STABILIZATION OF ESCHERICHIA-COLI PENICILLIN-G ACYLASE BY POLYETHYLENE GLYCOLS AGAINST THERMAL INACTIVATION

The effects of five polyethylene glycol (PEG) compounds of different m olecular weight on the thermal stability of penicillin G acylase (PGA) obtained from a mutant of Escherichia coli ATCC 11105 have been inves tigated. The molecular weights of PEG compounds were 400, 4000, 6000, 10,000, and 15,000. The thermal inactivation mechanisms of both native and PEG-containing PGA were considered to obey first order inactivati on kinetics during prolonged heat treatments. Optimal concentrations o f PEGs at molecular weights of 400, 4000, 6000, 10,000, and 15,000 wer e found to be 250, 150, 150, 100, and 50 mM, respectively. The greates t enhancement of thermostability was observed with PEG 4000 and PEG 60 00, as a nearly 20-fold increase above 50 degrees C. PGA showed almost the same temperature activity profile and optimal temperature values both in the presence and absence of PEG. The addition of PEGs did not cause any change in the optimal temperature value of PGA, but the para meters V-m, K-m, the activation energy, and the k(cat) values of enzym e were markedly decreased because of the mixed inhibition by PEG compo unds. The type of inhibition was found to be hyperbolic uncompetitive.