Cells switch enzymes off as often as on. Drugs and poisons often work the same way. An inhibitor is a molecule that slows an enzyme, and where it binds decides how it behaves on the rate curve.
- Competitive inhibitor: resembles the substrate and binds the active site. Substrate and inhibitor compete for the same spot.
- Noncompetitive (allosteric) inhibitor: binds elsewhere, at an allosteric site, and changes the enzyme's shape so the active site works poorly.
Key idea
Enough substrate outcompetes a competitive inhibitor: $V_{max}$ is unchanged, but more substrate is needed to reach it (the apparent $K_m$ rises). A noncompetitive inhibitor lowers $V_{max}$, and no amount of substrate brings it back.
Why it works
Picture a crowd of substrate molecules. With a competitive inhibitor, each empty active site gets whichever molecule bumps into it first. Flood the solution with substrate and the inhibitor almost never wins. With a noncompetitive inhibitor, the inhibited enzymes are out of action whatever the substrate does; they are like workers who went home. The rest still work at full speed, so the curve has the same shape but a lower ceiling.
Worked examples
Worked example: Reading the curves
Without inhibitor, an enzyme levels off at 40 µmol/min. With inhibitor X, the rate is low at small [S] but reaches nearly 40 at very high [S]. With inhibitor Y, it levels off at 20. Name each type.
- X: the ceiling is unchanged; high substrate overcomes it. That is competitive.
- Y: the ceiling is halved and stays down. That is noncompetitive.
Answer: X is competitive; Y is noncompetitive.
Worked example: Feedback inhibition
In a pathway A → B → C → D, product D binds an allosteric site on the enzyme that turns A into B. What happens when D builds up?
- D binds the first enzyme and changes its shape.
- Less A becomes B, so less C and D are made.
- When D is used up, it lets go and the pathway restarts.
Answer: The pathway slows itself: feedback inhibition keeps D from piling up and saves resources.
Common mistake
Assuming every inhibitor binds the active site, or that more substrate overcomes any inhibitor. Only competitive inhibition can be overcome by substrate. Test it on a graph: if the curve with inhibitor reaches the same top, it is competitive; if its top is lower, it is noncompetitive.