L-Aspartate 4-decarboxylase (Asd) catalyzes mainly the beta-decarboxylation of aspartate ahd also transamination with alpha-keto acids. To investigate residues that are critical in directing the reaction pathway, seven point mutations were designed based on the differences between Asd and arniontransferases in conservative amino acid residues. All mutant Asds were purified and characterized. The F204W mutant enhanced aminotransferase activity, and its ratio to beta-decarboxylase activity was 3.8-fold. Its K-m values for aspartate and alpha-ketoglutarate were 1.3 and 0.17 mM, respectively, representing a large increase in the binding affinity with substrates. The K347R mutation did not increase transaminase activity. The D360P mutation decreased transaminase activity and was more specific in catalyzing beta-decarboxylation reaction. This is the first study that successfully increased transaminase activity in Asd via site-directed mutagenesis. The modeled protein structure reveals how the residue may involve in reaction specificity, providing insights into comprehending the molecular evolution of this bifunctional enzyme. (c) 2007 Elsevier Inc. All rights reserved..