03.09 Enzyme Inhibitors

Question 1

Explain the difference between competitive and non-competitive enzyme inhibition. (6 marks)

Mark Scheme:

1. In competitive inhibition, the inhibitor resembles the substrate and binds to the active site of the enzyme. (1 mark)
2. This prevents substrate molecules from binding, reducing the rate of reaction temporarily. (1 mark)
3. Increasing substrate concentration reduces the effect of competitive inhibition as more substrate molecules outcompete the inhibitor. (1 mark)
4. In non-competitive inhibition, the inhibitor binds to a site other than the active site, causing a change in the enzyme’s 3D shape. (1 mark)
5. This alters the shape of the active site, reducing its ability to bind the substrate. (1 mark)
6. Non-competitive inhibition is unaffected by substrate concentration since substrate and inhibitor bind to different sites. (1 mark)

Question 2

How does competitive inhibition affect the rate of an enzyme-catalyzed reaction, and how can its effect be reduced? (5 marks)

Mark Scheme:

1. Competitive inhibitors bind to the active site, preventing substrate binding and reducing reaction rate. (1 mark)
2. The inhibition is temporary and reversible as inhibitors can be replaced by substrate molecules. (1 mark)
3. At low substrate concentrations, the effect of inhibition is more pronounced as inhibitors outcompete substrates. (1 mark)
4. Increasing substrate concentration reduces inhibition by increasing the chance of substrate molecules binding to active sites. (1 mark)
5. At very high substrate concentrations, the effect of competitive inhibition becomes negligible. (1 mark)

Question 3

Describe how non-competitive inhibitors affect enzyme activity and why their effect cannot be reduced by increasing substrate concentration. (5 marks)

Mark Scheme:

1. Non-competitive inhibitors bind to a site other than the active site, causing a conformational change in the enzyme. (1 mark)
2. This change alters the shape of the active site, reducing its ability to bind the substrate. (1 mark)
3. The inhibition remains effective while the inhibitor is bound, regardless of substrate concentration. (1 mark)
4. Increasing substrate concentration does not affect the inhibitor’s binding as they do not compete for the same site. (1 mark)
5. Non-competitive inhibitors lower the overall reaction rate and maximum reaction velocity (Vmax). (1 mark)

Question 4

What is end product inhibition, and how does it regulate metabolic pathways? (6 marks)

Mark Scheme:

1. End product inhibition occurs when the final product of a metabolic pathway acts as a non-competitive inhibitor for the first enzyme in the pathway. (1 mark)
2. This prevents excessive buildup of the product, maintaining its levels within an optimal range. (1 mark)
3. As the end product accumulates, it binds to enzyme 1, reducing its activity and slowing down the pathway. (1 mark)
4. When product levels decrease, the inhibition is reduced, allowing enzyme 1 to become active again. (1 mark)
5. This feedback mechanism ensures the pathway is self-regulating and efficient. (1 mark)
6. Example: End product inhibition in metabolic pathways like ATP production helps balance energy demand and supply. (1 mark)

Question 5

Compare the mechanisms of action of competitive and non-competitive inhibitors with examples. (6 marks)

Mark Scheme:

1. Competitive inhibitors resemble the substrate and bind to the active site, blocking substrate access. (1 mark)
2. Example: Ethanol acts as a competitive inhibitor of the enzyme that converts ethylene glycol to toxic oxalic acid. (1 mark)
3. Non-competitive inhibitors bind to a separate site on the enzyme, altering its shape and reducing substrate binding efficiency. (1 mark)
4. Example: In metabolic pathways, non-competitive inhibitors regulate enzyme activity to prevent excessive product formation. (1 mark)
5. The effect of competitive inhibition can be reduced by increasing substrate concentration, while non-competitive inhibition is unaffected by substrate levels. (1 mark)
6. Both types of inhibition can be reversible, allowing enzymes to regain functionality once the inhibitor is removed. (1 mark)

Question 6

Describe how increasing substrate concentration affects competitive and non-competitive inhibition differently. (5 marks)

Mark Scheme:

1. In competitive inhibition, increasing substrate concentration reduces inhibition because substrate molecules outcompete inhibitors for the active site. (1 mark)
2. At high substrate concentrations, the effect of competitive inhibitors becomes negligible. (1 mark)
3. In non-competitive inhibition, increasing substrate concentration has no effect on the level of inhibition. (1 mark)
4. This is because non-competitive inhibitors bind to a different site, altering enzyme shape and reducing active site functionality. (1 mark)
5. Non-competitive inhibition lowers Vmax, while competitive inhibition does not affect Vmax at high substrate concentrations. (1 mark)

Question 7

Explain how ethanol can act as a competitive inhibitor in the treatment of ethylene glycol poisoning. (4 marks)

Mark Scheme:

1. Ethanol competes with ethylene glycol for the active site of the enzyme that converts ethylene glycol into toxic oxalic acid. (1 mark)
2. By binding to the enzyme, ethanol prevents the conversion of ethylene glycol, reducing toxic product formation. (1 mark)
3. The inhibition is reversible, and increasing ethanol concentration increases its effectiveness. (1 mark)
4. This allows the ethylene glycol to be excreted without being converted into harmful metabolites. (1 mark)

Question 8

Why do non-competitive inhibitors reduce Vmax but not Km in enzyme-catalyzed reactions? (5 marks)

Mark Scheme:

1. Non-competitive inhibitors reduce the number of functional enzyme molecules by altering their active sites. (1 mark)
2. This lowers the maximum reaction rate (Vmax) since fewer enzymes are available to process substrates. (1 mark)
3. However, Km remains unchanged because the affinity of the remaining functional enzymes for the substrate is unaffected. (1 mark)
4. Substrate molecules can still bind to unaffected active sites, but the overall reaction rate is reduced. (1 mark)
5. Non-competitive inhibition is independent of substrate concentration, so it does not alter the substrate’s binding efficiency. (1 mark)

Question 9

Describe the role of feedback inhibition in regulating metabolic pathways. (5 marks)

Mark Scheme:

1. Feedback inhibition ensures that metabolic pathways are regulated to prevent overproduction of end products. (1 mark)
2. The final product acts as a non-competitive inhibitor, binding to the first enzyme in the pathway. (1 mark)
3. This reduces enzyme activity, slowing down the pathway as product levels rise. (1 mark)
4. When product levels fall, inhibition decreases, allowing the pathway to resume and produce more product. (1 mark)
5. This regulation maintains a balance between supply and demand for cellular products. (1 mark)

Question 10

How does non-competitive inhibition contribute to the regulation of enzyme activity in cells? (4 marks)

Mark Scheme:

• Non-competitive inhibitors bind to sites other than the active site, altering enzyme shape and reducing its activity. (1 mark)
• This allows cells to modulate enzyme activity without relying on substrate concentration. (1 mark)
• Non-competitive inhibition is commonly used in metabolic pathways to regulate the production of specific molecules. (1 mark)
• It ensures that enzymes are not overactive, preventing wasteful or harmful overproduction of products. (1 mark)