Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 211: Fumarase is classified as which type of enzyme?

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

Explanation: ### Explanation **Correct Answer: D. Lyase** **Why it is correct:** Fumarase (fumarate hydratase) is a key enzyme in the **TCA cycle** that catalyzes the reversible hydration of fumarate to L-malate. According to the International Union of Biochemistry (IUB) classification, **Lyases (Class 4)** are enzymes that catalyze the addition of groups to double bonds or the formation of double bonds by the removal of groups. Unlike hydrolases, lyases do not use water to "break" a bond; instead, they add water across a double bond (hydration) or remove it to create one (dehydration). Since fumarase adds a water molecule across the carbon-carbon double bond of fumarate without cleaving the molecule, it is classified as a Lyase. **Why the other options are incorrect:** * **A. Oxidoreductases (Class 1):** These catalyze oxidation-reduction reactions (e.g., Dehydrogenases). Fumarase does not involve the transfer of electrons or change in oxidation states. * **B. Transferases (Class 2):** These transfer functional groups (like methyl or phosphate groups) from one substrate to another (e.g., Kinases). * **C. Hydrolases (Class 3):** These catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water** (hydrolysis), effectively breaking a large molecule into two smaller ones (e.g., Digestive enzymes like Pepsin). **High-Yield Clinical Pearls for NEET-PG:** * **Metabolic Role:** Fumarase is essential for aerobic energy production. * **Clinical Correlation:** A deficiency of fumarase leads to **Fumaric Aciduria**, characterized by severe neurological impairment and encephalopathy. * **Oncology Link:** Mutations in the fumarate hydratase (FH) gene are associated with **Hereditary Leiomyomatosis and Renal Cell Cancer (HLRCC)**, where fumarate acts as an "oncometabolite." * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**ike **I**sland **L**overs (**O**xidoreductase, **T**ransferase, **H**ydrolase, **L**yase, **I**somerase, **L**igase).

Question 212: Which of the following is true regarding Nitric Oxide Synthase?

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

Explanation: **Explanation:** Nitric Oxide Synthase (NOS) is a complex enzyme responsible for the synthesis of Nitric Oxide (NO) from **L-arginine**. **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** (electron donor), **FAD**, **FMN**, **Heme iron (protoporphyrin IX)**, and **Tetrahydrobiopterin (BH4)**. The enzyme transfers electrons from NADPH through the flavins (FAD/FMN) to the heme center to activate molecular oxygen. **Analysis of Incorrect Options:** * **Option A:** Most isoforms of NOS (specifically eNOS and nNOS) are **activated by Ca++-Calmodulin** binding, not inhibited by it. * **Option B:** NOS is a **monooxygenase** (mixed-function oxidase), not a dioxygenase. It incorporates only one atom of molecular oxygen into the product (NO), while the other is reduced to water. * **Option C:** The primary electron donor for NOS is **NADPH**, which is generated via the Pentose Phosphate Pathway, not NADH. **High-Yield Clinical Pearls for NEET-PG:** * **Isoforms:** There are three types: **nNOS** (Neuronal/Type I), **iNOS** (Inducible/Type II - Ca++ independent), and **eNOS** (Endothelial/Type III). * **Substrate:** L-Arginine is the precursor; L-Citrulline is the byproduct. * **Inhibitor:** Asymmetric dimethylarginine (ADMA) acts as an endogenous inhibitor of NOS. * **Function:** NO is a potent vasodilator that acts by increasing **cGMP** levels via activation of soluble guanylyl cyclase.

Question 213: NAD+ is reduced to NADH + H+ by dehydrogenases of all the following substrates, except?

