What is the effect of changing the PH above/below the optimum PH?
what effect does it have on Rgroups in the active site?
I'll try to explain this without complicating things too much. But you have to know that the actual explanation goes much deeper.
A change in pH implies a change in the concentration of H+ ions. If the pH is too low (too acidic), there are too many H+ ions that form hydrogen bonds with the protein which affects the way it interacts with other chains.
If the pH is too low (too basic), there are too few H+ ions. This reduces the stability of the polypeptide, so some bonds will inevitably break.
Both cases will change the shape of the protein (hence denature it).