Preparation of d-threonine by biocatalytic kinetic resolution

D-Threonine is one of the important unnatural amino acids used as chiral building blocks in pharmaceutical drugs. Owing to the presence of two chiral centers, a synthetic protocol, either through chemocatalysis or biocatalysis, has not yet been available for one-step preparation of stereochemically pure d-threonine in terms of enantiomeric and diastereomeric excesses (i.e., both >99%). Here we demonstrate that facile production of d-threonine can be implemented using threonine deaminase (TD) via kinetic resolution of dl-threonine that can be readily prepared by conventional organic synthesis. TD catalyzes the dehydration/deamination of l-threonine, leading to generation of 2-oxobutyrate and ammonia. In contrast to mild substrate inhibition of the TD activity by l-threonine (i.e., apparent inhibition constant (KIapp) = 950 mM), d-threonine turned out to be a strong inhibitor (i.e., KIapp = 41 mM). In addition to the enzyme inhibitions by both enantiomers of threonine, cell lysis observed during small-scale kinetic resolutions of ≥1 M dl-threonine led us to carry out a preparative-scale reaction at 500 mM racemic substrate. The preparative-scale kinetic resolution in a 50 mL reaction mixture charged with 3 g dl-threonine and 3400 U whole cells was completed at 5 h with >99% ee of d-threonine. Product isolation by a cation-exchange chromatography led to white solid of d-threonine (1.36 g, 90.7% isolation yield). To explore whether our strategy could afford coproduction of another valuable unnatural amino acid, the pass-through solution from the cation-exchange column was further processed by a ω-transaminase (ω-TA) reaction where 2-oxobutyrate was converted to enantiopure homoalanine using isopropylamine as an amino donor. Addition of S- and R-selective ω-TA to the pass-through solution led to 93.2 and 90.9% reaction yield within 12 h with both >99% ee of the produced l- and d-homoalanine, respectively.