3.2.3—Reversible inhibitors
- Syllabus
- 9700–2028–2029
- Objective
- 3.2.3
- Level
- AS
Compare competitive and non-competitive inhibition
Reversible inhibitors reduce enzyme activity by limiting productive enzyme–substrate complexes. The binding site determines whether adding more substrate can recover the maximum rate.
| Feature | Competitive inhibitor | Non-competitive inhibitor |
|---|---|---|
| Binding site | Similar enough to the substrate to compete for the active site | Binds at an alternative site and changes the functional active-site shape |
| Vmax in the standard comparison | Unchanged when enough substrate eventually outcompetes the inhibitor | Decreased because some enzyme capacity remains functionally unavailable |
| Apparent Km in the standard comparison | Increased: more substrate is needed to reach half of the unchanged Vmax | Unchanged in the simple non-competitive model used here |
| Rate–substrate curve | Shifted to the right but approaches the same plateau | Lies below the uninhibited curve and approaches a lower plateau |
| Can more substrate recover the maximum? | Yes, in principle, because substrate and inhibitor compete for the same site | No, because extra substrate cannot restore the altered active site |
Active-site competition lowers the chance that a substrate occupies the site at a given concentration, so increasing substrate can restore productive binding and the original maximum. Alternative-site binding changes the active site on affected enzyme molecules, so increasing substrate cannot recover the lost catalytic capacity. Read Vmax and Km from matched initial-rate saturation curves, not from one inhibitor point.
End-product inhibition is a reversible control example: as the final product accumulates, it binds at an alternative site on an earlier enzyme and slows the pathway. When product concentration falls, inhibition is relieved and the pathway can resume.
The table uses the standard simplified comparison: real data must be judged from the whole curve and matched conditions. Do not identify inhibitor type from one rate value, call alternative-site binding harmless, or introduce irreversible inhibition into this objective.
Follow the limiting step across the topic
A rate change is evidence about the current bottleneck, not an explanation by itself. Ask whether the observation is best explained by collision frequency, substrate availability, usable active-site capacity, enzyme shape or diffusion.
- A substrate plateau shows that enzyme capacity now limits rate; the plateau approaches Vmax.
- Half of Vmax locates Km on the substrate-concentration axis.
- If more substrate restores the original maximum, competition for active sites is supported.
- If the maximum remains lower, functional enzyme capacity has been reduced.
- If immobilised enzyme appears slower, diffusion may be limiting even though reuse and separation improve.
To make a valid conclusion, change one factor, control the others, compare initial rates and stay within the tested range. A single point cannot establish a curve shape or inhibitor type.
ConceptA-Level CAIE Biology AS