Enzymes Practice Questions

Q: Glutathione reductase contains which of the following prosthetic groups?

FAD. **Explanation:** **Glutathione Reductase** is a critical enzyme in the antioxidant defense system, responsible for maintaining the pool of reduced glutathione (GSH) in the cell. **1. Why FAD is Correct:** Glutathione reductase is a flavoprotein. It utilizes **Flavin Adenine Dinucleotide (FAD)** as a tightly bound prosthetic group. The enzyme catalyzes the reduction of oxidized glutathione (GSSG) to reduced glutathione (GSH) using **NADPH** as a reducing equivalent. During this catalytic cycle, electrons are transferred from NADPH to the FAD prosthetic group, and then to the disulfide bond of GSSG. **2. Why Other Options are Incorrect:** * **NAD (Option B):** While NAD/NADH are common electron carriers, Glutathione Reductase specifically requires **NADPH** (derived from the HMP Shunt) as a co-substrate, not NAD. Furthermore, NADPH acts as a co-enzyme (dissociable), whereas FAD is the permanent prosthetic group. * **ATP (Option C):** ATP is the energy currency of the cell but does not participate in the redox reactions of the glutathione cycle. It is required for the *synthesis* of glutathione (via Glutamate-cysteine ligase), but not for its *regeneration* by the reductase. **3. Clinical Pearls & High-Yield Facts:** * **The HMP Shunt Connection:** The NADPH required for this reaction is primarily supplied by **Glucose-6-Phosphate Dehydrogenase (G6PD)**. * **G6PD Deficiency:** In G6PD deficiency, a lack of NADPH prevents Glutathione Reductase from functioning, leading to oxidative stress, Heinz body formation, and hemolytic anemia. * **Riboflavin Status:** Since FAD is derived from **Vitamin B2 (Riboflavin)**, the activity of erythrocyte glutathione reductase is used as a functional diagnostic marker to assess riboflavin deficiency. * **Active Site:** Besides FAD, the enzyme also contains a **selenocysteine** residue (in the case of glutathione peroxidase) or essential **thiol groups** (cysteine) at its active site.

Q: What is the relative Km of Hexokinase and Glucokinase?

Glucokinase has a higher Km than hexokinase.. **Explanation:** The correct answer is **B: Glucokinase has a higher Km than hexokinase.** **1. Underlying Concept:** The Michaelis constant (**Km**) represents the substrate concentration at which an enzyme works at half its maximum velocity ($V_{max}$). Km is **inversely proportional** to the affinity of the enzyme for its substrate. * **Hexokinase** has a **low Km** (high affinity) for glucose. This allows it to function at maximum capacity even during fasting states, ensuring that tissues like the brain and muscles can utilize glucose even when blood levels are low. * **Glucokinase (Hexokinase IV)** has a **high Km** (low affinity). It only becomes significantly active when blood glucose levels are high (e.g., after a meal). This allows the liver to "buffer" blood glucose by converting it to glycogen only when there is an excess. **2. Analysis of Incorrect Options:** * **Option A:** Incorrect. If hexokinase had a higher Km, it would be unable to trap glucose in tissues during fasting, leading to cellular energy failure. * **Option C:** Incorrect. These are distinct isoenzymes with different kinetic properties ($V_{max}$ and $Km$) suited to their specific physiological roles. * **Option D:** Incorrect. Km is an intrinsic property of the enzyme itself and does not change based on the amount of glucose ingested (though the *rate* of the reaction will change). **3. NEET-PG High-Yield Pearls:** * **Location:** Hexokinase is ubiquitous (all tissues); Glucokinase is primarily in the **Liver** and **Pancreatic Beta-cells**. * **Vmax:** Glucokinase has a **high Vmax**, allowing it to process large amounts of glucose rapidly post-prandially. * **Inhibition:** Hexokinase is inhibited by its product (**Glucose-6-Phosphate**); Glucokinase is **not**. * **Clinical Correlation:** Mutations in the Glucokinase gene are associated with **MODY type 2** (Maturity-Onset Diabetes of the Young).

Q: What does Km of an enzyme represent?

