Characteristic of L-threonine transaldolase for asymmetric synthesis of β-hydroxy-α-amino acids

L-threonine transaldoase (LTTA) was a putative serine hydroxymethyltransferase (SHMT) that could catalyze the trans-aldehyde reaction of L-threonine and aldehyde to produce L-threo-β-hydroxy-α-amino acids with excellent stereoselectivity. In present study, a L-threonine transaldolase from Pseudomonas sp. (PsLTTA) was mined and expressed in Escherichia coli BL21 (DE3). Substrate spectrum assay indicated that PsLTTA only consumed L-threonine as donor substrate and could accept a wide range of aromatic aldehydes as acceptor substrates. Among those substrates, PsLTTA could catalyze p-methylsulfonyl benzaldehyde and L-threonine to produce L-threo-p-methylsulfonylphenylserine with high conversion (74.4%) and high de value (79.9%). It was found that the conversion and stereoselectivity of PsLTTA were dramatically influenced by concentration of whole-cell, co-solvent and reaction temperature. By conditional optimization, L-threo-p-methylsulfonylphenylserine was obtained with 67.1% conversion and near perfect de value (94.5%), the highest stereoselectivity of L-threo-β-hydroxy-α-amino acids reported by enzymatic synthesis so far. Finally, synthesis of L-threo-p-methylsulfonylphenylserine in a 100 mL scare by a whole-cell biocatalyst was conducted. This was for the first time systemically reported that L-threonine transaldolase as a robust biocatalyst for preparation of β-hydroxy-α-amino acids which could provide new insights for β-hydroxy-α-amino acids synthesis.