Aminoacylase I from hog kidney: anion effects and the pH dependence of kinetic parameters

The hydrolysis of acetylamino acids by highly purified hog kidney aminoacylase I (N-acylamino acid amidohydrolase, EC 3.5.1.14) was investigated using flow injection analysis to determine reaction rates. We show that the distinctly bell-shaped pH versus activity profiles observed in previous studies do not reflect protonic equilibria in the enzyme, but were created by buffer effects. At low pH, anions such as phosphate, nitrate or chloride markedly increase Km. These effects are reversed at higher pH. In zwitterionic 'Good' buffers (Mes, Mops, and Bicine), maximal velocities are almost independent of pH between 6.5 and 9 for all substrates studied (Ac-LAla, Ac-LGlu, Ac-LMet, Ac-LPhe). Below pH 6.5, the catalytic constants decrease with pH, apparently due to the protonation of a carboxylate with a pKa of 5.5-6. The pH dependence of Km markedly varies among different substates. We conclude that the observed profiles all result from the dissociation of an active-site residue with a pKa of 8-8.5, which we tentatively identify as an active-site cysteine residue. A working model of aminoacylase catalysis is presented that accounts for most of the known facts.