Threonine aldolases: perspectives in engineering and screening the enzymes with enhanced substrate and stereo specificities

Threonine aldolases have emerged as a powerful tool for asymmetric carbon-carbon bond formation. These enzymes catalyse the unnatural aldol condensation of different aldehydes and glycine to produce highly valuable beta-hydroxy-alpha-amino acids with complete stereocontrol at the alpha-carbon and moderate specificity at the beta-carbon. A range of microbial threonine aldolases has been recently recombinantly produced by several groups and their biochemical properties were characterized. Numerous studies have been conducted to improve the reaction protocols to enable higher conversions and investigate the substrate scope of enzymes. However, the application of threonine aldolases in organic synthesis is still limited due to often moderate yields and low diastereoselectivities obtained in the aldol reaction. This review briefly summarizes the screening techniques recently applied to discover novel threonine aldolases as well as enzyme engineering and mutagenesis studies which were accomplished to improve the catalytic activity and substrate specificity. Additionally, the results from new investigations on threonine aldolases including crystal structure determinations and structural-functional characterization are reviewed.