Enzymes Practice Questions

Q: Which of the following is an allosteric inhibitor of pyruvate dehydrogenase?

Acetyl CoA. **Explanation:** The **Pyruvate Dehydrogenase (PDH) complex** is a multi-enzyme system that catalyzes the irreversible oxidative decarboxylation of pyruvate into Acetyl CoA, serving as the critical bridge between glycolysis and the TCA cycle. **1. Why Acetyl CoA is correct:** PDH is regulated primarily by **product inhibition**. The immediate products of the reaction are **Acetyl CoA** and **NADH**. When these accumulate, they act as potent allosteric inhibitors of the enzyme complex. High levels of Acetyl CoA signal that the cell’s energy needs are met or that fatty acid oxidation is providing sufficient fuel, thus slowing down the conversion of pyruvate. **2. Why the other options are incorrect:** * **AMP and ADP (Options A & C):** These are indicators of a "low energy state." They act as **allosteric activators** of the PDH complex (and inhibitors of PDH kinase) to promote ATP production. * **Citrate (Option D):** While citrate is an important allosteric inhibitor of **Phosphofructokinase-1 (PFK-1)** in glycolysis, it does not directly inhibit the PDH complex. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Covalent Modification:** PDH is also regulated by phosphorylation. **PDH Kinase** inactivates it (stimulated by Acetyl CoA/NADH), while **PDH Phosphatase** activates it (stimulated by $Ca^{2+}$ and Insulin). * **Co-factors:** PDH requires five co-enzymes: **T**hiamine pyrophosphate ($B_1$), **L**ipoic acid, **C**oenzyme A ($B_5$), **F**AD ($B_2$), and **N**AD ($B_3$). (Mnemonic: **T**ender **L**oving **C**are **F**or **N**o-one). * **Arsenic Poisoning:** Arsenite inhibits PDH by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and neurological symptoms.

Q: What is the principal serum enzyme used in the clinical diagnosis of Wilson disease?

Ceruloplasmin. **Explanation:** **Wilson Disease (Hepatolenticular Degeneration)** is an autosomal recessive disorder caused by a mutation in the **ATP7B gene** on chromosome 13. This defect impairs biliary copper excretion and prevents the incorporation of copper into apo-ceruloplasmin. **Why Ceruloplasmin is the correct answer:** Ceruloplasmin is the primary copper-carrying protein in the blood. In Wilson disease, the failure to form holoceruloplasmin leads to the release of unstable apo-ceruloplasmin, which is rapidly degraded in the circulation. Consequently, **low serum ceruloplasmin levels (<20 mg/dL)** are a hallmark diagnostic finding and the most commonly used serum biochemical marker for the disease. **Analysis of Incorrect Options:** * **A. Aspartate aminotransferase (AST):** While AST levels may rise due to liver damage in Wilson disease, it is a non-specific marker of hepatocellular injury found in many conditions (e.g., viral hepatitis, alcohol use). * **C. beta-Glucocerebrosidase:** This enzyme is deficient in **Gaucher disease**, a lysosomal storage disorder. It has no diagnostic role in copper metabolism. * **D. Lactate dehydrogenase isozyme 5 (LDH-5):** This isozyme is found primarily in the liver and skeletal muscle. While elevated in liver injury, it lacks the specificity required to diagnose Wilson disease. **NEET-PG High-Yield Pearls:** * **Gold Standard Diagnosis:** Liver biopsy showing increased copper content (>250 μg/g dry weight). * **Classic Triad:** Liver cirrhosis, Basal ganglia degeneration (Parkinsonian symptoms), and **Kayser-Fleischer (KF) rings** in the cornea (Descemet's membrane). * **Urinary Findings:** Increased 24-hour urinary copper excretion (>100 μg/day). * **Treatment:** Copper chelators like **D-Penicillamine** (first-line) or Trientine, and Zinc (to inhibit intestinal absorption).

