Enzymes Practice Questions

Q: Pyruvate dehydrogenase is inhibited allosterically by which of the following molecules?

NADH. **Explanation:** The **Pyruvate Dehydrogenase (PDH) complex** is a critical mitochondrial enzyme that converts Pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. Its regulation is a high-yield topic for NEET-PG, involving both allosteric control and covalent modification. **1. Why NADH is Correct:** PDH is regulated by **product inhibition**. The products of the reaction are **NADH** and **Acetyl-CoA**. When the energy status of the cell is high (high ATP/ADP ratio or high NADH/NAD+ ratio), these products bind allosterically to the enzyme complex to inhibit its activity. Specifically, NADH inhibits the E3 component (dihydrolipoyl dehydrogenase), while Acetyl-CoA inhibits the E2 component. **2. Analysis of Incorrect Options:** * **A. AMP:** This signifies a low-energy state. AMP (along with NAD+ and ADP) acts as an **activator** of PDH to promote energy production. * **B. Pyruvate:** This is the substrate. High concentrations of substrate drive the reaction forward and inhibit the PDH kinase, thereby keeping the PDH complex in its **active (dephosphorylated) state**. * **C. Insulin:** This is a hormonal regulator. In tissues like adipose, insulin activates PDH phosphatase, which dephosphorylates and **activates** the enzyme to promote lipogenesis. **Clinical Pearls for NEET-PG:** * **Covalent Modification:** PDH is **Active** when **Dephosphorylated** (mnemonic: **A**ctive = **A**way with phosphate). * **Cofactors:** PDH requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**T**ender **R**eeds **N**ever **P**ick **L**ilies). * **Arsenic Poisoning:** Arsenite inhibits PDH by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and neurological symptoms.

Q: Serum gamma glutamyl transpeptidase (GGT) is increased in which of the following conditions?

Alcoholism. **Explanation:** **Gamma-Glutamyl Transpeptidase (GGT)** is a microsomal enzyme primarily found in the liver, biliary tract, and kidneys. It plays a crucial role in the **GGT cycle (Meister cycle)** for glutathione metabolism and amino acid transport. **Why Alcoholism is the Correct Answer:** GGT is highly sensitive to alcohol consumption. It is a **microsomal enzyme**, and chronic alcohol intake leads to the **induction of microsomal enzymes** in the liver. Consequently, GGT levels rise significantly even in the absence of overt liver disease. It serves as a sensitive marker for chronic alcohol abuse and is often used to monitor abstinence in recovering patients. **Analysis of Incorrect Options:** * **Hepatitis (A):** While GGT can rise in hepatitis, it is not the most specific marker. Aminotransferases (ALT/AST) show much more dramatic elevations in hepatocellular injury. GGT is most significantly elevated in **obstructive jaundice** and cholestasis. * **Muscular Dystrophy (C):** GGT is **not present in skeletal muscle**. In muscular dystrophy, enzymes like Creatine Kinase (CK-MM), LDH, and AST are elevated, but GGT remains normal. This helps clinicians differentiate whether a raised AST is of hepatic or muscular origin. * **Myocardial Infarction (D):** GGT is not found in cardiac muscle. Markers for MI include Troponins (most specific), CK-MB, and LDH. **High-Yield NEET-PG Pearls:** * **GGT vs. ALP:** Both are elevated in obstructive jaundice. However, GGT is **normal in bone diseases**, whereas Alkaline Phosphatase (ALP) is elevated. Therefore, GGT is used to confirm if a high ALP is of hepatic origin. * **Meister Cycle:** GGT is the key enzyme that reacts with glutathione to transport amino acids across cell membranes. * **Sensitivity:** GGT is the most sensitive indicator of biliary tract disease, but its lack of specificity (due to induction by drugs like Phenytoin and Alcohol) limits its diagnostic utility.

Q: Which of the following enzyme activities increases in alcoholism?

