# Impact of pH? Change in Binding Affinity of protein-ligand complex based on pH and Kd?

When the $$\mathrm{pH}$$ was $$5.0$$, the temperature was $$\pu{25 °C}$$,the $$K_\mathrm{D}$$ was $$\pu{5 μM}$$, and [L] was equal to $$K_D$$, the protein(s) were half bound.

Question considers half bound condition where $$[L]= 𝐾_𝐷$$

If $$K_\mathrm{D} = \pu{20 μM}$$ and $$\mathrm{pH} = 6.5$$, what is the impact on binding affinity?

I understand that $$K_\mathrm{D}$$ is the inverse of $$K_\mathrm{a}$$ and that $$K_\mathrm{D}$$ describes the degree of dissociation at equilibrium for the reaction, thus a low $$K_\mathrm{D}$$ corresponds to a high binding affinity complex.

I don't understand how to account for $$\mathrm{pH}$$ change though.

• It would be nice to define what RL and $K_\mathrm{D}$ are. – andselisk Apr 30 '19 at 12:48
• Whoops. Question considers half bound condition where [L]= $K_D$ – ThermoRestart Apr 30 '19 at 12:52
• Since you are not given (or you don't provide) more information about how pH affects the protein, other than Kd, I would assume it's not really important. The point would be that the affinity goes down at the higher pH. – Buck Thorn Apr 30 '19 at 15:55
• @NightWriter - Yes, that was all the information I was given. But don't some proteins do better @ higher pH than lower pH? – ThermoRestart Apr 30 '19 at 15:57
• Sure, but this is presumably just an exercise for you to reveal that you understand that lower Kd means higher affiinity and vice-versa... – Buck Thorn Apr 30 '19 at 16:35