Noncompetitive Inhibitors (Information Interference)

[rooksai]

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In the Princeton Review, it states that:

Noncompetitive Inhibitors bind at an allosteric site and not at the active site. Hence, noncompetitive inhibition does diminish Vmax. But typically this does not alter Km because the substrate can still bind to the active site, but the inhibitor prevents the catalytic activity of the enzyme.

Doesn't the non competitive inhibitor that binds to the allosteric site change the shape of the active site? How can the substrate still bind to the active site? Wouldn't that change the Km?

 

[MedPR]

Km depends only on available enzymes.

 

[Lunasly]

Read my last post in that topic.

http://forums.studentdoctor.net/showthread.php?t=915936

 

[MedPR]

Remember that Km is the substrate concentration at which v=1/2vmax. It is dependent only on the affinity of substrate for enzyme.

So say you need 50 substrates to reach 1/2vmax when you have 100 available enzymes. Throw in a non-comp inhibitor and now you only have 80 available enzymes. vmax is lower, so you have a lower 1/2vmax, but because of the relationship (michaelis menten equation) the km remains the same.

 

[rooksai]

My question isn't answered. The princeton review book seems to assume that anything that interacts with the allosteric site won't affect the affinity of the substrate to the enzyme. To me it seems that the princeton book is saying that the noncompetitive inhibitor WILL NOT change the shape of the active site, which is contrary to what I've learned. Hence, learning interference.

 

[MedPR]

My question isn't answered. The princeton review book seems to assume that anything that interacts with the allosteric site won't affect the affinity of the substrate to the enzyme. To me it seems that the princeton book is saying that the noncompetitive inhibitor WILL NOT change the shape of the active site, which is contrary to what I've learned. Hence, learning interference.

As I've said before, the number of enzymes available is all Km cares about.

Think of the non-competitive inhibitor as disabling some enzymes, but not all of them. The still working enzymes have the same affinity for the substrate as before any non-competitive inhibitors were added, so the Km does not change.

Your question is answered, you just aren't comprehending it.

TPR is not saying the inhibitor will not change the shape of the active site. It is saying that if enzyme 1 is changed, but enzyme 2 is not, enzyme 2 is still working to catalyze the reaction at the same rate it always was. Km does not change in that case.