Gamma-glutamyltransferase. **Explanation:** **Gamma-glutamyltransferase (GGT)** is the most sensitive biochemical marker for chronic alcohol consumption. The increase in GGT activity in alcoholics occurs primarily due to **enzyme induction**. Alcohol acts as a potent inducer of the microsomal ethanol oxidizing system (MEOS) and stimulates the synthesis of GGT in the liver. Furthermore, alcohol causes structural damage to hepatocytes and can lead to cholestasis, causing the membrane-bound GGT to leak into the serum. A GGT level that is disproportionately higher than other liver enzymes is a classic diagnostic clue for alcohol abuse. **Why the other options are incorrect:** * **Lactate dehydrogenase (LDH):** While LDH can rise in non-specific liver damage or hemolysis, it is not a specific marker for alcoholism and does not undergo induction by ethanol. * **Acid phosphatase (ACP):** This enzyme is primarily a marker for prostatic carcinoma (prostatic ACP) or bone resorptive states (tartrate-resistant ACP). It has no clinical correlation with alcohol intake. * **Alkaline phosphatase (ALP):** ALP is a marker for obstructive jaundice and bone diseases. While it may rise slightly in alcoholic hepatitis or cirrhosis, it is not as sensitive or specific as GGT, nor is it directly induced by alcohol. **High-Yield Clinical Pearls for NEET-PG:** * **De Ritis Ratio:** In alcoholic liver disease, the **AST:ALT ratio is typically >2:1** (Recall: "S" in AST stands for "Scotch"). * **GGT vs. ALP:** GGT is used to differentiate the source of an elevated ALP. If both GGT and ALP are high, the origin is hepatobiliary. If ALP is high but GGT is normal, the origin is likely bone. * **MCV:** An increased Mean Corpuscular Volume (macrocytosis) is another common hematological finding in chronic alcoholics.

Q: Which enzyme is not regulated by phosphorylation/dephosphorylation?

Aspartate transcarboxylase. **Explanation:** The regulation of enzyme activity occurs through various mechanisms, primarily **covalent modification** (like phosphorylation/dephosphorylation) and **allosteric regulation**. **Why Aspartate Transcarboxylase (ATCase) is the correct answer:** ATCase is the classic example of an enzyme regulated exclusively by **allosteric modulation**, not covalent modification. It catalyzes the rate-limiting step in pyrimidine biosynthesis. It is inhibited by **CTP** (feedback inhibition) and activated by **ATP** (purine/pyrimidine balance). It follows a sigmoidal kinetics curve, characteristic of multi-subunit allosteric enzymes, rather than the standard Michaelis-Menten kinetics. **Analysis of Incorrect Options:** * **Glycogen Synthase:** Regulated by covalent modification. It is **active** in the dephosphorylated state and **inactive** when phosphorylated (by Protein Kinase A/Glycogen Synthase Kinase). * **Glycogen Phosphorylase:** The reciprocal of the synthase. It is **active** when phosphorylated (by Phosphorylase Kinase) and **inactive** when dephosphorylated. * **HMG CoA Reductase:** The rate-limiting enzyme of cholesterol synthesis. It is **active** in the dephosphorylated state and **inactive** when phosphorylated by AMP-activated protein kinase (AMPK). **NEET-PG High-Yield Pearls:** 1. **Rule of Thumb:** Most rate-limiting enzymes in carbohydrate and lipid metabolism are regulated by phosphorylation. Usually, they are **active when dephosphorylated** (except for Glycogen Phosphorylase and Hormone Sensitive Lipase). 2. **ATCase Structure:** It consists of 6 catalytic and 6 regulatory subunits. 3. **Key Allosteric Enzymes to Remember:** PFK-1 (Glycolysis), Acetyl CoA Carboxylase (Fatty acid synthesis), and ATCase (Pyrimidine synthesis).

Q: Which of the following reduces oxidative stress, except?

