Suppose that an enzyme uses a catalytic mechanism in which a functional group on the enzyme has to accept a proton from the intermediate formed in the first chemical step. The pKa of that basic functional group on the enzyme is 8.0, and is the only pKa apparent on a plot of kcat vs. pH for the activity of this enzyme. The kcat of the fully active form of the enzyme is 1 x 105 sec–1 .
What type of functional group might be responsible for the pH effect on kcat? Justify your answerbased on the “generic” pKa values of functional groups in peptides and proteins.