May/June 2026 Paper 22

2026 · 6 questions · 29 parts · 60 marks

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6(a)Enzymes Enzyme KineticsEasy2 marks
Lactate dehydrogenase (LDH) is an enzyme used in some blood tests to help diagnose liver disease. In liver disease, an enzyme known as ALT can leak out from damaged liver tissue and enter the blood. The presence of ALT can be confirmed by testing a blood sample at room temperature: - ALT reacts with substances that are added to the blood sample to produce substrates for the reaction catalysed by LDH - the rate of reaction of LDH can be used to estimate the concentration of ALT in the blood. Scientists investigated the activity of LDH extracted from the bacterium Lactobacillus delbrueckii at different pH values and at different temperatures. Figure 6.1 shows the effect of pH on the activity of LDH from L. delbrueckii. Figure 6.2 shows the effect of temperature on the activity of LDH from L. delbrueckii.
Figure 6.1, a line graph of the percentage of maximum activity of LDH from L. delbrueckii (y-axis, 0 to 100%) against pH (x-axis, 4 to 12): activity rises from 30% at pH 5 through 60% at pH 6 and 90% at pH 7 to a peak of 100% at pH 7.5, then falls through 75% at pH 8, 60% at pH 8.5 and 30% at pH 9 to 10% at pH 10 and 2% at pH 11.
Figure 6.2, a line graph of the percentage of maximum activity of LDH from L. delbrueckii (y-axis, 20 to 100%) against temperature in °C (x-axis, 20 to 80): activity rises from about 27% at 25°C through 40% at 35°C, 55% at 40°C, 68% at 45°C and 90% at 50°C to a peak of about 98% at 55°C, then falls to 84% at 60°C and 51% at 70°C.
With reference to Figure 6.1 and Figure 6.2, state the optimum pH and the optimum temperature for LDH.
6(b)Enzymes Enzyme KineticsMedium3 marks
Figure 6.1, a line graph of the percentage of maximum activity of LDH from L. delbrueckii (y-axis, 0 to 100%) against pH (x-axis, 4 to 12): activity rises from 30% at pH 5 through 60% at pH 6 and 90% at pH 7 to a peak of 100% at pH 7.5, then falls through 75% at pH 8, 60% at pH 8.5 and 30% at pH 9 to 10% at pH 10 and 2% at pH 11.
With reference to Figure 6.1, explain how pH can affect the activity of an enzyme such as LDH.

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6(c)Enzymes Enzyme KineticsEasy1 mark
Suggest why the recommended temperature for performing the test on the blood sample is not the same as the optimum temperature for LDH activity.

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