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Inhibition: how cells, poisons and medicines all turn enzymes down questions

Competitive and non-competitive inhibition and their effects on Vmax and Km, reversible against irreversible inhibitors, named examples including penicillin and statins, end-product inhibition as metabolic control, and immobilised enzymes in industry.

6 original questions · 20 marks · the inhibition: how cells, poisons and medicines all turn enzymes down notes · Enzymes and metabolic control

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• 4 marks · Compare

  Compare a competitive inhibitor with a non-competitive inhibitor, referring to where each one binds and what each does to the enzyme.

  Mark scheme

  1. B1 a competitive inhibitor binds to the active site, whereas a non-competitive inhibitor binds elsewhere, at an allosteric site
  2. B1 a competitive inhibitor has a shape similar to the substrate, whereas a non-competitive inhibitor has a shape unrelated to it
  3. B1 a competitive inhibitor blocks the site, whereas a non-competitive inhibitor alters the tertiary structure so the site is no longer complementary to the substrate
  4. B1 raising the substrate concentration overcomes competitive inhibition, but has no effect at all on non-competitive inhibition

• 4 marks · Explain

  Explain how end-product inhibition controls the rate of a metabolic pathway, and explain why it is the first enzyme of the pathway that is inhibited.

  Mark scheme

  1. B1 the final product of the pathway acts as a non-competitive inhibitor of the first enzyme, binding at an allosteric site
  2. B1 as product accumulates more of that first enzyme is inhibited, so the pathway slows
  3. B1 as product is used up the inhibitor comes off again, so the pathway speeds up and its rate matches demand
  4. B1 inhibiting the last enzyme instead would leave the earlier steps running, so substrate and ATP would be spent building intermediates the cell has no use for

• 4 marks · Evaluate

  A dairy producing lactose-free milk must decide whether to immobilise its lactase on a column or to add the free enzyme to each batch. Evaluate the case for immobilising it.

  Mark scheme

  1. B1 the immobilised enzyme stays on its support as the milk flows past, so it is recovered and reused instead of being lost with every batch
  2. B1 no enzyme leaves with the product, so no separation step is needed and the milk contains none of the enzyme protein
  3. B1 the support holds the tertiary structure, so the column tolerates a higher temperature and can be run continuously rather than in batches
  4. B1 against this, an immobilised enzyme works more slowly because part of the active site may be blocked and substrate must diffuse to it, and set-up costs are higher; since reuse repays those costs within days, immobilising is the better choice

• 3 marks · Explain

  Explain why penicillin kills growing bacteria but does no harm to human cells.

• 3 marks · Suggest

  Mercury ions inhibit many enzymes, and washing the mercury away does not restore activity. Suggest how mercury ions inhibit an enzyme, and suggest why the effect cannot be reversed.

• 2 marks · State

  State the effect a non-competitive inhibitor has on the Vmax of an enzyme, and state its effect on the Km.

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