Mixed inhibition

[akimhaneul]

I am confused about mixed inhibition. It says in the kaplan book that it can bind to either enzyme only or enzyme substrate complex. But it also says that it alters the Km value depending on the preference of the inhibitor for the enzyme vs enzyme-substrate complex. If it binds to enzyme-substrate complex, it lowers Km value whereas if it binds to enzyme itself, it increases Km. Vmax decreases in both cases.

But why does Km matter when the enzyme will become inactive due to the inhibitor? Since the Vmax decreases, doesn't this mean that there will be less active enzymes available to react?

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[rhinzaz]

The way I think of Km is in relation to substrate affinity to the enzyme. Specifically, there's an inverse relationship between them. For example, an enzyme with a higher affinity for S will have a lower Km value, while an enzyme with a lower affinity for S will have a higher Km value. The important point to remember is that enzyme affinity for S does not necessarily mean it has to be catalytically active; an enzyme can still bind S without a reaction. An example of mixed inhibition is when inhibitor binds only E, lowering its affinity for S, therefore increasing Km. Another example is where the inhibitor binds only ES, increasing its affinity for the S, and therefore lowering Km. Yet another example would be noncompetitive inhibitor (special type of mixed inhibitor) that can bind E or ES, without affecting E affinity for S, therefore Km remains constant.