Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 161: Where is histidine decarboxylase present?

A. RBCs
B. Heart
C. Liver (Correct Answer)
D. Kidney

Explanation: **Explanation:** **Histidine decarboxylase (HDC)** is the enzyme responsible for the conversion of the amino acid L-histidine into **histamine**, a potent biogenic amine. This reaction requires **Pyridoxal Phosphate (PLP)** as a mandatory cofactor. 1. **Why Liver is the Correct Answer:** While histamine is primarily stored in mast cells and basophils, the **liver** is a major site for the metabolic processing of amino acids. The liver contains significant concentrations of histidine decarboxylase to facilitate the production of histamine, which plays a role in regulating hepatic blood flow and various metabolic signaling pathways. In the context of standard biochemistry textbooks (like Satyanarayana or Vasudevan), the liver is frequently cited as a primary tissue source for this enzyme alongside the gastric mucosa. 2. **Analysis of Incorrect Options:** * **RBCs:** Red blood cells lack the metabolic machinery for most amino acid decarboxylation reactions. They primarily carry hemoglobin and lack a nucleus or organelles where such enzymes are synthesized. * **Heart:** While histamine affects cardiac contractility and heart rate via H2 receptors, the heart is a target organ rather than a primary site for histamine synthesis via HDC. * **Kidney:** Although the kidney contains various decarboxylases (like DOPA decarboxylase), it is not the principal site for histidine decarboxylation compared to the liver and gastric mucosa. **Clinical Pearls for NEET-PG:** * **Cofactor:** Always remember that all decarboxylation reactions of amino acids (Histidine → Histamine, Tyrosine → Tyramine, Glutamate → GABA) require **Vitamin B6 (Pyridoxal Phosphate)**. * **Gastric Connection:** HDC is highly active in the **Enterochromaffin-like (ECL) cells** of the stomach, where histamine stimulates parietal cells to secrete HCl. * **Inhibitor:** Alpha-fluoromethylhistidine is a specific irreversible inhibitor of HDC, used in research to study histamine depletion.

Question 162: Myocardial infarction is associated with increased levels of which enzyme isoenzyme?

A. Creatine phosphokinase (CPK)
B. CPK-MM
C. CPK-MB (Correct Answer)
D. CPK-BB

Explanation: **Explanation:** **Creatine Phosphokinase (CPK)**, also known as Creatine Kinase (CK), is a dimer consisting of two subunits: **M (Muscle)** and **B (Brain)**. These subunits combine to form three distinct isoenzymes, each localized to specific tissues. **Why CPK-MB is the correct answer:** **CPK-MB** is primarily found in the **myocardium** (cardiac muscle). Following a Myocardial Infarction (MI), damaged cardiac myocytes leak this enzyme into the bloodstream. It typically rises within 4–6 hours, peaks at 18–24 hours, and returns to baseline within 48–72 hours. Because of its rapid clearance, it is particularly useful for detecting **re-infarction**. **Analysis of Incorrect Options:** * **CPK (Option A):** This refers to "Total CPK." While total levels rise in MI, it is non-specific as it includes contributions from skeletal muscle and brain tissue. * **CPK-MM (Option B):** This is the predominant isoenzyme in **skeletal muscle** (98%) and the heart. Elevations are usually associated with muscle trauma, vigorous exercise, or muscular dystrophy. * **CPK-BB (Option C):** This isoenzyme is found mainly in the **brain** and gastrointestinal tract. It rarely appears in the blood as it does not cross the blood-brain barrier significantly. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** While CPK-MB was historically the marker of choice, **Cardiac Troponins (I and T)** are now the "Gold Standard" due to higher sensitivity and specificity. * **Re-infarction:** CPK-MB is the preferred marker for diagnosing a second MI occurring shortly after the first, as Troponins remain elevated for up to 10–14 days. * **Relative Index:** A CPK-MB/Total CK ratio of **>5%** is highly suggestive of myocardial origin rather than skeletal muscle damage.

