3 The enzyme glucose 6-phosphate dehydrogenase (G6PD) is composed of two identical polypeptide chains. (a) Students investigated the activity of two forms of G6PD, J and K, at different concentrations of substrate. K is a form of the enzyme that results from a mutation that changes one amino acid in the polypeptide. The results are shown in Fig. 3.1. [Figure 3.1]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

1. [3] one mark for units one mark per column
2. Vmax | Km
3. μmol min-1 | mmol dm-³;
4. J 60 | 0.2
5. K 50; | 0.2;
6. [4] K compared with J max 3 from mp4–mp8
7. any four from: 1. rate of reaction is lower at all substrate concentrations ;
8. 2. ref. to greater difference as substrate concentration increases ;
9. 3. rate of reactions for J and K compared at two substrate concentrations to support mp1 or mp2 or manipulated data at two substrate concentrations to support mp2;
10. 4. less efficient/acts more slowly / AW ;
11. 5. active site is still binding substrate ;
12. 6. idea that active site is less effective at catalysing reaction 7. or enzyme has lower affinity for substrate ;
13. 8. (replaced / changed) amino acid is important in, shape / structure, of active site ;
14. 9. AVP; e.g. how the mutation may have changed interaction between substrate and, active / catalytic, site e.g. ref. to hydrogen bonding / transfer of electrons
15. [2] each polypeptide shows primary, secondary and tertiary structure; A description of levels of structure
16. both (forms of G6PD / enzyme) composed of more than one polypeptide (so have quaternary structure) ;