Biotransamination of Furan-Based Aldehydes with Isopropylamine: Enzyme Screening and pH Influence

Furan-based amines are highly valuable compounds which can be directly obtained via reductive amination from easily accessible furfural, 5-(hydroxymethyl)furfural (HMF) and 2,5-diformylfuran (DFF). Herein the biocatalytic amination of these carbonyl derivatives is disclosed using amine transaminases (ATAs) and isopropylamine (IPA) as amine donors. Among the different biocatalysts tested, the ones from Chromobacterium violaceum (Cv-TA), Arthrobacter citreus (ArS-TA), and variants from Arthrobacter sp. (ArRmut11-TA) and Vibrio fluvialis (Vf-mut-TA), afforded high levels of product formation (>80 %) at 100-200 mM aldehyde concentration. The transformations were studied in terms of enzyme and IPA loading. The pH influence was found as a key factor and attributed to the imine/aldehyde equilibrium that can arise from the high reactivity of the carbonyl substrates with a nucleophilic amine such as IPA.

Computer Modeling Explains the Structural Reasons for the Difference in Reactivity of Amine Transaminases Regarding Prochiral Methylketones

Amine transaminases (ATAs) are pyridoxal-5′-phosphate (PLP)-dependent enzymes that catalyze the transfer of an amino group from an amino donor to an aldehyde and/or ketone. In the past decade, the enzymatic reductive amination of prochiral ketones catalyzed by ATAs has attracted the attention of researchers, and more traditional chemical routes were replaced by enzymatic ones in industrial manufacturing. In the present work, the influence of the presence of an α,β-unsaturated system in a methylketone model substrate was investigated, using a set of five wild-type ATAs, the (R)-selective from Aspergillus terreus (Atr-TA) and Mycobacterium vanbaalenii (Mva-TA), the (S)-selective from Chromobacterium violaceum (Cvi-TA), Ruegeria pomeroyi (Rpo-TA), V. fluvialis (Vfl-TA) and an engineered variant of V. fluvialis (ATA-256 from Codexis). The high conversion rate (80 to 99%) and optical purity (78 to 99% ee) of both (R)- and (S)-ATAs for the substrate 1-phenyl-3-butanone, using isopropylamine (IPA) as an amino donor, were observed. However, the double bond in the α,β-position of 4-phenylbut-3-en-2-one dramatically reduced wild-type ATA reactivity, leading to conversions of <10% (without affecting the enantioselectivity). In contrast, the commercially engineered V. fluvialis variant, ATA-256, still enabled an 87% conversion, yielding a corresponding amine with >99% ee. Computational docking simulations showed the differences in orientation and intermolecular interactions in the active sites, providing insights to rationalize the observed experimental results.

Improving the Thermostability and Activity of Transaminase From Aspergillus terreus by Charge-Charge Interaction

Transaminases that promote the amination of ketones into amines are an emerging class of biocatalysts for preparing a series of drugs and their intermediates. One of the main limitations of (R)-selective amine transaminase from Aspergillus terreus (At-ATA) is its weak thermostability, with a half-life (t _1/2) of only 6.9 min at 40°C. To improve its thermostability, four important residue sites (E133, D224, E253, and E262) located on the surface of At-ATA were identified using the enzyme thermal stability system (ETSS). Subsequently, 13 mutants (E133A, E133H, E133K, E133R, E133Q, D224A, D224H, D224K, D224R, E253A, E253H, E253K, and E262A) were constructed by site-directed mutagenesis according to the principle of turning the residues into opposite charged ones. Among them, three substitutions, E133Q, D224K, and E253A, displayed higher thermal stability than the wild-type enzyme. Molecular dynamics simulations indicated that these three mutations limited the random vibration amplitude in the two α-helix regions of 130-135 and 148-158, thereby increasing the rigidity of the protein. Compared to the wild-type, the best mutant, D224K, showed improved thermostability with a 4.23-fold increase in t _1/2 at 40°C, and 6.08°C increase in T 50 10 . Exploring the three-dimensional structure of D224K at the atomic level, three strong hydrogen bonds were added to form a special "claw structure" of the α-helix 8, and the residues located at 151-156 also stabilized the α-helix 9 by interacting with each other alternately.

