THE PRESENCE OF METHANOL EXERTS A STRONG AND COMPLEX MODULATION OF THE SYNTHESIS OF DIFFERENT ANTIBIOTICS BY IMMOBILIZED PENICILLIN-G ACYLASE

The yields in the synthesis of antibiotics catalyzed by Penicillin G a cylase under very mild experimental conditions by using esters of side chains as acylating agents depend on the ratio between synthetase and hydrolase activities of penicillin G acylase. The presence of low con centrations of methanol in the reaction medium exerts complex and inte resting changes in the catalytic properties of immobilized penicillin G acylase. When phenylglycine methyl ester is used as acylating agent, the presence of methanol has a double effect. It increased the ratio synthesis/hydrolysis of antibiotic, but decreased the ratio between sy nthesis of antibiotic and direct hydrolysis of the acyl donor ester. I n this way, synthetic yields are increased when using an excess of acy lating substrate (e.g., from 63% and 73%) because the key point in thi s reaction is to decrease the hydrolysis of the already synthesized an tibiotic. When using an excess of antibiotics nucleus, the hydrolysis of antibiotic is strongly inhibited and then the ratio synthesis of an tibiotic/hydrolysis of the acyl donor is the key point and the yields suffered a decrease (e.g., from 90 to 60%). On the contrary, when usin g mandelic acid methyl ester as acylating agent, methanol increases bo th ratios (hydrolysis/synthesis of the antibiotic and synthesis of ant ibiotic/hydrolysis of the acyl donor); thus, the yields increased by a lmost a twofold factor (e.g., from 38 to 70%). Finally, the contradict ory effects observed in the presence of methanol can be modulated by u sing different enzyme derivatives. In fact, the effect of methanol on the Penicillin G acylase structure could be cleared out by increasing the stability of the enzyme derivative or by ''fixing'' the enzyme con formation in the presence of the organic solvent during the immobiliza tion process. (C) 1998 Elsevier Science Inc.