A recombinant L-threonine aldolase with high catalytic efficiency for the asymmetric synthesis of L-threo-phenylserine and L-threo-4-fluorophenylserine

To develop robust variants of L-threonine aldolases (L-TAs), potent catalysts for synthesizing asymmetric β-hydroxy-α-amino acids, it is necessary to identify critical residues beyond the known active site residues.

Through virtual screening, a neglected residue Asn305, was identified as critical for catalytic efficiency. Subsequent site-saturation mutagenesis led to a potent variant N305R which exhibited excellent conversions of 88%_conv (87%_de) and 80%_conv (94%_de) for the synthesis of L-threo-phenylserine and L-threo-4-fluorophenylserine respectively. This variant not only outperformed the template enzyme, but also represented a promising L-TA for synthesizing the two β-hydroxy-α-amino acids. It was suggested that Arg305 of the variant N305R generated strong cation-arene interaction and electrostatic force with the intermediates, leading to strengthened binding, enhanced L-threo favored orientation and wider entrance.

Our work not only provided an excellent variant N305R, but also suggested the crucial function of a neglected residue Asn305, which offered valuable experiences for other L-TA studies.