Xanthine oxidase. ### Explanation The correct answer is **Xanthine oxidase (D)**. **1. Why Xanthine Oxidase is the correct answer:** Unlike the other options, Xanthine oxidase (XO) is a **pro-oxidant** enzyme, not an antioxidant. It catalyzes the oxidation of hypoxanthine to xanthine and xanthine to uric acid. During this process, molecular oxygen is reduced, leading to the generation of **Superoxide radicals ($O_2^{•–}$)** and **Hydrogen peroxide ($H_2O_2$)**. These Reactive Oxygen Species (ROS) increase oxidative stress and contribute to tissue injury, particularly during ischemia-reperfusion injury. **2. Why the other options are incorrect:** Options A, B, and C are the primary components of the body’s **enzymatic antioxidant defense system**: * **Superoxide dismutase (SOD):** Converts the highly reactive superoxide radical into less toxic hydrogen peroxide ($2O_2^{•–} + 2H^+ \rightarrow H_2O_2 + O_2$). * **Catalase:** A heme-containing enzyme found in peroxisomes that rapidly degrades hydrogen peroxide into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$). * **Glutathione peroxidase (GPx):** A selenium-dependent enzyme that neutralizes $H_2O_2$ by coupling it with the oxidation of reduced glutathione (GSH). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Allopurinol:** A suicide inhibitor of Xanthine oxidase used to treat Gout by reducing uric acid production and ROS generation. * **Selenium:** An essential cofactor for Glutathione peroxidase; its deficiency leads to Keshan disease. * **Zinc, Copper, and Manganese:** Essential cofactors for different isoforms of Superoxide dismutase (Cytosolic SOD uses Cu-Zn; Mitochondrial SOD uses Mn). * **Ischemia-Reperfusion Injury:** During ischemia, Xanthine dehydrogenase is converted to Xanthine oxidase, leading to a massive burst of free radicals upon reperfusion.

Q: Which of the following is a lyase?

Decarboxylase. ### Explanation Enzymes are classified into six major classes based on the International Union of Biochemistry (IUB) system, remembered by the mnemonic **OTH LIL**. **1. Why Decarboxylase is the Correct Answer:** **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. This often results in the formation of a double bond or the addition of a group to a double bond. **Decarboxylases** remove a carboxyl group ($CO_2$) from a substrate without using water or redox reactions, making them classic examples of Lyases (e.g., Pyruvate decarboxylase). **2. Analysis of Incorrect Options:** * **Synthetase (Class 6 - Ligases):** These enzymes catalyze the joining of two molecules coupled with the hydrolysis of a high-energy phosphate bond (ATP). *Note: "Synthases" are Lyases, but "Synthetases" are Ligases.* * **Kinase (Class 2 - Transferases):** Kinases specifically transfer a phosphate group from a high-energy donor (like ATP) to a substrate. * **Oxygenase (Class 1 - Oxidoreductases):** These enzymes catalyze oxidation-reduction reactions by incorporating oxygen into a substrate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dehydrogenases** are the most common Oxidoreductases (Class 1) encountered in the TCA cycle. * **Hydrolases (Class 3)** include digestive enzymes like Pepsin and Trypsin. * **Isomerases (Class 5)** catalyze structural rearrangements (e.g., Mutases, Epimerases). * **Key Distinction:** **Synthases** (Lyase) do NOT require ATP, whereas **Synthetases** (Ligase) DO require ATP. This is a frequent "trap" in PG entrance exams.

Q: Which metal is essential for the function of the enzyme enolase?

Magnesium. **Explanation:** **Enolase** (also known as phosphopyruvate hydratase) is a key glycolytic enzyme that catalyzes the reversible conversion of **2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP)**. 1. **Why Magnesium (Mg²⁺) is correct:** Enolase is a metalloenzyme that requires divalent metal ions for its catalytic activity. **Magnesium** is the natural physiological activator. It plays two roles: it helps stabilize the substrate binding at the active site and participates directly in the dehydration mechanism by coordinating with the carboxylate group of the substrate. 2. **Why other options are incorrect:** * **Calcium (Ca²⁺):** While chemically similar to magnesium, calcium does not activate enolase; in fact, it can act as a competitive inhibitor in certain contexts. * **Copper (Cu²⁺):** Copper is a cofactor for enzymes involved in redox reactions, such as *Cytochrome c oxidase* and *Superoxide dismutase*, but not for enolase. * **Iron (Fe²⁺/Fe³⁺):** Iron is essential for heme-containing enzymes (e.g., Catalase) and iron-sulfur cluster proteins (e.g., Aconitase), but it is not required for the dehydration step in glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Fluoride Inhibition:** Enolase is clinically significant because it is strongly inhibited by **Fluoride**. In clinical practice, fluoride (as sodium fluoride) is added to blood collection tubes (grey top) to inhibit glycolysis, ensuring accurate blood glucose estimation by preventing the breakdown of glucose by RBCs. * **Mechanism:** Fluoride forms a complex with magnesium and phosphate (**Magnesium-fluorophosphate**), which displaces the Mg²⁺ ion from the enzyme's active site, thereby inactivating it. * **Diagnostic Marker:** **Neuron-specific enolase (NSE)** is a high-yield tumor marker used for Small Cell Lung Carcinoma and Neuroblastoma.

