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

Abstract

The substrate stereospecificity of 1-aminocyclopropane-1-carboxylic acid synthase, a pyridoxal phosphate-containing enzyme, from the pericarp tissue of (tomatoes) was studied using the various stereoisomers of -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 [mu]M). Both (+/-)Ado-D-Met and (+)Ado-L-Met were inactive as substrates. The (+)Ado-L-Met (Ki = 15+/-5 [mu]M) was found to be a potent inhibitor of ACC synthase whereas (+/-)Ado-D-Met (Ki = 70+/-20 [mu]M) was less active as an inhibitor. This active isomer has the () configuration at both the sulfur and the [alpha]-carbon of the amino acid portion of AdoMet.