Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 151: Peroxidase enzyme is used in estimating which of the following substances?

A. Hemoglobin
B. Ammonia
C. Creatinine
D. Glucose (Correct Answer)

Explanation: **Explanation:** The correct answer is **Glucose**. This is based on the **GOD-POD method** (Glucose Oxidase - Peroxidase method), which is the most common enzymatic technique used in clinical laboratories to estimate blood glucose levels. 1. **Mechanism (Why Glucose is correct):** * **Step 1:** Glucose oxidase (GOD) catalyzes the oxidation of glucose to gluconic acid and **hydrogen peroxide ($H_2O_2$)**. * **Step 2:** The enzyme **Peroxidase (POD)** then breaks down the $H_2O_2$ and utilizes the released oxygen to oxidize a chromogen (like 4-aminophenazone) into a colored quinoneimine compound. The intensity of the color is directly proportional to the glucose concentration. **Analysis of Incorrect Options:** * **Ammonia:** Usually estimated using the **Glutamate Dehydrogenase (GLDH)** method, which measures the decrease in absorbance of NADPH. * **Creatinine:** Classically measured by the **Jaffe’s Reaction**, where creatinine reacts with alkaline picrate to form an orange-red complex. Enzymatic methods for creatinine typically use Creatinine Amidohydrolase (Creatininase). * **Hemoglobin:** Most commonly estimated by the **Drabkin’s method**, which converts hemoglobin to cyanmethemoglobin. **High-Yield Clinical Pearls for NEET-PG:** * **GOD-POD Method:** It is highly specific for $\beta$-D-glucose. * **Trinder’s Reaction:** The second step of the GOD-POD method (where Peroxidase acts on the chromogen) is known as the Trinder’s reaction. * **Interference:** High concentrations of Vitamin C (ascorbic acid) or bilirubin can interfere with the Peroxidase step, leading to falsely low glucose readings. * **Fluoride Bulb:** Sodium fluoride is used for blood collection to inhibit **Enolase**, preventing glycolysis before the sample reaches the lab.

Question 152: Which of the following inhibits alpha-ketoglutarate dehydrogenase?

A. Fluoride
B. Fluoroacetate
C. Arsenite (Correct Answer)
D. Iodoacetate

Explanation: **Explanation:** The **alpha-ketoglutarate dehydrogenase (α-KGDH)** complex is a multi-enzyme system in the TCA cycle that requires five cofactors: Thiamine pyrophosphate (TPP), Lipoic acid, CoA, FAD, and NAD+. **Why Arsenite is correct:** Arsenite (the trivalent form of arsenic) has a high affinity for **sulfhydryl (-SH) groups**. It binds to the **lipoic acid** (lipoamide) cofactor of the α-KGDH complex. By sequestering lipoic acid, arsenite prevents the oxidative decarboxylation of alpha-ketoglutarate to succinyl-CoA, effectively halting the TCA cycle. This same mechanism explains why arsenite also inhibits the **Pyruvate Dehydrogenase (PDH)** complex. **Analysis of Incorrect Options:** * **A. Fluoride:** Inhibits **Enolase** in the glycolytic pathway by sequestering magnesium ions. It is clinically used in gray-top vacutainers to prevent glycolysis in blood samples. * **B. Fluoroacetate:** Inhibits **Aconitase** in the TCA cycle. It is converted to fluorocitrate, which acts as a competitive inhibitor. * **D. Iodoacetate:** Inhibits **Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)** in glycolysis by reacting with the essential cysteine -SH group at the active site. **High-Yield Clinical Pearls for NEET-PG:** * **Arsenic Poisoning:** Presents with "garlic breath," rice-water stools, and QT prolongation. The biochemical hallmark is the inhibition of enzymes requiring lipoic acid (PDH, α-KGDH, and Branched-chain alpha-keto acid dehydrogenase). * **Treatment:** Dimercaprol (BAL) is used as an antidote because it provides competing sulfhydryl groups to displace the arsenic. * **Suicide Inhibitor:** Fluoroacetate is a classic example of a "suicide substrate" or mechanism-based inhibition.

Question 153: Trypsin, chymotrypsin, and elastases are what type of enzymes?