A. Pyruvate
B. Glyceraldehyde-3-phosphate
C. Malate
D. Succinate (Correct Answer)

Explanation: ### Explanation The core concept here is the specificity of electron acceptors in the mitochondrial respiratory chain and metabolic pathways. Dehydrogenases catalyze the removal of hydrogen atoms from substrates, but they utilize different coenzymes—typically **NAD+** or **FAD**—based on the energy change of the reaction. **1. Why Succinate is the Correct Answer:** Succinate is oxidized to Fumarate by the enzyme **Succinate Dehydrogenase** (Complex II of the Electron Transport Chain). This specific reaction uses **FAD** as the electron acceptor, reducing it to **FADH₂**, not NADH. This occurs because the free energy change associated with the oxidation of a C-C single bond to a C=C double bond is insufficient to reduce NAD+, but is enough to reduce FAD. **2. Analysis of Incorrect Options:** * **Pyruvate:** Converted to Acetyl-CoA by the *Pyruvate Dehydrogenase Complex*. This reaction involves the reduction of **NAD+ to NADH**. * **Glyceraldehyde-3-phosphate:** In glycolysis, *G3P Dehydrogenase* oxidizes G3P to 1,3-bisphosphoglycerate, reducing **NAD+ to NADH**. * **Malate:** In the TCA cycle, *Malate Dehydrogenase* oxidizes malate to oxaloacetate, reducing **NAD+ to NADH**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Succinate Dehydrogenase** is unique because it is the only TCA cycle enzyme that is **membrane-bound** (inner mitochondrial membrane) and functions as **Complex II** of the ETC. * **Riboflavin (Vitamin B2)** is the precursor for FAD; hence, B2 deficiency directly impairs succinate oxidation. * **Mnemonic:** Most "Dehydrogenases" in the TCA cycle produce NADH (Isocitrate, α-Ketoglutarate, Malate), except for Succinate Dehydrogenase, which produces FADH₂.

Question 214: What is the coenzyme for transamination reactions?

A. Pyridoxal phosphate (Correct Answer)
B. Vitamin C
C. Biotin
D. Thiamine

Explanation: **Explanation** **1. Why Pyridoxal Phosphate (PLP) is correct:** Transamination is the process where an $\alpha$-amino group is transferred from an amino acid to an $\alpha$-keto acid, catalyzed by enzymes called **aminotransferases (transaminases)**. **Pyridoxal phosphate (PLP)**, the active form of **Vitamin B6**, is the essential coenzyme for all transaminases. It acts as a temporary carrier of the amino group. During the reaction, PLP is converted to pyridoxamine phosphate (PMP) before transferring the amino group to the recipient keto acid. **2. Why other options are incorrect:** * **Vitamin C (Ascorbic Acid):** Acts as a reducing agent and is a coenzyme for hydroxylation reactions (e.g., prolyl hydroxylase in collagen synthesis). * **Biotin (Vitamin B7):** Serves as a coenzyme for **carboxylation** reactions (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). Remember: "Biotin carries $CO_2$." * **Thiamine (Vitamin B1):** In its active form, Thiamine Pyrophosphate (TPP), it is a coenzyme for **oxidative decarboxylation** (e.g., Pyruvate dehydrogenase) and the transketolase reaction in the HMP shunt. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Diagnostic Markers:** AST (SGOT) and ALT (SGPT) are transaminases used to assess liver function. ALT is more specific for liver injury. * **Exceptions:** All amino acids undergo transamination except **Lysine, Threonine, Proline, and Hydroxyproline**. * **Mechanism:** Transamination follows a **"Ping-Pong"** (Double Displacement) kinetic mechanism. * **Other PLP functions:** PLP is also required for decarboxylation (e.g., Histidine to Histamine), Heme synthesis ($\delta$-ALA synthase), and Cystathionine synthesis.

Question 215: The conversion of glutamate to glutamine is catalysed by which type of enzyme?

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

Explanation: **Explanation:** The conversion of glutamate to glutamine is catalyzed by the enzyme **Glutamine Synthetase**. This reaction involves the condensation of glutamate and ammonia, which requires the consumption of energy in the form of **ATP hydrolysis**. 1. **Why Ligase is correct:** According to the IUBMB classification, **Ligases (Class 6)** are enzymes that catalyze the joining of two molecules coupled with the breakdown of a high-energy phosphate bond (like ATP). Since Glutamine Synthetase joins glutamate and ammonia using ATP, it is a classic example of a ligase. 2. **Why other options are incorrect:** * **Lyases:** Catalyze the cleavage of bonds (C-C, C-O, C-N) by means other than hydrolysis or oxidation, often forming double bonds. * **Transferases:** Transfer a functional group (e.g., methyl or phosphate) from one substrate to another. * **Oxidoreductases:** Catalyze oxidation-reduction reactions (e.g., Dehydrogenases). **High-Yield Clinical Pearls for NEET-PG:** * **Ammonia Detoxification:** This reaction is the primary mechanism for ammonia detoxification in the **brain**. Glutamine is non-toxic and can safely cross the blood-brain barrier to be transported to the liver. * **Glutaminase vs. Synthetase:** Do not confuse these. *Glutaminase* (a Hydrolase) breaks down glutamine into glutamate and ammonia in the kidneys and liver, whereas *Glutamine Synthetase* (a Ligase) creates glutamine. * **Mnemonic:** "Ligases join things together like **Glue** (Glutamine Synthetase)."

Question 216: Flipped pattern of LDH is seen in which of the following conditions?

