Exercise | Chapter 3 : Enzymes | Class 11 Biology Notes (FBISE, Best for Exams)

Exercise Chapter 3: Enzymes Class 11

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MCQs with answers from Chapter 3 : Enzyme (Federal Board, 11th Class):

MCQs – Enzymes

1. The rate of an enzyme-catalyzed reaction depends on:

   (A) pH
   (B) Temperature
   (C) Substrate concentration
   (D) All of the above ✅

2. The enzyme responsible for converting pepsinogen into pepsin:

   (A) Trypsin
   (B) Enterokinase
   (C) Hydrochloric acid ✅
   (D) Lipase

3. Which enzyme is not affected by changes in pH?

   (A) Pepsin
   (B) Trypsin
   (C) DNA polymerase
   (D) None ✅

4. The function of an allosteric inhibitor is to:

   (A) Block the active site
   (B) Change enzyme shape ✅
   (C) Act as a cofactor
   (D) Enhance enzyme activity

5. The energy required to start a biochemical reaction is called:

   (A) Free energy
   (B) Activation energy ✅
   (C) Binding energy
   (D) Potential energy

6. Which enzyme is involved in DNA replication?

   (A) Helicase ✅
   (B) Ligase
   (C) Amylase
   (D) Catalase

7. Enzymes that catalyze oxidation-reduction reactions belong to:

   (A) Hydrolases
   (B) Oxidoreductases ✅
   (C) Transferases
   (D) Lyases

8. Which of the following is a characteristic of enzymes?

   (A) They increase activation energy
   (B) They are consumed in the reaction
   (C) They remain unchanged after the reaction ✅
   (D) They work at any temperature

9. The lock and key model explains:

   (A) Enzyme-substrate specificity ✅
   (B) Enzyme inhibition
   (C) Allosteric regulation
   (D) None of them

10. The enzyme that catalyzes the formation of ATP in oxidative phosphorylation:

    (A) ATP synthase ✅
    (B) Hexokinase
    (C) Phosphatase
    (D) Amylase

11. Which of the following is a zymogen?

    (A) Trypsin
    (B) Pepsinogen ✅
    (C) Urease
    (D) Catalase

12. A competitive inhibitor resembles:

    (A) Active site
    (B) Substrate ✅
    (C) Coenzyme
    (D) Cofactor

13. Which factor does not affect enzyme activity?

    (A) Temperature
    (B) pH
    (C) Substrate concentration
    (D) Light ✅

2. What are ribozymes?

Ribozymes are RNA molecules that act as enzymes and catalyze biochemical reactions, such as peptide bond formation during protein synthesis. Example: Peptidyl transferase in ribosomes.

3. What is the structure of an enzyme?

• Globular proteins with tertiary or quaternary structure.
• Have an active site where the substrate binds.
• Some require cofactors (metal ions or coenzymes).
• Example: Pepsin, DNA polymerase.

4. Explain the enzyme pepsin which does not require a cofactor.

• Pepsin is a protease enzyme secreted as inactive pepsinogen.
• Activated by HCl in the stomach.
• Unlike many enzymes, it does not require a cofactor to function.

5. What is a prosthetic group? Give an example.

• A prosthetic group is a permanently attached non-protein part of an enzyme.
• Example: Heme group in cytochrome oxidase.

6. What is the mechanism of enzyme action?

• Step 1: Substrate binds to the enzyme’s active site (forming the enzyme-substrate complex).
• Step 2: Enzyme lowers activation energy and catalyzes the reaction.
• Step 3: Product is released, and the enzyme remains unchanged.

7. What is the role of free energy of activation in a chemical reaction?

• Activation energy is the minimum energy needed to start a reaction.
• Enzymes lower activation energy, making reactions faster without needing excessive heat.

8. List the external conditions which affect the rate of enzyme reaction.

1. Temperature
2. pH
3. Substrate concentration
4. Enzyme concentration
5. Presence of inhibitors or activators

9. Compare the optimum temperatures of enzymes of humans and thermophilic bacteria.

• Human enzymes: Optimum at 37-38°C.
• Thermophilic bacteria: Optimum at ~70°C or higher (used in high-temperature environments like hot springs).

