Study of E. coli penicillin amidase. The pH dependence of the equilibrium constant of the enzymatic hydrolysis of benzylpenicillin]

The equilibrium constant for penicillin amidase-catalyzed hydrolysis of benzylpenicillin(Keg =3.00 +/- 0.24 x 10(-3) M at pH 5.0) and the ionization constants for phenylacetic acid (PAA) and the amino groups of 6-aminopenicillanic acid (6-APA) were determined (4.20 and 4.60 under conditions of the kinetic experiments respectively). The experimental data at pH 6.0 satisfactorily correlated with the theoretical pH-dependence for Keg constructed according to the hypothesis that benzylpenicillin synthesis has a thermodynamic optimum at pH 4.4 equal to a half-sum of the pK values for the carboxylic and amino groups of the PAA and 6-APA respectively.