Enzymes — MCQs
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In the reaction catalyzed by alcohol dehydrogenase (ADH), an alcohol is oxidized to an aldehyde as NAD+ is reduced to NADH and dissociates from the enzyme. ADH requires NAD+ for catalytic activity. In this reaction, NAD+ is functioning as a:
Which of the following enzymes is activated in its dephosphorylated state?
All of the following are tightly bound to component enzymes of pyruvate dehydrogenase, EXCEPT?
What is the primary function of Glucose-6-phosphate dehydrogenase (G6PD)?
Which class of enzymes catalyzes the transfer of electrons?
Which of the following statements about Cytochrome P450 is FALSE?
What is true regarding isozymes?
Which of the following statements regarding isoenzymes is false?
In non-competitive inhibition, where do inhibitors bind?
Peptidyl transferase is a/an
Enzymes Indian Medical PG Practice Questions and MCQs
Question 271: In the reaction catalyzed by alcohol dehydrogenase (ADH), an alcohol is oxidized to an aldehyde as NAD+ is reduced to NADH and dissociates from the enzyme. ADH requires NAD+ for catalytic activity. In this reaction, NAD+ is functioning as a:
- A. Apoenzyme
- B. Coenzyme-cosubstrate (Correct Answer)
- C. Coenzyme-prosthetic group
- D. Cofactor
Explanation: **Explanation:** The correct answer is **B. Coenzyme-cosubstrate.** **1. Why it is correct:** Enzymes often require non-protein components called **cofactors** for activity. Cofactors are divided into inorganic ions and organic molecules called **coenzymes**. Coenzymes are further classified based on their binding affinity: * **Cosubstrates:** These are loosely and transiently bound to the enzyme. They bind, undergo a chemical change (like NAD+ being reduced to NADH), and then **dissociate** from the enzyme to be regenerated by a different reaction. * In this case, NAD+ acts as a cosubstrate because it shuttles electrons from the alcohol and leaves the alcohol dehydrogenase (ADH) complex as NADH. **2. Why other options are incorrect:** * **A. Apoenzyme:** This refers to the protein portion of the enzyme that is catalytically inactive without its cofactor. * **C. Coenzyme-prosthetic group:** A prosthetic group is a coenzyme that is **tightly or covalently bound** to the enzyme and does not dissociate during the reaction (e.g., FAD in Succinate Dehydrogenase or Heme in Cytochrome c). * **D. Cofactor:** While NAD+ is technically a cofactor, "Coenzyme-cosubstrate" is the more specific and accurate description required for NEET-PG level biochemistry. **3. High-Yield Clinical Pearls for NEET-PG:** * **Alcohol Metabolism:** ADH is the rate-limiting enzyme in alcohol metabolism (Zero-order kinetics). It is inhibited by **Fomepizole** (used in methanol/ethylene glycol poisoning). * **Niacin (Vitamin B3):** NAD+ and NADP+ are derived from Niacin. Deficiency leads to **Pellagra** (4 Ds: Dermatitis, Diarrhea, Dementia, Death). * **Key Cosubstrates:** NAD+, NADP+, and Coenzyme A. * **Key Prosthetic Groups:** FAD, FMN, Biotin, and Pyridoxal Phosphate (PLP).
Question 272: Which of the following enzymes is activated in its dephosphorylated state?
- A. HMG Co A reductase (Correct Answer)
- B. Glycogen phosphorylase
- C. Glycogen phosphorylase kinase
- D. Citrate lyase
Explanation: ### Explanation In metabolic regulation, many key enzymes are controlled via **reversible covalent modification**, specifically phosphorylation and dephosphorylation. **1. Why HMG-CoA Reductase is Correct:** HMG-CoA reductase is the rate-limiting enzyme of cholesterol synthesis. It follows the general rule for **anabolic (synthetic) pathways**: they are typically **active in the dephosphorylated state**. * **Mechanism:** Insulin promotes the dephosphorylation of HMG-CoA reductase via protein phosphatase, thereby activating it. Conversely, Glucagon and AMPK trigger phosphorylation, which inactivates the enzyme to conserve energy. **2. Analysis of Incorrect Options:** * **B & C (Glycogen phosphorylase and Phosphorylase kinase):** These are key enzymes in **glycogenolysis** (a catabolic pathway). Catabolic enzymes are generally **active in their phosphorylated state**. This ensures that during fasting (high glucagon) or stress (high epinephrine), glycogen is broken down to release glucose. * **D (Citrate lyase):** ATP-citrate lyase, which provides acetyl-CoA for fatty acid synthesis, is actually **activated by phosphorylation** (specifically by Akt/PKB in response to insulin), making it an exception to the general rule that anabolic enzymes are dephosphorylated. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Thumb":** Most rate-limiting enzymes of **Anabolic** pathways (e.g., Glycogen synthase, HMG-CoA reductase, Acetyl-CoA carboxylase) are **Active when Dephosphorylated**. * Most **Catabolic** enzymes (e.g., Glycogen phosphorylase, Hormone-sensitive lipase) are **Active when Phosphorylated**. * **Statins:** These drugs are competitive inhibitors of HMG-CoA reductase, mimicking the structure of the HMG-CoA substrate. * **AMPK:** This "energy sensor" inhibits HMG-CoA reductase by phosphorylating it when cellular ATP levels are low.
