Stereochemical Course of Biotin-Independent Malonate Decarboxylase Catalysis

Malonate decarboxylases, which catalyze the conversion of malonate to acetate, can be classified into biotin-dependent and biotin-independent enzymes. In order to reveal the stereochemical course of the reactions catalyzed by the biotin-independent enzymes from Acinetobacter calcoaceticus and Pseudomonas fluorescens, a chiral substrate, malonate carrying 13C in one carboxyl group and 3H at one of the methylene positions, was prepared and used in the reactions catalyzed by these two enzymes. The decarboxylation of (R)-[1-13C1, 2-3H]malonate in 2H2O gave a pseudo-racemate of chiral acetate which was converted via acetyl-CoA into malate with malate synthase. From the relative proportions of the isotopomers of malate present, determined by 3H NMR analysis, it was concluded that in the decarboxylation of malonate by these two biotin-independent enzymes COOH is replaced by H with retention of configuration. The same stereochemical outcome had been previously observed for the reaction catalyzed by the biotin-dependent malonate decarboxylase from Malonomonas rubra (J. Micklefield et al. J. Am. Chem. Soc. 117, 1153–1154, 1995).