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Preparation of d-threonine by biocatalytic kinetic resolution B Enzymatic

Han, Sang-Woo, Shin, Jong-Shik
Journal of molecular catalysis 2015 v.122 pp. 227-232
ammonia, biocatalysis, cation exchange, cation exchange chromatography, deamination, drugs, enantiomers, stereochemistry, threonine
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)=950mM), d-threonine turned out to be a strong inhibitor (i.e., KIapp=41mM). In addition to the enzyme inhibitions by both enantiomers of threonine, cell lysis observed during small-scale kinetic resolutions of ⿥1M dl-threonine led us to carry out a preparative-scale reaction at 500mM racemic substrate. The preparative-scale kinetic resolution in a 50mL reaction mixture charged with 3g dl-threonine and 3400U whole cells was completed at 5h with >99% ee of d-threonine. Product isolation by a cation-exchange chromatography led to white solid of d-threonine (1.36g, 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 Ͽ-transaminase (Ͽ-TA) reaction where 2-oxobutyrate was converted to enantiopure homoalanine using isopropylamine as an amino donor. Addition of S- and R-selective Ͽ-TA to the pass-through solution led to 93.2 and 90.9% reaction yield within 12h with both >99% ee of the produced l- and d-homoalanine, respectively.