Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 181: Which of the following enzymes does NOT possess antioxidant property?

A. Catalase
B. Glutathione peroxidase
C. Phosphorylase (Correct Answer)
D. Superoxide dismutase

Explanation: **Explanation:** The correct answer is **Phosphorylase**. This question tests the distinction between enzymes involved in metabolic pathways and those involved in the cellular defense against oxidative stress. **1. Why Phosphorylase is the correct answer:** Phosphorylase (specifically Glycogen Phosphorylase) is a key enzyme in **glycogenolysis**. It catalyzes the rate-limiting step of breaking down glycogen into glucose-1-phosphate by adding an inorganic phosphate. It has no role in neutralizing Reactive Oxygen Species (ROS) or protecting the cell from oxidative damage. **2. Why the other options are incorrect:** The other three enzymes constitute the primary **enzymatic antioxidant defense system** of the body: * **Superoxide Dismutase (SOD):** This is the "first line" of defense. It converts the highly reactive superoxide radical ($O_2^{\bullet-}$) into hydrogen peroxide ($H_2O_2$) and oxygen. * **Catalase:** Located primarily in peroxisomes, it catalyzes the decomposition of $H_2O_2$ into water and oxygen, preventing the formation of the toxic hydroxyl radical. * **Glutathione Peroxidase (GPx):** This selenium-dependent enzyme reduces $H_2O_2$ and lipid hydroperoxides to water and alcohols, respectively, using reduced glutathione (GSH) as a donor. **High-Yield Clinical Pearls for NEET-PG:** * **Selenium Connection:** Glutathione peroxidase requires **Selenium** as a cofactor (in the form of selenocysteine). Deficiency can lead to Keshan disease. * **SOD Isoforms:** Cytosolic SOD requires **Copper and Zinc**, while mitochondrial SOD requires **Manganese**. * **G6PD Deficiency:** This is clinically relevant because G6PD is essential for generating **NADPH**, which is required by Glutathione Reductase to regenerate GSH, maintaining the antioxidant capacity of RBCs.

Question 182: Alcohol dehydrogenase is a/an:

A. Transferase
B. Hydrolase
C. Ligase
D. Oxidoreductase (Correct Answer)

Explanation: **Explanation:** **Alcohol dehydrogenase (ADH)** belongs to the **Oxidoreductase** class of enzymes (EC 1). This is because it catalyzes the oxidation of alcohols to aldehydes or ketones by transferring hydrogen atoms to a coenzyme, typically **NAD+**. In the metabolism of ethanol, ADH removes two hydrogen atoms from ethanol to form acetaldehyde, while reducing NAD+ to NADH. **Analysis of Options:** * **Oxidoreductase (Correct):** These enzymes catalyze oxidation-reduction reactions (transfer of H atoms or electrons). ADH fits this definition perfectly as it facilitates the redox reaction between ethanol and NAD+. * **Transferase:** These enzymes transfer functional groups (e.g., methyl, phosphate) from one substrate to another. ADH does not transfer a group; it changes the oxidation state of the substrate. * **Hydrolase:** These enzymes catalyze the cleavage of bonds (C-O, C-N, C-C) by the addition of water. ADH does not involve water-mediated bond cleavage. * **Ligase:** These enzymes catalyze the joining of two molecules, usually coupled with ATP hydrolysis. ADH is a degradative/transformative enzyme, not a synthetic one. **Clinical Pearls for NEET-PG:** 1. **Rate-Limiting Step:** Alcohol dehydrogenase is the rate-limiting enzyme in alcohol metabolism and follows **zero-order kinetics** (metabolizes a constant amount of alcohol per unit time). 2. **Cofactor:** It is a **zinc-containing metalloenzyme** and requires **NAD+** as a coenzyme. 3. **Inhibition:** **Fomepizole** is a competitive inhibitor of ADH, used as an antidote in methanol and ethylene glycol poisoning to prevent the formation of toxic metabolites (formaldehyde and glycolic acid). 4. **Location:** It is primarily found in the **cytosol** of hepatocytes.

