Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 361: Pyruvate dehydrogenase is inhibited allosterically by which of the following molecules?

A. AMP
B. Pyruvate
C. NADH (Correct Answer)
D. Insulin

Explanation: **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.

Question 362: Serum gamma glutamyl transpeptidase (GGT) is increased in which of the following conditions?

A. Hepatitis
B. Alcoholism (Correct Answer)
C. Muscular dystrophy
D. Myocardial infarction

Explanation: **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.

Question 363: What coenzyme is required for the transketolase reaction?

A. Ca2+
B. Mg2+ (Correct Answer)
C. H+
D. PO4-

Explanation: **Explanation:** The correct answer is **Mg²⁺**. Transketolase is a key enzyme in the **Pentose Phosphate Pathway (PPP)**, specifically in the non-oxidative phase. It catalyzes the transfer of a two-carbon unit from a ketose donor to an aldose acceptor. For its catalytic activity, transketolase requires two essential components: **Thiamine Pyrophosphate (TPP)** as a prosthetic group and **Magnesium ions (Mg²⁺)** as a cofactor. Mg²⁺ acts as a bridge, stabilizing the binding of the negatively charged pyrophosphate group of TPP to the enzyme’s active site. **Analysis of Options:** * **Mg²⁺ (Correct):** It is the mandatory divalent cation cofactor for transketolase. Without Mg²⁺, the enzyme cannot bind TPP effectively. * **Ca²⁺:** While calcium is a vital signaling molecule and cofactor for enzymes like α-ketoglutarate dehydrogenase, it does not play a role in the transketolase reaction. * **H⁺ and PO₄⁻:** These are involved in acid-base balance and phosphorylation reactions, respectively, but do not function as specific cofactors for this enzyme. **Clinical Pearls for NEET-PG:** 1. **Wernicke-Korsakoff Syndrome:** This condition is characterized by a genetic predisposition where transketolase has a low affinity for TPP. Symptoms are exacerbated by Thiamine (Vitamin B1) deficiency. 2. **Diagnostic Utility:** Measuring **Erythrocyte Transketolase Activity (ETKA)** is the gold standard biochemical test to diagnose Thiamine deficiency. An increase in enzyme activity upon adding TPP in vitro indicates a deficiency state. 3. **Pathway Link:** Transketolase provides a reversible link between the PPP and Glycolysis (via Glyceraldehyde-3-phosphate and Fructose-6-phosphate).

Question 364: Which enzyme is deficient in chronic alcoholics?

A. Aconitase
B. Citrate synthase
C. Isocitrate dehydrogenase
D. Alpha ketoglutarate dehydrogenase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Alpha-ketoglutarate dehydrogenase (α-KGDH)**. **1. Why Alpha-ketoglutarate dehydrogenase is correct:** Chronic alcoholics often suffer from malnutrition and impaired absorption, leading to a deficiency of **Thiamine (Vitamin B1)**. Alpha-ketoglutarate dehydrogenase is a multi-enzyme complex in the TCA cycle that requires **Thiamine Pyrophosphate (TPP)** as a mandatory co-enzyme. In the absence of thiamine, the activity of α-KGDH is significantly reduced, impairing aerobic metabolism and ATP production. This is a critical factor in the pathogenesis of Wernicke-Korsakoff syndrome. **2. Why other options are incorrect:** * **Aconitase:** This enzyme converts Citrate to Isocitrate. It requires **Iron (Fe²⁺)** in the form of an iron-sulfur cluster, not thiamine. * **Citrate synthase:** This is the first regulatory enzyme of the TCA cycle. It does not require thiamine; its activity is primarily regulated by substrate availability (Acetyl-CoA and Oxaloacetate) and ATP/NADH levels. * **Isocitrate dehydrogenase:** This is the rate-limiting step of the TCA cycle. It requires **NAD⁺ or NADP⁺** and Mg²⁺/Mn²⁺, but it is not thiamine-dependent. **3. Clinical Pearls for NEET-PG:** * **The "Tender Loving Care For No-one" Mnemonic:** TPP-dependent enzymes include **T**ransketolase (HMP shunt), **L**eucine (Branched-chain α-ketoacid dehydrogenase), **C**itric acid cycle (α-KGDH), and **P**yruvate dehydrogenase. * **Diagnostic Marker:** Erythrocyte transketolase activity is used to diagnose thiamine deficiency. * **Clinical Warning:** Never give intravenous glucose to a chronic alcoholic before thiamine supplementation, as it can precipitate acute Wernicke encephalopathy by consuming the remaining thiamine stores during glycolysis.

Question 365: Which reaction requires HMG–COA reductase activity?

A. β-hydroxy–β-methyl glutaryl COA → Mevalonic acid (Correct Answer)
B. Acetyl COA + CO2 + ATP → Malonyl COA + ADP + Pi
C. L-methylmalonyl-CoA → Succinyl CoA
D. Lactose + H2O → Glucose + Galactose