The substrate concentration at half maximum velocity. **Explanation:** The **Michaelis constant (Km)** is a fundamental parameter in enzyme kinetics derived from the Michaelis-Menten equation. **1. Why the Correct Answer is Right:** Km is defined as the **substrate concentration [S] at which the reaction velocity is exactly half of the maximum velocity (½ Vmax)**. It reflects the **affinity** of an enzyme for its substrate [1]. There is an inverse relationship: a **low Km** indicates high affinity (the enzyme reaches half-maximal velocity at low substrate levels), while a **high Km** indicates low affinity [1]. **2. Analysis of Incorrect Options:** * **Option A:** Km is not a simple dissociation constant ($K_d$), though it equals $K_d$ only when the rate of product formation is much slower than the rate of substrate dissociation. * **Option B:** Km is a constant characteristic of the enzyme-substrate pair; it does not necessarily represent the "normal" physiological concentration of a substrate in a cell. * **Option C:** **Isozymes** (e.g., Hexokinase and Glucokinase) catalyze the same reaction but typically have **different Km values** [1]. For example, Glucokinase has a much higher Km for glucose than Hexokinase, allowing it to function specifically when blood glucose levels are high [1]. **3. NEET-PG High-Yield Pearls:** * **Lineweaver-Burk Plot:** On a double-reciprocal plot, the **x-intercept is -1/Km**. * **Competitive Inhibition:** Km **increases** (affinity decreases), but Vmax remains unchanged. * **Non-competitive Inhibition:** Km **remains unchanged**, but Vmax decreases. * **Clinical Example:** **Methanol poisoning** is treated with Ethanol because Ethanol has a much lower Km (higher affinity) for Alcohol Dehydrogenase, effectively outcompeting methanol and preventing the formation of toxic formaldehyde.

Q: The predominant isozyme of lactate dehydrogenase (LDH) in cardiac muscle is:

LD-1. **Explanation:** Lactate dehydrogenase (LDH) is a tetrameric enzyme composed of two types of subunits: **H (Heart)** and **M (Muscle)**. These subunits combine in five different ways to form isozymes (LD1 to LD5), which are tissue-specific. **Why LD-1 is correct:** LD-1 is composed of four 'H' subunits (**H4**). It is the predominant isozyme found in **cardiac muscle** and **erythrocytes**. Because the heart relies on aerobic metabolism, LD-1 is specialized to favor the conversion of lactate to pyruvate for energy production. **Analysis of incorrect options:** * **LD-2 (H3M1):** Found primarily in the **Reticuloendothelial system** and serum. In a healthy individual, LD-2 is the most abundant isozyme in the blood. * **LD-3 (H2M2):** Predominantly found in the **Lungs** and spleen. * **LD-5 (M4):** Predominantly found in the **Liver** and **Skeletal muscle**. It favors the conversion of pyruvate to lactate under anaerobic conditions. **High-Yield Clinical Pearls for NEET-PG:** 1. **LDH Flip:** Normally, serum LD-2 > LD-1. However, in **Myocardial Infarction (MI)**, LD-1 levels rise significantly, leading to a "flipped pattern" where **LD-1 > LD-2**. 2. **Diagnostic Window:** LDH levels begin to rise 12–24 hours after an MI, peak at 48 hours, and remain elevated for 7–10 days (useful for late diagnosis). 3. **Hemolysis:** Since LD-1 is high in RBCs, any hemolytic condition will cause a significant rise in LD-1. 4. **LD-4:** Found in the kidneys and pancreas.

Q: A drug that competes for an active binding site is called:

Competitive inhibitor. ### Explanation **1. Why Option A is Correct:** In **competitive inhibition**, the inhibitor molecule structurally resembles the substrate. Because of this similarity, it "competes" with the substrate for the same **active site** on the enzyme. * **Mechanism:** When the inhibitor binds to the active site, it prevents the substrate from binding. * **Kinetics:** This can be overcome by increasing the substrate concentration ($[S]$); therefore, the **$V_{max}$ remains unchanged**, but the **$K_m$ increases** (affinity appears to decrease). **2. Why Other Options are Incorrect:** * **B. Non-competitive inhibitor:** These inhibitors bind to an **allosteric site** (a site other than the active site). They do not compete for the active binding site; instead, they change the enzyme's conformation, reducing its catalytic activity. Here, $V_{max}$ decreases, but $K_m$ remains unchanged. * **C. Covalent inhibitor:** These form strong, irreversible covalent bonds with the enzyme (often at the active site), permanently inactivating it. While they may target the active site, the term "competes" specifically refers to the reversible process defined in Option A. **3. NEET-PG High-Yield Clinical Pearls:** * **Classic Examples of Competitive Inhibitors:** * **Statins** (e.g., Atorvastatin) compete with HMG-CoA for HMG-CoA reductase. * **Methanol poisoning treatment:** Ethanol or Fomepizole competes for Alcohol Dehydrogenase. * **Methotrexate** competes with Dihydrofolate for Dihydrofolate Reductase. * **Sulfonamides** compete with PABA for Dihydropteroate synthase. * **Lineweaver-Burk Plot:** In competitive inhibition, the lines intersect on the **Y-axis** ($1/V_{max}$ is the same).