Question 183: Peroxidases belong to which enzyme group?

A. Oxidase-Reductase (Correct Answer)
B. Lipase
C. Hydrolase
D. Transferase

Explanation: **Explanation:** **1. Why Oxidase-Reductase is Correct:** Peroxidases belong to the **Oxidoreductase (EC 1)** class of enzymes. These enzymes catalyze oxidation-reduction reactions, where electrons are transferred from a donor (reductant) to an acceptor (oxidant). Specifically, peroxidases catalyze the breakdown of hydrogen peroxide ($H_2O_2$) or organic peroxides, using them as electron acceptors to oxidize various substrates. A classic example is **Glutathione Peroxidase**, which reduces $H_2O_2$ to water while oxidizing glutathione (GSH to GSSG), protecting cells from oxidative damage. **2. Why the Other Options are Incorrect:** * **Lipases (EC 3):** These are a sub-class of hydrolases that specifically catalyze the hydrolysis of ester bonds in lipid substrates (e.g., Triacylglycerol lipase). * **Hydrolases (EC 3):** These enzymes catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. Examples include digestive enzymes like pepsin and urease. * **Transferases (EC 2):** These enzymes transfer a functional group (e.g., methyl, phosphate, or amino groups) from one molecule to another. Examples include Kinases and Transaminases (ALT/AST). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Heme Protein:** Most peroxidases (like Myeloperoxidase) contain **heme** as a prosthetic group. * **Myeloperoxidase (MPO):** Found in neutrophil granules; it produces hypochlorous acid (HOCl) to kill bacteria. Its deficiency leads to impaired microbial killing. * **Glutathione Peroxidase:** This is a **Selenium-dependent** enzyme. Selenium deficiency can lead to Keshan disease (cardiomyopathy). * **Catalase:** A specific type of peroxidase that converts $H_2O_2$ into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$), protecting the cell from reactive oxygen species (ROS).

Question 184: Carbonic anhydrase is which type of enzyme?

A. Coenzyme
B. Metalloenzyme (Correct Answer)
C. Serine protease
D. Endopeptidase

Explanation: **Explanation:** **1. Why Metalloenzyme is Correct:** Carbonic anhydrase is a classic example of a **metalloenzyme**. Metalloenzymes are proteins that contain a tightly bound metal ion as an integral part of their structure, which is essential for their catalytic activity. In the case of carbonic anhydrase, a **Zinc ($Zn^{2+}$) ion** is coordinated at the active site. This zinc ion facilitates the nucleophilic attack of water on carbon dioxide, catalyzing the rapid interconversion of $CO_2$ and water into bicarbonate ($HCO_3^-$) and $H^+$. **2. Why Other Options are Incorrect:** * **Coenzyme:** These are non-protein organic molecules (often derived from vitamins like B-complex) that loosely bind to enzymes to assist in catalysis (e.g., NAD+, FAD). Carbonic anhydrase uses a metal ion, not an organic molecule. * **Serine Protease:** These are enzymes that use a serine residue in their active site to cleave peptide bonds (e.g., Trypsin, Chymotrypsin). Carbonic anhydrase does not cleave proteins. * **Endopeptidase:** These are proteolytic enzymes that break internal peptide bonds within a polypeptide chain (e.g., Pepsin). Carbonic anhydrase is a lyase/hydrolase-type enzyme, not a protease. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Speed:** Carbonic anhydrase is one of the **fastest known enzymes**, with a turnover number ($K_{cat}$) of $10^6$ reactions per second. * **Location:** Found in high concentrations in RBCs (for $CO_2$ transport), gastric mucosa (HCL secretion), and renal tubules (acid-base balance). * **Inhibitor:** **Acetazolamide** is a potent inhibitor used clinically to treat glaucoma, altitude sickness, and as a weak diuretic. * **Other Metalloenzymes:** Remember **Carboxypeptidase (Zinc)**, **Superoxide Dismutase (Copper/Zinc)**, and **Tyrosinase (Copper)** for exam comparisons.

