Specificity and pH dependence for acylproline cleavage by prolidase

Catalytic pH dependence for the hydrolytic activity of the enzyme prolidase with a series of dipeptide substrates is found to be generally bell-shaped (kcat/Km) or simple sigmoidal (kcat). An enzymic residue with a pKa value of 6.6 is found to be critically involved in the catalytic mechanism, as is the substrate amino group. Significant catalysis at a pH of 6.6 is also observed for prolidase with (alkylthio)acetylprolines and with haloacetylprolines. A reverse-protonation state mechanism for substrate binding and activation is postulated, involving a chelative interaction of the aminoacylamide portion of substrate with a strongly Lewis-acidic active site metal ion.