The inhibitors considered here are reversible, so their effects depend on temporary interactions with enzyme molecules.
A competitive inhibitor has a shape sufficiently similar to the substrate that it can bind to the enzyme's active site.
The inhibitor and substrate therefore compete for the same binding region. When an inhibitor occupies the active site, the substrate cannot bind there and an enzyme-substrate complex cannot form at that enzyme molecule.
Effect on reaction rate
- Competitive inhibitors bind temporarily to active sites.
- Occupied active sites are unavailable to substrate molecules.
- Fewer enzyme-substrate complexes form per unit time.
- The rate of the enzyme-controlled reaction therefore decreases.
A greater concentration of competitive inhibitor produces more competition for active sites and reduces the reaction rate further.
Increasing substrate concentration
The effect of competitive inhibition can be reduced by increasing substrate concentration. With more substrate molecules present, substrates encounter active sites more frequently and become increasingly likely to bind instead of inhibitor molecules.
At sufficiently high substrate concentration, the enzyme can still achieve the same maximum rate as it would without the competitive inhibitor.
Exam Tip: For competitive inhibition, use the terms similar shape, active site, competition and fewer enzyme-substrate complexes. If substrate concentration is increased, explain that substrate molecules become more likely to occupy the active sites.