A partially purified protein showed three pH optimas. For the enzyme assay same substrate and coenzyme were used at all pH values. What can be the reason for it ?
It would be very unusual to have more than one pH optimum for a single active site enzyme. The most likely cause of your strange pH-activity profile is the presence of more than one enzyme - I do note that you are working with a partially purified protein preparation. Add another purification step, check the purity (by SDS PAGE) and see if the problem resolves itself.