| Date | November 2017 | Marks available | 2 | Reference code | 17N.3.hl.TZ0.11 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Determine | Question number | 11 | Adapted from | N/A |
Question
An enzyme catalyses the conversion of succinate to fumarate ions in a cell, as part of the process of respiration.
The rate of the reaction was monitored and the following graph was plotted.
Determine the value of the Michaelis constant, Km, by annotating the graph.
The malonate ion acts as an inhibitor for the enzyme.
Suggest, on the molecular level, how the malonate ion is able to inhibit the enzyme.
Draw a curve on the graph above showing the effect of the presence of the malonate ion inhibitor on the rate of reaction.
Markscheme
Km labelled on x-axis as the [succinate ion] at ½Vmax
OR
horizontal line at ½Vmax AND vertical line down to x-axis
«Km ＝» 6.5 x 10–3 «mol dm–3»
Annotation of graph required for M1.
Accept any specific value in the range 6.0 x 10–3 to 7.5 x 10–3 «mol dm–3».
similar shape/size/structure «as succinate ion/substrate»
competes for the active site «with the succinate ion/substrate»
Accept “competitive inhibitor” for M2.
Award [1 max] if non-competitive inhibition is correctly described.
same Vmax reached at higher [substrate]
