Enzymes Practice Questions

Q: What is the cofactor of glutathione peroxidase?

Selenium. **Explanation:** **1. Why Selenium is Correct:** Glutathione Peroxidase (GPx) is a critical antioxidant enzyme that protects cells from oxidative damage by reducing lipid hydroperoxides and free hydrogen peroxide ($H_2O_2$) into water. The functional activity of this enzyme is strictly dependent on **Selenium**. Specifically, selenium is incorporated into the enzyme's active site as the 21st amino acid, **Selenocysteine**. This makes GPx a "selenoprotein." Without selenium, the enzyme cannot neutralize reactive oxygen species, leading to cellular membrane damage. **2. Why the Other Options are Incorrect:** * **Magnesium (B):** Acts as a cofactor for enzymes involving ATP (kinases) and DNA/RNA polymerases. It is not involved in the glutathione redox cycle. * **Manganese (C):** This is the cofactor for **Manganese Superoxide Dismutase (Mn-SOD)**, found in the mitochondria. It is also a cofactor for Arginase and Pyruvate Carboxylase. * **Zinc (D):** A structural component of "Zinc fingers" in transcription factors and a cofactor for **Cytosolic SOD (Cu-Zn SOD)**, Carbonic Anhydrase, and Alcohol Dehydrogenase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Keshan Disease:** A cardiomyopathy caused by Selenium deficiency, leading to decreased Glutathione Peroxidase activity. * **Glutathione Reductase:** Unlike the peroxidase, the *reductase* enzyme requires **Riboflavin (Vitamin B2)** in the form of FAD. * **The Redox Cycle:** Glutathione peroxidase uses **Reduced Glutathione (GSH)** as an electron donor, converting it to **Oxidized Glutathione (GSSG)**. * **Other Selenoproteins:** Deiodinase (converts T4 to T3) and Thioredoxin reductase also require Selenium.

Q: Which of the following causes hydrolysis of peptidoglycans?

Lysozyme. **Explanation:** **Lysozyme (Option A)** is the correct answer. It is a glycoside hydrolase enzyme found in secretions like tears, saliva, and mucus. Its primary mechanism of action is the **hydrolysis of the β(1→4) glycosidic bond** between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in the bacterial **peptidoglycan** cell wall. This disruption leads to osmotic lysis of the bacteria, making lysozyme a critical component of the innate immune system. **Analysis of Incorrect Options:** * **Lactoferrin (Option B):** This is an iron-binding protein found in secretions. It acts as a bacteriostatic agent by sequestering free iron, which is essential for bacterial growth, rather than degrading the cell wall. * **Protease (Option C):** These enzymes catalyze the proteolysis of peptide bonds in proteins. While peptidoglycan contains peptide cross-links, the primary backbone is a polysaccharide chain targeted by lysozymes, not general proteases. * **Aflatoxin (Option D):** This is a potent hepatocarcinogenic toxin produced by *Aspergillus flavus*. It is not an enzyme and does not cause hydrolysis; instead, it causes DNA damage and is linked to hepatocellular carcinoma. **NEET-PG High-Yield Pearls:** * **Target Site:** Lysozyme specifically cleaves the **β-1,4 linkage** between NAM and NAG. * **Gram-Positive vs. Negative:** Lysozyme is more effective against Gram-positive bacteria because their peptidoglycan layer is exposed, whereas Gram-negative bacteria have an outer membrane protecting the cell wall. * **Clinical Relevance:** Deficiencies in lysozyme are associated with increased susceptibility to conjunctival and pulmonary infections. It is often used as a marker in the diagnosis of sarcoidosis and certain leukemias (monocytic leukemia).

Q: Which of the following enzymes requires zinc as a cofactor?

Carbonic anhydrase. **Explanation:** **Carbonic anhydrase** is a classic example of a metalloenzyme that requires **Zinc ($Zn^{2+}$)** as a mandatory cofactor for its catalytic activity. The zinc ion is coordinated to three histidine residues and a water molecule/hydroxyl group at the enzyme's active site. This arrangement facilitates the rapid interconversion of carbon dioxide and water into bicarbonate and protons ($H^+$), a process vital for $CO_2$ transport in RBCs and acid-base regulation in the kidneys. **Analysis of Incorrect Options:** * **Tyrosinase:** This is a **Copper (Cu)** containing enzyme. It is essential for melanin synthesis; its deficiency leads to Albinism. * **Hexokinase:** This glycolytic enzyme requires **Magnesium ($Mg^{2+}$)**. Magnesium acts by complexing with ATP (Mg-ATP) to shield negative charges, facilitating the nucleophilic attack. * **Oxidoreductase:** This is a broad class of enzymes. While some may use zinc (like Alcohol Dehydrogenase), the term is too general. Many use NAD/FAD or Heme (Iron) as cofactors. **High-Yield Clinical Pearls for NEET-PG:** * **Other Zinc-containing enzymes:** Alcohol dehydrogenase, Alkaline phosphatase (ALP), Carboxypeptidase, and RNA/DNA polymerases. * **Zinc Finger Motifs:** Zinc is crucial for the structural stability of DNA-binding proteins (transcription factors). * **Clinical Deficiency:** Zinc deficiency leads to **Acrodermatitis enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. It also causes poor wound healing and hypogeusia (decreased taste acuity).