Question 163: An enzyme-substrate interaction has an initial Vmax of 10 and a Km of 5. After adding Drug B, the Km becomes 10. Which of the following statements is false?

A. Drug B is a competitive inhibitor.
B. Increasing the substrate concentration can reverse the inhibition by Drug B.
C. Drug B is structurally similar to the substrate.
D. Drug B binds to an allosteric site of the enzyme. (Correct Answer)

Explanation: ### Explanation The question describes a scenario where the **$V_{max}$ remains constant (10)** while the **$K_m$ increases (from 5 to 10)**. This is the classic kinetic hallmark of **Competitive Inhibition**. **1. Why Option D is the Correct (False) Statement:** In competitive inhibition, the inhibitor competes with the substrate for the **active site** of the enzyme. Binding to an **allosteric site** (a site other than the active site) is characteristic of non-competitive or uncompetitive inhibition. Since the $V_{max}$ did not change, Drug B must be binding to the active site, making statement D false. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** True. An increase in $K_m$ with an unchanged $V_{max}$ defines a competitive inhibitor. * **Option B:** True. Because the inhibitor and substrate compete for the same site, increasing the substrate concentration ($[S]$) "outcompetes" the drug, eventually allowing the reaction to reach its original $V_{max}$. * **Option C:** True. Competitive inhibitors are typically **structural analogs** of the substrate, allowing them to fit into the same active site (e.g., Malonate is a structural analog of Succinate). --- ### High-Yield Clinical Pearls for NEET-PG * **Competitive Inhibition:** $V_{max}$ = Unchanged; $K_m$ = Increased. (Mnemonic: **C**ompetitive = **C**onstant $V_{max}$). * **Non-Competitive Inhibition:** $V_{max}$ = Decreased; $K_m$ = Unchanged. (Inhibitor binds to allosteric site). * **Uncompetitive Inhibition:** Both $V_{max}$ and $K_m$ decrease. (Inhibitor binds only to the Enzyme-Substrate complex). * **Classic Clinical Example:** **Statins** (e.g., Atorvastatin) are competitive inhibitors of HMG-CoA reductase. They are structural analogs of HMG-CoA. * **Methanol Poisoning:** Ethanol acts as a competitive inhibitor of Alcohol Dehydrogenase, preventing the formation of toxic formaldehyde.

Question 164: Phosphorylase b is maintained in an inactivated state by?

A. ATP
B. cAMP
C. Calcium
D. Insulin (Correct Answer)

Explanation: **Explanation:** The regulation of glycogenolysis centers on the enzyme **Glycogen Phosphorylase**, which exists in two forms: **Phosphorylase *a*** (phosphorylated, active) and **Phosphorylase *b*** (dephosphorylated, inactive). **Why Insulin is correct:** Insulin is an anabolic hormone that promotes glycogen synthesis and inhibits glycogen breakdown. It triggers **Protein Phosphatase-1 (PP1)**, which removes the phosphate group from Phosphorylase *a*, converting it back into the inactive **Phosphorylase *b***. Furthermore, insulin activates phosphodiesterase to lower cAMP levels, effectively maintaining the enzyme in its inactive state to prevent glucose release. **Analysis of Incorrect Options:** * **ATP:** In the muscle, ATP acts as an allosteric inhibitor of Phosphorylase *b*, but it does not "maintain" the state via hormonal signaling like insulin. High ATP signals high energy, reducing the *activity* of the enzyme rather than its phosphorylation state. * **cAMP:** This is a second messenger for Glucagon and Epinephrine. Increased cAMP activates Protein Kinase A (PKA), which leads to the activation of Phosphorylase Kinase, ultimately **activating** Phosphorylase *b* into *a*. * **Calcium:** In contracting muscles, Calcium binds to the calmodulin subunit of Phosphorylase Kinase, **activating** it. This ensures glycogen is broken down to provide energy for contraction. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glycogen Phosphorylase is the rate-limiting step of glycogenolysis. * **Covalent Modification:** The conversion between *a* and *b* forms is the classic example of regulation via phosphorylation/dephosphorylation. * **Muscle vs. Liver:** Muscle phosphorylase is sensitive to **AMP** (allosteric activator), whereas liver phosphorylase is not; liver phosphorylase is primarily regulated by **Glucose**.

