Preparation of d -threonine by biocatalytic kinetic resolution

Abstract

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 (K app I ) = 950 mM), d-threonine turned out to be a strong inhibitor (i.e., K app I = 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 -transaminase (-TA) reaction where 2-oxobutyrate was converted to enantiopure homoalanine using isopropylamine as an amino donor. Addition of S-and R-selective -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.

Discovery, Engineering, and Synthetic Application of Transaminase Biocatalysts

“…anthropi and the ( R )-TA from Arthrobacter sp. KNK168 , Park et al , synthesized a series of α-amino acids in high conversions and excellent ees from the corresponding α-ketoacids . Furthermore, l - and d -homoalanine were accessed from the readily available l -threonine via a threonine deaminase/ω-transaminase cascade, on a 100 mM scale, conversions of 90% and >99.9% ee being achieved after 90 min (Scheme a).…”

Overview on Multienzymatic Cascades for the Production of Non-canonical α-Amino Acids

“…A similar system allowed for concomitant production of D-Thr and D-or L-ABA starting from racemic Thr (>99% e.e. ; Han , and Shin, 2015). An alternative approach for the production of D-or L-ABA consisted in coupling an L-methioninase with a D-TA from Bacillus sp or an L-TA from E. coli, using methionine as substrate (Silva et al, 2019).…”

A facile method to determine intrinsic kinetic parameters of ω-transaminase displaying substrate inhibition

“…Analysis of (S)-D1 was carried out using a Crownpak CR(−) column (Daicel Co., Osaka, Japan) under isocratic elution of water (pH adjusted to 2 by perchloric acid) at 0.8 mL/min with a UV detector tuned at 200 nm. L-alanine was analyzed after derivatization with a Marfey's reagent as previously described [26]. A Sunfire C18 column (Waters Co.) was used with isocratic elution of 45% (v/v) methanol and 55% (v/v) water, each containing 0.1% (v/v) trifluoroacetic acid, at 0.7 mL/min.…”