Enzyme optimal PH question

[Odi]

I have a decent grasp of this concept but I just need to know "why" this is so.

I understand that the optimal operating pH of an enzyme depends on the pKa of it's side chains.

So let's say enzyme X works best at pH of 4. This means its side chain should have a pKa of 4.

Since the pKa is 4... that means at a pH of 4 we have an equilibrium state between protonated and deprotonated forms of the side chain. Why does this equilibrium state enhance an enzymes function to its maximum capacity???

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[blackmarble]

because outside of the optimum pH the active site isn't in its most favored conformation to catalyze the reaction it is specific to due to protonated/deprotonated side chines. also enzymes most often have multiple subunits which are held together via covalent/disulfide/hydrophobic interactions which are also affected outside its optimum pH.

 

[Odi]

I understand that.

I'm asking why is the pKa of the side chain the optimal pH?

Why can't you have a side chain pKa of 9 and an optimal pH of 4?

I think this is one of those things I should just accept. pKa = optimal pH. Probably requires an in depth answer with unnecessary knowledge to know.

 

[kraskadva]

Because if it's (for example) acid-base catalysis that happens in the active site, then the side chain that facilitates that needs to be able to gain and lose that spare proton in order to make the reaction happen.
If the pH is way below the pKa, then the side chain will remain protonated and no reaction will happen.
If the pH is way above the pKa, then the side chain has no proton to catalyze with.
If pH=pKa, then that proton is readily exchangeable, allowing the reaction to proceed optimally.

 

[Odi]

Thanks!