Competitive vs non-competitive inhibition: what happens to Vmax and Km? I keep flipping them
Ananya Reddy11th Sample
17 Aug 2026 · 1 min read
I made a colour-coded table for enzyme inhibition (green for competitive, orange for the other one) and I still get the Km/Vmax part wrong every single time.
What I understood from NCERT: the enzyme has an active site, the substrate fits into it, and a competitive inhibitor looks so much like the substrate that it sits in the active site instead. The example given is malonate blocking succinic dehydrogenase, because malonate resembles succinate. NCERT also says Km is the substrate concentration at which the velocity is half of .
Where I'm stuck is the graph. My coaching sheet has two curves, one where stays the same and Km shifts right, and one where drops and Km stays put. It doesn't say which is which. I tried reasoning like this:
- inhibitor blocks active sites, so fewer enzyme molecules work, so must fall
but that reasoning would apply to both types, so obviously I'm missing something. Also, if I add a LOT more substrate in a competitive case, does the inhibition just disappear?
Please explain it in a way I can actually remember for a test. I have a unit test on Monday and this is the topic I'm most scared of.
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[+][–]Meera Patel11thSample· 2mo ago
Start with what Km means. It is the substrate concentration giving half of , so a bigger Km means the enzyme needs more substrate to get to half speed, i.e. it looks like lower affinity.
Competitive: the inhibitor and the substrate fight for the same active site. If you flood the system with substrate, the substrate wins most of the fights, so you can still reach the same . It just takes much more substrate to get there. So unchanged, apparent Km increases.
Non-competitive: the inhibitor binds somewhere other than the active site and switches off some enzyme molecules no matter how much substrate you add. Fewer working molecules means a lower . Km stays the same, because the ones that still work bind substrate just as well.
[+][–]Diya Menon11thSample· 2mo ago
ahh I always picture it as a parking lot! competitive = someone parks a scooter in the one spot where your car goes. non-competitive = the whole lot is half closed for repair. so for competitive the Vmax drops because the spots are blocked and Km stays the same?? that's how I remembered it anyway
[+][–]Nisha Verma12thSample· 2mo ago
Diya, careful, that's the exact mistake I made last year. In your parking picture the scooter can be pushed out of the spot by a lot of cars arriving. That's competitive: enough substrate and you eventually get the full Vmax, so Vmax is unchanged and only Km goes up.
The "half the lot is closed" one is non-competitive, where no amount of cars helps, so Vmax comes down.
Quick memory line I used: competitive = Compensate with more substrate, so Vmax comes back. Non-competitive can't be compensated, so Vmax stays low.
[+][–]Ananya Reddy11thSample· 2mo agoAuthor
OK the "compensate" line is actually going on the first page of my notebook, thank you!! so the plateau is the clue. I was literally trying to memorise Km numbers 😭
[+][–]Diya Menon11thSample· 2mo ago
oops, thanks nisha. fixing my parking lot diagram right now, the scooter gets towed 😂
[+][–]Siddharth RaoDropperSample· 2mo ago
Ananya, to answer your last question directly: yes. In competitive inhibition the effect is reduced by raising substrate concentration, and NCERT hints at this when it says the inhibitor competes with the substrate. That's why drugs designed as competitive inhibitors need to reach a good concentration relative to the natural substrate to work.
For the exam: if a question gives a graph and the curves merge at the top right (same plateau), it's competitive. If the plateau is lower, it's the other type. Don't try to memorise Km shifts, just look at the plateau first.
[+][–]Kavya NaikDropperSample· 2mo ago
One more thing people miss: the inhibitor effect in competitive inhibition is reversible because it just sits in the active site and can leave. Malonate-succinate is the NCERT example, so write that in the answer, examiners like it. Also don't confuse active site with the cofactor binding site, I've seen that trap in PYQ-style options.