Question 185: Which of the following enzymes acts by a 'Ping-Pong' mechanism?

A. Alanine amino transferase (Correct Answer)
B. Hexokinase
C. Lactate Dehydrogenase
D. Pyruvate Dehydrogenase

Explanation: ### Explanation **1. Why Alanine Aminotransferase (ALT) is Correct:** Alanine Aminotransferase (ALT) follows the **Ping-Pong (Double-Displacement) mechanism**. In this mechanism, the first substrate (Alanine) binds to the enzyme and transfers its amino group to the prosthetic group, **Pyridoxal Phosphate (PLP)**, forming Pyridoxamine Phosphate (PMP). The first product (Pyruvate) is released *before* the second substrate (α-Ketoglutarate) binds. The second substrate then binds, picks up the amino group from PMP, and is released as the second product (Glutamate). The key feature is that the enzyme exists in a modified intermediate state (PMP-enzyme) between the release of the first product and the binding of the second substrate. **2. Why the Other Options are Incorrect:** * **Hexokinase:** Follows a **Random Bi-Bi mechanism**. Substrates (Glucose and ATP) can bind in any order, and both must be present on the enzyme before any product is released. * **Lactate Dehydrogenase (LDH):** Follows an **Ordered Bi-Bi mechanism**. There is a mandatory sequence: NADH must bind first before Pyruvate can bind, and Lactate must be released before NAD+ can leave. * **Pyruvate Dehydrogenase (PDH):** This is a multi-enzyme complex involving five cofactors. While it involves a series of transfers, it is classified as a complex oxidative decarboxylation rather than a classic Ping-Pong bisubstrate kinetic model. **3. High-Yield Clinical Pearls for NEET-PG:** * **PLP Dependency:** All aminotransferases (ALT, AST) require **Vitamin B6 (Pyridoxine)** as a cofactor. * **Ping-Pong Rule:** If a question mentions "transamination" or "covalent enzyme intermediate," think Ping-Pong mechanism. * **Diagnostic Marker:** ALT is more specific for liver cell damage than AST, as AST is also found in cardiac and skeletal muscle.

Question 186: Lactate dehydrogenase enzyme requires which metal?

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

Explanation: **Explanation:** Lactate Dehydrogenase (LDH) is a key glycolytic enzyme that catalyzes the reversible conversion of lactate to pyruvate. It belongs to the class of **Oxidoreductases** and requires **Zinc ($Zn^{2+}$)** as a structural and functional cofactor. Zinc is essential for maintaining the enzyme's quaternary structure and stabilizing the transition state during the hydride transfer between NADH and the substrate. **Analysis of Options:** * **Zinc (Correct):** Zinc is a constituent of several important enzymes including LDH, Carbonic Anhydrase, Alcohol Dehydrogenase, and DNA/RNA Polymerases. * **Selenium:** This is a vital cofactor for **Glutathione Peroxidase**, which protects cells from oxidative damage, and Thioredoxin reductase. * **Copper:** Copper is required for enzymes involved in redox reactions and connective tissue formation, such as **Cytochrome c Oxidase**, Tyrosinase, and Lysyl Oxidase. * **Magnesium:** Magnesium is the most common cofactor for enzymes involving ATP (kinases), such as **Hexokinase** and Phosphofructokinase, as it stabilizes the negative charges on phosphate groups. **Clinical Pearls for NEET-PG:** 1. **LDH Isoenzymes:** LDH is a tetramer with two subunits (H and M). There are 5 isoenzymes; **LDH-1 (H4)** is elevated in Myocardial Infarction, while **LDH-5 (M4)** is elevated in liver disease and skeletal muscle injury. 2. **Zinc Deficiency:** Clinically manifests as **Acrodermatitis enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. 3. **Diagnostic Marker:** LDH is used as a non-specific marker of tissue breakdown (hemolysis, malignancy, or inflammation).