Q: What is the term for the combination of a main supporting enzyme and its cofactor?

Holoenzyme. ### Explanation **1. Why Holoenzyme is Correct:** In biochemistry, many enzymes are not active on their own and require a non-protein component to function. A **Holoenzyme** represents the complete, catalytically active unit. It is formed by the combination of the protein part (**Apoenzyme**) and its required non-protein component (**Cofactor**). * **Formula:** Apoenzyme + Cofactor = Holoenzyme. * The cofactor can be a metal ion (e.g., $Zn^{2+}$, $Mg^{2+}$) or an organic molecule (Coenzyme). **2. Analysis of Incorrect Options:** * **Apoenzyme:** This is the protein portion of the enzyme alone. It is catalytically inactive because it lacks the necessary cofactor. * **Coenzyme:** This is a specific type of cofactor—a non-protein, organic molecule (often derived from vitamins like B-complex) that binds to the apoenzyme. It is only one part of the holoenzyme, not the whole complex. * **Constitutive enzyme:** These are "housekeeping" enzymes that are produced at a constant rate by the cell regardless of the physiological demand or substrate concentration (e.g., enzymes of glycolysis), unlike inducible enzymes. **3. NEET-PG High-Yield Pearls:** * **Prosthetic Group:** If a coenzyme is covalently or very tightly bound to the apoenzyme (e.g., Heme in Cytochrome P450 or Biotin in Carboxylases), it is called a prosthetic group. * **Metalloenzymes:** Enzymes that require a specific metal ion for their activity (e.g., Carbonic Anhydrase requires Zinc). * **Clinical Correlation:** Many vitamin deficiencies manifest as metabolic blocks because the body cannot form the necessary **Coenzymes** to complete the **Holoenzyme** complex (e.g., Thiamine deficiency leads to decreased activity of Pyruvate Dehydrogenase).

Q: Which enzyme is primarily stored in muscle?

SGOT. **Explanation:** The correct answer is **SGOT (Serum Glutamic Oxaloacetic Transaminase)**, also known as **AST (Aspartate Aminotransferase)**. While many enzymes are found in muscle tissue, the question asks which is "primarily stored" or highly concentrated there. AST is found in high concentrations in the **cardiac muscle, skeletal muscle**, and liver. In the context of muscle metabolism, AST plays a crucial role in the malate-aspartate shuttle and amino acid metabolism. When muscle tissue is damaged (e.g., rhabdomyolysis or myocardial infarction), AST levels rise significantly in the serum. **Analysis of Incorrect Options:** * **A. Alkaline Phosphatase (ALP):** Primarily found in the liver (bile duct epithelium), bone (osteoblasts), placenta, and intestine. It is a marker for obstructive jaundice and bone turnover, not muscle. * **C. SGPT (ALT):** While present in muscles, ALT is considered **liver-specific**. Its concentration in the liver is much higher than in any other tissue, making it the preferred marker for hepatocellular injury. * **D. CPK (Creatine Phosphokinase):** Although CPK is highly abundant in muscle (specifically the CK-MM and CK-MB isoenzymes), the NEET-PG pattern often distinguishes between "storage" and "activity." While CPK is a more *sensitive* marker for muscle damage, AST is historically categorized in biochemistry as being stored in high quantities across both cardiac and skeletal muscle groups. **NEET-PG High-Yield Pearls:** * **De Ritis Ratio:** AST/ALT ratio > 2 is suggestive of Alcoholic Liver Disease. * **Myocardial Infarction (MI):** AST was historically used as a cardiac marker; it rises 6–8 hours after an MI, peaks at 24 hours, and returns to normal by day 5. * **Tissue Distribution of AST:** Heart > Liver > Skeletal Muscle > Kidney > RBCs.