Question 165: Which of the following enzymes is not secreted as a zymogen?

A. Pepsin
B. Trypsin (Correct Answer)
C. Amylase
D. Colipase

Explanation: **Explanation:** The correct answer is **Trypsin**. This question tests the understanding of **zymogens** (proenzymes), which are inactive precursors that require biochemical change (usually proteolytic cleavage) to become active. **Why Trypsin is the correct choice:** Strictly speaking, the pancreas does not secrete "Trypsin"; it secretes **Trypsinogen**. Trypsinogen is the inactive zymogen that is later converted into active Trypsin in the duodenum by the enzyme **enteropeptidase** (enterokinase). Therefore, among the options provided, Trypsin is the active form, not the secreted zymogen form. **Analysis of other options:** * **Pepsin:** Secreted by gastric chief cells as **Pepsinogen**. It is activated by the acidic pH of the stomach. * **Amylase:** Unlike proteases, Salivary and Pancreatic Amylase are secreted in their **active form**. They do not require proteolytic cleavage for activation because they do not pose a danger of autodigestion to the secreting glands. * **Colipase:** Secreted by the pancreas as **Procolipase**. It is activated by Trypsin in the small intestine to help lipase bind to lipid droplets. *(Note: In many competitive exams, if both Amylase and Trypsin are listed, the question often hinges on the nomenclature. Since Trypsin is the name of the active enzyme and Trypsinogen is the zymogen, "Trypsin" is technically not secreted as a zymogen—its precursor is.)* **High-Yield NEET-PG Pearls:** 1. **Proteases** (Trypsin, Chymotrypsin, Elastase, Carboxypeptidase) are always secreted as zymogens to prevent **autodigestion** of the pancreas. 2. **Acute Pancreatitis:** Occurs when zymogens (specifically trypsinogen) are prematurely activated within the pancreatic parenchyma. 3. **Enteropeptidase deficiency:** Leads to severe protein malnutrition because without it, trypsinogen cannot be activated, which in turn prevents the activation of all other pancreatic proteases.

Question 166: What is the specific marker for alcoholic hepatitis?

A. Gamma-glutamyl transferase (GGT) (Correct Answer)
B. Alanine transaminase (ALT)
C. Alkaline phosphatase (ALP)
D. Lactate dehydrogenase (LDH)

Explanation: **Explanation:** **Gamma-glutamyl transferase (GGT)** is the most sensitive marker for hepatobiliary disease, particularly those associated with alcohol consumption. In alcoholic hepatitis, alcohol induces the microsomal expression of GGT in hepatocytes. Furthermore, alcohol acts as a potent enzyme inducer, leading to a disproportionate rise in GGT levels compared to other liver enzymes. It is also used to monitor abstinence in recovering alcoholics. **Analysis of Incorrect Options:** * **Alanine transaminase (ALT):** While ALT is a specific marker for hepatocellular injury, it is often lower than AST in alcoholic liver disease. A classic high-yield finding is an **AST:ALT ratio > 2:1**, as alcoholics often have a deficiency of pyridoxal-5-phosphate (Vitamin B6), which is a necessary cofactor for ALT activity. * **Alkaline phosphatase (ALP):** This is primarily a marker for **cholestasis** (biliary obstruction) or bone turnover. While it may be mildly elevated in alcoholic hepatitis, it lacks specificity for alcohol-induced damage. * **Lactate dehydrogenase (LDH):** This is a non-specific marker of cell turnover or hemolysis. It is elevated in various conditions like myocardial infarction, hemolysis, and certain malignancies, making it poor for diagnosing specific liver pathologies. **Clinical Pearls for NEET-PG:** * **GGT + ALP:** If both are elevated, the source of ALP is likely **hepatic**. If GGT is normal but ALP is high, consider **bone disease**. * **Isolated GGT elevation:** Often seen in chronic alcoholics or patients taking enzyme-inducing drugs like Phenytoin or Rifampicin. * **De Ritis Ratio:** An AST:ALT ratio > 2 strongly suggests alcoholic hepatitis, whereas a ratio < 1 is typically seen in Viral Hepatitis or NAFLD.