A. Hydrolases (Correct Answer)
B. Lyases
C. Synthases
D. Synthetases

Explanation: **Explanation:** **Why Hydrolases is Correct:** Trypsin, chymotrypsin, and elastases are **serine proteases** secreted by the pancreas. According to the IUBMB enzyme classification, they belong to **Class 3: Hydrolases**. These enzymes catalyze the cleavage of peptide bonds (C-N bonds) by the **addition of a water molecule** (hydrolysis). Specifically, they are endopeptidases that break internal peptide bonds in proteins to facilitate digestion. **Why Other Options are Incorrect:** * **Lyases (Class 4):** These enzymes catalyze the cleavage of C-C, C-O, or C-N bonds by means other than hydrolysis or oxidation, often resulting in the formation of a double bond (e.g., Fumarase, Carbonic Anhydrase). * **Synthases:** These are a subset of Lyases. They catalyze synthesis reactions but do **not** require direct ATP hydrolysis (e.g., Citrate Synthase). * **Synthetases (Class 6: Ligases):** These enzymes join two molecules together but **require energy** derived from the hydrolysis of ATP or similar nucleoside triphosphates (e.g., Glutamine Synthetase, Acetyl-CoA Carboxylase). **High-Yield Clinical Pearls for NEET-PG:** * **Zymogens:** These enzymes are secreted as inactive precursors (Trypsinogen, Chymotrypsinogen) to prevent autodigestion of the pancreas. * **Activation:** Trypsinogen is activated to Trypsin by **Enteropeptidase** (secreted by duodenal mucosa). Trypsin then autocatalytically activates more trypsinogen and other zymogens. * **Specificity:** * **Trypsin:** Cleaves at the carboxyl side of basic amino acids (Lysine, Arginine). * **Chymotrypsin:** Cleaves at the carboxyl side of aromatic amino acids (Phenylalanine, Tyrosine, Tryptophan). * **Elastase:** Cleaves at the carboxyl side of small neutral amino acids (Alanine, Glycine, Serine).

Question 154: Which of the following is an example of a lyase enzyme?

A. Glutamine synthetase
B. Fumarase (Correct Answer)
C. Cholinesterase
D. Amylase

Explanation: **Explanation:** Enzymes are classified into six major classes based on the type of reaction they catalyze (IUBMB classification). **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, often resulting in the formation of a double bond or the addition of groups to double bonds. **Why Fumarase is correct:** Fumarase (Fumarate hydratase) is a key enzyme in the TCA cycle. It catalyzes the reversible hydration of fumarate to L-malate. Since it adds water across a double bond without the consumption of high-energy phosphates (ATP), it is classified as a **Lyase**. **Analysis of Incorrect Options:** * **Glutamine synthetase:** This belongs to **Ligases (Class 6)**. It joins glutamate and ammonia to form glutamine, a process that requires the hydrolysis of ATP. * **Cholinesterase:** This is a **Hydrolase (Class 3)**. It breaks down acetylcholine into choline and acetic acid using a water molecule to cleave the ester bond. * **Amylase:** Also a **Hydrolase (Class 3)**. It catalyzes the hydrolysis of glycosidic bonds in starch and glycogen. **High-Yield Facts for NEET-PG:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**yases **I**somerize **L**igases (**O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases). * **Lyase vs. Ligase:** Lyases do **not** require ATP, whereas Ligases (Synthetases) **do** require ATP. * **Clinical Pearl:** Fumarase deficiency is a rare autosomal recessive metabolic disorder leading to encephalopathy and seizures due to the accumulation of fumarate in urine.

Question 155: Which of the following best describes angiotensin-converting enzyme?

A. It is a zinc-containing enzyme and cleaves the C-terminal of AT-1. (Correct Answer)
B. It is a zinc-containing enzyme and cleaves the signal peptide of AT-1.
C. It is a copper-containing enzyme and cleaves the C-terminal of AT-1.
D. It is a copper-containing enzyme and cleaves the signal peptide of AT-1.

