| Date | November 2010 | Marks available | 1 | Reference code | 10N.3.SL.TZ0.7 |
| Level | Standard level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Distinguish | Question number | 7 | Adapted from | N/A |
Question
Metabolic pathways can be controlled by end-product inhibition of the enzyme-catalysed reactions.
KAS III is the initial enzyme of fatty acid production in plants and bacteria. The substrates for this reaction are acetyl CoA and malonyl-ACP.
Three different strains of plant were generated, each with a different mutated KAS III gene: M1, M2 and M3. The enzyme activity of the normal (wild-type) and the three mutant strains was tested without and with the addition of the inhibitor, dodecanoyl-ACP. Dodecanoyl-ACP has a similar structure to malonyl-ACP. The graph shows the mean activity of the enzymes.
State the activity of the wild-type enzyme without the inhibitor and with the inhibitor.
Without inhibitor:
With inhibitor:
Distinguish between the enzyme activity without the inhibitor in the wild-type and the mutant strains.
Explain why the activity of the enzyme from wild-type plants changes when the inhibitor is added.
The scientists concluded that the enzymes of the mutant plants had a reduced activity, but were insensitive to the inhibition by dodecanoyl-ACP. Evaluate these conclusions.
Markscheme
without inhibitor: 5.2 μmol min-1 mg-1 (units required)
with inhibitor: 1.4 μmol min-1 mg-1 (units required)
Both needed to award the mark.
wild-type enzyme has greater activity than the mutant enzymes
a. inhibitor is similar in shape/structure to malonyl-ACP which is a substrate of the reaction;
b. inhibitor is competing for the active site / competitive inhibition;
c. attaches to active site and does not let reaction occur;
d. if more substrate is added then the inhibition will be less;
a. activity of mutant enzymes without the inhibitor is always lower than wild-type;
b. activity of mutant enzymes with the inhibitor is always less than without;
c. but the differences are not as great as in the wild-type enzyme / the mutant enzymes were less sensitive to the inhibitor;
d. the activity of the mutant enzymes with the inhibitor are always higher than the activity of the wild-type enzyme with the inhibitor;
e. the data does not indicate whether these differences are significant or not / difference between wild-type enzyme and M1 enzyme not great;
Examiners report
Candidates were able to correctly identify the enzyme activity using the graphs.
Most saw that the wild-type enzyme had greater activity than the mutant form.
There were some good responses earning 2 out of 3 marks but few responses earned full marks.
This question was difficult for the candidates and very few answered it correctly. A few candidates earned one mark for stating that the activity of the mutant enzymes without the inhibitor is always lower than the wild-type but they were not able to evaluate the data to identify differences in sensitivity to the inhibitor.
