MCAT® ExamChemical and Physical Foundations of Biological SystemsMedium
A biochemist is studying a novel enzyme that catalyzes a specific reaction. They find that the enzyme's activity is significantly reduced in the presence of a molecule that binds to a site distinct from the active site, inducing a conformational change that lowers the enzyme's affinity for its substrate. Which type of inhibition is being described?
- AUncompetitive inhibition
- BNon-competitive inhibition
- CCompetitive inhibition
- DIrreversible inhibition
Show answer & explanationAnswer & explanation
Correct answer: B. Non-competitive inhibition
Non-competitive inhibition occurs when an inhibitor binds to an allosteric site (a site distinct from the active site) on the enzyme. This binding causes a conformational change that reduces the enzyme's catalytic efficiency (Vmax is lowered) and often affects substrate binding (Km may or may not change, but often appears to change due to the Vmax effect).
Why the other options are wrong
- A. Uncompetitive inhibitors bind only to the enzyme-substrate complex, not the free enzyme.
- C. Competitive inhibitors bind directly to the active site, competing with the substrate.
- D. Irreversible inhibitors form a strong, often covalent, bond with the enzyme, permanently inactivating it.
Non-Competitive Inhibition
Non-competitive inhibition is a type of enzyme inhibition where the inhibitor binds to an allosteric site on the enzyme, altering its conformation and reducing its catalytic activity, regardless of substrate concentration.
- Inhibitor binds to an allosteric site, not the active site.
- Vmax is decreased.
- Km typically remains unchanged (pure non-competitive) or appears to change (mixed non-competitive).
Memory trick: ACE Inhibitors: Allosteric, Competitive, Enzyme-Substrate