Neutrase® is an enzyme that is used to hydrolyse proteins in solution. When the enzyme is mixed with a 2% protein solution the reaction mixture changes from white to colourless. A student carried out an experiment to find the effect of copper sulfate and potassium sulfate on the activity of Neutrase®. The student made four reaction mixtures in test-tubes A to D. Test-tubes A to C contained equal volumes of protein solution and 0.1cm³ of solutions of copper sulfate or potassium sulfate. Test-tube D contained the same volume of protein solution and 0.1 cm³ of water. 0.5cm³ of a 1% Neutrase® solution was added to test-tube A and immediately placed into a colorimeter. The colorimeter was used to measure the intensity of light that is absorbed by the solution (absorbance) over 100 seconds. The procedure was repeated with the other reaction mixtures, B, C and D. The results are shown in [Figure 3.1].

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

1. any two from:
2. (Neutrase® breaks down / hydrolyses) protein to, peptides / amino acids / smaller molecules; A idea of increase in solubility
3. during the reaction / AW, more light passes through / more light is transmitted / less light is absorbed ;
4. idea that 100 s is long enough to see the progress of the reactions; I 'the rate of reaction can be calculated' I 'allow time for reaction to complete'
5. accept ora where appropriate
6. 1 copper sulfate, decreases / AW, the activity of Neutrase (ref. to A or B) ;
7. 2 0.01 (mol dm⁻³) / low concentration, CuSO₄ has less of an effect than, 0.05 (mol dm⁻³) / high concentration (ref. to A and B);
8. 3 potassium sulfate has, little / no, effect on activity (ref. to C);
9. 4 data quote to show absorbance for two different lines at the same time; one time and two absorbance readings from different lines on the graph with the unit for time used once anywhere in answer, allow 'at the end' for 100 s
10. 5 copper sulfate is an inhibitor of Neutrase;
11. 6 potassium sulfate is not an inhibitor; A 'less of...'
12. 7 copper sulfate binds to Neutrase ; A anywhere
13. 8 substrate cannot enter active site / ESCs do not form (so protein not hydrolysed); A fewer ESCs
14. higher productivity / higher yield and fewer costs because
15. enzyme can be re-used;
16. enzyme can be easily recovered ;
17. downstream processing is easier ;
18. product, not / less, contaminated; A less purification needed
19. longer shelf-life of enzyme ;
20. reduces product inhibition ;
21. enzyme is, more stable / less likely to denature or described;
22. A thermostable / can work at high temperatures
23. A in context of change in pH
24. I 'can withstand changes in temperature'