Question 273: All of the following are tightly bound to component enzymes of pyruvate dehydrogenase, EXCEPT?
- A. Thiamine pyrophosphate
- B. Lipoic acid
- C. Flavin adenine dinucleotide
- D. Coenzyme A (Correct Answer)
Explanation: The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme cluster that converts pyruvate to Acetyl-CoA. It requires five distinct cofactors. The distinction between "tightly bound" (prosthetic groups) and "mobile" (transient) cofactors is a high-yield NEET-PG concept. ### 1. Why Coenzyme A is the Correct Answer **Coenzyme A (CoA)** and **NAD+** are considered **mobile carriers** or "dissociable" cofactors. They are not permanently attached to the enzyme complex. Instead, they enter the reaction, pick up the products (the Acetyl group and electrons, respectively), and then diffuse away to participate in other metabolic pathways (like the TCA cycle or Electron Transport Chain). ### 2. Why the Other Options are Incorrect The remaining three cofactors are **prosthetic groups**, meaning they are covalently or very tightly bound to their respective sub-enzymes: * **Thiamine Pyrophosphate (TPP):** Tightly bound to **E1** (Pyruvate decarboxylase). It is essential for the decarboxylation step. * **Lipoic Acid (Lipoamide):** Tightly bound to **E2** (Dihydrolipoyl transacetylase) via a lysine residue. It swings between active sites to transfer the acetyl group. * **Flavin Adenine Dinucleotide (FAD):** Tightly bound to **E3** (Dihydrolipoyl dehydrogenase). It accepts electrons from lipoamide to become FADH₂. ### 3. Clinical Pearls for NEET-PG * **The "Tender Loving Care For No One" Mnemonic:** TPP, Lipoic Acid, CoA, FAD, NAD. * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the **SH groups of Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Leads to Beriberi and Wernicke-Korsakoff syndrome because PDH and Alpha-ketoglutarate dehydrogenase cannot function without TPP. * **Location:** The PDH complex is located in the **mitochondrial matrix**.
Question 274: What is the primary function of Glucose-6-phosphate dehydrogenase (G6PD)?
- A. Oxidizes NADPH
- B. Reduces NADP+ (Correct Answer)
- C. Oxidizes NAD
- D. Reduces NAD
Explanation: **Explanation:** Glucose-6-phosphate dehydrogenase (G6PD) is the **rate-limiting enzyme** of the Pentose Phosphate Pathway (Hexose Monophosphate Shunt). Its primary function is to catalyze the oxidation of Glucose-6-phosphate to 6-phosphogluconolactone. During this reaction, electrons are transferred to the coenzyme **NADP+**, thereby **reducing it to NADPH**. **Why Option B is correct:** The reaction catalyzed by G6PD is: *Glucose-6-P + NADP+ → 6-phosphogluconolactone + NADPH + H+* By reducing NADP+, G6PD ensures a steady supply of NADPH, which is essential for reductive biosynthesis (e.g., fatty acids) and for maintaining the pool of **reduced glutathione** to protect cells against oxidative stress. **Why other options are incorrect:** * **Option A:** G6PD does not oxidize NADPH; it produces it. NADPH is oxidized back to NADP+ by enzymes like Glutathione Reductase or in biosynthetic pathways. * **Options C & D:** G6PD is highly specific for the NADP+/NADPH cofactor system. It does not utilize the NAD+/NADH system, which is primarily involved in ATP-generating catabolic pathways (like Glycolysis or the TCA cycle). **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **Non-immune Hemolytic Anemia** because RBCs lack mitochondria and rely solely on G6PD for NADPH to neutralize reactive oxygen species (ROS). * **Heinz Bodies:** Denatured hemoglobin precipitates seen in G6PD deficiency. * **Bite Cells:** Result from splenic macrophages removing Heinz bodies. * **Triggers:** Hemolysis is typically triggered by infections, Fava beans, or drugs (e.g., Primaquine, Sulphonamides, Dapsone).
