An Efficient Route to Both Enantiomers of allo-Threonine by Simultaneous Amino Acid Racemase-Catalyzed Isomerization of Threonine and Crystallization

We present an efficient method for the production of D- and L-allo-threonine (allo-Thr) with very high purity by enzymatic isomerization of L- or D-threonine (Thr) and simultaneous crystallization. Isomerization of Thr to allo-Thr is catalyzed by a purified amino acid racemase (AArac12996) from Pseudomonas putida NBRC12996, which can easily be obtained from a recombinant E. coli strain by secretion into the medium and subsequent anion exchange chromatography. Crystallization of D- and L-allo-Thr was performed in a repetitive batch mode over a period of up to 55 days at 30 °C. Total amounts of 30.8 g D-allo-Thr and 32.4 g L-allo-Thr were obtained with a very good diastereomeric excess of deD-allo>99.2% and deL-allo>98.4%, respectively, and in good yields (D-allo-Thr: 83%, L-allo-Thr: 79%). The enzyme’s remarkable high stability under process conditions resulted in enzyme specific yields of 2.56 g D-allo-Thr per mg AArac12996 and 1.62 g L-allo-Thr per mg AArac12996. In contrast to chemical multi-step syntheses of allo-Thr, our process consists of only one enzyme-catalyzed reaction step with simultaneous product crystallization. The process is performed under low energy consumption (30 °C, atmospheric pressure) in water and avoids the use and production of any toxic or harmful compounds. Recovery of the enantiomerically pure products is performed by simple filtration which reduces downstream processing significantly compared to chromatographic methods which are usually applied.