Enzymes Practice Questions

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

Glucose. **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.

Q: Which of the following inhibits alpha-ketoglutarate dehydrogenase?

Arsenite. **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.

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

Hydrolases. **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).

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

Fumarase. **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.

Q: Which enzyme is responsible for the respiratory burst?

Oxidase. **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:** $O_2 \xrightarrow{\text{NADPH Oxidase}} O_2^{\bullet-} \xrightarrow{\text{Superoxide Dismutase}} H_2O_2 \xrightarrow{\text{MPO}} HOCl$.

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

Alcohol dehydrogenase. **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.

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

Glycogen phosphorylase. ### 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.

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

Neutrophils. **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 1

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

Accepted Answer

Glucose

Answer

Hemoglobin

Answer

Ammonia

Answer

Creatinine

Question 2

Which of the following inhibits alpha-ketoglutarate dehydrogenase?

Accepted Answer

Arsenite

Answer

Fluoride

Answer

Fluoroacetate

Answer

Iodoacetate

Question 3

Trypsin, chymotrypsin, and elastases are what type of enzymes?

Accepted Answer

Hydrolases

Answer

Lyases

Answer

Synthases

Answer

Synthetases

Question 4

Which of the following is an example of a lyase enzyme?

Accepted Answer

Fumarase

Answer

Glutamine synthetase

Answer

Cholinesterase

Answer

Amylase

Question 5

Which of the following best describes angiotensin-converting enzyme?

Accepted Answer

It is a zinc-containing enzyme and cleaves the C-terminal of AT-1.

Answer

It is a zinc-containing enzyme and cleaves the signal peptide of AT-1.

Answer

It is a copper-containing enzyme and cleaves the C-terminal of AT-1.

Answer

It is a copper-containing enzyme and cleaves the signal peptide of AT-1.

Question 6

Which enzyme is responsible for the respiratory burst?

Accepted Answer

Oxidase

Answer

Dehydrogenase

Answer

Peroxidase

Answer

Catalase

Question 7

Zinc is a cofactor for which of the following enzymes?

Accepted Answer

Alcohol dehydrogenase

Answer

Pyruvate dehydrogenase

Answer

Pyruvate decarboxylase

Answer

alpha-keto glutarate dehydrogenase

Question 8

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

Accepted Answer

Glycogen phosphorylase

Answer

Glycogen synthase

Answer

Acetyl CoA Carboxylase

Answer

Glucose-6-phosphate dehydrogenase (G-6-PD)

Question 9

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

Accepted Answer

Neutrophils

Answer

Eosinophils

Answer

Polymorphs

Answer

Basophils

Question 10

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

Accepted Answer

Superoxide dismutase (SOD)

Answer

NADPH oxidase

Answer

Catalase

Answer

Glutathione peroxidase

Enzymes — MCQs

Enzymes — MCQs

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