A. Muscular dystrophy
B. Acute myocardial infarction (Correct Answer)
C. Lymphoma
D. Hemolytic anemia

Explanation: **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme with five isoenzymes (LDH1 to LDH5). In a healthy individual, the concentration of **LDH2 (H3M1)** is higher than **LDH1 (H4)**. **1. Why Acute Myocardial Infarction (AMI) is correct:** LDH1 is primarily found in cardiac muscle and RBCs. Following an AMI, damaged myocardial cells release large amounts of LDH1 into the bloodstream. When the levels of LDH1 rise to exceed LDH2, the normal ratio is reversed (LDH1 > LDH2). This phenomenon is known as the **"Flipped Pattern."** While Troponins are the current gold standard for AMI, the LDH flip remains a classic biochemical marker, typically appearing 24–48 hours after the event and persisting for 7–10 days. **2. Why other options are incorrect:** * **Muscular Dystrophy:** Characterized by an increase in **LDH5** (found in skeletal muscle). * **Lymphoma:** Associated with a general increase in LDH levels (often LDH2, LDH3, and LDH4) due to high cell turnover, but not specifically a flipped LDH1/LDH2 ratio. * **Hemolytic Anemia:** While LDH1 increases (as it is present in RBCs), the "flipped pattern" is a term classically used in medical literature to describe the diagnostic shift specifically associated with **myocardial injury**. **High-Yield Clinical Pearls for NEET-PG:** * **LDH Isoenzymes:** LDH1 (Heart/RBC), LDH2 (Reticuloendothelial), LDH3 (Lungs), LDH4 (Kidney/Pancreas), LDH5 (Liver/Skeletal Muscle). * **Total LDH:** A non-specific marker of tissue injury or inflammation. * **Mega-tip:** If a question mentions "Flipped LDH" and both AMI and Hemolytic Anemia are options, **AMI** is the preferred clinical association for this specific terminology.

Question 217: Which enzyme is utilized by the Na/K pump?

A. ATPase (Correct Answer)
B. GTPase
C. Acetyl CoA
D. None of these

Explanation: **Explanation:** The **Na+/K+ pump** (also known as the Na+/K+-ATPase) is a classic example of **Primary Active Transport**. It moves 3 Na+ ions out of the cell and 2 K+ ions into the cell against their respective concentration gradients. 1. **Why ATPase is correct:** To move ions against a gradient, the pump requires energy. This energy is derived from the hydrolysis of **ATP into ADP and inorganic phosphate (Pi)**. The pump itself acts as an enzyme—specifically a **P-type ATPase**—which undergoes autophosphorylation to facilitate the conformational changes needed for ion transport. 2. **Why other options are incorrect:** * **GTPase:** These enzymes hydrolyze GTP (e.g., G-proteins in signal transduction or Ras proteins), but they are not the energy source for the Na+/K+ pump. * **Acetyl CoA:** This is a key metabolic intermediate in the TCA cycle and fatty acid synthesis, not an enzyme or a direct energy source for membrane pumps. **High-Yield Clinical Pearls for NEET-PG:** * **Stoichiometry:** 3 Na+ Out / 2 K+ In. This creates an **electrogenic** effect, contributing to the negative resting membrane potential. * **Inhibitors:** The pump is specifically inhibited by **Cardiac Glycosides** (e.g., **Digoxin** and Ouabain). Digoxin binds to the extracellular side of the pump, leading to increased intracellular Na+, which subsequently slows the Na+/Ca2+ exchanger, increasing intracellular Ca2+ and myocardial contractility. * **Energy Consumption:** In a resting state, this pump can consume up to 30-40% of a cell's total ATP, particularly in neurons.

Question 218: The provided graph illustrates the effect of substrate concentration on the initial velocity of an enzyme-catalyzed reaction. Identify the INCORRECT statement regarding this graph.

A. The curve is hyperbolic in shape.
B. The value represented by '?' on the x-axis is the Km of the enzyme.
C. At point C, only a small amount of the enzyme is present as the enzyme-substrate complex. (Correct Answer)
D. At point C, the initial velocity (Vi) is independent of substrate concentration.

