(a) Describe the induced-fit hypothesis of enzyme action. (b) Many marine organisms can become attached to hard surfaces such as rocks or the surfaces of ships. These organisms are known as fouling organisms. The larva of the acorn barnacle, Amphibalanus amphitrite, is an example of a fouling organism. One of these barnacle larvae is shown in Fig. 3.1. The larvae of A. amphitrite use a protein to attach themselves to the surfaces of ships. It is expensive to remove fouling organisms from ships. Scientists have developed substances to prevent the attachment of larvae. However, some of these substances are toxic and have been responsible for a decrease in marine biodiversity. Scientists investigated the effect of using an immobilised protease, subtilisin A, to prevent the attachment of the larvae of A. amphitrite to surfaces. The scientists used 4 different concentrations of subtilisin A which had been immobilised onto the surface of a polymer film. As a control they used denatured subtilisin A immobilised onto the surface of the same polymer. Glass slides were also used as a control. The larvae were released into 6 tanks of artificial sea water: • 4 tanks, each with a polymer surface and a different concentration of immobilised subtilisin A • 1 tank with a polymer surface and denatured immobilised subtilisin A • 1 tank with glass slides instead of a polymer surface. The number of larvae that attached to the different surfaces in the tanks was counted after 24 hours and again after 48 hours. The number of larvae attached in each tank was expressed as the percentage of the total number of larvae released in each tank. The results are shown in Fig. 3.2.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

1. [1] any three from:
2. [1] to award MPs 2, 3 and 6 there must be reference to shape or conformation at least once in the answer
3. [1] if no reference to shape or conformation in the answer mark to max 2
4. [1] if lock and key described mark to max 2 from MP4 onwards
5. [1] idea that active site (of inactive enzyme) and substrate are, (only) partially / not, complementary ;
6. [1] as substrate enters active site there is a, change in shape / conformational change, (of active site / enzyme) ; A active site moulds around substrate
7. [1] R substrate changes shape / substrate active site
8. [1] so shape of active site becomes complementary to substrate ; A idea that there is a good fit between active site and substrate
9. [1] enzyme-substrate complex forms ;
10. [1] lowers activation energy ;
11. [1] product(s) leave and, enzyme / active site, returns to original, shape / conformation ;
12. [1] AVP; e.g. catalytic sites move to be in correct position
13. [1] e.g. detail of lowering activation energy – putting strain on substrate / stress on bonds
14. [1] allow enzyme or subtilisin for immobilised subtilisin A
15. [1] allow attachment for attachment of larvae
16. [1] I comparisons between controls
17. [1] any four from:
18. [1] effective because (much) lower percentage attachment with, the enzyme / AW, compared with the control(s) / AW ; ora for (both) controls
19. [1] any comparison between 24 and 48 hrs in the same experimental tank (1 to 4) ; e.g. tank 2 only one showing decrease at 48 hours
20. [1] any comparison between any two different tanks (1 to 4 or any experimental with control) at either 24 or 48 hours ; e.g. less attachment in tank 2 than in the others at 48 hours
21. [1] 48 hours is a short period of time / may not be effective after a few days ;
22. [1] not 100% effective ;
23. [1] (generally) becomes less effective at 48 hours (with exception of tank 2) ;
24. [1] immobilised enzyme, not stable / detached from surface ;
25. [1] ref. to no trend with increasing subtilisin concentration ;
26. [1] no repeats so data may not be valid / AW ;
27. [1] AVP;
28. [1] I ref. to cost
29. [1] any two from:
30. [1] density / concentration of, enzyme / subtilisin A / polymer ; A way of attaching enzyme to polymer
31. [1] type of polymer ;
32. [1] check that all organisms are removed from ships before applying, enzyme / polymer ;
33. [1] way to fix polymer to surfaces of ships / material used to make surface of ships ;
34. [1] number of larvae put into tanks / experiment carried out at time of year when larvae are in sea water; A idea that different places have different numbers of larvae
35. [1] length of time before taking measurements ;
36. [1] length of time, enzyme / anti-fouling agent, remains active ; A ref. to stability of immobilised enzyme
37. [1] idea of any interaction with other, fouling / marine, organisms ; A 'affect' as an interaction
38. [1] e.g. that could, graze / decompose, subtilisin A / polymer
39. [1] that could compete with A. amphitrite for attachment sites
40. [1] effectiveness against fouling organisms other than, A. amphitrite / barnacles
41. [1] subtilisin A / product of enzyme, may be, toxic / harmful, to marine organisms
42. [1] effect of immersion in sea water on, enzyme / polymer (not in artificial sea water);
43. [1] effect of different types of sea water, e.g. salt content / mineral content / roughness ; A different, concentrations of sea water / salinities
44. [1] effect of marine pollutants / something in sea water, as enzyme inhibitors;
45. [1] AVP; e.g. ref. to surface area of ship exposed to sea water
46. [1] types of antifouling agents present in the seawater