Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 481: A 30-year-old woman with hemolytic anemia is found to have an elevated reticulocyte count and Heinz bodies in red blood cells. Which enzyme deficiency is most likely?

A. Hexokinase deficiency
B. Phosphofructokinase deficiency
C. Pyruvate kinase deficiency
D. Glucose-6-phosphate dehydrogenase deficiency (Correct Answer)

Explanation: ***Glucose-6-phosphate dehydrogenase*** - **G6PD deficiency** impairs the **hexose monophosphate shunt**, reducing NADPH production. - This leads to increased oxidative stress, causing **hemoglobin denaturation** and the formation of **Heinz bodies**, visible as precipitates in red blood cells. *Hexokinase deficiency* - This is a rare cause of **congenital hemolytic anemia**, but it typically does not present with **Heinz bodies**. - Hexokinase is the first enzyme in glycolysis, and its deficiency primarily affects energy production (ATP), leading to premature red blood cell destruction. *Phosphofructokinase deficiency* - This deficiency primarily affects **muscle and red blood cell glycolysis**, causing muscle weakness and fatigue, and sometimes mild hemolytic anemia. - It does not typically lead to the formation of **Heinz bodies**, as it's not directly involved in oxidative stress protection. *Pyruvate kinase deficiency* - This is the most common enzyme deficiency in the **glycolytic pathway** causing chronic hemolytic anemia. - It causes a build-up of upstream glycolytic intermediates but does not lead to the formation of **Heinz bodies**, which are characteristic of oxidative damage.

Question 482: How does the Lineweaver-Burk plot change with a non-competitive inhibitor?

A. Increased y-intercept, same x-intercept (Correct Answer)
B. Increased x-intercept, same y-intercept
C. Both intercepts increase
D. No change

Explanation: ***Increased y-intercept, same x-intercept*** - A **non-competitive inhibitor** binds to an allosteric site on the enzyme, reducing its catalytic efficiency (Vmax). - On a Lineweaver-Burk plot, a decrease in **Vmax** results in an **increased y-intercept (1/Vmax)**, while the **x-intercept (-1/Km)** remains unchanged because the inhibitor does not affect substrate binding affinity. *Increased x-intercept, same y-intercept* - This pattern is characteristic of a **competitive inhibitor**, which increases the apparent **Km** (shifting the x-intercept toward zero). - The **Vmax** remains unchanged, so the y-intercept does not change. *Both intercepts increase* - This does not accurately describe any standard inhibition pattern. - In **uncompetitive inhibition**, both Vmax and Km decrease proportionally, creating parallel lines (both intercepts increase, but the slope remains constant). - In **mixed inhibition**, both Km and Vmax are affected, but the pattern varies depending on whether the inhibitor binds preferentially to the enzyme or enzyme-substrate complex. *No change* - No change would indicate that the substance is **not an inhibitor** or has no effect on enzyme kinetics. - Inhibitors, by definition, alter the enzyme's activity and therefore impact the Lineweaver-Burk plot.

Question 483: In the context of enzyme inhibition, which approach is generally considered more effective for targeting highly conserved metabolic enzymes?

A. Targeting active sites directly
B. Both methods are equally effective in all cases
C. Effectiveness depends on the specific enzyme and context
D. Targeting allosteric sites for better specificity (Correct Answer)

