Scientists investigated the progress of reactions catalysed by two enzymes: dopa oxidase and neutrase. The reactions catalysed by these enzymes result in changes to the appearance of the reaction mixtures. The reactions are shown in Fig. 3.1. The changes in appearance of the reaction mixtures make it possible to follow the reactions using a colorimeter. Fig. 3.2 shows the progress of the reaction catalysed by dopa oxidase as recorded from a colorimeter. Fig. 3.3 shows the progress of the reaction catalysed by neutrase as recorded from a colorimeter. Scientists searching for a suitable enzyme to use in an industrial process isolated the bacterium Vibrio parahaemolyticus from the mouth of the Mediterranean eel, Muraena helena. The scientists discovered an enzyme in the bacterium that was suitable for the industrial process. The scientists named the enzyme VpSP37. The scientists investigated how the rate of reaction catalysed by VpSP37 is affected by the concentration of its substrate. The results of the investigation are shown in Fig. 3.4. [Figure 3.1] [Figure 3.2] [Figure 3.3] [Figure 3.4]

Mark scheme

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

1. [1] (a)(i) 1 change in, absorbance / rate of reaction, and (then), reaches a time when there is no further change / reaches a plateau / reaction stops / absorbance becomes constant ; A for change absorbance increases, in Fig. 3.2/ for dopa oxidase, and decreases, in Fig. 3.3 / for neutrase A decrease in rate of reaction R if rate of reaction increases
2. [1] (a)(i) 2 due to change in (intensity / shade, of) colour (of reaction mixture) ;
3. [1] (a)(i) 3 due to, collisions between substrate and enzyme / enzyme-substrate complexes form(ing) / product being made ;
4. [1] (a)(i) 4 idea that, no/little, change in absorbance because, all / most, of the substrate is used up / AW ; A substrate is limiting factor I reactants are used up
5. [1] (a)(ii) 1 can take, quantitative / numerical, readings / results;
6. [1] (a)(ii) 2 idea that can use values from the colorimeter to plot graph(s) ;
7. [1] (a)(ii) 3 can take readings continuously/do not have to take samples ; I continuous data
8. [1] (a)(ii) 4 results are not subjective/no judgements by eye/no bias in the results/AW ; A results are objective/more accurate I precise
9. [1] (a)(ii) 5 can determine / AW, rates of reactions ;
10. [1] (a)(ii) 6 AVP; e.g. ref. to use of, standards / calibration curve, to obtain actual concentrations e.g. can detect, small differences in, colour / cloudiness / AW
11. [1] (b)(i) Vmax = 80 (µmol mg⁻¹ min⁻¹) and half Vmax = 40 (µmol mg⁻¹ min⁻¹) ; A 80 and 40/ horizontal lines shown at 80 and 40 on Fig. 3.4
12. [1] (b)(i) Km = 0.014 mmol dm⁻³/ 14 µmol dm⁻³; unit must be given on answer line or in working A in range 0.012 to 0.016 mmol dm⁻³
13. [1] (b)(ii) lower concentration of substrate to reach (1/2) Vmax / AW ; A faster rate at, the same concentration / lower concentration
14. [1] (b)(ii) VpSP37 has a higher affinity for its substrate (than the other enzymes) ; ora explained ; e.g. better fit between substrate and active site / AW ; I ESCs formed more efficiently / any ref. to cost, etc.