Four types of threonine aldolases: Similarities and differences in kinetics/thermodynamics

Four types of threonine aldolases (TAs) with different stereospecificities were tested on the aldol synthesis of phenylserine (PS) starting from benzaldehyde and glycine under kinetic and thermodynamic control. At short time of reaction the chosen enzymes show different stereoselectivities (l-syn, l-anti, d-syn) compared to the reaction at equilibrium where syn-PS is obtained as the major product (d.e. ∼20%) for all types of TAs. A new aspect of the catalytic mechanism involved and a sight on the relative energy barriers for the possible rate-determining steps were obtained, based on data from the 13C-label distribution between components at the conditions close to equilibrium.