10. Describe the range of pH at which human enzymes function.

• Most human enzymes work at pH 6-8.
• Exceptions:
  • Pepsin (stomach) → pH 2 (acidic)

Trypsin (intestine) → pH 8 (alkaline)

11. What are enzyme inhibitors? Name the molecules that act as enzyme inhibitors.

• Enzyme inhibitors are substances that reduce or stop enzyme activity.
• Types of inhibitors:
  • Competitive inhibitors (bind to the active site).
  • Non-competitive inhibitors (bind to an allosteric site).
• Examples:
  • Poisons (cyanide, heavy metals).
  • Drugs (penicillin, sulfonamides).
  • Metabolic regulators (feedback inhibitors).

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12. What is the importance of competitive enzyme inhibitors?

• Block enzyme activity by competing with the substrate.
• Used in medicine (e.g., sulfa drugs inhibit bacterial enzymes).
• Help study enzyme specificity (support the Lock and Key model).

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13. Describe cyanides as irreversible non-competitive inhibitors.

• Cyanide (CN⁻) binds to cytochrome oxidase, blocking cellular respiration.
• Irreversible: It permanently deactivates the enzyme.
• Causes death by preventing ATP production.

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14. Describe ions of heavy metals as irreversible non-competitive inhibitors.

• Heavy metals (Hg²⁺, Ag⁺, Cu²⁺) bind to sulfhydryl (-SH) groups in enzymes.
• Break disulfide bonds, altering enzyme shape.
• Irreversibly denature enzymes, stopping function.

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15. Differences

(a) Binding site vs. Catalytic site

Binding Site	Catalytic Site
Attaches substrate via weak interactions.	Carries out the reaction (converts substrate to product).
Ensures specificity of substrate binding.	Contains key amino acids for catalysis.

(b) Apoenzyme vs. Holoenzyme

Apoenzyme	Holoenzyme
Inactive enzyme (without a cofactor).	Active enzyme (with a cofactor).
Example: Pepsinogen.	Example: Pepsin (activated form).

(c) Prosthetic group vs. Coenzyme

Prosthetic Group	Coenzyme
Permanently attached to the enzyme.	Loosely attached and detachable.
Example: Heme in cytochromes.	Example: NAD⁺, FAD.

(d) Inorganic cofactor vs. Organic cofactor

Inorganic Cofactor	Organic Cofactor
Metal ions (Fe²⁺, Mg²⁺, Zn²⁺).	Vitamin-derived molecules (NAD⁺, FAD).
Example: Mg²⁺ in Hexokinase.	Example: NAD⁺ in dehydrogenases.

(e) Lock and Key Model vs. Induced Fit Model

Lock and Key Model	Induced Fit Model
Active site has a fixed shape (substrate must fit exactly).	Active site is flexible and molds around the substrate.
Example: Sucrase.	Example: Hexokinase.

(f) Competitive vs. Non-Competitive Inhibitors

Competitive Inhibitors	Non-Competitive Inhibitors
Bind to active site (compete with substrate).	Bind to allosteric site (not active site).
Can be overcome by increasing substrate concentration.	Cannot be reversed by increasing substrate.
Example: Malonate inhibits Succinate Dehydrogenase.	Example: Cyanide inhibits Cytochrome Oxidase.

(g) Reversible vs. Irreversible Non-Competitive Inhibitors

Reversible Non-Competitive Inhibitors	Irreversible Non-Competitive Inhibitors
Temporarily inactivates enzyme.	Permanently destroys enzyme function.
Example: Feedback inhibition.	Example: Cyanide, heavy metals.

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16. Properties of Enzymes

1. Biological catalysts → Speed up reactions without being consumed.
2. Highly specific → Work on a particular substrate.
3. Sensitive to pH & temperature → Optimal conditions required.
4. Reusable → Not consumed in the reaction.
5. Lower activation energy → Facilitate reaction progress.
6. Can work in vivo & in vitro → Function in both living systems and lab conditions.
7. May require cofactors → Some need metal ions or coenzymes.

