Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 371: The pyruvate dehydrogenase multienzyme complex is responsible for all the following reactions EXCEPT:

A. A decarboxylation reaction
B. Production of a molecule of ATP (Correct Answer)
C. Formation of an acetyl group from pyruvate
D. Combination of the acetyl group with a cofactor

Explanation: **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a mitochondrial multienzyme system that serves as the bridge between glycolysis and the TCA cycle. It converts pyruvate into Acetyl-CoA through a process known as **oxidative decarboxylation**. **Why Option B is correct:** The PDH complex does **not** produce ATP directly. Instead, it reduces $NAD^+$ to **NADH**. This NADH later enters the Electron Transport Chain (ETC) to generate ATP via oxidative phosphorylation. Direct ATP (or GTP) production in the carbohydrate metabolic pathway occurs via substrate-level phosphorylation in glycolysis and the TCA cycle (Succinate thiokinase reaction), but not in the PDH reaction. **Analysis of Incorrect Options:** * **Option A:** PDH catalyzes the removal of a carboxyl group from pyruvate as $CO_2$. This is the "decarboxylation" step performed by the $E_1$ subunit (Pyruvate dehydrogenase). * **Option C:** The remaining two-carbon unit after decarboxylation is oxidized to form an **acetyl group** (hydroxyethyl group initially). * **Option D:** The acetyl group is eventually transferred to **Coenzyme A (CoA-SH)** to form Acetyl-CoA, which is the final product that enters the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Coenzymes (The "Tender Loving Care For Nancy" Mnemonic):** PDH requires five cofactors: **T**hiamine pyrophosphate ($B_1$), **L**ipoic acid, **C**oenzyme A ($B_5$), **F**AD ($B_2$), and **N**AD ($B_3$). * **Deficiency:** Thiamine deficiency (Beriberi/Wernicke-Korsakoff) leads to PDH failure, causing lactic acidosis and neurological symptoms. * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to similar clinical features as thiamine deficiency.

Question 372: Sodium fluoride is added to blood, as it inhibits which enzyme?

A. Hexokinase
B. Glucokinase
C. Glucose-6-phosphatase
D. Enolase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Enolase**. Sodium fluoride (NaF) is routinely added to blood collection tubes (grey-top tubes) used for glucose estimation to prevent **in vitro glycolysis**. **Why Enolase?** Enolase is the ninth enzyme in the glycolytic pathway, responsible for converting 2-phosphoglycerate to phosphoenolpyruvate (PEP). Fluoride ions inhibit this enzyme by forming a complex with **magnesium (Mg²⁺)** and phosphate. Since Enolase requires magnesium as a cofactor, the formation of the **magnesium-fluorophosphate complex** effectively removes the available magnesium, thereby competitively inhibiting the enzyme and halting glycolysis. This ensures that the glucose concentration in the sample remains stable until it reaches the laboratory. **Analysis of Incorrect Options:** * **A & B (Hexokinase/Glucokinase):** These enzymes catalyze the first step of glycolysis (glucose to glucose-6-phosphate). While they are essential for the pathway, they are not the targets of fluoride inhibition. * **C (Glucose-6-phosphatase):** This enzyme is involved in gluconeogenesis and glycogenolysis (converting glucose-6-phosphate back to glucose), primarily in the liver and kidneys. It is not a target for fluoride in blood collection tubes. **Clinical Pearls for NEET-PG:** * **The 1:3 Ratio:** In grey-top tubes, Sodium Fluoride is often combined with **Potassium Oxalate** (an anticoagulant) in a 1:3 ratio. * **Time Sensitivity:** Fluoride inhibition of enolase is not instantaneous; it takes about 1–2 hours to fully take effect. * **Other Inhibitors:** Remember that **Iodoacetate** is another inhibitor of glycolysis, specifically targeting **Glyceraldehyde-3-phosphate dehydrogenase**.

Question 373: Which of the following is a lyase?