Q: Which one of the following isoenzyme variants of Creatine kinase is elevated in myocardial infarction?

CK-MB. **Explanation:** Creatine Kinase (CK) is a dimeric enzyme composed of two subunits: **M** (Muscle) and **B** (Brain). These subunits combine to form three distinct isoenzymes, each specific to different tissues. **Why CK-MB is the correct answer:** **CK-MB (CK-2)** is primarily found in the **myocardium** (heart muscle). When myocardial cells are damaged during a Myocardial Infarction (MI), this enzyme leaks into the bloodstream. It typically rises within 4–6 hours of chest pain, peaks at 18–24 hours, and returns to baseline within 48–72 hours. While Troponins are now the preferred biomarkers due to higher sensitivity, CK-MB remains clinically significant for detecting **re-infarction** because of its rapid clearance from the blood. **Analysis of Incorrect Options:** * **CK-BB (CK-1):** Predominantly found in the **Brain** and lungs. Elevated levels are seen in CNS damage, strokes, or certain tumors, but not typically in MI. * **CK-MM (CK-3):** The major isoenzyme in **Skeletal Muscle** (comprising 99% of muscle CK). It rises in conditions like muscular dystrophy, rhabdomyolysis, or strenuous exercise. * **All the above:** Incorrect, as the elevation is specific to the tissue involved. **High-Yield Clinical Pearls for NEET-PG:** * **CK-MB Index:** If the CK-MB/Total CK ratio is **>5%**, it strongly suggests myocardial origin rather than skeletal muscle damage. * **Total CK:** CK-MM is the most abundant isoenzyme in the serum of healthy individuals. * **Troponin T/I:** These are the "Gold Standard" markers for MI due to their high cardiac specificity. * **Re-infarction:** If a patient has a second MI within 3–4 days, CK-MB is the marker of choice for diagnosis (as Troponins remain elevated for up to 10–14 days).

Q: Serum alkaline phosphatase level is reduced in which of the following conditions?

Hypothyroidism. **Explanation:** Alkaline Phosphatase (ALP) is a group of isoenzymes primarily found in the liver, bone, placenta, and intestine. While ALP is most commonly elevated in clinical practice, certain conditions lead to its deficiency. **Why Hypothyroidism is the correct answer:** Thyroid hormones (T3 and T4) play a crucial role in bone turnover and the stimulation of osteoblastic activity. In **hypothyroidism**, there is a significant reduction in bone metabolism and turnover. This leads to decreased production of the bone-specific isoenzyme of ALP, resulting in low serum levels. Additionally, zinc and magnesium are essential cofactors for ALP activity; deficiencies in these (sometimes seen in metabolic slowing) can further contribute to low levels. **Analysis of Incorrect Options:** * **Paget’s Disease:** Characterized by excessive and disorganized bone remodeling. It shows the **highest** elevations of serum ALP due to intense osteoblastic activity. * **Hyperparathyroidism:** Increased Parathyroid Hormone (PTH) stimulates osteoclastic bone resorption, which is coupled with compensatory osteoblastic activity, leading to **elevated** ALP. * **Fibrous Dysplasia:** A bone disorder where normal bone is replaced by fibrous tissue. It is associated with increased bone turnover and **elevated** ALP levels. **High-Yield Clinical Pearls for NEET-PG:** * **Causes of Low ALP (Mnemonic: "H-H-H-H-Z"):** **H**ypothyroidism, **H**ypophosphatasia (genetic deficiency), **H**alivudism (Vitamin D toxicity), **H**ernicious anemia (B12 deficiency), and **Z**inc deficiency. * **ALP as a Marker:** It is a primary marker for **cholestasis** (liver) and **osteoblastic activity** (bone). * **Normal Physiological Increase:** ALP is naturally elevated during **pregnancy** (placental isoenzyme) and during **puberty** (rapid bone growth).