Question 187: Glycogen phosphorylase is active in which form?

A. Phosphorylated form (Correct Answer)
B. Dephosphorylated form
C. Both phosphorylated and dephosphorylated form
D. No rate of phosphorylation

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Glycogen phosphorylase is the rate-limiting enzyme of **glycogenolysis** (the breakdown of glycogen into glucose-1-phosphate). Its activity is regulated by reversible covalent modification: **phosphorylation**. * **Phosphorylase a:** The phosphorylated form, which is **active**. * **Phosphorylase b:** The dephosphorylated form, which is **inactive**. The enzyme **Phosphorylase Kinase** adds a phosphate group to the enzyme (converting b to a) in response to hormonal signals like Glucagon (in the liver) or Epinephrine (in the muscle), ensuring glucose is released when the body needs energy. **2. Why Incorrect Options are Wrong:** * **Option B:** The dephosphorylated form (Phosphorylase b) is the resting/inactive state of the enzyme. Dephosphorylation is catalyzed by **Protein Phosphatase-1**, which occurs under the influence of Insulin (the "fed state" hormone). * **Option C:** The enzyme exists in a reciprocal relationship; it cannot be equally active in both states. This dual sensitivity allows for precise metabolic control. * **Option D:** Phosphorylation is the primary mechanism of regulation for this enzyme; saying there is "no rate" is physiologically incorrect. **3. High-Yield Clinical Pearls for NEET-PG:** * **Reciprocal Regulation:** Remember the "Rule of Thumb"—most catabolic enzymes (like Glycogen Phosphorylase) are **active when phosphorylated**, while most anabolic enzymes (like Glycogen Synthase) are **inactive when phosphorylated**. * **Allosteric Activators:** In muscles, Glycogen Phosphorylase b can be activated *without* phosphorylation by high levels of **AMP** (signaling low energy) and **Ca²⁺** (signaling muscle contraction). * **Clinical Correlation:** A deficiency in liver glycogen phosphorylase leads to **Hers Disease (GSD Type VI)**, characterized by hepatomegaly and mild hypoglycemia.

Question 188: The protein part of an enzyme is called:

A. Holoenzyme
B. Coenzyme
C. Cofactor
D. Apoenzyme (Correct Answer)

Explanation: ### Explanation **1. Why Apoenzyme is Correct:** In biochemistry, many enzymes are **conjugated proteins** (holoenzymes) consisting of a protein portion and a non-protein portion. The **apoenzyme** refers specifically to the **protein part** of the enzyme. It is catalytically inactive on its own and requires the binding of a specific cofactor to become functional. The apoenzyme determines the substrate specificity of the reaction. **2. Analysis of Incorrect Options:** * **Holoenzyme (A):** This is the complete, catalytically active enzyme system. It is the sum of the protein part (apoenzyme) and the non-protein part (cofactor). Formula: *Holoenzyme = Apoenzyme + Cofactor*. * **Coenzyme (B):** This is a type of cofactor. Specifically, it is a **non-protein, organic molecule** (often derived from vitamins like B-complex) that binds loosely to the apoenzyme to assist in catalysis. * **Cofactor (C):** This is a broad term for any non-protein component required for enzyme activity. It includes both inorganic metal ions (like $Mg^{2+}$, $Zn^{2+}$) and organic molecules (coenzymes). **3. NEET-PG High-Yield Pearls:** * **Prosthetic Group:** If a coenzyme is **covalently or very tightly bound** to the apoenzyme (e.g., FAD, Heme in peroxidase), it is called a prosthetic group. * **Zymogen (Proenzyme):** An inactive precursor of an enzyme that requires biochemical change (like proteolysis) to become active (e.g., Pepsinogen to Pepsin). * **Metalloenzymes:** Enzymes that hold a metal ion tightly (e.g., Carbonic Anhydrase contains $Zn^{2+}$). * **Key Mnemonic:** **"Apo"** means "away" or "separate"—think of it as the protein part standing alone, waiting for its partner.