A. Decarboxylase (Correct Answer)
B. Synthetase
C. Kinase
D. Oxygenase

Explanation: ### Explanation Enzymes are classified into six major classes based on the International Union of Biochemistry (IUB) system, remembered by the mnemonic **OTH LIL**. **1. Why Decarboxylase is the Correct Answer:** **Lyases (Class 4)** are enzymes that catalyze the cleavage of C-C, C-O, C-N, and other bonds by means other than hydrolysis or oxidation. This often results in the formation of a double bond or the addition of a group to a double bond. **Decarboxylases** remove a carboxyl group ($CO_2$) from a substrate without using water or redox reactions, making them classic examples of Lyases (e.g., Pyruvate decarboxylase). **2. Analysis of Incorrect Options:** * **Synthetase (Class 6 - Ligases):** These enzymes catalyze the joining of two molecules coupled with the hydrolysis of a high-energy phosphate bond (ATP). *Note: "Synthases" are Lyases, but "Synthetases" are Ligases.* * **Kinase (Class 2 - Transferases):** Kinases specifically transfer a phosphate group from a high-energy donor (like ATP) to a substrate. * **Oxygenase (Class 1 - Oxidoreductases):** These enzymes catalyze oxidation-reduction reactions by incorporating oxygen into a substrate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dehydrogenases** are the most common Oxidoreductases (Class 1) encountered in the TCA cycle. * **Hydrolases (Class 3)** include digestive enzymes like Pepsin and Trypsin. * **Isomerases (Class 5)** catalyze structural rearrangements (e.g., Mutases, Epimerases). * **Key Distinction:** **Synthases** (Lyase) do NOT require ATP, whereas **Synthetases** (Ligase) DO require ATP. This is a frequent "trap" in PG entrance exams.

Question 374: Serum alkaline phosphatase is greatly increased in which of the following conditions?

A. Haemolytic jaundice
B. Hepatic jaundice
C. Obstructive jaundice (Correct Answer)
D. None of these

Explanation: ### Explanation **Correct Answer: C. Obstructive jaundice** **Mechanism:** Alkaline Phosphatase (ALP) is an enzyme found primarily in the cells lining the biliary canaliculi of the liver. In **obstructive (post-hepatic) jaundice**, the flow of bile is blocked (e.g., by gallstones or tumors). This obstruction triggers increased synthesis of ALP by the canalicular cells and causes the enzyme to leak into the bloodstream. Because ALP is highly sensitive to biliary pressure, its levels typically rise significantly (**3 to 10 times the upper limit of normal**) in obstructive conditions compared to other liver diseases. **Analysis of Incorrect Options:** * **A. Haemolytic jaundice:** This is a pre-hepatic condition caused by excessive breakdown of RBCs. The liver's biliary system remains intact; therefore, ALP levels are usually normal or only minimally elevated. * **B. Hepatic jaundice:** In hepatocellular diseases like viral hepatitis, the primary damage is to the hepatocytes. While ALP may rise slightly due to inflammation, the hallmark is a massive increase in transaminases (ALT/AST). ALP elevation here is usually mild (less than 3 times normal). **High-Yield Clinical Pearls for NEET-PG:** * **Marker of Cholestasis:** ALP and GGT (Gamma-Glutamyl Transferase) are the primary biochemical markers for cholestasis. * **Bone vs. Liver:** ALP is also found in osteoblasts. To differentiate if a high ALP is from bone or liver, check **GGT** or **5'-nucleotidase** (these are NOT elevated in bone disease). * **Heat Stability:** A common mnemonic for ALP isoenzymes is *"Regan is the most heat-stable"* (Regan isoenzyme is a marker for certain cancers). * **Other causes of high ALP:** Pregnancy (placental isoenzyme), bone growth in children, and Paget’s disease of the bone.

Question 375: Which of the following is NOT a mechanism by which enzymes act?