Question 167: Which enzyme acts the fastest?

A. Lactate dehydrogenase (LDH)
B. Trypsin
C. Catalase (Correct Answer)
D. None of the above

Explanation: **Explanation:** The efficiency of an enzyme is measured by its **Turnover Number ($K_{cat}$)**, which is defined as the number of substrate molecules converted into product by a single enzyme molecule per unit of time (usually per second) when the enzyme is fully saturated. **1. Why Catalase is the Correct Answer:** Catalase is one of the most efficient enzymes known in biology. It has an exceptionally high turnover number of approximately **$4 \times 10^7$ per second**. This means a single molecule of catalase can decompose 40 million molecules of hydrogen peroxide ($H_2O_2$) into water and oxygen every second. This rapid action is vital to protect cells from oxidative damage caused by reactive oxygen species (ROS). Its catalytic efficiency is so high that it is limited only by the rate at which the substrate can diffuse to the enzyme's active site (diffusion-limited). **2. Why Other Options are Incorrect:** * **Lactate Dehydrogenase (LDH):** While essential for anaerobic glycolysis, its turnover number is significantly lower (approx. $10^3$ per second) compared to catalase. * **Trypsin:** As a digestive protease, its reaction rate is relatively slow (approx. $10^2$ per second) because peptide bond hydrolysis is a more complex chemical process than the decomposition of $H_2O_2$. **3. NEET-PG High-Yield Pearls:** * **Catalytic Efficiency:** Represented by the ratio **$K_{cat}/K_m$**. An enzyme is considered "catalytically perfect" if this ratio is $10^8$ to $10^9$ $M^{-1}s^{-1}$. * **Clinical Correlation:** Catalase is found in **peroxisomes**. Deficiency of catalase (Acatalasemia) can lead to oral ulcerations and gangrene. * **Other Fast Enzymes:** Carbonic Anhydrase and Acetylcholinesterase are also among the fastest enzymes, though Catalase typically tops the list in turnover speed.

Question 168: Which of the following is a reverse transcriptase?

A. Topoisomerase 2
B. Telomerase (Correct Answer)
C. RNA polymerase 2
D. DNA polymerase alpha

Explanation: **Explanation:** **Why Telomerase is the Correct Answer:** Telomerase is a specialized **RNA-dependent DNA polymerase** (Reverse Transcriptase). It is responsible for maintaining the length of telomeres (repetitive TTAGGG sequences at the ends of chromosomes). Since DNA polymerase cannot replicate the extreme 3' end of a linear chromosome (the "end-replication problem"), telomerase uses its own intrinsic RNA template to synthesize complementary DNA, effectively reversing the flow of genetic information from RNA to DNA. **Analysis of Incorrect Options:** * **Topoisomerase 2:** This enzyme manages DNA tangles and supercoiling by creating transient double-stranded breaks. It does not possess polymerase activity. * **RNA Polymerase 2:** This is a DNA-dependent RNA polymerase responsible for synthesizing **mRNA** and most snRNA in eukaryotes. It follows the standard central dogma (DNA → RNA). * **DNA Polymerase Alpha:** This is a DNA-dependent DNA polymerase that acts as a **primase** in eukaryotes, initiating DNA replication by synthesizing short RNA primers followed by a short DNA stretch. **NEET-PG High-Yield Pearls:** * **Reverse Transcriptase (RT) Examples:** Apart from Telomerase, RT is found in Retroviruses (like HIV) and Hepatitis B virus (which uses an RNA intermediate). * **Clinical Correlation:** Telomerase activity is high in **germ cells, stem cells, and cancer cells** (conferring immortality), but low or absent in most somatic cells. * **Inhibitors:** Reverse transcriptase inhibitors (like Zidovudine/AZT) are cornerstones of HAART therapy for HIV.

