Question

The active site of lysozyme contains two amino acid residues essential for catalysis: Glu³⁵ and Asp⁵². The pKa values of the carboxyl side chains of these residues are 5.9 and 4.5, respectively. What is the ionization state (protonated or deprotonated) of each residue at pH 5.2, the pH optimum of lysozyme? How can the ionization states of these residues explain the pH-activity profile of lysozyme.

Answer 1

At a pH between the two pKa values (pH 5.2), the side-chain carboxyl group of Asp52, with the lower pKa(4.5), is mainly deprotonated (OCOOϪ), whereas Glu35, with the higher pKa(5.9; the stronger base), is protonated (OCOOH). At pH values below 5.2, Asp52becomes protonated and the activity decreases. Similarly, at pH values above 5.2, Glu35 becomes deprotonated and the activity also decreases. The pH-activity profile suggests that maximum catalytic activity occurs at a pH midway between the pKa values of the two acidicgroups, when Glu35 is protonated and Asp52 is deprotonated.