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Enzymes: what a catalyst can and cannot do questions

Enzymes as globular protein catalysts, activation energy and what lowering it does and does not change, the active site and the enzyme-substrate complex, the lock-and-key model and the induced fit model that replaced it, and where specificity comes from.

5 original questions · 16 marks · the enzymes: what a catalyst can and cannot do notes · Enzymes and metabolic control

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• 4 marks · Explain

  Explain why a given enzyme catalyses only one reaction, using the terms active site, complementary and enzyme-substrate complex in your answer.

  Mark scheme

  1. B1 the active site is a region formed by the tertiary structure of the enzyme, with a particular shape
  2. B1 only a substrate whose shape is complementary to the active site can bind to it
  3. B1 when it binds, an enzyme-substrate complex forms and the reaction is catalysed
  4. B1 a molecule that is not complementary cannot enter the site, so no complex forms and that reaction is not catalysed

• 4 marks · Compare

  Compare the lock-and-key model of enzyme action with the induced fit model.

  Mark scheme

  1. B1 in lock and key the active site is already exactly complementary to the substrate, whereas in induced fit it is only close to complementary before binding
  2. B1 in lock and key nothing moves, whereas in induced fit the active site changes shape as the substrate binds
  3. B1 both models account for specificity, but only induced fit accounts for the rise in rate, because the closing of the site strains the substrate's bonds
  4. B1 induced fit is supported by enzymes photographed with and without their substrate, such as hexokinase closing around glucose, which lock and key is not

• 3 marks · Explain

  Explain why every enzyme is a globular protein, and why a fibrous protein such as collagen cannot be one.

  Mark scheme

  1. B1 a globular protein folds with hydrophilic side chains facing outwards, so it dissolves in cytoplasm or plasma where the reaction happens
  2. B1 its amino acid sequence is irregular, so the surface it presents is irregular and can carry an active site of a particular shape
  3. B1 a fibrous protein has a repetitive sequence and forms long insoluble strands, so it is neither soluble nor able to present such a pocket

• 3 marks · Suggest

  A molecule built to resemble the strained, half-reacted form of a substrate binds to its enzyme far more tightly than the substrate itself does. Suggest what this tells us about the shape of the active site, and suggest why such a molecule would slow the enzyme down.

• 2 marks · State

  State what an enzyme does to the activation energy of a reaction, and state one quantity that an enzyme does not change.

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