Q: What is the effect of non-competitive inhibition on enzyme kinetics?

Normal Km and decreased Vmax. In **Non-competitive inhibition**, the inhibitor binds to an allosteric site (a site other than the active site) on both the free enzyme (E) and the enzyme-substrate complex (ES). ### Why Option B is Correct: * **Vmax (Decreased):** Because the inhibitor binds to a different site, it effectively reduces the total concentration of functional enzyme available to convert substrate to product. No amount of additional substrate can "outcompete" the inhibitor; therefore, the maximum velocity of the reaction is permanently lowered. * **Km (Normal/Unchanged):** Since the inhibitor does not compete for the active site, the affinity of the remaining functional enzymes for the substrate remains unchanged. Thus, the Michaelis constant ($K_m$) stays the same. ### Why Other Options are Incorrect: * **Option A & C:** A **decreased $K_m$** (increased affinity) is characteristic of **Uncompetitive inhibition**, where the inhibitor binds only to the ES complex. * **Option D:** An **increased Vmax** is physiologically impossible in the presence of an inhibitor; inhibitors by definition decrease the rate or efficiency of a reaction. * **Note on Competitive Inhibition:** In competitive inhibition (the most common distractor), $V_{max}$ remains **normal** while $K_m$ **increases**. ### High-Yield NEET-PG Pearls: 1. **Lineweaver-Burk Plot:** In non-competitive inhibition, the plots intersect on the **negative X-axis** ($-1/K_m$ is constant). 2. **Reversibility:** Non-competitive inhibition is usually reversible. If the binding is irreversible (e.g., Lead poisoning or Organophosphates), it is often termed "Irreversible inhibition," which kinetically mimics non-competitive inhibition. 3. **Classic Example:** Cyanide inhibition of Cytochrome Oxidase.

Question 1

Pyruvate dehydrogenase is inhibited allosterically by which of the following molecules?

Accepted Answer

NADH

Answer

AMP

Answer

Pyruvate

Answer

Insulin

Question 2

Serum gamma glutamyl transpeptidase (GGT) is increased in which of the following conditions?

Accepted Answer

Alcoholism

Answer

Hepatitis

Answer

Muscular dystrophy

Answer

Myocardial infarction

Question 3

Which of the following enzyme activities increases in alcoholism?

Accepted Answer

Gamma-glutamyltransferase

Answer

Lactate dehydrogenase

Answer

Acid phosphatase

Answer

Alkaline phosphatase

Question 4

Which enzyme is not regulated by phosphorylation/dephosphorylation?

Accepted Answer

Aspartate transcarboxylase

Answer

Glycogen synthase

Answer

Glycogen phosphorylase

Answer

HMG CoA reductase

Question 5

Which of the following reduces oxidative stress, except?

Accepted Answer

Xanthine oxidase

Answer

Superoxide dismutase

Answer

Catalase

Answer

Glutathione peroxidase

Question 6

Which of the following is a lyase?

Accepted Answer

Decarboxylase

Answer

Synthetase

Answer

Kinase

Answer

Oxygenase

Question 7

Which metal is essential for the function of the enzyme enolase?

Accepted Answer

Magnesium

Answer

Calcium

Answer

Copper

Answer

Iron

Question 8

Which one of the following isoenzyme variants of Creatine kinase is elevated in myocardial infarction?

Accepted Answer

CK-MB

Answer

CK-BB

Answer

CK-MM

Answer

All the above

Question 9

Serum alkaline phosphatase level is reduced in which of the following conditions?

Accepted Answer

Hypothyroidism

Answer

Paget's disease

Answer

Hyperparathyroidism

Answer

Fibrous dysplasia

Question 10

What is the effect of non-competitive inhibition on enzyme kinetics?

Accepted Answer

Normal Km and decreased Vmax

Answer

Decreased Km and decreased Vmax

Answer

Decreased Km and increased Vmax

Answer

Normal Km and increased Vmax

Enzymes — MCQs

Enzymes — MCQs

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