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Mixed Inhibitor vs Noncompetitive Inhibitor
#1 Posted : Friday, May 21, 2021 6:52:10 PM
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On page 26 of the EK biochemistry book it says that: mixed inhibitors will increase or decrease the Km depending on whether they bind to the enzyme or E-S complex. Noncompetitive inhibitors are a special type of mixed yet the book says that Km is not affected. Could you please elaborate on the explanation the book is trying to give? I find it to be a bit contradictory. Thank you in advance.
#2 Posted : Sunday, May 23, 2021 2:40:49 AM
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Hi Abhinav,

Mixed inhibitors are known to bind at any site on the enzyme, besides the active site. They are known to preferentially bind on an enzyme without a substrate or bind on an enzyme-substrate complex. Whichever the inhibitor decides to bind to, the Vmax of the reaction would always decrease. Mixed inhibitors also cause a conformational change on the active site, which is guaranteed to either affect substrate binding.

Noncompetitive inhibitors are mixed inhibitors because they (1) bind to a site other than the active site, (2) bind to an enzyme alone or an E-S complex, and (3) decrease Vmax. However, unlike other mixed inhibitors, noncompetitive inhibitors don't have preferential binding and don't affect the substrate's affinity for enzyme's active site (i.e. still allows substrates to bind with same affinity, hence the unaffected Km). Pretty much, mixed inhibitors and noncompetitive inhibitors greatly subdue the conversion of E-S complex to product.

I feel like knowing that mix inhibition exists is good enough for the MCAT. However, don't stress too much about it. I suggest focusing your attention on competitive, uncompetitive, and noncompetitive inhibition, especially on how their Michaelis-Menten graph vs Lineweaver-Burke Plot would look like.

Hope these help!
#3 Posted : Thursday, May 27, 2021 5:32:33 PM
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Mixed inhibition is when the inhibitor binds to the enzyme at a location distinct from the substrate binding site. The binding of the inhibitor alters the KM and Vmax.

Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the activity of the enzyme and binds equally well to the enzyme whether or not it has already bound the substrate. In non-competitive inhibition, the Km does not change as Km is a measure of the affinity of the enzyme for its substrate and this can only be measured by an active enzyme. The fixed amount of inactive enzyme in non-competitive inhibition does not affect the Km.

I would try to not overcomplicate the topic. For your MCAT I would just understand the mechanism of action of each inhibition, and the effect on KM and VMAX.
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