Q: Lactate dehydrogenase is:

Isoenzyme. **Explanation:** **Lactate Dehydrogenase (LDH)** is a classic example of an **Isoenzyme** (or Isozyme) [3]. Isoenzymes are physically distinct forms of the same enzyme that catalyze the same chemical reaction but differ in their amino acid sequence, physical properties (like electrophoretic mobility), and kinetic parameters ($K_m$ and $V_{max}$) [3]. LDH is a tetramer composed of two types of subunits: **H (Heart)** and **M (Muscle)** [3]. These combine in five different ways to form the five isoenzymes (LDH1 to LDH5), which are distributed tissue-specifically [3]. **Why other options are incorrect:** * **Coenzyme:** These are non-protein organic molecules (like $NAD^+$ or $FAD$) that assist enzymes in catalysis [1]. LDH *uses* $NAD^+$ as a coenzyme but is not one itself [1]. * **Antienzyme:** These are substances (often antibodies or inhibitors) that inhibit enzymatic activity (e.g., Trypsin inhibitors). * **Zymogen:** Also known as proenzymes, these are inactive precursors that require cleavage to become active (e.g., Pepsinogen, Trypsinogen). LDH is synthesized in its active form. **High-Yield Clinical Pearls for NEET-PG:** 1. **LDH Composition:** * **LDH1 ($H_4$):** Found in Heart and RBCs [3]. * **LDH2 ($H_3M_1$):** Predominant form in normal serum [2]. * **LDH4 & LDH5 ($M_4$):** Found in Liver and Skeletal muscle [3]. 2. **Diagnostic Significance:** In Myocardial Infarction (MI), the **"LDH Flip"** occurs where LDH1 levels exceed LDH2 (normally LDH2 > LDH1) [2]. 3. **Cancer Marker:** LDH is a non-specific marker of high cell turnover (e.g., Lymphoma, Germ cell tumors).

Q: Which of the following covalent modifications does NOT regulate enzyme kinetics?

Glycosylation. **Explanation:** Enzyme activity is regulated through various mechanisms, including **covalent modification**, where the addition or removal of a chemical group alters the enzyme's conformation and kinetics. **Why Glycosylation is the Correct Answer:** While glycosylation (the addition of carbohydrate chains) is a vital post-translational modification, its primary roles are **protein folding, stability, cell-cell recognition, and trafficking** (e.g., targeting enzymes to lysosomes via Mannose-6-Phosphate). Unlike phosphorylation, it is generally not used as a "molecular switch" to acutely turn enzyme catalytic activity on or off in response to metabolic signals. **Analysis of Incorrect Options:** * **Phosphorylation (Option A):** The most common covalent modification. It occurs on Serine, Threonine, or Tyrosine residues. Example: **Glycogen phosphorylase** is activated by phosphorylation, while **Glycogen synthase** is inactivated. * **Acetylation (Option B):** Common in histones and metabolic enzymes. Acetylation of lysine residues can alter the charge and affinity of enzymes for DNA or substrates. * **ADP-Ribosylation (Option C):** Involves the transfer of ADP-ribose from NAD+. This is a key mechanism for several bacterial toxins (e.g., **Cholera toxin** ADP-ribosylates Gs proteins, and **Diphtheria toxin** inhibits Elongation Factor-2). **High-Yield Clinical Pearls for NEET-PG:** * **Zymogen Activation:** A form of irreversible covalent modification (proteolysis), e.g., Pepsinogen to Pepsin. * **Master Regulator:** Protein Kinase A (PKA) mediates most phosphorylation events triggered by cAMP. * **Lysosomal Targeting:** Deficiency in the glycosylation step that adds Mannose-6-Phosphate leads to **I-Cell Disease**, where enzymes are secreted extracellularly instead of being sent to lysosomes.

Q: Which liver enzyme elevation suggests bile duct obstruction?