A. Forming non-covalent interactions
B. Catalyzing the reaction
C. Increasing activation energy (Correct Answer)
D. Increasing the rate of reaction

Explanation: **Explanation:** The fundamental principle of enzyme kinetics is that enzymes act as biological catalysts to speed up chemical reactions without being consumed in the process. **Why "Increasing activation energy" is the correct answer:** Activation energy ($E_a$) is the energy barrier that reactants must overcome to be converted into products. Enzymes work by **lowering** the activation energy, not increasing it. By providing an alternative reaction pathway with a lower energy threshold, a larger fraction of molecules can reach the transition state at body temperature, thereby accelerating the reaction. Increasing the activation energy would make the reaction slower and more difficult to occur. **Analysis of incorrect options:** * **Forming non-covalent interactions:** Enzymes stabilize the transition state through weak, non-covalent forces (hydrogen bonds, ionic interactions, and Van der Waals forces). This release of "binding energy" is what lowers the activation energy. * **Catalyzing the reaction:** This is the definition of an enzyme. They increase the velocity of a reaction toward equilibrium. * **Increasing the rate of reaction:** By lowering the $E_a$, enzymes significantly increase the reaction rate (often by factors of $10^6$ to $10^{12}$). **High-Yield NEET-PG Pearls:** * **Thermodynamics:** Enzymes change the **rate** of reaction but do **not** change the equilibrium constant ($K_{eq}$) or the standard free-energy change ($\Delta G$). * **Transition State:** Enzymes have the highest affinity for the **transition state** of the substrate, rather than the substrate itself (Linus Pauling’s principle). * **Active Site Models:** The "Induced Fit Model" (Koshland) is more accurate than the "Lock and Key Model" as it accounts for the conformational flexibility of enzymes.

Question 376: Which metal is essential for the function of the enzyme enolase?

A. Magnesium (Correct Answer)
B. Calcium
C. Copper
D. Iron

Explanation: **Explanation:** **Enolase** (also known as phosphopyruvate hydratase) is a key glycolytic enzyme that catalyzes the reversible conversion of **2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP)**. 1. **Why Magnesium (Mg²⁺) is correct:** Enolase is a metalloenzyme that requires divalent metal ions for its catalytic activity. **Magnesium** is the natural physiological activator. It plays two roles: it helps stabilize the substrate binding at the active site and participates directly in the dehydration mechanism by coordinating with the carboxylate group of the substrate. 2. **Why other options are incorrect:** * **Calcium (Ca²⁺):** While chemically similar to magnesium, calcium does not activate enolase; in fact, it can act as a competitive inhibitor in certain contexts. * **Copper (Cu²⁺):** Copper is a cofactor for enzymes involved in redox reactions, such as *Cytochrome c oxidase* and *Superoxide dismutase*, but not for enolase. * **Iron (Fe²⁺/Fe³⁺):** Iron is essential for heme-containing enzymes (e.g., Catalase) and iron-sulfur cluster proteins (e.g., Aconitase), but it is not required for the dehydration step in glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Fluoride Inhibition:** Enolase is clinically significant because it is strongly inhibited by **Fluoride**. In clinical practice, fluoride (as sodium fluoride) is added to blood collection tubes (grey top) to inhibit glycolysis, ensuring accurate blood glucose estimation by preventing the breakdown of glucose by RBCs. * **Mechanism:** Fluoride forms a complex with magnesium and phosphate (**Magnesium-fluorophosphate**), which displaces the Mg²⁺ ion from the enzyme's active site, thereby inactivating it. * **Diagnostic Marker:** **Neuron-specific enolase (NSE)** is a high-yield tumor marker used for Small Cell Lung Carcinoma and Neuroblastoma.

Question 377: Which one of the following isoenzyme variants of Creatine kinase is elevated in myocardial infarction?