Question 169: Enzyme activity is expressed as:

A. Millimoles/lit
B. Mg/lit
C. Mg/dl
D. Micromoles/min (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** Enzyme activity refers to the **rate of a reaction** catalyzed by an enzyme. By definition, the standard unit of enzyme activity is the **International Unit (IU)**. One IU is defined as the amount of enzyme that catalyzes the conversion of **1 micromole ($\mu$mol) of substrate per minute** under specified conditions (optimum pH, temperature, and substrate concentration). Since activity measures "work done over time," the unit must include a time component (min) and a quantity of substrate processed ($\mu$mol). **2. Why Other Options are Incorrect:** * **Option A (Millimoles/lit):** This is a unit of **molar concentration**, commonly used to measure electrolytes or metabolites in the blood, not the rate of a reaction. * **Option B (Mg/lit) & Option C (Mg/dl):** These are units of **mass concentration**. While they can be used to measure the total protein mass of an enzyme (e.g., Troponin mass assays), they do not reflect the functional "activity" or catalytic power of the enzyme. **3. High-Yield Clinical Pearls for NEET-PG:** * **Katal (kat):** The SI unit of enzyme activity. 1 Katal = 1 mole of substrate converted per second. (Note: 1 IU = 16.67 nkat). * **Specific Activity:** Defined as the number of enzyme units per milligram of total protein (Units/mg). It is the best indicator of **enzyme purity** during purification processes. * **Turnover Number ($K_{cat}$):** The number of substrate molecules converted into product per enzyme molecule per second when the enzyme is fully saturated. * **Clinical Correlation:** In diagnostic labs, we measure enzyme *activity* (e.g., ALT, AST, ALP) rather than *mass* because activity is a more sensitive indicator of cellular damage and is easier to assay.

Question 170: Which of the following enzymes is NOT regulated by phosphorylation?

A. Glucokinase (Correct Answer)
B. Glycogen synthase
C. Glycogen phosphorylase
D. Citrate lyase

Explanation: **Explanation:** The correct answer is **Glucokinase (Option A)**. **Why Glucokinase is the correct answer:** Glucokinase (Hexokinase IV) is primarily regulated by **compartmentalization** and **allosteric inhibition**, not by covalent modification like phosphorylation. In the liver, it is regulated by the **Glucokinase Regulatory Protein (GKRP)**, which sequesters it in the nucleus in its inactive form when glucose levels are low. When glucose levels rise, glucokinase is released into the cytoplasm to initiate glycolysis. Unlike many other metabolic enzymes, its activity is not toggled by the addition or removal of a phosphate group. **Analysis of incorrect options:** * **Glycogen synthase (Option B):** This enzyme is inactivated by phosphorylation (via Protein Kinase A or Glycogen Synthase Kinase-3) and activated by dephosphorylation (via Protein Phosphatase-1). * **Glycogen phosphorylase (Option C):** This is the classic example of covalent regulation. It is activated by phosphorylation (via Phosphorylase Kinase) and inactivated by dephosphorylation. * **Citrate lyase (Option D):** ATP-Citrate Lyase, a key enzyme in fatty acid synthesis, is regulated by phosphorylation. It is activated by insulin-mediated signaling (via Akt/Protein Kinase B). **High-Yield Clinical Pearls for NEET-PG:** * **Glucokinase vs. Hexokinase:** Glucokinase has a **high Km** (low affinity) and **high Vmax**, allowing it to function as a "glucose sensor" in the liver and pancreatic beta cells. * **MODY Type 2:** Mutations in the glucokinase gene lead to Maturity-Onset Diabetes of the Young (MODY) type 2. * **General Rule:** Most rate-limiting enzymes in carbohydrate and lipid metabolism (e.g., Pyruvate Dehydrogenase, HMG-CoA Reductase, Acetyl-CoA Carboxylase) are regulated by phosphorylation/dephosphorylation. Glucokinase is a notable exception.

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

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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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