Q: Leucine aminopeptidase is elevated in obstruction of which of the following?

Common bile duct. **Explanation:** **Leucine Aminopeptidase (LAP)** is a hydrolytic enzyme found in various tissues, with the highest concentrations located in the **liver, pancreas, and small intestine**. In the liver, it is specifically localized to the biliary canalicular membrane. **Why Common Bile Duct is correct:** LAP serves as a sensitive marker for **cholestasis** (interference with bile flow). When there is an obstruction in the **Common Bile Duct (CBD)**, such as by a gallstone or a tumor, the pressure within the biliary system increases. This leads to the regurgitation of LAP into the bloodstream. Its clinical significance is similar to **Alkaline Phosphatase (ALP)** and **Gamma-Glutamyl Transferase (GGT)**. It is particularly useful in differentiating the source of an elevated ALP; if both ALP and LAP are elevated, the pathology is likely hepatobiliary rather than bone-related. **Why incorrect options are wrong:** * **Ureter and Urethra:** Obstruction in the urinary tract (uropathy) leads to hydronephrosis or urinary retention. While enzymes like LDH or NAG might be studied in renal pathology, LAP is not a marker for urinary obstruction. * **Spermatic Cord:** Obstruction here (e.g., torsion or vasectomy) affects sperm transport or blood flow to the testes but has no correlation with hepatobiliary enzymes like LAP. **High-Yield Clinical Pearls for NEET-PG:** * **LAP vs. ALP:** LAP levels remain **normal in bone diseases** (e.g., Paget’s, rickets), making it a specific tool to confirm that an elevated ALP is of hepatic origin. * **Pregnancy:** LAP levels rise significantly during the third trimester of pregnancy (produced by the placenta), so it is less specific for liver disease in pregnant patients. * **Other Cholestatic Markers:** Always correlate LAP with **GGT** and **5'-Nucleotidase** for hepatobiliary questions.

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.

Question 1

Glutathione reductase contains which of the following prosthetic groups?

Accepted Answer

FAD

Answer

NAD

Answer

ATP

Answer

None of the above

Question 2

What is the effect of competitive inhibition on Vmax and Km?

Accepted Answer

Km increased, Vmax unchanged

Answer

Km unchanged

Answer

Vmax increased, Km unchanged

Answer

Km and Vmax increased

Question 3

What is the relative Km of Hexokinase and Glucokinase?

Accepted Answer

Glucokinase has a higher Km than hexokinase.

Answer

Hexokinase has a higher Km than glucokinase.

Answer

The Km is the same for both enzymes.

Answer

The Km depends on the glucose ingested.

Question 4

Inhibition of placental alkaline phosphatase by phenylalanine is an example of which type of enzyme inhibition?

Accepted Answer

Uncompetitive inhibition

Answer

Competitive inhibition

Answer

Noncompetitive inhibition

Answer

Allosteric inhibition

Question 5

What does Km of an enzyme represent?

Accepted Answer

The substrate concentration at half maximum velocity

Answer

Dissociation constant

Answer

Normal substrate concentration

Answer

Numerically identical for all isozymes that catalyze a given reaction

Question 6

The predominant isozyme of lactate dehydrogenase (LDH) in cardiac muscle is:

Accepted Answer

LD-1

Answer

LD-2

Answer

LD-3

Answer

LD-5

Question 7

A drug that competes for an active binding site is called:

Accepted Answer

Competitive inhibitor

Answer

Non-competitive inhibitor

Answer

Covalent inhibitor

Answer

None of these

Question 8

Leucine aminopeptidase is elevated in obstruction of which of the following?

Accepted Answer

Common bile duct

Answer

Ureter

Answer

Urethra

Answer

Spermatic cord

Question 9

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 10

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)

Enzymes — MCQs

Enzymes — MCQs

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