Explanation: ***Targeting allosteric sites for better specificity*** - **Allosteric sites** are distinct from the active site and are generally less conserved between different species or even isoforms of the same enzyme, providing better opportunities for **selective inhibition** of specific enzymes over others. - Modulators binding to allosteric sites induce **conformational changes** that can either activate or inhibit enzyme activity, allowing for a more nuanced and **tunable control** over metabolic pathways. *Targeting active sites directly* - Inhibiting the **active site** of highly conserved metabolic enzymes can lead to significant **off-target effects** due to the structural similarity of active sites across various enzymes. - This lack of **specificity** can result in toxicity and undesirable side effects, making it less ideal for differentiating between host and pathogen enzymes or between different metabolic contexts. *Both methods are equally effective in all cases* - The effectiveness of targeting approaches is highly **context-dependent**, influenced by factors such as the specific enzyme, the desired therapeutic outcome, and the potential for off-target effects. - Generalizing that both methods are equally effective overlooks the inherent differences in **selectivity and mechanism of action** offered by active versus allosteric site targeting. *Effectiveness depends on the specific enzyme and context* - While it's true that effectiveness depends on specific enzyme and context, the question asks which approach is *generally* more effective for *highly conserved* metabolic enzymes. - For such enzymes, the **inherent specificity advantage** of allosteric targeting makes it a generally preferred approach over active site targeting.

Question 484: Which enzyme is responsible for converting pyruvate to acetyl-CoA, thereby linking glycolysis and the TCA cycle?

A. Pyruvate dehydrogenase (Correct Answer)
B. Pyruvate carboxylase
C. Pyruvate kinase
D. Lactate dehydrogenase

Explanation: ***Pyruvate dehydrogenase*** - The **pyruvate dehydrogenase complex** catalyzes the oxidative decarboxylation of **pyruvate** to **acetyl-CoA**, releasing carbon dioxide. - This crucial reaction is the committed step that links glycolysis (which produces pyruvate) to the **TCA cycle** (which consumes acetyl-CoA). *Pyruvate carboxylase* - This enzyme converts **pyruvate** to **oxaloacetate**, an important anaplerotic reaction that replenishes intermediates of the **TCA cycle**. - It does not produce acetyl-CoA and is typically involved in **gluconeogenesis**. *Pyruvate kinase* - **Pyruvate kinase** is the final enzyme in **glycolysis**, catalyzing the transfer of a phosphate group from **phosphoenolpyruvate (PEP)** to ADP, generating ATP and pyruvate. - It is an ATP-generating step within glycolysis and does not convert pyruvate to acetyl-CoA. *Lactate dehydrogenase* - This enzyme catalyzes the reversible conversion of **pyruvate** to **lactate**, primarily under anaerobic conditions. - It regenerates NAD+ for glycolysis to continue in the absence of oxygen but does not link glycolysis to the TCA cycle.

Question 485: What role does the enzyme catalase play in cellular metabolism?

A. It converts hydrogen peroxide into water and oxygen (Correct Answer)
B. It synthesizes fatty acids from acetyl-CoA
C. It catalyzes the transfer of phosphate groups in ATP production
D. It breaks down excess amino acids

Explanation: ***It converts hydrogen peroxide into water and oxygen*** - **Catalase** is an enzyme that specifically catalyzes the decomposition of **hydrogen peroxide (H2O2)**, a harmful reactive oxygen species generated during various metabolic processes. - This conversion into **water (H2O)** and **oxygen (O2)** is crucial for protecting cells from **oxidative damage**. *It synthesizes fatty acids from acetyl-CoA* - The synthesis of fatty acids from **acetyl-CoA** is primarily carried out by **fatty acid synthase**, a multi-enzyme complex, not catalase. - This process is part of **anabolic metabolism**, while catalase is involved in detoxification. *It catalyzes the transfer of phosphate groups in ATP production* - The transfer of phosphate groups for **ATP production** (e.g., during oxidative phosphorylation or glycolysis) is catalyzed by enzymes like **ATP synthase** or **kinases**, respectively. - These enzymes are distinct from catalase, which has a specific role in peroxide breakdown. *It breaks down excess amino acids* - The breakdown of excess amino acids, including their deamination and conversion into other metabolic intermediates, is managed by a variety of enzymes such as **transaminases** and **dehydrogenases**. - Catalase is not involved in amino acid catabolism.

Question 486: Which of the following enzymes is critical for relieving DNA supercoiling during replication and transcription?

