Competitive vs. Non-Competitive Inhibition

[agp]

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I'm reading The Princeton Review's biological science review and have a question regarding competitive and non-competitive inhibition.

The book says that competitive inhibitors can be overcome by adding more substrate and the rate of product formation can be equal to the rate when there's no inhibitors. This part I understand - if there are 20 units of substrate and 10 units of competitive inhibitors, only 10 units of substrate can react in a certain period of time. But when you add another 10 units of substrate, there are now 20 units of react-able substrate, so therefore the rate will become 20 units in the same period of time.

For noncompetitive inhibitors, shouldn't it be exactly the same as competitive inhibitors? For example, if there are 20 units of substrate and 10 units of NC inhibitor that binds 10 units of substrate at allosteric site, then you're only left with 10 units of substrate that can react. But when you add another 10 units of substrate to the original 20, you should then have 20 units of uninhibited substrates right? So, if you add enough substrate, the rate of formation should be the same with the rate when no inhibitors are present, right? The book says I'm wrong, but I don't see why...

 

[loveoforganic]

I'm not exactly sure what's going on with your number reasoning so I'll just explain it how I know it.

Competitive inhibitors are just compounds with an affinity for the same active site that the desired substrate binds to. Either the competitive inhibitor or the substrate has a chance to make the contact with the active site. When one of them binds, the other has a chance to knock it out.

Noncompetitive inhibitors bind to a site on the enzyme other than the active site. When they do so, they deform the enzyme, and thus the active site. This inhibits binding of the substrate to the active site.

 

[capn jazz]

noncompetitive inhibitors usually to an allosteric site on the ENZYME not the substrate. so adding more substrate doesn't help if the enzymes are altered by the NC inhibitors in such a way that they can no longer bind the substrate at the active site.

theoretically, if you had a huge excess of enzyme and a minimal amount of NC inhibitor, it could be overcome, but I think that's not really applicable to the MCAT.

 

[Omni]

I understand what you're trying to say with your numbers, but here's a better way to say it:
(This is simplifying the whole thing, but you'll get the idea)
If you have one enzyme with 10 molecules of substrates, there is a 100% chance that the substrate is bound to the enzyme
But, if we add 10 molecules of competitive inhibitor molecules, than the chance for the desired substrate to be on the enzyme is reduced to 50%. The time it will require for the 10 molecules of substrate to bind to the enzyme would be twice as long as it would be without the inhibitor molecule
And as for Non-competitive inhibitor, as organic said, it binds to different region of the enzyme to deactivate it. Its like a key that is locking up the enzyme

 

[agp]

noncompetitive inhibitors usually to an allosteric site on the ENZYME not the substrate.

That's what's causing the confusion. I thought competitors bind to the substrates instead of the enzymes that the substrates bind to.
Thanks! 👍