Stereochemical course of the biosynthesis of 1-aminocyclopropane-1-carboxylic acid. I. Role of the asymmetric sulfonium pole and the alpha-amino acid center.

The substrate stereospecificity of 1- aminocyclopropane -1-carboxylic acid synthase, a pyridoxal phosphate-containing enzyme, from the pericarp tissue of Lycopersicon esculentum (tomatoes) was studied using the various stereoisomers of S-adenosylmethionine (AdoMet) at both the sulfonium pole and the amino acid center. The data indicate that only the naturally occurring isomer (-)Ado-L-Met acts as substrate (Km = 20 +/- 5 microM). Both (+/-)Ado-D-Met and (+)Ado-L-Met were inactive as substrates. The (+)Ado-L-Met (Ki = 15 +/- 5 microM) was found to be a potent inhibitor of ACC synthase whereas (+/-)Ado-D-Met (Ki = 70 +/- 20 microM) was less active as an inhibitor. This active isomer has the (S) configuration at both the sulfur and the alpha-carbon of the amino acid portion of AdoMet.