A. Helicase
B. Topoisomerase (Correct Answer)
C. Ligase
D. Polymerase

Explanation: ***Topoisomerase*** - **Topoisomerases** are enzymes essential for **relieving torsional stress** by cutting and rejoining DNA strands ahead of the replication fork or transcription machinery - This activity **prevents DNA from becoming excessively supercoiled**, which would otherwise impede replication and transcription processes - Type I topoisomerases create single-strand breaks; Type II create double-strand breaks to manage DNA topology *Helicase* - **Helicase** unwinds the DNA double helix by breaking hydrogen bonds between base pairs, creating the replication fork - While helicase action **creates supercoiling downstream**, it does not relieve this torsional stress - Topoisomerases work ahead of helicase to prevent accumulation of positive supercoils *Ligase* - **DNA ligase** catalyzes phosphodiester bond formation to join DNA fragments, particularly Okazaki fragments on the lagging strand - Functions in DNA repair and replication completion - **No role in managing DNA topology or supercoiling** *Polymerase* - **DNA polymerase** synthesizes new DNA strands during replication; **RNA polymerase** synthesizes RNA during transcription - Both enzymes require topoisomerases to function efficiently by relieving supercoiling - **Do not directly address DNA supercoiling** themselves

Question 487: A 45-year-old man presents with muscle weakness and cramps. Blood tests reveal hypokalemia. Which enzyme deficiency could result in this condition?

A. Glucose-6-phosphate dehydrogenase
B. Aldolase
C. Phosphofructokinase
D. Pyruvate kinase (Correct Answer)

Explanation: ***Pyruvate kinase*** - **Pyruvate kinase deficiency** causes **chronic hemolytic anemia** with compensatory increased erythropoiesis and high metabolic demand. - In the context of chronic hemolysis, patients may develop **secondary hypokalemia** due to: increased renal potassium losses from chronic illness, poor nutrition, or associated gastrointestinal losses. - The **muscle weakness and cramps** result from both the chronic anemia (tissue hypoxia, reduced ATP) and the electrolyte disturbance (hypokalemia affects muscle membrane potential). - Among the glycolytic enzyme deficiencies listed, pyruvate kinase deficiency is most commonly associated with chronic systemic complications. *Phosphofructokinase* - **Phosphofructokinase deficiency** (Tarui's disease, glycogenosis type VII) causes **exercise-induced muscle symptoms** including myalgia, cramps, and fatigue. - Results from impaired glycolysis in muscle with glycogen accumulation. - Does not typically cause hypokalemia; may show elevated lactate and myoglobinuria after exercise. *Glucose-6-phosphate dehydrogenase* - **G6PD deficiency** causes **episodic hemolytic anemia** triggered by oxidative stress (infections, drugs, fava beans). - Presents with acute hemolytic crises rather than chronic muscle weakness. - Not associated with chronic hypokalemia as a primary feature. *Aldolase* - **Aldolase deficiency** is extremely rare and can cause **hemolytic anemia**. - May present with myopathy in some variants, but hypokalemia is not a characteristic feature. - Clinical presentation varies depending on which aldolase isoenzyme is affected.

Question 488: Which of the following enzymes directly catalyzes the decomposition of hydrogen peroxide to water and oxygen without requiring an additional reducing substrate?

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

Explanation: ***Catalase*** - **Catalase** directly catalyzes the breakdown of **hydrogen peroxide (H2O2)** into **water (H2O)** and **oxygen (O2)** without requiring any additional reducing substrate. - The reaction is: **2 H2O2 → 2 H2O + O2** - This unique mechanism makes catalase extremely efficient at detoxifying high concentrations of H2O2, particularly in peroxisomes and red blood cells. - Catalase has one of the highest turnover rates of all enzymes (millions of molecules per second). *Superoxide dismutase* - **Superoxide dismutase (SOD)** converts the **superoxide radical (O2•−)** into oxygen and **hydrogen peroxide**. - The reaction is: **2 O2•− + 2 H+ → H2O2 + O2** - While it detoxifies superoxide, it produces hydrogen peroxide, which then requires other enzymes like catalase for further detoxification. *Glutathione peroxidase* - **Glutathione peroxidase (GPx)** reduces **hydrogen peroxide** to water using **reduced glutathione (GSH)** as a reducing substrate. - The reaction is: **H2O2 + 2 GSH → 2 H2O + GSSG** - Unlike catalase, GPx requires glutathione as a co-substrate and works in concert with glutathione reductase to regenerate GSH. *Peroxiredoxin* - **Peroxiredoxins** reduce **hydrogen peroxide** to water using **thioredoxin or glutaredoxin** as reducing substrates. - The reaction requires: **H2O2 + thioredoxin(red) → 2 H2O + thioredoxin(ox)** - They are abundant antioxidant enzymes but depend on the thioredoxin/thioredoxin reductase system, unlike catalase which acts independently.

