A competitive inhibitor of an enzyme:

Question

BlackTom AI · Accepted Answer

A competitive inhibitor occupies the enzyme's active site, directly blocking substrate binding.

1. Relevant knowledge points and concepts
- Enzyme active site: region where substrate binds and catalysis occurs.
- Competitive inhibition: inhibitor resembles substrate and competes for the active site.
- Michaelis–Menten effects: competitive inhibitors increase apparent Km (lower apparent affinity) but do not change Vmax; inhibition can be overcome by increasing substrate concentration.
- Other inhibition types: noncompetitive/allosteric (binds elsewhere) and uncompetitive (binds only to enzyme–substrate complex).

2. Step-by-step reasoning
- A competitive inhibitor must prevent the substrate from binding by occupying the same binding region.
- Because it competes with substrate for the same site, binding is mutually exclusive: either enzyme–inhibitor (EI) or enzyme–substrate (ES) complex forms, not both.
- If substrate concentration is increased sufficiently, substrate can outcompete the inhibitor for the active site, restoring activity toward Vmax.

3. Calculations and intermediate steps (Michaelis–Menten context)
- In presence of competitive inhibitor, apparent Km (Km,app) = Km × (1 + [I]/Ki), where [I] is inhibitor concentration and Ki is inhibitor dissociation constant.
- Vmax remains unchanged because at high [S] the inhibitor effect is overcome.
- These expressions derive from substituting competitive inhibition into the steady-state derivation of velocity.

4. Why the correct answer is correct
- Because competitive inhibitors physically occupy the active site, they directly prevent substrate binding, producing the characteristic change in Km but not Vmax. This matches the definition of competitive inhibition.

5. Why the other options are incorrect
- "binds at another site on the enzyme to the active site": describes noncompetitive or allosteric inhibitors, which bind a different site and typically alter Vmax without changing Km in the same way.
- "binds at a site on the enzyme only when the substrate has been bound": describes uncompetitive inhibition, where the inhibitor binds only to the ES complex and lowers both Km and Vmax.

6. Conclusion
Therefore, the correct option is "binds at the active site," which matches the provided answer.

A competitive inhibitor of an enzyme:单项选择题

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