A. CK-BB
B. CK-MB (Correct Answer)
C. CK-MM
D. All the above

Explanation: **Explanation:** Creatine Kinase (CK) is a dimeric enzyme composed of two subunits: **M** (Muscle) and **B** (Brain). These subunits combine to form three distinct isoenzymes, each specific to different tissues. **Why CK-MB is the correct answer:** **CK-MB (CK-2)** is primarily found in the **myocardium** (heart muscle). When myocardial cells are damaged during a Myocardial Infarction (MI), this enzyme leaks into the bloodstream. It typically rises within 4–6 hours of chest pain, peaks at 18–24 hours, and returns to baseline within 48–72 hours. While Troponins are now the preferred biomarkers due to higher sensitivity, CK-MB remains clinically significant for detecting **re-infarction** because of its rapid clearance from the blood. **Analysis of Incorrect Options:** * **CK-BB (CK-1):** Predominantly found in the **Brain** and lungs. Elevated levels are seen in CNS damage, strokes, or certain tumors, but not typically in MI. * **CK-MM (CK-3):** The major isoenzyme in **Skeletal Muscle** (comprising 99% of muscle CK). It rises in conditions like muscular dystrophy, rhabdomyolysis, or strenuous exercise. * **All the above:** Incorrect, as the elevation is specific to the tissue involved. **High-Yield Clinical Pearls for NEET-PG:** * **CK-MB Index:** If the CK-MB/Total CK ratio is **>5%**, it strongly suggests myocardial origin rather than skeletal muscle damage. * **Total CK:** CK-MM is the most abundant isoenzyme in the serum of healthy individuals. * **Troponin T/I:** These are the "Gold Standard" markers for MI due to their high cardiac specificity. * **Re-infarction:** If a patient has a second MI within 3–4 days, CK-MB is the marker of choice for diagnosis (as Troponins remain elevated for up to 10–14 days).

Question 378: What is the effect of non-competitive inhibition on enzyme kinetics?

A. Decreased Km and decreased Vmax
B. Normal Km and decreased Vmax (Correct Answer)
C. Decreased Km and increased Vmax
D. Normal Km and increased Vmax

Explanation: In **Non-competitive inhibition**, the inhibitor binds to an allosteric site (a site other than the active site) on both the free enzyme (E) and the enzyme-substrate complex (ES). ### Why Option B is Correct: * **Vmax (Decreased):** Because the inhibitor binds to a different site, it effectively reduces the total concentration of functional enzyme available to convert substrate to product. No amount of additional substrate can "outcompete" the inhibitor; therefore, the maximum velocity of the reaction is permanently lowered. * **Km (Normal/Unchanged):** Since the inhibitor does not compete for the active site, the affinity of the remaining functional enzymes for the substrate remains unchanged. Thus, the Michaelis constant ($K_m$) stays the same. ### Why Other Options are Incorrect: * **Option A & C:** A **decreased $K_m$** (increased affinity) is characteristic of **Uncompetitive inhibition**, where the inhibitor binds only to the ES complex. * **Option D:** An **increased Vmax** is physiologically impossible in the presence of an inhibitor; inhibitors by definition decrease the rate or efficiency of a reaction. * **Note on Competitive Inhibition:** In competitive inhibition (the most common distractor), $V_{max}$ remains **normal** while $K_m$ **increases**. ### High-Yield NEET-PG Pearls: 1. **Lineweaver-Burk Plot:** In non-competitive inhibition, the plots intersect on the **negative X-axis** ($-1/K_m$ is constant). 2. **Reversibility:** Non-competitive inhibition is usually reversible. If the binding is irreversible (e.g., Lead poisoning or Organophosphates), it is often termed "Irreversible inhibition," which kinetically mimics non-competitive inhibition. 3. **Classic Example:** Cyanide inhibition of Cytochrome Oxidase.

Question 379: All of the following are true about allosteric enzymes, EXCEPT:

A. Allosteric enzymes can have quaternary structure. (Correct Answer)
B. Allosteric enzymes have a regulatory site other than the active site.
C. Allosteric enzymes do not show Michaelis-Menten kinetics.
D. Allosteric modulators bind noncovalently to the allosteric site.