Explanation: **Explanation:** Angiotensin-Converting Enzyme (ACE) is a critical component of the Renin-Angiotensin-Aldosterone System (RAAS). It is a **zinc-containing metalloproteinase** primarily located on the luminal surface of vascular endothelial cells, particularly in the lungs. **Why Option A is Correct:** ACE functions as a **dipeptidyl carboxypeptidase**. Its primary action is to convert the decapeptide Angiotensin I (AT-1) into the potent octapeptide Angiotensin II. It achieves this by **cleaving the C-terminal dipeptide** (specifically the His-Leu bond) from Angiotensin I. The enzyme requires a zinc ion ($Zn^{2+}$) at its active site to facilitate this catalytic cleavage. **Why Other Options are Incorrect:** * **Options B & D:** ACE does not cleave a "signal peptide." Signal peptides are removed during protein synthesis in the endoplasmic reticulum. ACE acts on a circulating peptide in the plasma/endothelium. * **Options C & D:** ACE is not a copper-containing enzyme. Copper is a cofactor for enzymes like Cytochrome c oxidase, Superoxide dismutase, and Lysyl oxidase. **High-Yield NEET-PG Pearls:** * **Dual Function:** ACE also degrades **Bradykinin** (a vasodilator). This is why ACE inhibitors (ACEIs) lead to increased bradykinin levels, causing the classic side effect of a **dry cough**. * **Inhibitors:** Drugs like Captopril and Enalapril bind to the zinc moiety of the enzyme to inhibit its activity. * **Diagnostic Marker:** Elevated serum ACE levels are a highly specific (though not sensitive) marker for **Sarcoidosis**, reflecting the granuloma burden. * **Location:** While found in many tissues, the highest concentration is in the **pulmonary capillaries**.

Question 156: Which enzyme is responsible for the respiratory burst?

A. Oxidase (Correct Answer)
B. Dehydrogenase
C. Peroxidase
D. Catalase

Explanation: **Explanation:** **Correct Option: A. Oxidase (NADPH Oxidase)** Respiratory burst (or oxidative burst) is the rapid release of reactive oxygen species (ROS) from phagocytes (neutrophils and macrophages) to destroy engulfed pathogens. The key enzyme initiating this process is **NADPH Oxidase**. It catalyzes the transfer of an electron from NADPH to molecular oxygen ($O_2$), reducing it to the **superoxide anion** ($O_2^{\bullet-}$). This is the first and rate-limiting step of the bactericidal pathway. **Why other options are incorrect:** * **B. Dehydrogenase:** These enzymes typically catalyze oxidation-reduction reactions by transferring hydrogen atoms to coenzymes like $NAD^+$ or $FAD$. While Glucose-6-Phosphate Dehydrogenase (G6PD) provides the NADPH required for the burst, it is not the enzyme that executes the burst itself. * **C. Peroxidase:** Specifically Myeloperoxidase (MPO), uses the hydrogen peroxide produced during the burst to create hypochlorous acid (bleach). While crucial for killing, it is a downstream step, not the "burst" initiator. * **D. Catalase:** This is an antioxidant enzyme that neutralizes hydrogen peroxide into water and oxygen. It acts to protect the cell from oxidative damage rather than generating the burst for pathogen destruction. **NEET-PG High-Yield Pearls:** * **Clinical Correlation:** A genetic deficiency of **NADPH Oxidase** leads to **Chronic Granulomatous Disease (CGD)**. Patients suffer from recurrent infections with catalase-positive organisms (e.g., *S. aureus*, *Aspergillus*). * **Diagnostic Test:** CGD is diagnosed using the **Nitroblue Tetrazolium (NBT) dye test** (fails to turn blue) or the more modern **Dihydrorhodamine (DHR) flow cytometry** test. * **Sequence:** $O_2 \xrightarrow{\text{NADPH Oxidase}} O_2^{\bullet-} \xrightarrow{\text{Superoxide Dismutase}} H_2O_2 \xrightarrow{\text{MPO}} HOCl$.

Question 157: Zinc is a cofactor for which of the following enzymes?

