Enzymes

Question 1

Describe the induced-fit model of enzyme action (2 marks)

Answer 1

• Before the reaction, the active site is not complementary to the substrate
• The shape of the active site changes as an enzyme substrate complex forms

Question 2

Explain how a competitive inhibitor stops an enzyme from working (3 marks)

Answer 2

• Inhibitor is a similar shape to the substrate
• Inhibitor blocks the active site
• Substrate can’t bind to active site so fewer ES complexes formed

Question 3

Describe how proteins are digested in the human gut (4 marks)

Answer 3

• Hydrolysis of peptide bonds;
• Endopeptidases break polypeptides into smaller peptide chains;
• Exopeptidases remove terminal amino acids;
• Dipeptidases hydrolyse dipeptides into amino acids

Question 4

Explain why enzymes only catalyse certain reactions (3 marks)

Answer 4

• The active site of an enzyme is complementary to the substrate
• So only the substrate can fit
• To form an enzyme substrate complex

Question 5

Explain why maltase:
• only breaks down maltose
• allows this reaction to take place at normal body temperature (5 marks)

Answer 5

• Tertiary structure of enzyme means
• Active site is only complementary to maltose
• The shape of the active site changes as an enzyme substrate complex forms
• Enzyme lowers activation energy
• By forming enzyme-substrate complex

Question 6

Describe competitive and non-competitive inhibition of an enzyme (7 marks)

Answer 6

• Inhibitors reduce binding of enzyme to substrate

Competitive inhibition:

• Inhibitor similar shape to substrate
• Binds to the active site of enzyme
• Inhibition can be overcome by more substrate

Non-competitive inhibition:

• Inhibitor binds to site on enzyme other than active site
• Prevents formation of active site
• Cannot be overcome by adding more substrate