Alkaline phosphatase (ALP). **Explanation:** The correct answer is **Alkaline phosphatase (ALP)**. **1. Why ALP is the correct answer:** Alkaline phosphatase is an enzyme found primarily in the cells lining the biliary canaliculi of the liver. In the setting of **cholestasis** (bile duct obstruction), the increased pressure from bile stasis triggers the de novo synthesis of ALP and its subsequent release into the bloodstream. Therefore, a disproportionate rise in ALP compared to aminotransferases is the hallmark of an **obstructive (post-hepatic) jaundice** pattern. **2. Why the other options are incorrect:** * **ALT (Alanine aminotransferase) & AST (Aspartate aminotransferase):** These are markers of **hepatocellular injury**. They are located within the hepatocytes. While they may rise slightly in obstruction, their primary elevation occurs in conditions like viral hepatitis or toxin-induced liver damage. ALT is more specific to the liver than AST. * **Aminotransferase:** This is a general term for the group containing ALT and AST; it is not specific for biliary obstruction. **3. NEET-PG High-Yield Clinical Pearls:** * **Confirmatory Test:** Since ALP is also found in bone, placenta, and intestine, **GGT (Gamma-glutamyl transferase)** is measured to confirm the hepatic origin of an elevated ALP. GGT is *not* elevated in bone disease. * **The "3-Fold Rule":** In obstructive jaundice, ALP typically rises to >3 times the upper limit of normal. * **Mnemonic:** **A**LP = **A**way (Obstruction/Bile flow blocked); **A**LT = **A**live (Hepatocyte damage). * **Other Biliary Markers:** 5'-Nucleotidase is another specific marker for biliary obstruction, often used alongside ALP.

Q: Molybdenum is a component of which enzyme?

Xanthine oxidase. **Explanation:** **Xanthine Oxidase** is a complex metalloenzyme that requires **Molybdenum** (as a molybdopterin cofactor), Iron, and FAD for its catalytic activity. It plays a critical role in purine catabolism, catalyzing the oxidation of hypoxanthine to xanthine and xanthine to uric acid. **Analysis of Options:** * **Cytochrome Oxidase (Option A):** This is the terminal enzyme of the electron transport chain (Complex IV). It contains **Copper (Cu)** and **Iron (Fe)** (in heme groups), not molybdenum. * **Glutathione Peroxidase (Option C):** This enzyme protects cells from oxidative damage by reducing lipid hydroperoxides. It is a well-known **Selenium-dependent** enzyme. * **Urease (Option D):** Found in bacteria (like *H. pylori*) and plants, this enzyme catalyzes the hydrolysis of urea. It requires **Nickel (Ni)** for its activity. **Clinical Pearls & High-Yield Facts:** 1. **Gout Connection:** Allopurinol, a drug used to treat chronic gout, acts as a suicide inhibitor of Xanthine Oxidase, thereby lowering serum uric acid levels. 2. **Molybdenum-dependent enzymes:** Besides Xanthine Oxidase, other human enzymes requiring Molybdenum include **Sulfite Oxidase** (deficiency leads to neurological symptoms) and **Aldehyde Oxidase**. 3. **Genetic Deficiency:** Hereditary Xanthinuria is caused by a deficiency of Xanthine Oxidase, leading to hypouricemia and potential xanthine stones in the urinary tract.

Q: Which of the following is most specific for cholestasis?

5' nucleotidase. **Explanation:** The correct answer is **5' nucleotidase**. **1. Why 5' nucleotidase is the most specific:** 5' nucleotidase is an enzyme located in the canalicular and sinusoidal membranes of hepatocytes. While both **Alkaline Phosphatase (ALP)** and **Gamma-glutamyl transferase (GGT)** are elevated in cholestasis, ALP can also originate from bone, placenta, and intestine. 5' nucleotidase is highly specific to the liver; its elevation almost exclusively indicates hepatobiliary disease. In clinical practice, it is primarily used to confirm that an elevated ALP is of hepatic (cholestatic) origin rather than bone origin. **2. Analysis of Incorrect Options:** * **ALT (Alanine Aminotransferase):** This is a marker of **hepatocellular injury** (necrosis/inflammation) rather than cholestasis. It is found primarily in the cytosol of hepatocytes. * **AST (Aspartate Aminotransferase):** Similar to ALT, this indicates hepatocellular damage. It is less specific than ALT as it is also found in cardiac muscle, skeletal muscle, and RBCs. * **GGT (Gamma-glutamyl transferase):** While GGT is a very **sensitive** marker for cholestasis and biliary obstruction, it lacks specificity. It is induced by alcohol consumption and various drugs (e.g., phenytoin) even in the absence of cholestasis. **3. NEET-PG High-Yield Pearls:** * **Best initial test for cholestasis:** Alkaline Phosphatase (ALP). * **Most specific test for cholestasis:** 5' nucleotidase. * **Marker for chronic alcoholism:** GGT (often shows a GGT/ALP ratio > 2.5). * **De Ritis Ratio:** AST/ALT ratio > 2 suggests alcoholic liver disease; < 1 suggests viral hepatitis. * **Bone vs. Liver ALP:** If ALP is high but GGT and 5' nucleotidase are normal, the source is likely bone (e.g., Paget’s disease).