Explanation: **Explanation:** The correct answer is **A**, but it is important to note that in the context of this question, Option A is a **true statement**, whereas the question asks for the **exception**. In NEET-PG biochemistry, allosteric enzymes are characterized by their complex regulatory mechanisms which differ significantly from simple enzymes. 1. **Why Option A is the "Exception" (Contextual Note):** Most allosteric enzymes are **multimeric** (possess quaternary structure) because they require multiple subunits to exhibit cooperativity. While Option A is scientifically true, in many MCQ formats, if all options are true, the question may be testing the most defining characteristic. However, strictly speaking, all options (A, B, C, and D) are true statements regarding allosteric enzymes. If this were a "find the false statement" question, there may be a typographical error in the source; however, for learning purposes, all four points define allosteric behavior. 2. **Analysis of Other Options:** * **Option B:** True. By definition, "allosteric" means "other site." These enzymes have a **regulatory (allosteric) site** distinct from the catalytic (active) site. * **Option C:** True. Allosteric enzymes show a **Sigmoidal (S-shaped) curve** rather than the Hyperbolic curve seen in Michaelis-Menten kinetics. They do not follow the standard $K_m$ and $V_{max}$ equations. * **Option D:** True. Modulators (activators or inhibitors) generally bind **noncovalently**, allowing for rapid, reversible metabolic physiological control. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting steps:** Allosteric enzymes usually catalyze the committed/rate-limiting step of a pathway (e.g., **PFK-1** in Glycolysis). * **Cooperativity:** They exhibit "T" (Tense/low affinity) and "R" (Relaxed/high affinity) states. * **K-series vs. V-series:** K-series enzymes change their affinity ($K_{0.5}$), while V-series enzymes change their maximal velocity ($V_{max}$) in response to modulators. * **Aspartate Transcarbamoylase (ATCase):** The classic example of an allosteric enzyme.

Question 380: Which of the following justifies the statement that not all enzymes are proteins?

A. Ribonucleic acids (RNAs) can function as enzymes (ribozymes). (Correct Answer)
B. Antibodies participate in the catalysis of many reactions.
C. Not all enzymes adhere to the Michaelis-Menten kinetics.
D. Metals are involved in the binding to enzymes and act as catalysts.

Explanation: **Explanation:** The fundamental definition of an enzyme has evolved from "all enzymes are proteins" to "most enzymes are proteins" due to the discovery of **Ribozymes**. **1. Why Option A is Correct:** Ribozymes are catalytic **RNA molecules** that can facilitate biochemical reactions, such as peptide bond formation and RNA splicing, without being proteins. The most significant example is the **23S rRNA** (in prokaryotes) or **28S rRNA** (in eukaryotes) of the large ribosomal subunit, which acts as a peptidyl transferase to link amino acids during translation. This proves that biological catalysis is not exclusive to protein structures. **2. Analysis of Incorrect Options:** * **Option B:** While **Abzymes** (antibody enzymes) exist, they are essentially specialized proteins (immunoglobulins) with catalytic activity. They do not disprove the protein nature of enzymes. * **Option C:** Michaelis-Menten kinetics describe the rate of reaction for many enzymes, but **allosteric enzymes** follow a sigmoid curve instead. This relates to the *kinetics* of the enzyme, not its chemical composition. * **Option D:** Metals act as **cofactors** (metalloenzymes or metal-activated enzymes). They assist the protein part (apoenzyme) but do not function as independent non-protein enzymes themselves. **High-Yield Clinical Pearls for NEET-PG:** * **Peptidyl Transferase:** The most important ribozyme in humans; it is a component of the ribosome. * **RNase P:** A ribozyme involved in processing tRNA molecules. * **Spliceosomes:** Utilize small nuclear RNAs (snRNAs) to catalyze the removal of introns. * **Telomerase:** A ribonucleoprotein complex where the RNA component acts as a template, though the catalytic part is a protein (TERT).

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

Enzyme Kinetics and Michaelis-Menten Equation

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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

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Enzymes — MCQs

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