03.02 Enzymes: Mode of Action
Enzyme Function and Structure
- Enzyme Shape:
- Globular proteins with precise 3D structure and active site.
- Hydrophilic R groups outward-facing, making enzymes soluble in water.
- Active Site:
- Region on enzyme where substrate binds.
- Specific shape ensures substrate fits exactly.
- Temporary bonds form between substrate and R groups of enzyme’s amino acids, creating an enzyme-substrate complex.
Mechanisms of Enzyme Action
Lock-and-Key Hypothesis:
- Substrate (key) fits exactly into the enzyme’s active site (lock).
- Specificity: Enzyme’s active site fits only a specific substrate shape.
Induced-Fit Hypothesis:
- Flexible active site: Enzyme or substrate can slightly adjust shape for a perfect fit.
- Improves catalytic efficiency by enhancing enzyme-substrate interaction.
Enzyme Action Steps
- Substrate binds to enzyme’s active site → enzyme-substrate complex.
- Reaction occurs:
- Breakdown: Substrate splits into two or more products.
- Joining: Two substrates combine to form a product.
- Enzyme-product complex briefly forms → products released.
- Enzyme unchanged and ready for a new substrate.
Example of Enzyme Action: Lysozyme
- Function: Breaks down bacterial cell walls by splitting polysaccharide chains in bacteria.
- Location: Found in tears and saliva; natural defense against bacteria.
- Action: Binds to polysaccharides, causing cell wall degradation → bacteria burst due to osmotic pressure.
Enzymes and Activation Energy
- Activation Energy:
- Energy needed to initiate a reaction.
- Typically requires heat for non-catalyzed reactions.
- Enzyme Function:
- Lowers activation energy by holding substrate in position that facilitates reaction.
- Allows reactions to occur rapidly at lower temperatures.
Example
- Catalase: Breaks down hydrogen peroxide at a rate of 10 million molecules per second.
Enzyme-substrate:
- Enzyme-substrate binding: Substrate (key) fits or adjusts to active site (lock).
- Reaction occurs (e.g., breakdown or joining).
- Products released, enzyme ready for reuse.
Figure A: Stages of substrate binding and product release.
- According to the induced-fit model, the active site of the enzyme undergoes conformational changes upon binding with the substrate.
- (a) Substrates approach active sites on enzyme.
- (b) Substrates bind to active sites, producing an enzyme–substrate complex.
- (c) Changes internal to the enzyme–substrate complex facilitate interaction of the substrates.
- (d) Products are released and the enzyme returns to its original form, ready to facilitate another enzymatic reaction.
Figure B: Lysozyme action breaking polysaccharide chains.
- Lysozyme breaking a polysaccharide chain. This is a hydrolysis reaction.
- a Diagram showing the formation of enzyme-substrate and enzyme–product complexes before the release of the products.
- b Space-filling model showing the substrate in the active site of the enzyme. The substrate is a polysaccharide chain that slides neatly into the groove (active site) and is split by the enzyme. Many such chains give the bacterial cell wall rigidity. When the chains are broken, the wall loses its rigidity and the bacterial cell explodes as a result of osmosis.
Figure C: Activation energy with vs. without enzyme:
- The energies of the stages of a chemical reaction.
- Uncatalysed (dashed line), substrates need a lot of activation energy to reach a transition state, which then decays into lower-energy products.
- When enzyme catalysed (solid line), the enzyme binds the substrates (ES), then stabilizes the transition state (ES‡) to reduce the activation energy required to produce products (EP) which are finally released.
(a) Higher activation energy without enzyme.
(b) Reduced activation energy with enzyme.