A Single Mutation Increases the Thermostability and Activity of Aspergillus terreus Amine Transaminase

Enhancing the thermostability of (R)-selective amine transaminases (AT-ATA) will expand its application in the asymmetric synthesis of chiral amines. In this study, mutual information and coevolution networks of ATAs were analyzed by the Mutual Information Server to Infer Coevolution (MISTIC). Subsequently, the amino acids most likely to influence the stability and function of the protein were investigated by alanine scanning and saturation mutagenesis. Four stabilized mutants (L118T, L118A, L118I, and L118V) were successfully obtained. The best mutant, L118T, exhibited an improved thermal stability with a 3.7-fold enhancement in its half-life (t_1/2) at 40 °C and a 5.3 °C increase in T₅₀¹⁰ compared to the values for the wild-type protein. By the differential scanning fluorimetry (DSF) analysis, the best mutant, L118T, showed a melting temperature (T_m) of 46.4 °C, which corresponded to a 5.0 °C increase relative to the wild-type AT-ATA (41.4 °C). Furthermore, the most stable mutant L118T displayed the highest catalytic efficiency among the four stabilized mutants.

Improving thermostability of (R)-selective amine transaminase from Aspergillus terreus through introduction of disulfide bonds

To improve the thermostability of (R)-selective amine transaminase from Aspergillus terreus (AT-ATA), we used computer software Disulfide by Design and Modelling of Disulfide Bonds in Proteins to identify mutation sites where the disulfide bonds were most likely to form. We obtained three stabilized mutants (N25C-A28C, R131C-D134C, M150C-M280C) from seven candidates by site-directed mutagenesis. Compared to the wild type, the best two mutants N25C-A28C and M150C-M280C showed improved thermal stability with a 3.1- and 3.6-fold increase in half-life (t_1/2 ) at 40 °C and a 4.6 and 5.1 °C increase in T₅₀¹⁰ . In addition, the combination of mutant R131C-D134C and M150C-M280C displayed the largest shift in thermostability with a 4.6-fold increase in t_1/2 at 40 °C and a 5.5 °C increase in T₅₀¹⁰ . Molecular dynamics simulation indicated that mutations of N25C-A28C and M150C-M280C lowered the overall root mean square deviation for the overall residues at elevated temperature and consequently increased the protein rigidity. The stabilized mutation of R131C-D134C was in the region of high mobility and on the protein surface, and the disulfide bond constraints the flexibility of loop 121-136.

Amine Transaminase Engineering for Spatially Bulky Substrate Acceptance

Amine transaminase (ATA) catalyzing stereoselective amination of prochiral ketones is an attractive alternative to transition metal catalysis. As wild-type ATAs do not accept sterically hindered ketones, efforts to widen the substrate scope to more challenging targets are of general interest. We recently designed ATAs to accept aromatic and thus planar bulky amines, with a sequence-based motif that supports the identification of novel enzymes. However, these variants were not active against 2,2-dimethyl-1-phenyl-propan-1-one, which carries a bulky tert-butyl substituent adjacent to the carbonyl function. Here, we report a solution for this type of substrate. The evolved ATAs perform asymmetric synthesis of the respective R amine with high conversions by using either alanine or isopropylamine as amine donor.

Alteration of the Donor/Acceptor Spectrum of the (S)-Amine Transaminase from Vibrio fluvialis

To alter the amine donor/acceptor spectrum of an (S)-selective amine transaminase (ATA), a library based on the Vibrio fluvialis ATA targeting four residues close to the active site (L56, W57, R415 and L417) was created. A 3DM-derived alignment comprising fold class I pyridoxal-5′-phosphate (PLP)-dependent enzymes allowed identification of positions, which were assumed to determine substrate specificity. These positions were targeted for mutagenesis with a focused alphabet of hydrophobic amino acids to convert an amine:α-keto acid transferase into an amine:aldehyde transferase. Screening of 1200 variants revealed three hits, which showed a shifted amine donor/acceptor spectrum towards aliphatic aldehydes (mainly pentanal), as well as an altered pH profile. Interestingly, all three hits, although found independently, contained the same mutation R415L and additional W57F and L417V substitutions.

Structural and biochemical characterization of the dual substrate recognition of the (R)-selective amine transaminase from Aspergillus fumigatus

Chiral amines are important precursors for the pharmaceutical and fine-chemical industries. Because of this, the demand for enantiopure amines is currently increasing. Amine transaminases can produce a large spectrum of chiral amines in the (R)- or (S)-configuration, depending on their substrate scope and stereo-preference, by converting a prochiral ketone into the chiral amine while using alanine as the amine donor producing pyruvate as an α-keto acid product. In order to guide the protein engineering of transaminases to improve substrate specificity and enantioselectivity, we carried out a crystal structure analysis at 1.6 Å resolution of the (R)-amine transaminase from Aspergillus fumigatus with the bound inhibitor gabaculine. This revealed that Arg126 has an important role in the dual substrate recognition of this enzyme because mutating this residue to alanine reduced substantially the ability of the enzyme to use pyruvate as an amino acceptor.