Characterization of a Novel Methylaspartate Ammonia Lyase from E. coli O157:H7 for Efficient Asymmetric Synthesis of Unnatural Amino Acids

The asymmetric addition of ammonia to unsaturated acids using engineered methylaspartate ammonia lyase (MAL) is a particularly attractive and atom-economic method for the synthesis of unnatural amino acids. However, owing to insufficient enzyme gene mining of MALs, the catalytic performances of MALs have only been characterized in a few organisms. Herein, we describe a novel MAL from Escherichia coli (E. coli) O157:H7, whose gene was derived from a genome mining strategy. The enzyme (designated as El-MAL) has been successfully expressed in E. coli BL21(DE3) and isolated and purified to homogeneity by using 6 × polyhistidine tag. El-MAL existed as a dimer in solution, consisting of two identical subunits (ca. 45 kDa). Enzymatic properties indicated that the enzyme performed maximum activity in the presence of Mg2+ at pH 8.5 and 25 °C. El-MAL accepted fumarate, mesaconate, maleate, citraconic acid, and itaconic acid as substrates in the amination reaction. To the best of our knowledge, such catalytic activity toward citraconic acid and itaconic acid has not been reported previously. Therefore, this novel MAL displayed with high stereoselectivity in an asymmetric amination reaction for the synthesis of unnatural amino acids may become a promising biocatalyst in further exploitation.