[Study of E. coli penicillin amidase. The pH-dependence of the enzymatic inactivation kinetics]

The pH-dependence of the inactivation rate constant of penicillin amidase at a temperature of 40 degrees C was studied. It was shown that in all cases the enzyme inactivation corresponded to the kinetics of the reaction of the 1st order. The pH-dependence profile was found to be bell-shaped, the effect of transfer from the highest to the lowest values of the inactivation rate constants increasing more than 100 times. On the basis of the data obtained and published earlier it was concluded that the enzyme inactivation proceeded in accordance with the scheme in which out of 3 equilibrium ionic forms of penicillin amidase, i.e. "acid", "neutral" and "alkaline" the neutral form of the active enzyme was most stable. Kinetic analysis of the scheme was carried out and it was shown that the dependence found was in accordance with the theoretical curve in which the pK values of the ionogenic groups controlling the interconvertions between the penicillin amidase forms were equal to 2.4 and 10.1 at a temperature of 40 degrees C. The value of the inactivation rate constant of the "acid" or "alkaline" form was equal to 5.95 min-1, while the "neutral" form of the enzyme was characterized by the inactivation rate constant equal to 5.1.10(-4) min-1. A mechanism for the enzyme inactivation was proposed. According to this mechanism, destruction of the salt bridge in the native structure of penicillin amidase resulted in production of extremely labile forms of the enzyme as compared to the native form.