Explanation: ***At point C, only a small amount of the enzyme is present as the enzyme-substrate complex.*** - This statement is **INCORRECT** because at point C (near **Vmax**), the substrate concentration is very high, meaning nearly **all enzyme molecules** are saturated and present as **enzyme-substrate (ES) complex**. - At high substrate concentrations, the enzyme is **maximally saturated**, not minimally bound as suggested by this statement. *The curve is hyperbolic in shape.* - This statement is **correct** as enzyme kinetics follows the **Michaelis-Menten equation**, which produces a characteristic **rectangular hyperbola**. - The curve shows rapid initial rise at low substrate concentrations, then plateaus as it approaches **Vmax**. *The value represented by '?' on the x-axis is the Km of the enzyme.* - This statement is **correct** as **Km** represents the substrate concentration at which the initial velocity is **half of Vmax**. - **Km** is a measure of enzyme **affinity** for its substrate - lower Km indicates higher affinity. *At point C, the initial velocity (Vi) is independent of substrate concentration.* - This statement is **correct** because at very high substrate concentrations (point C), the reaction follows **zero-order kinetics**. - The enzyme is **saturated** with substrate, so increasing substrate concentration further does not increase the reaction rate.

Question 219: Which of the following statements is true about allosteric inhibition?

A. Allosteric inhibitors are substrates of the enzyme.
B. Citrate and ATP inhibiting PFK-1 is a classical example of allosteric inhibition. (Correct Answer)
C. Allosteric inhibitors bind directly to the active site of the enzyme.
D. Allosteric regulation is only homotropic.

Explanation: **Explanation:** **1. Why Option B is Correct:** Allosteric inhibition occurs when an effector molecule binds to a site other than the active site (the **allosteric site**), causing a conformational change that reduces the enzyme's affinity for its substrate. Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme of glycolysis. It is inhibited by high levels of **ATP** and **Citrate**, which signal that the cell has sufficient energy and biosynthetic precursors. This is a classic example of feedback inhibition via allosteric regulation. **2. Why Other Options are Incorrect:** * **Option A:** Allosteric inhibitors are typically end-products or metabolic intermediates, not the substrates themselves. Substrates bind to the active site. * **Option C:** Allosteric inhibitors bind to a **regulatory (allosteric) site**, distinct from the active site. Inhibitors that bind directly to the active site are called **competitive inhibitors**. * **Option D:** Allosteric regulation can be **homotropic** (where the substrate itself acts as the effector, e.g., Oxygen binding to Hemoglobin) or **heterotropic** (where a different molecule acts as the effector, e.g., ATP inhibiting PFK-1). **3. High-Yield Clinical Pearls for NEET-PG:** * **Kinetics:** Allosteric enzymes do not follow Michaelis-Menten kinetics; they show a **Sigmoidal (S-shaped)** curve rather than a hyperbolic one. * **PFK-1 Activators:** While ATP/Citrate inhibit PFK-1, **Fructose-2,6-bisphosphate** and **AMP** are its most potent allosteric activators. * **Key Allosteric Enzymes:** Aspartate transcarbamoylase (inhibited by CTP) and Acetyl-CoA Carboxylase (activated by Citrate).

Question 220: All of the following can be attributed to enzymatic catalysis, EXCEPT:

A. Entropy reduction
B. Solvation of active site (Correct Answer)
C. Reduction of activation energy
D. Catalysis by strain

Explanation: **Explanation:** Enzymes function by lowering the activation energy of a reaction, thereby increasing the reaction rate. The correct answer is **Option B (Solvation of active site)** because enzymes actually achieve catalysis through **desolvation**. 1. **Why Option B is correct:** In an aqueous environment, substrates are surrounded by a "hydration shell" of water molecules. For a reaction to occur, this water must be removed so the substrate can interact directly with the enzyme. Enzymes displace these water molecules (**Desolvation**), which replaces the strong hydrogen bonds between the substrate and water with weaker, more specific interactions between the substrate and the enzyme's active site. This increases the reactivity of the substrate. 2. **Why other options are incorrect:** * **Entropy reduction (A):** Enzymes hold substrates in a specific orientation and proximity. This reduces the random translational and rotational motion (entropy) of the substrates, making the formation of the transition state more favorable. * **Reduction of activation energy (C):** This is the fundamental mechanism of all catalysts. By stabilizing the transition state, enzymes lower the energy barrier required for the reaction to proceed. * **Catalysis by strain (D):** Also known as the "Induced Fit" or "Induced Strain" model, the enzyme undergoes conformational changes that physically strain the bonds of the substrate, pushing it toward the transition state. **High-Yield NEET-PG Pearls:** * **Transition State Stabilization:** The most important way enzymes lower activation energy is by having a higher affinity for the *transition state* than for the substrate itself. * **Acid-Base Catalysis:** Often involves amino acids like **Histidine** (due to its pKa near physiological pH) acting as proton donors or acceptors. * **Covalent Catalysis:** Involves the formation of a transient covalent bond (e.g., Serine proteases).

Practice by Chapter

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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