Practise Questions
Question 1
Describe the structure of an enzyme and explain how its structure relates to its solubility in water. (4 marks)
Mark Scheme:
- Enzymes are globular proteins with a precise 3D structure. (1 mark)
- The structure includes hydrophilic R groups positioned outward-facing. (1 mark)
- These hydrophilic groups interact with water, making the enzyme soluble. (1 mark)
- The solubility is important for the enzyme to function in aqueous environments like the cytoplasm. (1 mark)
Question 2
Differentiate between the lock-and-key hypothesis and the induced-fit hypothesis for enzyme action. (5 marks)
Mark Scheme:
- Lock-and-key hypothesis: The substrate (key) fits exactly into the enzyme’s active site (lock) without any change in shape. (1 mark)
- Specificity arises because the active site’s shape matches only a specific substrate. (1 mark)
- Induced-fit hypothesis: The enzyme’s active site is flexible and can adjust its shape slightly to better fit the substrate. (1 mark)
- This adjustment improves catalytic efficiency and enhances substrate-enzyme interaction. (1 mark)
- Both models involve substrate binding, reaction facilitation, and product release, but the induced-fit model explains flexibility in binding. (1 mark)
Question 3
Outline the steps of enzyme action using lysozyme as an example. (6 marks)
Mark Scheme:
- The substrate (polysaccharide chain) binds to the enzyme’s active site, forming an enzyme-substrate complex. (1 mark)
- Lysozyme catalyzes the hydrolysis reaction, breaking the polysaccharide chain. (1 mark)
- This reaction weakens the bacterial cell wall, causing it to lose rigidity. (1 mark)
- Osmotic pressure leads to the bacterial cell bursting. (1 mark)
- The products are released, and the enzyme returns to its original form. (1 mark)
- Lysozyme is a natural defense found in tears and saliva. (1 mark)
Question 4
Explain how enzymes lower the activation energy of a reaction. Use the example of catalase to illustrate your answer. (5 marks)
Mark Scheme:
- Enzymes lower the activation energy by holding the substrate in a favorable position for the reaction to occur. (1 mark)
- They stabilize the transition state, reducing the energy required to convert substrates into products. (1 mark)
- This allows reactions to occur rapidly at lower temperatures. (1 mark)
- Example: Catalase breaks down hydrogen peroxide into water and oxygen, protecting cells from oxidative damage. (1 mark)
- Catalase’s efficiency: it catalyzes the reaction at a rate of 10 million molecules per second. (1 mark)
Question 5
Label and explain the features shown in a diagram of enzyme-substrate binding and product release. (5 marks)
Mark Scheme:
- Substrate binding: Substrates bind to the enzyme’s active site to form the enzyme-substrate complex. (1 mark)
- Reaction: The enzyme facilitates a reaction, either breaking down or combining substrates. (1 mark)
- Product formation: Products are formed and released from the enzyme. (1 mark)
- The enzyme remains unchanged and ready for a new substrate. (1 mark)
- Reference to the induced-fit model: The enzyme undergoes conformational changes during binding. (1 mark)
Question 6
Explain the importance of hydrophilic R groups in enzyme functionality. (3 marks)
Mark Scheme:
- Hydrophilic R groups face outward, making the enzyme soluble in water. (1 mark)
- This solubility allows enzymes to function effectively in aqueous environments like the cytoplasm. (1 mark)
- Hydrophilic interactions enable the enzyme to remain suspended and interact with substrates in solution. (1 mark)
Question 7
Describe the differences in activation energy for catalyzed and uncatalyzed reactions, referencing a diagram. (4 marks)
Mark Scheme:
- Uncatalyzed reaction: Requires a high activation energy to reach the transition state. (1 mark)
- Enzyme-catalyzed reaction: The enzyme stabilizes the transition state, lowering the activation energy. (1 mark)
- Reference to the diagram: A dashed line indicates higher energy (uncatalyzed) and a solid line shows reduced energy (catalyzed). (1 mark)
- Lower activation energy allows reactions to occur faster and at lower temperatures. (1 mark)
Question 8
Explain how the induced-fit model enhances enzyme specificity. (3 marks)
Mark Scheme:
- The active site is flexible and adjusts to fit the substrate perfectly. (1 mark)
- This ensures a precise fit, increasing enzyme-substrate interaction. (1 mark)
- Enhanced interaction improves the efficiency and specificity of catalysis. (1 mark)
Question 9
Explain how enzymes are reusable in chemical reactions. (4 marks)
Mark Scheme:
- During the reaction, the enzyme binds the substrate to form an enzyme-substrate complex. (1 mark)
- The enzyme facilitates the reaction, forming products. (1 mark)
- After releasing the products, the enzyme returns to its original structure. (1 mark)
- This structural recovery allows the enzyme to catalyze subsequent reactions without being consumed. (1 mark)
Question 10
Discuss the role of lysozyme in protecting the human body. (5 marks)
Mark Scheme:
- Lysozyme is an enzyme found in tears and saliva. (1 mark)
- It breaks down polysaccharide chains in bacterial cell walls. (1 mark)
- This weakens the cell wall, causing bacterial cells to burst due to osmotic pressure. (1 mark)
- Lysozyme acts as a natural defense mechanism against bacterial infections. (1 mark)
- Its presence in bodily secretions helps prevent infections by eliminating harmful bacteria. (1 mark)