Q: Trypsin is an activator of all of the following enzymes EXCEPT?

Pepsinogen. **Explanation:** The activation of pancreatic enzymes is a critical step in digestion, initiated by a cascade mechanism. **Trypsin** acts as the common activator for almost all proteolytic and lipolytic zymogens secreted by the pancreas. **Why Pepsinogen is the correct answer:** Pepsinogen is a gastric proenzyme secreted by the **Chief cells** of the stomach. Its activation into **Pepsin** is mediated by the acidic environment (low pH) of the stomach and by **auto-activation** (pepsin activating more pepsinogen). Trypsin, which operates in the alkaline environment of the duodenum, has no role in the activation of gastric pepsinogen. **Analysis of incorrect options:** * **Chymotrypsinogen:** Trypsin cleaves the peptide bond between Arginine-15 and Isoleucine-16 to convert it into active $\pi$-chymotrypsin. * **Proelastase:** Trypsin activates proelastase into **Elastase**, which breaks down elastin in connective tissues. * **Procolipase:** Trypsin converts procolipase into **Colipase**, which is essential for the action of pancreatic lipase in the presence of bile salts. **High-Yield NEET-PG Pearls:** 1. **The Master Switch:** **Enteropeptidase** (Enterokinase), secreted by the duodenal brush border, is the initial trigger that converts Trypsinogen to Trypsin. 2. **Pancreatitis Link:** Premature activation of trypsinogen within the pancreas (due to ductal obstruction or injury) leads to autodigestion of the gland, resulting in acute pancreatitis. 3. **Protective Mechanism:** The pancreas secretes **PSTI (Pancreatic Secretory Trypsin Inhibitor)** to prevent accidental internal activation of trypsin.

Q: Gamma Glutamyl Transferase is increased in which of the following conditions?

Alcoholic hepatitis. **Explanation:** **Gamma-Glutamyl Transferase (GGT)** is a microsomal enzyme found primarily in the liver, bile ducts, and kidneys. While it is a sensitive marker for hepatobiliary disease, its most significant clinical utility in NEET-PG stems from its role as a marker for **chronic alcohol consumption.** 1. **Why Alcoholic Hepatitis is Correct:** Alcohol acts as a potent **enzyme inducer** of GGT in the hepatic microsomes. Even in the absence of significant liver cell damage, alcohol consumption leads to an isolated rise in GGT. In alcoholic hepatitis, GGT levels are disproportionately elevated compared to other liver enzymes (often with an AST:ALT ratio > 2:1), making it the most specific indicator for alcohol-induced liver injury among the given options. 2. **Analysis of Incorrect Options:** * **Pancreatitis:** While GGT is present in the pancreas, it is not a primary or specific marker for pancreatitis. Lipase and Amylase are the preferred diagnostic enzymes. * **Hepatocellular Carcinoma (HCC):** While GGT can be elevated in HCC due to cholestasis or tumor mass effect, it is non-specific. **Alpha-fetoprotein (AFP)** is the classic marker for HCC. * **Infective Hepatitis:** Viral hepatitis primarily causes a massive rise in transaminases (ALT/AST). While GGT may rise slightly due to inflammation, it is not the hallmark or the most characteristic finding compared to alcoholic etiology. **High-Yield Clinical Pearls for NEET-PG:** * **GGT vs. Alkaline Phosphatase (ALP):** Both are elevated in obstructive jaundice. However, GGT is **normal in bone diseases**, whereas ALP is elevated. Therefore, GGT is used to confirm that an elevated ALP is of hepatic origin. * **Inducers:** Besides alcohol, drugs like **Phenytoin** and **Phenobarbitone** also induce GGT. * **Sensitivity:** GGT is the most sensitive marker for detecting **cholestasis** and early biliary cirrhosis.

Q: Which one of the following is a lyase enzyme?

Aldolase B. **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, or 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 Aldolase B is the correct answer:** Aldolase B (Fructose-1,6-bisphosphate aldolase) catalyzes the reversible cleavage of Fructose-1,6-bisphosphate into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate. In the liver, it also cleaves Fructose-1-phosphate. Since it breaks a C-C bond without the use of water or redox cofactor, it is a classic example of a **Lyase**. **Analysis of Incorrect Options:** * **Hexokinase (Option A):** This is a **Transferase (Class 2)**. It transfers a phosphate group from ATP to a hexose sugar (glucose). * **Pyruvate Kinase (Option B):** Despite the name "kinase," it is also a **Transferase (Class 2)**, transferring a phosphate group from phosphoenolpyruvate (PEP) to ADP. * **Propionyl CoA Carboxylase (Option C):** This is a **Ligase (Class 6)**. Carboxylases use ATP energy to join a CO₂ molecule to a substrate. **High-Yield Clinical Pearls for NEET-PG:** * **Hereditary Fructose Intolerance (HFI):** Caused by a deficiency of **Aldolase B**. It leads to the accumulation of Fructose-1-Phosphate, causing hypoglycemia and liver damage. * **Mnemonic for Enzyme Classes:** **"O T H L I L"** (Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases). * **Synthases vs. Synthetases:** *Synthases* are Lyases (do not require ATP), whereas *Synthetases* are Ligases (require ATP).

