manganese-requiring enzymes.

Question 437: Which enzyme protects the brain from free radical injury?

A. Myeloperoxidase
B. Monoamine oxidase (MAO)
C. Superoxide dismutase (Correct Answer)
D. Hydroxylase

Explanation: ### Explanation **Correct Option: C. Superoxide dismutase (SOD)** Superoxide dismutase is a critical antioxidant enzyme that protects cells from oxidative stress. It catalyzes the dismutation of the highly reactive **superoxide radical ($O_2^-$)** into oxygen and hydrogen peroxide ($H_2O_2$). In the brain, which is highly susceptible to oxidative damage due to its high oxygen consumption and lipid content, SOD acts as the first line of defense against reactive oxygen species (ROS). **Analysis of Incorrect Options:** * **A. Myeloperoxidase:** Found primarily in neutrophils, this enzyme produces hypochlorous acid (HOCl) from $H_2O_2$ and chloride ions. It is involved in the respiratory burst to kill bacteria, actually *generating* free radicals rather than scavenging them. * **B. Monoamine oxidase (MAO):** This enzyme breaks down neurotransmitters like dopamine and serotonin. A byproduct of this reaction is $H_2O_2$, which can contribute to oxidative stress if not neutralized; thus, MAO is a source of potential injury, not a protector. * **D. Hydroxylase:** These enzymes (e.g., Phenylalanine hydroxylase) add hydroxyl groups to substrates. They are metabolic enzymes and do not possess specific antioxidant properties. **High-Yield Clinical Pearls for NEET-PG:** * **SOD Isoforms:** SOD1 (Cytosolic, contains Cu-Zn), SOD2 (Mitochondrial, contains Mn), and SOD3 (Extracellular, contains Cu-Zn). * **Clinical Link:** Mutations in the **SOD1 gene** are associated with **Amyotrophic Lateral Sclerosis (ALS)**, highlighting the enzyme's vital role in protecting motor neurons. * **Antioxidant Trio:** Remember the sequence: **SOD** converts $O_2^-$ to $H_2O_2$, then **Catalase** or **Glutathione Peroxidase** converts $H_2O_2$ to water.

Question 438: Which of the following is RNA with catalytic activity?

A. Ribonuclease P
B. Peptidyl transferase
C. Sn RNA
D. All of the above (Correct Answer)

Explanation: ### Explanation The central concept here is the **Ribozyme**, which refers to RNA molecules that possess catalytic activity, challenging the traditional view that all enzymes are proteins. **Why "All of the above" is correct:** All three options listed are classic examples of ribozymes essential for cellular function: * **Ribonuclease P (RNase P):** This is one of the first ribozymes discovered. It is a ribonucleoprotein responsible for the processing of tRNA precursors by cleaving the 5' end of the pre-tRNA. * **Peptidyl transferase:** Located within the large ribosomal subunit (23S rRNA in prokaryotes, 28S rRNA in eukaryotes), this ribozyme catalyzes the formation of peptide bonds during protein synthesis. This confirms that the ribosome is essentially a giant ribozyme. * **Small Nuclear RNA (snRNA):** These are components of the spliceosome (e.g., U2, U6). They catalyze the removal of introns from pre-mRNA through transesterification reactions. **High-Yield Clinical Pearls for NEET-PG:** 1. **Discovery:** Thomas Cech and Sidney Altman won the Nobel Prize for discovering the catalytic properties of RNA. 2. **Other Examples:** * **Self-splicing introns** (Group I and Group II introns). * **Hammerhead ribozymes** (found in viroids and satellite RNAs). 3. **Medical Significance:** Ribozymes are being researched as therapeutic agents (gene silencing) to specifically cleave viral RNA or oncogene mRNA. 4. **Key Distinction:** While most enzymes are proteins, ribozymes prove that nucleic acids can also lower activation energy and increase reaction rates.

Question 439: What is the function of a coenzyme?

A. To enhance the specificity of an apoenzyme.
B. To accept one of the cleavage products.
C. To activate the substrate. (Correct Answer)
D. To increase the number of active sites on an apoenzyme.

Explanation: **Explanation:** In biochemistry, an **Holoenzyme** consists of a protein part (**Apoenzyme**) and a non-protein part (**Cofactor**). When the cofactor is an organic molecule, it is termed a **Coenzyme**. **Why Option C is Correct:** The primary role of a coenzyme is to act as a transient carrier of specific functional groups, protons, or electrons. By binding to the enzyme-substrate complex, the coenzyme effectively **activates the substrate** by lowering the activation energy required for the reaction. It facilitates the conversion of the substrate into a more reactive intermediate state, allowing the chemical transformation (like decarboxylation or phosphorylation) to proceed efficiently. **Analysis of Incorrect Options:** * **Option A:** Specificity is a property of the **Apoenzyme** (protein part), determined by the unique 3D configuration of its active site. Coenzymes are often "promiscuous" and work with multiple different enzymes. * **Option B:** While coenzymes may temporarily carry a group removed from a substrate, their role is catalytic facilitation, not merely acting as a "sink" for cleavage products. * **Option D:** The number of active sites is determined by the tertiary and quaternary structure of the apoenzyme; coenzymes do not create new sites but rather function within existing ones. **High-Yield NEET-PG Pearls:** * **Prosthetic Group:** A coenzyme that is covalently or very tightly bound to the apoenzyme (e.g., FAD, Biotin). * **Vitamin Precursors:** Most coenzymes are derivatives of B-complex vitamins (e.g., NAD+ from Niacin/B3, TPP from Thiamine/B1). * **Metalloenzymes:** If the cofactor is a metal ion (like Zn²⁺ in Carbonic Anhydrase), it is a metalloenzyme, not a coenzyme.

Question 440: One of the following groups of enzymes does not exhibit stereospecificity?

A. Oxidoreductases
B. Isomerases (Correct Answer)
C. Lyases
D. Transferases

Explanation: ### Explanation **Why Isomerases are the Correct Answer:** Stereospecificity refers to the ability of an enzyme to act exclusively on one specific stereoisomer (e.g., L-amino acids or D-sugars). **Isomerases** are unique because their primary function is to catalyze the **interconversion** of optical or geometric isomers. For an enzyme to convert a D-isomer into an L-isomer (or vice versa), it must interact with both configurations. Therefore, by definition, isomerases lack the absolute stereospecificity seen in other enzyme classes, as they must accommodate and transform different stereochemical arrangements of the same molecule. **Analysis of Incorrect Options:** * **Oxidoreductases:** These are highly stereospecific. For example, Lactate Dehydrogenase (LDH) acts specifically on L-lactate, not D-lactate. Alcohol dehydrogenase also distinguishes between the pro-R and pro-S hydrogens of ethanol. * **Lyases:** These enzymes catalyze the addition or removal of groups to form double bonds. They exhibit high stereospecificity; for instance, Fumarase adds water to Fumarate (trans) but cannot act on Maleate (cis). * **Transferases:** These enzymes transfer functional groups (like phosphate or methyl groups) with high spatial precision. Hexokinase, for example, is specific for D-glucose and will not phosphorylate L-glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Racemases and Epimerases:** These are sub-classes of Isomerases. A classic example is **Mutarotase**, which converts α-D-glucose to β-D-glucose. * **Absolute Specificity:** Most enzymes exhibit "Optical Specificity," meaning they only work on one isomer (e.g., human enzymes almost exclusively utilize **L-amino acids** and **D-sugars**). * **Exception to the Rule:** While most enzymes are proteins, **Ribozymes** (RNA-based enzymes) also exhibit high specificity.

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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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