Characteristics of l -threonine transaldolase for asymmetric synthesis of β-hydroxy-α-amino acids

• Published in: Catalysis science & technology 2019, Vol.9 (21), p.5943-5952
• Main Authors: Xu, Lian, Wang, Li-Chao, Xu, Xin-Qi, Lin, Juan
• Format: Article
• Language: English
• Subjects: Aldehydes; Amino acids; Benzaldehyde; Conversion; E coli; Optimization; Stereoselectivity; Substrates; Synthesis
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/C9CY01608B

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