MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A research team is investigating a novel enzyme, 'Enzyme X,' which catalyzes a reaction with a Michaelis constant (Km) of 50 µM. They then introduce a competitive inhibitor to the reaction mixture. Which of the following changes would be observed in the kinetic parameters of Enzyme X in the presence of this competitive inhibitor?
- ANo change in Vmax and an increase in Km.
- BA decrease in Vmax and an increase in Km.
- CNo change in Vmax and a decrease in Km.
- DAn increase in Vmax and a decrease in Km.
Show answer & explanationAnswer & explanation
Correct answer: A. No change in Vmax and an increase in Km.
Competitive inhibitors bind to the enzyme's active site, competing with the substrate. This effectively increases the apparent Km because more substrate is needed to reach half Vmax. However, if enough substrate is added, the inhibitor can be outcompeted, allowing the enzyme to reach its original Vmax.
Why the other options are wrong
- B. A decrease in Vmax is characteristic of non-competitive or uncompetitive inhibition, not competitive. An increase in Km is correct.
- C. A decrease in Km suggests increased affinity, which is incorrect for competitive inhibition. Vmax is unaffected.
- D. An increase in Vmax is not characteristic of competitive inhibition, and a decrease in Km suggests increased affinity, which is incorrect.
Competitive Inhibition
A type of enzyme inhibition where the inhibitor binds reversibly to the enzyme's active site, competing with the substrate. This increases the apparent Km but does not change Vmax.
- Binds at active site.
- Km increases (apparent).
- Vmax remains unchanged.
- Can be overcome by increasing substrate concentration.
Memory trick: Competitive Crusaders: Vmax stays, Km climbs!