Question 275: Which class of enzymes catalyzes the transfer of electrons?
- A. Oxidoreductases (Correct Answer)
- B. Transferases
- C. Lyases
- D. Ligases
Explanation: ### Explanation **1. Why Oxidoreductases is Correct:** Oxidoreductases (EC Class 1) are enzymes that catalyze **oxidation-reduction (redox) reactions**. Oxidation involves the loss of electrons (or hydrogen), while reduction involves the gain of electrons. These enzymes facilitate the transfer of electrons from a reductant (electron donor) to an oxidant (electron acceptor). They typically utilize cofactors like **NAD⁺/NADH, FAD/FADH₂, or NADP⁺**. Common examples include dehydrogenases, oxidases, and reductases. **2. Why the Other Options are Incorrect:** * **Transferases (EC 2):** These enzymes catalyze the transfer of a **functional group** (e.g., methyl, phosphate, or amino groups) from one molecule to another. They do not primarily involve electron transfer. *Example: Hexokinase (transfers phosphate).* * **Lyases (EC 4):** These enzymes catalyze the **cleavage of bonds** (C-C, C-O, C-N) by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of groups to double bonds. *Example: Aldolase.* * **Ligases (EC 6):** These enzymes catalyze the **joining of two large molecules** by forming new chemical bonds, a process that requires energy input, usually from **ATP hydrolysis**. *Example: DNA Ligase, Pyruvate carboxylase.* **3. NEET-PG High-Yield Pearls:** * **IUBMB Classification:** Remember the mnemonic **"O.T.H.L.I.L."** to recall the six classes in order: **O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases. * **Dehydrogenases:** These are the most clinically significant oxidoreductases in the TCA cycle and Glycolysis (e.g., Lactate Dehydrogenase/LDH). * **Clinical Correlation:** LDH isoenzymes are diagnostic markers; LDH-1 is elevated in myocardial infarction, while LDH-5 is elevated in liver disease.
Question 276: Which of the following statements about Cytochrome P450 is FALSE?
- A. It is exclusively seen in the ER of cells of the liver and intestines.
- B. Peroxidase converts H2O2 at the expense of electron acceptors. (Correct Answer)
- C. Catalase uses H2O2 as both electron acceptor and donor.
- D. Superoxide ion is the most toxic form.
Explanation: ### Explanation This question tests the fundamental understanding of **Heme-containing enzymes** and their roles in oxidative stress and drug metabolism. **Why Option B is the correct answer (The FALSE statement):** Peroxidases (like Glutathione Peroxidase) reduce $H_2O_2$ to water, but they do so at the expense of **electron donors** (reducing agents), not electron acceptors. For example, Glutathione Peroxidase requires **Reduced Glutathione (GSH)** as a donor. The statement incorrectly identifies the role of the cofactor. **Analysis of other options:** * **Option A:** While Cytochrome P450 (CYP450) is most abundant in the liver and intestines, it is primarily located in the **Smooth Endoplasmic Reticulum** (microsomal fraction). *Note: Some sources mention mitochondrial CYP450 in steroidogenic tissues, but in the context of drug metabolism, the ER is the classic site.* * **Option C:** **Catalase** is unique because it performs a dismutation reaction where two molecules of $H_2O_2$ react; one acts as an electron donor (oxidized to $O_2$) and the other as an acceptor (reduced to $H_2O$), making the statement true. * **Option D:** While the hydroxyl radical ($\cdot OH$) is often cited as the most reactive, the **Superoxide ion ($O_2^{\cdot -}$)** is considered the "primary" ROS that initiates the chain of oxidative damage and is highly toxic, often leading to the formation of other lethal radicals. **High-Yield Clinical Pearls for NEET-PG:** * **CYP450 System:** It is a **Monooxygenase** (Mixed Function Oxidase). It incorporates one atom of oxygen into the substrate and reduces the other into water. * **Inducers vs. Inhibitors:** Remember **GP CELL** (Griseofulvin, Phenytoin, Carbamazepine, Ethanol, Rifampicin, Phenobarbitone) as Inducers and **VITAMIN K** (Valproate, INH, Timetidine, Amiodarone, Macrolides, Ketoconazole) as Inhibitors. * **Glutathione Peroxidase:** Contains **Selenium** at its active site—a frequent NEET-PG fact.
