Enzyme inhibition: types (competitive, noncompetitive, uncompetitive, mixed, covalent) & evidence
Enzyme inhibitors:
* In the presence of an inhibitor, the inherent “goodness” of an enzyme doesn’t change - it still actually has the same constants, KM, kcat (and thus vmax), etc. BUT what we measure may change - so we call what we measure the “apparent” KM (KMapp) & apparent Vmax (Vmaxapp)
* Irreversible covalent inhibitors
* Often “mechanism-based,” aka “suicide” inhibitors → take advantage of covalent catalysis mechanisms → enzyme attacks inhibitor just like it would attack a substrate, but the inhibitor has a poor leaving group, so gets stuck on, preventing the enzyme from regenerating
blog: http://bit.ly/enzymeinhibition
* There are 4 main types of reversible inhibitors:
* Competitive
* Binds E in active site
* Competes with S for binding site
* Resembles substrate/TS
* ↑ apparent KM (KMapp) but doesn’t affect apparent Vmax (Vmaxapp)
* Noncompetitive (form of mixed)
* Binds E & ES at allosteric site with equal affinity & prevents conversion from S to P
* Does not affect KMapp, but ↓ Vmaxapp
* Uncompetitive
* Binds ES (but NOT E) at allosteric site & prevents conversion from S to P
* ↓ KMapp & ↓ Vmaxapp
* Mixed
* Binds E & ES at allosteric site with different affinities & prevents conversion from S to P
* May ↑ or ↓ KMapp, & will ↓ Vmaxapp
* Competitive:
* Binds to active site
* Competes with substrate for binding to active site
* Binds E only (because its binding spot is already taken in ES!)
* Can be competed out
* Remember that each time the enzyme lets go of a substrate or an inhibitor, it then has a chance of picking up a substrate or an inhibitor
* The relative concentrations of the 2 will determine which it is more likely to bump into first
* If you have enough of the substrate it can “win”
* The relative affinities of the 2 will determine how likely it is to stick & stay stuck to what it bumps into
* You’ll need more substrate to compete out a stronger inhibitor
* ↑ KMapp but doesn’t affect Vmaxapp (you’ll just take more S to reach it)
* Typically look like substrate &/or TS
* Strongest inhibitors will look more like TS because that’s what the enzyme binds tightest
* Ki is a measure of inhibitor affinity - it’s basically the dissociation constant, Kd, for the inhibitor
* Lower Ki → stronger affinity
* Noncompetitive:
* Binds to an allosteric site (not active site, or at least not in the same spot as the substrate) & prevents conversion of substrate to product
* Typically will not resemble substrate/TS because it’s binding to a different spot
* Does not compete with substrate for binding, so cannot be competed out, but can be diluted out
* If there’s barely any around, the enzyme’s unlikely to find another copy to pick up if it lets go, so if you dilute it a ton, then let it reach equilibrium, there’d basically be none bound
* Binds E & ES with equal affinity
* Does not affect KMapp, but ↓ Vmaxapp
* substrate binding isn’t affected, but the conversion to product is so the apparent affinity won’t change, but the apparent rate of conversion will slow down since those bindings aren’t productive
* Memory tip: it’s noncompetitive because its binding is basically independent of S binding, so S binding isn’t affected by its presence
* Uncompetitive:
* Similarly to noncompetitive…
* it binds to an allosteric site & prevents conversion of substrate to product
* Typically will not resemble substrate/TS because it’s binding to a different spot
* Does not compete with substrate for binding, so cannot be competed out, but can be diluted out
* But as opposed to noncompetitive, it only binds ES (not E)
* ↓KMapp & ↓ Vmaxapp
* substrate binding is increased because binding to ES (forming ESI) draws away ES from the equilibrium binding equation E + S ⇆ ES, leading to a rightward shift (more S binding) thanks to the whole Le Chatlier’s principle thing
* Memory tip: it’s uncompetitive because it’s binding to basically the “opposite” thing as the substrate, and actually increasing the KMapp, as opposed to a competitive inhibitor, which raises it
* Mixed inhibition:
* Really similar to noncompetive, except that it has different affinities for E & ES
* Noncompetitive is really just an “idealized” form of mixed inhibition that isn’t very realistic
* Binds E & ES, but with different affinities
* Can ↓ or ↑ KMapp , but will always ↓ Vmaxapp
* Will ↑ KMapp if has higher affinity for E (more like competitive except it’s binding to a different site)
* Will ↓ KMapp if it has higher affinity for ES (more like uncompetitive)
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