EFFECT OF PH IN THE SYNTHESIS OF AMPICILLIN BY PENICILLIN ACYLASE

Recombinant Escherichia coli cells with high penicillin acylase (PA) a ctivity were immobilized by gel entrapment with agar. This biocatalyst was used to study the effect of pH on the synthesis of ampicillin fro m phenylglycine methylester (PGME) and 6-aminopenicillanic acid (6-APA ). The parallel hydrolysis reactions of PGME and ampicillin were also studied. A selective inhibition of the hydrolysis of the ester was pos sible by controlling the pH at 6.0. At such conditions, and using 6-AP A solutions ranging from 50-200 mM, a 75% conversion to ampicillin was obtained. This yield was higher than obtained with other strategies. The reaction kinetics was described by a second-order model for ampici llin synthesis with experimentally determined Michaelis-Menten constan ts of 27 and 25 mM for 6-APA nd PGME, respectively. In addition, ampic illin and PGME were hydrolyzed by the enzyme following Michaelis-Mente n kinetics with K-m values of 40.5 and 30 mM, respectively. A good cor relation was found between experimental results of synthesis reactions and the kinetic model derived from initial rate experiments with only slight deviations at high substrate concentrations. This is the first report where the specific effect of pH on the synthesis of ampicillin was studied in detail. It is shown that, by controlling the pH, it is possible to inhibit the lateral undesirable reactions increasing the yield of the main reaction.