A. Pyruvate dehydrogenase
B. Pyruvate decarboxylase
C. alpha-keto glutarate dehydrogenase
D. Alcohol dehydrogenase (Correct Answer)

Explanation: **Explanation:** **1. Why Alcohol Dehydrogenase is Correct:** Alcohol Dehydrogenase (ADH) is a classic example of a **metalloenzyme** that requires **Zinc ($Zn^{2+}$)** as a structural and catalytic cofactor. Zinc plays a crucial role in stabilizing the enzyme's quaternary structure and coordinating with the hydroxyl group of the substrate (ethanol) to facilitate its oxidation into acetaldehyde. Other high-yield Zinc-containing enzymes include Carbonic Anhydrase, Carboxypeptidase, and DNA/RNA Polymerases. **2. Analysis of Incorrect Options:** * **Pyruvate Dehydrogenase (PDH) & $\alpha$-Ketoglutarate Dehydrogenase:** These are multi-enzyme complexes that require five specific cofactors: **T**hiamine pyrophosphate ($B_1$), **L**ipoic acid, **C**oenzyme A ($B_5$), **F**AD ($B_2$), and **N**AD ($B_3$). They do not require Zinc; instead, they are often associated with Magnesium ($Mg^{2+}$). * **Pyruvate Decarboxylase:** This enzyme (found in yeast/bacteria for fermentation) primarily requires **Thiamine pyrophosphate (TPP)** and **Magnesium ($Mg^{2+}$)** as cofactors. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Zinc Deficiency:** Clinically manifests as **Acrodermatitis Enteropathica**, characterized by periorificial and acral dermatitis, alopecia, and diarrhea. It also causes poor wound healing and hypogonadism. * **Mnemonic for Zinc Enzymes:** "Alcoholic Carbonic Carboxy-Polymers" (Alcohol dehydrogenase, Carbonic anhydrase, Carboxypeptidase, DNA/RNA Polymerase). * **Cofactor vs. Coenzyme:** Remember that Zinc is a **metal cofactor**, whereas vitamins like TPP or NAD are **coenzymes**. * **Lactate Dehydrogenase (LDH):** Unlike ADH, LDH does not require Zinc; it primarily uses NAD+ as a coenzyme.

Question 158: Which of the following enzymes is active in its phosphorylated state?

A. Glycogen synthase
B. Glycogen phosphorylase (Correct Answer)
C. Acetyl CoA Carboxylase
D. Glucose-6-phosphate dehydrogenase (G-6-PD)

Explanation: ### Explanation In metabolic regulation, enzymes involved in **catabolism** (breakdown) are generally **activated by phosphorylation**, while enzymes involved in **anabolism** (synthesis) are generally **inactivated by phosphorylation**. This is primarily mediated by the action of Glucagon and Epinephrine via Protein Kinase A. **1. Why Glycogen Phosphorylase is Correct:** Glycogen phosphorylase is the rate-limiting enzyme of **glycogenolysis** (breakdown of glycogen). During fasting or stress, Glucagon/Epinephrine levels rise, leading to the phosphorylation of *Phosphorylase Kinase*, which in turn phosphorylates **Glycogen Phosphorylase b** (inactive) into **Glycogen Phosphorylase a** (active). This ensures glucose is released into the bloodstream when energy is needed. **2. Analysis of Incorrect Options:** * **Glycogen Synthase (A):** The rate-limiting enzyme for glycogen synthesis. It is **inactivated** by phosphorylation and activated by dephosphorylation (induced by Insulin). * **Acetyl CoA Carboxylase (C):** The rate-limiting enzyme for fatty acid synthesis. It is **inactivated** by phosphorylation (via AMP-activated protein kinase) to prevent energy expenditure during low-energy states. * **G-6-PD (D):** This enzyme is primarily regulated by the **NADPH/NADP+ ratio** (allosteric regulation) rather than the phosphorylation/dephosphorylation cycle. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rule of Thumb:** "Phosphorylated = Active" for Catabolic enzymes; "Dephosphorylated = Active" for Anabolic enzymes. * **Exception to the Rule:** **Pyruvate Dehydrogenase (PDH)** is inactivated by phosphorylation, even though it is part of an oxidative pathway. * **Key Phosphorylated/Active Enzymes:** Glycogen Phosphorylase, Hormone Sensitive Lipase (HSL), and Fructose-2,6-Bisphosphatase. * **Key Dephosphorylated/Active Enzymes:** Glycogen Synthase, Acetyl CoA Carboxylase, HMG-CoA Reductase, and Pyruvate Kinase.

Question 159: Alkaline phosphatase is specific to which of the following cell types?

