Inactivation of penicillin acylase from Kluyvera citrophila by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline: a case of time-dependent non-covalent enzyme inhibition.

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Penicillin acylase (PA) from Kluyvera citrophila was inhibited by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), a specific carboxy-group-reactive reagent. Enzyme activity progressively decreased to a residual value depending on EEDQ concentration. Neither enzymic nor non-enzymic decomposition of EEDQ is concomitant with PA inactivation. Moreover, enzyme re-activation is achieved by chromatographic removal of EEDQ, pH increase or displacement of the reagent with penicillin G. It was then concluded that PA inactivation is due to an equilibrium reaction. The kinetics of enzyme inactivation was analysed by fitting data to theoretical equations derived in accordance with this mechanism. Corrections for re-activation during the enzyme assay were a necessary introduction. The pH-dependence of the rate constant for EEDQ hydrolysis either alone or in the presence of enzyme was studied by u.v. spectroscopy. It turned out to be coincident with the pH-dependence of the forward and reverse rate constants for the inactivation process. It is suggested that previous protonation of the EEDQ molecule is required for these reactions to occur. The thermodynamic values associated with the overall reaction showed little change. Finally it is proposed that the inactivation of PA by EEDQ proceeds through a two-step reaction. The initial and rapid reversible binding is followed by a slow, time-dependent, non-covalent, reversible inactivating step. The expected behaviour in the case of enzyme modification by covalent activation of carboxy residues is also reviewed.