Question 277: What is true regarding isozymes?
- A. Forms of the same enzymes that catalyze different reactions
- B. Forms of the same enzymes that catalyze the same reaction (Correct Answer)
- C. Forms of different enzymes that catalyze different reactions
- D. Forms of different enzymes that catalyze the same reaction
Explanation: **Explanation:** **Isozymes (or Isoenzymes)** are physically distinct forms of the same enzyme. The core concept is that they possess **different amino acid sequences** (encoded by different gene loci) but **catalyze the same chemical reaction**. 1. **Why Option B is correct:** Isozymes are "iso-functional." While they differ in their primary structure, physical properties (like electrophoretic mobility), and kinetic parameters ($K_m$ and $V_{max}$), they act upon the same substrate to produce the same product. This allows for fine-tuned metabolic regulation in different tissues or organelles. 2. **Why incorrect options are wrong:** * **Options A & C:** If enzymes catalyze different reactions, they are simply different enzymes (e.g., Hexokinase vs. Glucose-6-Phosphatase), not isozymes. * **Option D:** Isozymes are considered variants of the *same* enzyme family, not entirely "different" enzymes, as they share the same EC (Enzyme Commission) number. **High-Yield Clinical Pearls for NEET-PG:** * **Lactate Dehydrogenase (LDH):** A tetramer with 5 isoforms. **LDH-1 (H4)** is predominant in the heart, while **LDH-5 (M4)** is found in skeletal muscle and liver. A "flipped pattern" (LDH-1 > LDH-2) is a classic marker for Myocardial Infarction. * **Creatine Kinase (CK):** A dimer with 3 isoforms. **CK-MB** is specific for cardiac muscle; **CK-MM** for skeletal muscle; **CK-BB** for the brain. * **Hexokinase vs. Glucokinase:** These are functional isozymes. Glucokinase (Hexokinase IV) is found in the liver/pancreas and has a **high $K_m$** (low affinity), allowing it to respond to high post-prandial glucose levels.
Question 278: Which of the following statements regarding isoenzymes is false?
- A. They differ in tissue localization
- B. They catalyze the same reaction
- C. They can be separated by electrophoresis
- D. Isoenzymes have the same Km values (Correct Answer)
Explanation: ### Explanation **Isoenzymes** (or isozymes) are physically distinct forms of the same enzyme. The core concept is that while they perform the exact same biochemical function, they are structurally different because they are encoded by different genes or gene loci. #### Why Option D is the Correct (False) Statement: Isoenzymes differ in their **amino acid sequences**, which results in different three-dimensional conformations at their active sites. Consequently, they possess **different kinetic properties**, specifically different **Km (Michaelis constant)** and **Vmax** values. * *Example:* **Glucokinase** (Liver) has a high Km for glucose (low affinity), while **Hexokinase** (Muscle) has a low Km (high affinity). This allows the liver to only process glucose when blood levels are high. #### Why Other Options are Incorrect (True Statements): * **A. Tissue Localization:** Isoenzymes are often tissue-specific. For instance, LDH-1 is predominant in the heart, while LDH-5 is found in the liver and skeletal muscle. * **B. Catalyze the same reaction:** By definition, isoenzymes catalyze the same chemical transformation (e.g., all LDH isoenzymes convert pyruvate to lactate). * **C. Separated by electrophoresis:** Because they have different amino acid compositions, they carry different net charges. This allows them to be separated based on their electrophoretic mobility. #### High-Yield Clinical Pearls for NEET-PG: 1. **LDH (Lactate Dehydrogenase):** A tetramer with 5 isoforms. **"Flipped Pattern"** (LDH1 > LDH2) is a classic (though older) marker for Myocardial Infarction. 2. **CK (Creatine Kinase):** * **CK-MB:** Cardiac muscle (Marker for MI). * **CK-MM:** Skeletal muscle. * **CK-BB:** Brain. 3. **Alkaline Phosphatase (ALP):** Isoenzymes help differentiate the source of pathology (e.g., **Regan isoenzyme** is a carcinofetal marker seen in some cancers).
Question 279: In non-competitive inhibition, where do inhibitors bind?
- A. The inhibitor binds to the enzyme at the substrate binding site.
