An enzyme lowers activation energy by stabilising the transition state.
Activation energy The energy barrier a reaction must cross to proceed.
Step 1: Something to Notice
Watch first. The explanation comes later.
Two inhibited enzyme samples behave identically at low substrate. Flooding both with substrate rescues one completely and the other not at all.
What differs between the two inhibitors?
Step 2: Find Out
Isolate the inhibitor type and read Km and Vmax separately at each setting.
Step 1 — Predict
Does adding more substrate overcome an enzyme inhibitor?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
The curve flattens because sites fill
Vmax is the rate when every active site is occupied.
Km is the substrate giving half of Vmax
A low Km means the enzyme works well at low substrate concentration.
Km The substrate concentration at which an enzyme reaches half its maximum rate.
A competitive inhibitor raises Km
It occupies the active site, and enough substrate outcompetes it.
A non-competitive inhibitor lowers Vmax
It binds elsewhere and changes the shape, so no amount of substrate helps.
Km and Vmax say different things
Vmax is the maximum rate when the enzyme is saturated; Km is the substrate concentration giving half that rate, and it measures affinity. A low Km means the enzyme works well at low substrate.
Inhibitors distinguished by their effect
A competitive inhibitor raises Km and leaves Vmax unchanged, because more substrate outcompetes it. A non-competitive inhibitor lowers Vmax. The kinetics identify the mechanism.
Step 4: A Common Mistake
Lots of people think
“Adding more substrate always overcomes an enzyme inhibitor.”
Step 5: Why Enzymes Do Not Change Equilibrium
The same idea somewhere new.
A catalyst lowers the activation barrier in both directions equally, so it accelerates the forward and reverse reactions by the same factor and the equilibrium position is untouched. That is why an enzyme cannot make an unfavourable reaction happen — cells solve that by coupling it to ATP hydrolysis instead, which is a thermodynamic solution rather than a kinetic one. Students frequently merge the two ideas, and the test is simple: if a reaction will not go, an enzyme cannot help; if it goes too slowly, an enzyme is exactly the answer.
Step 6: Think It Through
Practice makes it stick.
Flood It With Substrate
Problem 1 of 2
Substrate rescues one inhibited sample completely and the other not at all. Why?
Ethanol as Treatment
Problem 2 of 2
Why is ethanol given for methanol poisoning?
Two Constants
1 of 5
What does an enzyme lower?
2 of 5
Why does the rate curve flatten?
3 of 5
What does a low Km indicate?
4 of 5
A competitive inhibitor changes which constant?
5 of 5
Can an enzyme make an unfavourable reaction happen?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Does adding more substrate always overcome an inhibitor?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain what Km and Vmax measure and how the two inhibitor types affect each.
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Enzymes lower activation energy without changing equilibrium.
- Vmax is the saturated rate; Km is the substrate concentration giving half of it.
- A competitive inhibitor raises Km and leaves Vmax alone.
- A non-competitive inhibitor lowers Vmax and cannot be outcompeted.