Stereochemical course of the decarboxylation of 2-amino-2-methylmalonic acid by serine hydroxymethyltransferase

Abstract

2-Amino-2-methylmalonic acid has been shown to be a slow decarboxylation substrate for the enzyme serine hydroxymethyltransferase from both rabbit liver cytosol and E. coli. For both enzymes the amino acid product was (2R)-alanine. The enantiomers of 2-amino-2-methyl[1-¹³C]malonic acid have been synthesized and used to probe the stereochemical course and mechanism of the reaction. The pro-R carboxy group of 2-amino-2-methylmalonic acid was removed during the decarboxylation reaction catalysed by each enzyme and was replaced by a proton with retention of configuration at C-2. Similar results were also obtained with the H228N mutant of the E. coli enzyme. These results contrast earlier findings where it was suggested that 2-aminomalonic acid, a fast substrate for serine hydroxymethyltransferase, was decarboxylated non-stereospecifically.