How to use the simulator
The graph plots reaction rate v against substrate concentration [S] from 0 to 200. Units are arbitrary. With no inhibitor the enzyme has a maximum rate of 100 and a Km of 20. There are two controls:
- Inhibitor menu: None, Competitive or Noncompetitive. The simulator opens on None.
- substrate [S] slider. It runs from 0 to 200 in steps of 1 and starts at 20.
The readout box updates as you change either control:
- Vmax (apparent): the maximum rate of the current curve.
- Km (apparent): the [S] at which the current curve reaches half of its own Vmax.
- at this [S], rate v: the rate at the slider's [S], to one decimal place.
- % of Vmax: the rate as a percentage of the uninhibited Vmax of 100, so it always matches v (rounded).
On the graph, the coral line is the current curve and a teal dot marks your [S]. A dashed horizontal line labelled Vmax shows the apparent maximum, and a dotted vertical line labelled Km drops from the half-maximum point to the [S] axis. When an inhibitor is selected, the uninhibited curve stays on the graph as a faded dashed line, so you can see what the inhibitor has changed.
Each inhibitor is modelled at one fixed strength. Competitive inhibition doubles Km to 40 and leaves Vmax at 100. Noncompetitive inhibition halves Vmax to 50 and leaves Km at 20.
The equations
The curve follows the Michaelis-Menten equation, shown in the simulator's caption:
- is the reaction rate at substrate concentration .
- is the rate when every active site is occupied, so the enzyme is saturated.
- is the substrate concentration that gives half of . A lower means the enzyme reaches half speed at lower [S].
You will not have to use this equation on the AP Biology exam, and it is not on the formula sheet. What you do need is the shape it produces. At low [S] most active sites are empty, so adding substrate raises the rate almost in proportion. At high [S] nearly every active site is busy, so extra substrate adds very little and the curve flattens toward .
- Competitive inhibitor: resembles the substrate and binds the active site. Substrate and inhibitor compete for it, so a high enough [S] outcompetes the inhibitor. Vmax is unchanged, but more substrate is needed to reach it, so Km rises.
- Noncompetitive inhibitor: binds an allosteric site, away from the active site, and changes the enzyme's shape so it works less well. Adding substrate cannot undo that, so Vmax falls. In this simple model Km stays the same.
Worked example
Problem: A student measures an enzyme's rate with and without a drug. Without the drug: rate 50.0 at [S] = 20 and 90.0 at [S] = 180. With the drug: rate 33.3 at [S] = 20 and 81.8 at [S] = 180. Is the drug a competitive or a noncompetitive inhibitor?
Step 1: Compare at low [S]. . The drug cuts the rate by about a third.
Step 2: Compare at high [S]. . The drug now cuts the rate by less than a tenth.
Step 3: Interpret. The inhibition weakens as [S] rises, so substrate is outcompeting the drug for the active site. That is competitive inhibition. A noncompetitive inhibitor would remove the same fraction of activity at every [S], because extra substrate cannot reverse the shape change.
Check it in the simulator: choose Competitive and set [S] to 20; the rate reads 33.3. Move to 180 and it reads 81.8, close to the faded uninhibited curve. Now choose Noncompetitive. At [S] = 20 the rate is 25.0 and at 180 it is 45.0, exactly half the uninhibited values of 50.0 and 90.0 at both concentrations.
Common mistakes on the AP exam
- Saying more substrate always speeds the reaction up. Once the enzyme is saturated, adding substrate barely changes the rate. To go faster you need more enzyme.
- Saying a competitive inhibitor lowers Vmax. It does not. At very high [S] the rate still approaches the same maximum.
- Saying a noncompetitive inhibitor binds the active site. It binds elsewhere (an allosteric site) and changes the enzyme's shape.
- Saying the enzyme is used up. An enzyme is not consumed by the reaction. A plateau means all active sites are busy, not that the enzyme has run out.
- Mixing up inhibition and denaturation. Extreme pH or temperature can denature an enzyme by disrupting its structure, which is not the same as an inhibitor binding to it.
When the AP exam uses this
Enzyme inhibition belongs to Unit 3 (Cellular Energetics), topic 3.3, Environmental Impacts on Enzyme Function, along with the effects of temperature and pH, which this simulator does not model. Expect to interpret a rate-versus-[S] graph, identify an inhibitor type from data, and explain changes in terms of the active site and enzyme shape. Free-response questions may ask you to identify the variables in an enzyme experiment.