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17. Role and Components of the Active Site of an Enzyme

• Role → Binds to the substrate & catalyzes the reaction.
• Components:
  1. Binding Site → Holds substrate via weak bonds.
  2. Catalytic Site → Converts substrate into product.

Example: Aldolase’s active site contains Glycine, Histidine & Alanine.

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18. Cofactors and Their Types

• Definition → Non-protein molecules that assist enzyme function.
• Types:
  1. Inorganic Cofactors → Metal ions like Fe²⁺, Mg²⁺, Zn²⁺.
     • Example: Mg²⁺ in Hexokinase.
  2. Organic Cofactors (Coenzymes & Prosthetic Groups)
     • Coenzymes (loosely attached) → NAD⁺, FAD.
     • Prosthetic groups (permanently attached) → Heme in Cytochromes.

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19. Mechanism of Enzyme Action (Induced Fit Model)

• Proposed by Koshland (1959).
• Active site is flexible and molds around the substrate.
• After reaction, active site regains its original shape.
• Example: Hexokinase modifies its shape when binding to glucose.

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20. Mechanism of Enzyme Action (Lock and Key Model)

• Proposed by Emil Fischer (1894).
• Active site has a fixed shape → Only specific substrates fit.
• Example: Sucrase binds only to sucrose.

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21. How an Enzyme Catalyzes Specific Reactions

• Specificity due to active site shape.
• Substrate binds → Reaction occurs → Product released.
• Example: DNA polymerase acts only on DNA nucleotides.

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22. Enzyme Action & Energy of Activation (Graph Explanation)

• Enzymes lower activation energy, making reactions faster.
• Graph shows two curves:
  • Without enzyme → Higher activation energy.
  • With enzyme → Lower activation energy.

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23. Effect of Temperature on Enzyme Action

• Increases with temperature (more collisions).
• Optimal at 37-38°C (humans), ~70°C (thermophiles).
• Above optimum → Denaturation (loss of function).
• Below minimum → Inactive but not denatured.

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24. Effect of Enzyme Concentration on Reaction Rate

• Higher enzyme concentration → More active sites → Faster reaction.
• At equilibrium, increasing enzymes has no further effect.

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25. Effect of Substrate Concentration on Reaction Rate

• Increases reaction rate up to Vmax.
• At saturation, all active sites are occupied, so adding more substrate has no effect.

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26. Enzymatic Inhibition, Types, and Significance

• Inhibition → Process where enzyme activity is reduced/stopped.
• Types:
  1. Competitive Inhibition → Inhibitor competes for active site.
     • Example: Malonate inhibits Succinate Dehydrogenase.
  2. Non-Competitive Inhibition → Inhibitor binds to allosteric site, changing enzyme shape.
     • Example: Cyanide inhibits Cytochrome Oxidase.

Significance: Used in drug design, metabolic regulation, and poison control.

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27. Feedback Inhibition & Enzymes

• End-product inhibits the first enzyme in a pathway.
• Prevents overproduction & maintains balance.
• Example: Threonine inhibits its own synthesis by binding to the first enzyme.

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28. Classification of Enzymes by Reaction Type (IUB 1961)

1. Oxidoreductases → Catalyze oxidation-reduction.
   • Example: Cytochrome oxidase.
2. Transferases → Transfer functional groups.
   • Example: Hexokinase (transfers phosphate from ATP to glucose).
3. Hydrolases → Break bonds using water.
   • Example: Pepsin (breaks proteins).
4. Lyases → Break bonds without hydrolysis.
   • Example: Histidine decarboxylase.
5. Isomerases → Rearrange molecular structure.
   • Example: Phosphohexose isomerase.
6. Ligases (Synthetases) → Join molecules using ATP.
   • Example: DNA ligase (joins DNA fragments).

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29. Classification of Enzymes by Substrate Type

• Named after the substrate + "-ase".

1. Protease (Peptidase) → Acts on proteins (e.g., Trypsin, Pepsin).
2. Lipase → Acts on lipids (e.g., Pancreatic lipase).
3. Amylase → Acts on starch (e.g., Salivary amylase).
4. Cellulase → Acts on cellulose (e.g., in bacteria & fungi).
5. Nuclease → Acts on nucleic acids (e.g., DNase, RNase).

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