| Date | May 2012 | Marks available | 1 | Reference code | 12M.1.HL.TZ1.26 |
| Level | Higher level | Paper | Paper 1 | Time zone | Time zone 1 |
| Command term | Predict | Question number | 26 | Adapted from | N/A |
Question
A substrate undergoes a series of enzyme-catalysed reactions to form intermediate substances X, Y and then the final product.
What would be the effect on the reaction of adding a competitive inhibitor to enzyme 2?
A. The substrate would not react to form intermediate X.
B. The concentration of intermediate X would increase.
C. The activity of enzyme 3 would increase to compensate.
D. No final product would be formed.
Markscheme
B
Examiners report
Fewer than half of candidates answered question 26 correctly. Many thought that a competitive inhibitor of an intermediate enzyme in a pathway would prevent any final product from being formed. This is not likely because whatever the quantity of inhibitor, the substrate of the inhibited enzyme would still sometimes manage to bind to the active site and there would be some final product. The expected answer was that the substrate of the inhibited enzyme would accumulate.