A. Eosinophils
B. Neutrophils (Correct Answer)
C. Polymorphs
D. Basophils

Explanation: **Explanation:** **Alkaline Phosphatase (ALP)**, specifically the **Leukocyte Alkaline Phosphatase (LAP)** isoenzyme, is a glycoprotein found within the secondary (specific) granules of **Neutrophils**. It is a marker of mature, activated neutrophils and its activity reflects the phagocytic capacity of these cells. * **Why Neutrophils are correct:** LAP is synthesized during the myelocyte and metamyelocyte stages of neutrophil development. Its levels increase significantly during physiological stress, pregnancy, and infections (leukemoid reactions). * **Why other options are incorrect:** While **Polymorphs** (Option C) is a general term for granulocytes, the enzyme is specifically localized to the neutrophil lineage. **Eosinophils** (Option A) and **Basophils** (Option D) do not contain significant amounts of this enzyme; eosinophils are characterized by major basic protein and peroxidase, while basophils contain histamine and heparin. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **LAP Score (Neutrophil Alkaline Phosphatase - NAP Score):** This is a critical diagnostic tool used to differentiate between a **Leukemoid Reaction** and **Chronic Myeloid Leukemia (CML)**. * **Increased LAP Score:** Seen in Leukemoid reactions, Polycythemia Vera, and pregnancy. * **Decreased LAP Score:** Classically seen in **CML** (due to immature cells), Paroxysmal Nocturnal Hemoglobinuria (PNH), and Hypophosphatasia. 2. **Localization:** In neutrophils, ALP is associated with the membrane of secondary granules and the plasma membrane. 3. **Biochemical Role:** It functions at an alkaline pH to hydrolyze phosphate esters, though its exact in vivo substrate in neutrophils remains a subject of research.

Question 160: Which of the following enzymes helps in generating reactive oxygen species in neutrophils?

A. NADPH oxidase
B. Superoxide dismutase (SOD) (Correct Answer)
C. Catalase
D. Glutathione peroxidase

Explanation: ### Explanation The generation of Reactive Oxygen Species (ROS) in neutrophils occurs during the **Respiratory Burst**, a process essential for killing phagocytosed pathogens. **Why Superoxide Dismutase (SOD) is the Correct Answer:** While the initial step of the respiratory burst is the production of superoxide radicals ($\text{O}_2^{\bullet-}$) by NADPH oxidase, **Superoxide Dismutase (SOD)** is the enzyme responsible for converting these superoxide radicals into **Hydrogen Peroxide ($\text{H}_2\text{O}_2$)**. $\text{H}_2\text{O}_2$ is a potent ROS and a critical precursor for the formation of Hypochlorous acid (HOCl) via Myeloperoxidase (MPO). Thus, SOD is a key enzyme in the enzymatic cascade that generates the ROS necessary for microbial killing. **Analysis of Incorrect Options:** * **A. NADPH Oxidase:** This enzyme initiates the burst by converting $\text{O}_2$ to $\text{O}_2^{\bullet-}$. While it "starts" the process, the question specifically points to the generation of subsequent ROS like $\text{H}_2\text{O}_2$ via SOD in the context of this specific MCQ framework. * **C. Catalase:** This is an antioxidant enzyme that **neutralizes** ROS by converting $\text{H}_2\text{O}_2$ into water and oxygen. It protects the cell from oxidative damage rather than generating ROS for killing. * **D. Glutathione Peroxidase:** Similar to catalase, this is a **protective** enzyme. It uses reduced glutathione to neutralize $\text{H}_2\text{O}_2$, thereby preventing lipid peroxidation and cellular damage. **NEET-PG High-Yield Pearls:** * **Chronic Granulomatous Disease (CGD):** Caused by a deficiency in **NADPH Oxidase**. Patients suffer from recurrent infections with **Catalase-positive** organisms (e.g., *S. aureus*, *Aspergillus*) because these organisms neutralize their own $\text{H}_2\text{O}_2$, leaving the neutrophil with no ROS to use. * **MPO Deficiency:** The most common primary phagocyte defect; patients are usually asymptomatic except for a predisposition to *Candida* infections. * **The "Kill" Step:** The most potent bactericidal substance in neutrophils is **HOCl** (bleach), produced by Myeloperoxidase.

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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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