Alternate substrates and inhibitors of 1-aminocyclopropane-1-carboxylic acid synthase

A kinetic study of simultaneous suicide inactivation and irreversible inhibition of an enzyme. Application to 1-aminocyclopropane-1-carboxylate (ACC) synthase inactivation by its substrate S-adenosylmethionine

This paper deals with the development of an experimental method for the kinetic study of the inactivation of an enzyme by a racemic mixture of an inhibitor, whose isomers operate as suicide substrate and irreversible inhibitor respectively. The ratio between the isomer concentration in the biological or commercial source must be determined, but no separation of them is required. The method involves a kinetic analysis and an experimental design that enables the affinity (1/Km), rate of catalysis (kcat), rate of inactivation (lambda max), efficiency of catalysis (kcat/Km) and efficiency of inactivation (lambda max/Km) to be determined. The method has been applied to the kinetic characterization of the inactivation of 1-aminocyclopropane-1-carboxylate (ACC) synthase from tomato fruits by its substrate, S-adenosylmethionine (AdoMet). The ratio between AdoMet isomers with respect to its sulfonium centre, namely (-)-AdoMet and (+)-AdoMet, present in the commercial sample used, has been determined by 1H nuclear magnetic resonance.