Q: A patient took a medication to combat altitude sickness. A graph depicting this medication's mechanism of enzyme inhibition is shown. What type of inhibitor is this medication?

Non-competitive. ***Non-competitive*** - **Acetazolamide** (the altitude sickness medication) binds to a site other than the active site of **carbonic anhydrase**, causing conformational changes that reduce enzyme activity. - On the **Lineweaver-Burk plot**, non-competitive inhibition shows the same **Km** (lines cross at x-axis) but decreased **Vmax** (increased y-intercept), indicating reduced maximum velocity without affecting substrate binding affinity. *Competitive* - Inhibitor competes with substrate for the **active site**, causing increased **Km** but unchanged **Vmax**. - On Lineweaver-Burk plot, lines would cross at the **y-axis**, not the x-axis as shown for acetazolamide. *Irreversible* - Inhibitor forms **covalent bonds** with the enzyme, permanently inactivating it and showing decreased **Vmax** with no change in **Km**. - Unlike acetazolamide, irreversible inhibition cannot be overcome by **washing** or **dialysis** and shows time-dependent inhibition. *Allosteric* - This is a **regulatory mechanism** rather than a type of inhibition kinetics, involving binding to regulatory sites that affect enzyme activity. - **Allosteric inhibition** is actually a subset of non-competitive inhibition, making this option less specific than the correct answer.

Q: Which enzyme serves as a marker for the pancreas?

Amylase. **Explanation:** **Amylase** is the correct answer because it is one of the primary digestive enzymes synthesized and secreted by the exocrine pancreas. In clinical practice, serum amylase is a classic biochemical marker used to diagnose **Acute Pancreatitis**. When pancreatic acinar cells are damaged or the pancreatic duct is obstructed, amylase leaks into the interstitial space and subsequently into the bloodstream. While **Lipase** is considered more specific for the pancreas, Amylase remains a standard diagnostic marker. **Analysis of Incorrect Options:** * **Creatine phosphokinase (CPK):** This enzyme is primarily found in the heart (CK-MB), skeletal muscle (CK-MM), and brain (CK-BB). It is a marker for myocardial infarction or muscular dystrophies, not pancreatic pathology. * **SGOT (AST) and ALT:** These are aminotransferases. **ALT** is a specific marker for **liver** cell injury (hepatocytes), while **SGOT (AST)** is found in the liver, heart, and skeletal muscle. They are used to evaluate hepatic function and biliary diseases. **High-Yield Clinical Pearls for NEET-PG:** * **Lipase vs. Amylase:** Lipase is more specific for the pancreas and remains elevated longer (7–14 days) than amylase (2–5 days). * **P-Amylase:** The S-type (salivary) and P-type (pancreatic) isoenzymes of amylase exist; P-type is specific to pancreatic injury. * **Hypertriglyceridemia:** High lipid levels can falsely lower serum amylase levels (spurious normalcy) in acute pancreatitis. * **Macroamylasemia:** A benign condition where amylase binds to immunoglobulins, causing elevated serum amylase but normal urinary amylase levels.

Q: Which of the following enzymes protects the brain from free radical injury?