Q: Enolase is inhibited by which of the following?

Fluoride. **Explanation:** **Enolase** is a key glycolytic enzyme that catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). It is a metalloenzyme that requires **Magnesium (Mg²⁺)** ions for its catalytic activity. 1. **Why Fluoride is Correct:** Fluoride acts as a potent competitive inhibitor of Enolase. It reacts with inorganic phosphate and magnesium to form a complex called **Magnesium-Fluorophosphate**. This complex binds to the active site of the enzyme, displacing the essential Mg²⁺ ions and effectively halting glycolysis. 2. **Why Other Options are Incorrect:** * **Fumarate:** This is an intermediate in the TCA cycle and not a classic enzyme inhibitor in this context. * **Iodoacetate:** This inhibits **Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)** by binding to the essential –SH (sulfhydryl) groups at its active site. * **Arsenite:** Trivalent arsenic (Arsenite) inhibits enzymes requiring **Lipoic acid** as a cofactor, such as the Pyruvate Dehydrogenase (PDH) complex and α-ketoglutarate dehydrogenase. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood Glucose Estimation:** In clinical practice, blood samples for glucose estimation are collected in **Fluoride bulbs (Grey top)** containing Sodium Fluoride (NaF) and Potassium Oxalate. NaF inhibits Enolase to prevent "in vitro" glycolysis by RBCs, ensuring the glucose level remains stable until analysis. * **Oxalate's Role:** While Fluoride inhibits glycolysis, Potassium Oxalate acts as an anticoagulant by chelating calcium. * **Arsenate vs. Arsenite:** Note that *Arsenate* (pentavalent) competes with inorganic phosphate in the GAPDH reaction, leading to the bypass of ATP synthesis (substrate-level phosphorylation), whereas *Arsenite* (trivalent) inhibits PDH.

Question 1

What is the cofactor of glutathione peroxidase?

Accepted Answer

Selenium

Answer

Magnesium

Answer

Manganese

Answer

Zinc

Question 2

Which of the following causes hydrolysis of peptidoglycans?

Accepted Answer

Lysozyme

Answer

Lactoferrin

Answer

Protease

Answer

Aflatoxin

Question 3

Which of the following enzymes requires zinc as a cofactor?

Accepted Answer

Carbonic anhydrase

Answer

Tyrosinase

Answer

Hexokinase

Answer

Oxidoreductase

Question 4

What is the term for the combination of a main supporting enzyme and its cofactor?

Accepted Answer

Holoenzyme

Answer

Apoenzyme

Answer

Coenzyme

Answer

Constitutive enzyme

Question 5

Which enzyme is primarily stored in muscle?

Accepted Answer

SGOT

Answer

Alkaline phosphatase

Answer

SGPT

Answer

CPK

Question 6

Trypsin is an activator of all of the following enzymes EXCEPT?

Accepted Answer

Pepsinogen

Answer

Chymotrypsinogen

Answer

Proelastase

Answer

Procolipase

Question 7

A transporter has a Km for glucose of 45 mM and a Vmax of 36 micromoles glucose/sec/mg of transporter. If the glucose concentration in peripheral blood is 15 mM, what will the rate of glucose transport (in micromoles glucose/sec/mg transporter) be?

Accepted Answer

12

Answer

6

Answer

3

Answer

9

Question 8

Gamma Glutamyl Transferase is increased in which of the following conditions?

Accepted Answer

Alcoholic hepatitis

Answer

Pancreatitis

Answer

Hepatocellular carcinoma

Answer

Infective hepatitis

Question 9

Which one of the following is a lyase enzyme?

Accepted Answer

Aldolase B

Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Proponyl CoA carboxylase

Question 10

Enolase is inhibited by which of the following?

Accepted Answer

Fluoride

Answer

Fumarate

Answer

Iodoacetate

Answer

Arsenite

Enzymes — MCQs

Enzymes — MCQs

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