Explanation: **Explanation:** **1. Why Option A is Correct:** The conversion of **$\beta$-hydroxy-$\beta$-methylglutaryl CoA (HMG-CoA)** to **Mevalonic acid** is the committed and **rate-limiting step** in cholesterol biosynthesis. This reaction is catalyzed by the enzyme **HMG-CoA Reductase**, which utilizes 2 molecules of NADPH as a reducing agent. This step occurs in the cytosol/endoplasmic reticulum of cells, primarily in the liver. **2. Analysis of Incorrect Options:** * **Option B:** This describes the conversion of Acetyl CoA to Malonyl CoA, catalyzed by **Acetyl-CoA Carboxylase (ACC)**. This is the rate-limiting step of **Fatty Acid Synthesis** and requires Biotin (Vitamin B7) as a cofactor. * **Option C:** This is the isomerization of L-methylmalonyl-CoA to Succinyl CoA, catalyzed by **Methylmalonyl-CoA Mutase**. This reaction is critical for the metabolism of odd-chain fatty acids and requires **Vitamin B12 (Cobalamin)**. * **Option D:** This is the hydrolysis of lactose into its constituent monosaccharides, catalyzed by the enzyme **Lactase** (a $\beta$-galactosidase) found in the intestinal brush border. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Pharmacology Link:** **Statins** (e.g., Atorvastatin, Rosuvastatin) are competitive inhibitors of HMG-CoA Reductase, used to treat hypercholesterolemia. * **Regulation:** HMG-CoA Reductase is inhibited by high levels of cholesterol (feedback inhibition) and glucagon (via phosphorylation), while it is stimulated by **Insulin**. * **Location:** Do not confuse this with HMG-CoA **Lyase**, which is involved in **Ketogenesis** (occurring in the mitochondria). HMG-CoA Reductase is for cholesterol synthesis (cytosol).

Question 366: What does Km represent in enzyme kinetics?

A. Substrate concentration at which the velocity of reaction is maximum
B. Substrate concentration at which the velocity of reaction is half of the maximum (Correct Answer)
C. Corresponds to the Y-axis in an enzyme kinetics graph
D. Denotes the catalytic efficiency of an enzyme

Explanation: ### Explanation **1. Why Option B is Correct:** The Michaelis-Menten constant (**Km**) is defined as the specific substrate concentration $[S]$ at which the reaction velocity ($v$) is exactly **half of the maximum velocity** ($V_{max}/2$). Mathematically, when $v = \frac{1}{2} V_{max}$, the Michaelis-Menten equation ($v = \frac{V_{max}[S]}{Km + [S]}$) simplifies to $Km = [S]$. * **Medical Concept:** Km is an intrinsic property of an enzyme that reflects its **affinity** for a substrate. A **low Km** indicates high affinity (the enzyme reaches half-saturation at low substrate levels), while a **high Km** indicates low affinity. **2. Why Other Options are Incorrect:** * **Option A:** The velocity is maximum ($V_{max}$) only when the enzyme is fully saturated with substrate; this is a theoretical limit, not Km. * **Option C:** In a Michaelis-Menten plot, Km is found on the **X-axis** (substrate concentration). In a Lineweaver-Burk (double reciprocal) plot, the X-intercept is $-1/Km$. * **Option D:** Catalytic efficiency is represented by the ratio **Kcat/Km**, not Km alone. **3. NEET-PG High-Yield Pearls:** * **Competitive Inhibition:** Km **increases** (affinity decreases), but $V_{max}$ remains unchanged. (Classic exam question). * **Non-competitive Inhibition:** Km remains **unchanged**, but $V_{max}$ decreases. * **Glucokinase vs. Hexokinase:** Glucokinase has a **high Km** for glucose (low affinity), allowing it to function only when blood glucose is high (e.g., post-prandial), whereas Hexokinase has a **low Km** (high affinity) to ensure glucose uptake even during fasting. * **Lineweaver-Burk Plot:** Remember that the **Y-intercept** is $1/V_{max}$ and the **X-intercept** is $-1/Km$.

Question 367: Which of the following enzyme activities increases in alcoholism?

A. Lactate dehydrogenase
B. Acid phosphatase
C. Alkaline phosphatase
D. Gamma-glutamyltransferase (Correct Answer)

Explanation: **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.

Question 368: Which enzyme is not regulated by phosphorylation/dephosphorylation?

A. Glycogen synthase
B. Glycogen phosphorylase
C. Aspartate transcarboxylase (Correct Answer)
D. HMG CoA reductase

Explanation: **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).

Question 369: What is the unit of Km?

A. second⁻¹
B. Moles second⁻¹
C. Millimoles
D. Millimoles Litre⁻¹ (Correct Answer)

Explanation: **Explanation:** The **Michaelis constant ($K_m$)** is defined as the substrate concentration at which the reaction velocity is exactly half of the maximum velocity ($V_{max}/2$). Since $K_m$ represents a concentration, its units must be expressed in terms of molarity (moles per unit volume). **1. Why Option D is correct:** Concentration is measured in Moles/Litre. Therefore, **Millimoles Litre⁻¹** (mM) is a standard unit of concentration used to denote $K_m$. It reflects the affinity of an enzyme for its substrate: a low $K_m$ indicates high affinity, while a high $K_m$ indicates low affinity. **2. Why other options are incorrect:** * **Option A (second⁻¹):** This is the unit for the first-order rate constant or the turnover number ($k_{cat}$), representing how many substrate molecules one enzyme molecule converts to product per unit time. * **Option B (Moles second⁻¹):** This represents the **Reaction Velocity ($V$)**, which is the amount of product formed per unit time. * **Option C (Millimoles):** This is a unit of quantity (amount), not concentration. Concentration must account for volume (Litre⁻¹). **High-Yield NEET-PG Pearls:** * **Lineweaver-Burk Plot:** $K_m$ is determined by the **x-intercept** ($-1/K_m$). * **Competitive Inhibition:** $K_m$ increases (affinity decreases), but $V_{max}$ remains unchanged. * **Non-competitive Inhibition:** $K_m$ remains unchanged, but $V_{max}$ decreases. * **Hexokinase vs. Glucokinase:** Hexokinase has a low $K_m$ (high affinity for glucose), allowing it to function even at low blood glucose levels, whereas Glucokinase has a high $K_m$ (low affinity) and functions primarily after meals.

Question 370: Which of the following reduces oxidative stress, except?

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

Explanation: ### 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.

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Enzymes — MCQs

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