Question 189: Which of the following is a functional enzyme?

A. LDH
B. Amylase
C. Prothrombin (Correct Answer)
D. Acid phosphatase

Explanation: ### Explanation In biochemistry, plasma enzymes are categorized into two groups: **Functional** and **Non-functional** plasma enzymes. **1. Why Prothrombin is the Correct Answer:** **Functional plasma enzymes** are those that are actively present in the blood at all times and perform their primary physiological function within the circulation. **Prothrombin** (Factor II) is synthesized in the liver and secreted into the plasma, where it plays a critical role in the blood coagulation cascade. Other examples include pseudocholinesterase, lipoprotein lipase, and various clotting factors. These enzymes are present in higher concentrations in plasma than in tissues. **2. Why the Other Options are Incorrect:** Options A, B, and D are **Non-functional plasma enzymes**. These enzymes have no known physiological function in the blood. They are normally present in very low concentrations in the plasma because they perform their work intracellularly or within specific tracts (like the GI tract). * **LDH (Lactate Dehydrogenase):** An intracellular enzyme; elevated levels indicate tissue damage (e.g., myocardial infarction or hemolysis). * **Amylase:** Secreted by the pancreas and salivary glands into the digestive tract. Its presence in high amounts in the blood indicates pathology like acute pancreatitis. * **Acid Phosphatase:** Primarily found in the prostate; elevated levels serve as a marker for prostatic carcinoma. **3. High-Yield Clinical Pearls for NEET-PG:** * **Functional Enzymes:** Substrates are always present in the blood. Their deficiency usually indicates liver dysfunction (decreased synthesis). * **Non-functional Enzymes:** Their elevation in plasma is a diagnostic marker for organ-specific damage (e.g., ALT/AST for liver, CK-MB for heart). * **Key Distinction:** Functional enzymes are synthesized in the liver and act *in* the blood; Non-functional enzymes act *inside* cells and leak into the blood during injury.

Question 190: Transaldolase is a type of enzyme that belongs to which class?

A. Hydrolase
B. Lyase
C. Transferase (Correct Answer)
D. Ligase

Explanation: **Explanation:** **1. Why Transferase is Correct:** Transaldolase is a key enzyme in the **non-oxidative phase of the Pentose Phosphate Pathway (PPP)**. Its primary function is to catalyze the transfer of a three-carbon dihydroxyacetone unit from a ketose (Sedoheptulose-7-phosphate) to an aldose (Glyceraldehyde-3-phosphate). According to the IUBMB enzyme classification, enzymes that move a functional group from one substrate to another belong to **Class 2: Transferases**. Specifically, transaldolase facilitates the interconversion of sugars to generate ribose-5-phosphate and NADPH. **2. Why Other Options are Incorrect:** * **Hydrolases (Class 3):** These enzymes catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. Transaldolase does not involve water in its mechanism. * **Lyases (Class 4):** These enzymes catalyze the cleavage of bonds by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of groups to double bonds (e.g., Aldolase in glycolysis). While the names are similar, Transaldolase transfers a group rather than simply splitting a molecule. * **Ligases (Class 6):** These enzymes catalyze the joining of two large molecules, usually coupled with the **hydrolysis of ATP** (e.g., Pyruvate carboxylase). Transaldolase does not require ATP. **3. High-Yield Clinical Pearls for NEET-PG:** * **PPP Link:** Transaldolase and Transketolase (also a transferase) link the PPP to Glycolysis. * **Cofactor Distinction:** Unlike Transketolase, which requires **Thiamine Pyrophosphate (TPP)** as a cofactor, Transaldolase does **not** require any cofactor. * **Clinical Significance:** Transaldolase deficiency is a rare metabolic disorder presenting with liver dysfunction, hepatosplenomegaly, and hemolytic anemia. * **Mnemonic:** Remember the "Three T's" of the PPP: **T**ransketolase and **T**ransaldolase are **T**ransferases.

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Enzyme Classification and Nomenclature

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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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Enzymes — MCQs

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