- B. The inhibitor binds to the enzyme at a site distinct from the substrate binding site. (Correct Answer)
- C. The inhibitor can bind to either the substrate binding site or a distinct site.
- D. None of the above.
Explanation: **Explanation:** **Non-competitive inhibition** occurs when an inhibitor binds to an enzyme at a site other than the active site (the **allosteric site**). 1. **Why Option B is Correct:** In non-competitive inhibition, the inhibitor has no structural similarity to the substrate. Therefore, it does not compete for the active site. It binds to a distinct site on either the free enzyme or the enzyme-substrate (ES) complex. This binding induces a conformational change that reduces the catalytic activity ($V_{max}$) of the enzyme, regardless of how much substrate is added. 2. **Why Other Options are Incorrect:** * **Option A:** This describes **Competitive Inhibition**, where the inhibitor mimics the substrate and competes for the active site. This can be overcome by increasing substrate concentration ($K_m$ increases, $V_{max}$ remains unchanged). * **Option C:** This describes **Mixed Inhibition**, a more complex model where the inhibitor's binding affects the affinity for the substrate, altering both $K_m$ and $V_{max}$. **High-Yield Clinical Pearls for NEET-PG:** * **Kinetics:** In non-competitive inhibition, **$V_{max}$ decreases** (the enzyme is effectively "poisoned"), but **$K_m$ remains unchanged** (the affinity for the substrate at the active site is unaffected). * **Classic Examples:** * Cyanide inhibition of Cytochrome Oxidase. * Heavy metal poisoning (e.g., Lead, Mercury) affecting various enzymes. * Fluoride inhibiting Enolase (used in blood collection vials to prevent glycolysis). * **Lineweaver-Burk Plot:** The plots for inhibited and uninhibited enzymes intersect on the **negative X-axis** (same $-1/K_m$).
Question 280: Peptidyl transferase is a/an
- A. Enzyme
- B. Catalyst
- C. Ribozyme (Correct Answer)
- D. Elongation factor
Explanation: **Explanation:** **1. Why Ribozyme is the correct answer:** Peptidyl transferase is the primary enzyme responsible for peptide bond formation during protein synthesis (translation). Unlike most enzymes which are proteins, peptidyl transferase is a **Ribozyme**—an RNA molecule with catalytic activity. Specifically, in the large ribosomal subunit (60S in eukaryotes, 50S in prokaryotes), it is the **23S rRNA** (prokaryotes) or **28S rRNA** (eukaryotes) that catalyzes the transfer of the amino acid from the tRNA in the P-site to the aminoacyl-tRNA in the A-site. **2. Why other options are incorrect:** * **Enzyme:** While peptidyl transferase functions as an enzyme, "Ribozyme" is the more specific and accurate biochemical classification required for NEET-PG. Most enzymes are proteins; ribozymes are the notable exception. * **Catalyst:** This is a broad term. While all ribozymes are biological catalysts, the question tests the specific structural nature of this molecule. * **Elongation factor:** Elongation factors (like EF-Tu or EF-G) are proteins that facilitate the translation process (e.g., bringing tRNA to the ribosome or translocation), but they do not possess the catalytic activity to form peptide bonds. **3. Clinical Pearls & High-Yield Facts:** * **Mechanism of Action:** Peptidyl transferase catalyzes the nucleophilic attack of the A-site amino group on the P-site ester linkage. * **Antibiotic Link:** **Chloramphenicol** is a high-yield antibiotic that acts by inhibiting the peptidyl transferase activity of the bacterial 50S ribosomal subunit. * **Other Ribozymes:** Apart from the ribosome, other examples include **SnRNAs** (involved in splicing) and **Ribonuclease P** (involved in tRNA processing).
Practice by Chapter
Enzyme Classification and Nomenclature
Practice Questions
Enzyme Kinetics and Michaelis-Menten Equation
Practice Questions
Enzyme Inhibition: Competitive and Non-competitive
Practice Questions
Allosteric Regulation
Practice Questions
Coenzymes and Cofactors
Practice Questions
Isoenzymes and Clinical Significance
Practice Questions
Enzyme Regulation: Covalent Modification
Practice Questions
Enzyme Regulation: Zymogen Activation
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Enzyme Induction and Repression
Practice Questions
Ribozymes and Catalytic RNA
Practice Questions
Enzyme Diagnostic Applications
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Enzyme Therapy and Inhibitors as Drugs
Practice Questions
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