Question 489: Which of the following is a serine protease?

A. Caspases
B. Chymotrypsin (Correct Answer)
C. Carboxypeptidase
D. Pepsin

Explanation: ***Chymotrypsin*** - **Chymotrypsin** is a digestive enzyme that belongs to the class of **serine proteases**, meaning it uses a serine residue in its active site to catalyze peptide bond hydrolysis. - It specifically cleaves peptide bonds adjacent to **aromatic amino acids** (e.g., phenylalanine, tryptophan, tyrosine). *Caspases* - **Caspases** are a family of cysteine proteases, meaning they use a **cysteine residue** in their active site for catalysis. - They play crucial roles in **apoptosis** (programmed cell death) and inflammation. *Carboxypeptidase* - **Carboxypeptidases** are exopeptidases that cleave the **C-terminal amino acid** from a polypeptide chain. - While they are proteases, they do not primarily rely on a serine residue in their active site in the same way as serine proteases; some are metalloproteases (e.g., carboxypeptidase A uses zinc). *Pepsin* - **Pepsin** is an aspartic protease, meaning it uses two **aspartic acid residues** in its active site to cleave peptide bonds. - It functions optimally in the **acidic environment of the stomach** and is responsible for initial protein digestion.

Question 490: What is the primary mechanism of action of 5-alpha reductase?

A. Reduction of C4-C5 double bond (Correct Answer)
B. Breakage of C-N bond
C. Breakage of amide bond
D. Breakage of N-N bond

Explanation: ***Reduction of C4-C5 double bond*** - 5-alpha reductase catalyzes the **reduction of testosterone** to dihydrotestosterone (DHT) by adding hydrogen atoms across the **C4-C5 double bond** in the steroid A-ring. - This reaction involves the **saturation of this specific double bond** through a reduction reaction, which is critical for its biological action. - The enzyme uses **NADPH as a cofactor** to donate hydrogen atoms, converting the double bond to a single bond. *Breakage of C-N bond* - The breaking of a **carbon-nitrogen bond** is not the mechanism of 5-alpha reductase. - This type of bond cleavage is characteristic of other enzymatic reactions, such as those involving **peptide hydrolysis** or certain types of **deamination**. *Breakage of amide bond* - An **amide bond** (-CO-NH-) is typically cleaved by enzymes like **amidases** or **proteases**. - This type of bond is not present in the substrate (testosterone) and is therefore not targeted by 5-alpha reductase. *Breakage of N-N bond* - The breaking of an **nitrogen-nitrogen bond** is a rare enzymatic process and is not involved in the metabolism of steroid hormones. - This type of reaction is seen in certain specialized enzymes involved in **nitrogen fixation** or **redox reactions** of nitrogen-containing compounds.

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

Enzyme Kinetics and Michaelis-Menten Equation

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Enzyme Inhibition: Competitive and Non-competitive

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Allosteric Regulation

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Coenzymes and Cofactors

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Isoenzymes and Clinical Significance

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Enzyme Regulation: Covalent Modification

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Enzyme Regulation: Zymogen Activation

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Enzyme Induction and Repression

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Ribozymes and Catalytic RNA

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Enzyme Diagnostic Applications

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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