Explain how non-competitive inhibitors reduce the rate of an enzyme-catalysed reaction.

IB DP Biology Standard Level (2023 syllabus) — C1.1 Enzymes and metabolism · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A non‑competitive inhibitor binds to an allosteric site on the enzyme, not the active site. This binding changes the enzyme’s tertiary structure, distorting the active site. The distorted active site cannot bind substrate effectively, so the enzyme–substrate complex forms less readily. Consequently the rate of the reaction falls and cannot be restored by raising substrate concentration.

Examiner tips

Use the term "allosteric site" and "tertiary structure". Show the chain: inhibitor → conformational change → altered active site → reduced substrate binding → lower rate. Mention that the effect is independent of substrate concentration.

Common mistakes

Confusing non‑competitive with competitive inhibition. Failing to mention the allosteric site or the change in tertiary structure. Saying the rate can be restored by more substrate, which is incorrect.

Mark scheme (4 marks)

A non-competitive inhibitor binds to a site on the enzyme other than the active site (an allosteric site).
Binding of the inhibitor changes the tertiary structure / shape of the enzyme, including the active site.
The altered active site can no longer bind substrate effectively / the enzyme–substrate complex cannot form (or forms less readily).
The rate of reaction is reduced and cannot be restored by increasing substrate concentration (unlike competitive inhibition).

Key terms in this question

non-competitive inhibitor