Can someone explain noncompetitive inhibition not changing Km?

[anbuitachi]

I see how they increase Vmax but I'm not too clear on why they don't change Km. Is it because substrate can still bind to enzymes that are inhibited so the affinity doesn't really change? But since those enzymes conformations have been changed they are now slower so Vmax lowers but Km stays same? Or did I understand this completely wrong.

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[Isoprop]

Pretty much. Km tells you the affinity of the enzyme is to the substrate. Vmax tells you how many active sites you have. So the enzyme still has the same affinity, but the number of active sites went down.

 

[jiayo]

Doesn't Km = 1/2(V max)?
Then shouldn't they follow the same trend?
I'm always confused on this, I hear so many different explanations.