Superoxide dismutase. **Explanation:** **Correct Option: C. Superoxide dismutase (SOD)** The brain is highly susceptible to oxidative stress due to its high oxygen consumption and lipid-rich content. **Superoxide dismutase (SOD)** is a primary antioxidant enzyme that protects cells from free radical injury. It catalyzes the **dismutation** of the highly reactive superoxide radical ($O_2^{\bullet-}$) into oxygen ($O_2$) and the less reactive hydrogen peroxide ($H_2O_2$). This $H_2O_2$ is subsequently neutralized by Catalase or Glutathione peroxidase. Without SOD, superoxide radicals would react with nitric oxide to form peroxynitrite, leading to significant neuronal damage. **Why Incorrect Options are Wrong:** * **A. Hydroxylase:** These enzymes (e.g., Phenylalanine hydroxylase) are involved in biosynthetic pathways, such as the synthesis of neurotransmitters, rather than antioxidant defense. * **B. Myeloperoxidase (MPO):** Found primarily in neutrophils, MPO actually *produces* reactive oxygen species (specifically hypochlorous acid, HOCl) to kill bacteria. It is a pro-oxidant, not a protective antioxidant. * **D. Monoamine oxidase (MAO):** MAO breaks down neurotransmitters like dopamine and serotonin. This metabolic process actually generates $H_2O_2$ as a byproduct, potentially contributing to oxidative stress rather than preventing it. **Clinical Pearls for NEET-PG:** * **SOD Isoforms:** SOD1 (Cytosolic, requires Cu-Zn), SOD2 (Mitochondrial, requires Mn), and SOD3 (Extracellular). * **Clinical Correlation:** Mutations in the **SOD1 gene** are a known cause of **Amyotrophic Lateral Sclerosis (ALS)**, highlighting the enzyme's critical role in neuronal protection. * **Glutathione Peroxidase:** This is the other major brain antioxidant; it requires **Selenium** as a cofactor.

Question 1

Which of the following is an allosteric inhibitor of pyruvate dehydrogenase?

Accepted Answer

Acetyl CoA

Answer

AMP

Answer

ADP

Answer

Citrate

Question 2

What is the principal serum enzyme used in the clinical diagnosis of Wilson disease?

Accepted Answer

Ceruloplasmin

Answer

Aspartate aminotransferase

Answer

beta-Glucocerebrosidase

Answer

Lactate dehydrogenase isozyme 5

Question 3

Lactate dehydrogenase is:

Accepted Answer

Isoenzyme

Answer

Coenzyme

Answer

Antienzyme

Answer

Zymogen

Question 4

Which of the following covalent modifications does NOT regulate enzyme kinetics?

Accepted Answer

Glycosylation

Answer

Phosphorylation

Answer

Acetylation

Answer

ADP-Ribosylation

Question 5

Which liver enzyme elevation suggests bile duct obstruction?

Accepted Answer

Alkaline phosphatase (ALP)

Answer

Aspartate aminotransferase (AST)

Answer

Alanine aminotransferase (ALT)

Answer

Aminotransferase

Question 6

Molybdenum is a component of which enzyme?

Accepted Answer

Xanthine oxidase

Answer

Cytochrome oxidase

Answer

Glutathione peroxidase

Answer

Urease

Question 7

Which of the following is most specific for cholestasis?

Accepted Answer

5' nucleotidase

Answer

ALT

Answer

AST

Answer

GGT

Question 8

A patient took a medication to combat altitude sickness. A graph depicting this medication's mechanism of enzyme inhibition is shown. What type of inhibitor is this medication?

Accepted Answer

Non-competitive

Answer

Competitive

Answer

Irreversible

Answer

Allosteric

Question 9

Which enzyme serves as a marker for the pancreas?

Accepted Answer

Amylase

Answer

Creatine phosphokinase (CPK)

Answer

Serum glutamic-oxaloacetic transaminase (SGOT)

Answer

Alanine transaminase (ALT)

Question 10

Which of the following enzymes protects the brain from free radical injury?

Accepted Answer

Superoxide dismutase

Answer

Hydroxylase

Answer

Myeloperoxidase

Answer

Monoamine oxidase

Enzymes — MCQs

Enzymes — MCQs

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