Continuous Flow Biocatalytic Reductive Amination by Co-Entrapping Dehydrogenases with Agarose Gel in a 3D-Printed Mould Reactor

Herein, we show how the merge of biocatalysis with flow chemistry aided by 3D-printing technologies can facilitate organic synthesis. This concept was exemplified for the reductive amination of benzaldehyde catalysed by co-immobilised amine dehydrogenase and formate dehydrogenase in a continuous flow micro-reactor. For this purpose, we investigated enzyme co-immobilisation by covalent binding, or ion-affinity binding, or entrapment. Entrapment in an agarose hydrogel turned out to be the most promising solution for this biocatalytic reaction. Therefore, we developed a scalable and customisable approach whereby an agarose hydrogel containing the co-entrapped dehydrogenases was cast in a 3D-printed mould. The reactor was applied to the reductive amination of benzaldehyde in continuous flow over 120 h and afforded 47 % analytical yield and a space-time yield of 7.4 g L day^-1 using 0.03 mol% biocatalysts loading. This work also exemplifies how rapid prototyping of enzymatic reactions in flow can be achieved through 3D-printing technology.

Generation of Oxidoreductases with Dual Alcohol Dehydrogenase and Amine Dehydrogenase Activity

The l-lysine-ϵ-dehydrogenase (LysEDH) from Geobacillus stearothermophilus naturally catalyzes the oxidative deamination of the ϵ-amino group of l-lysine. We previously engineered this enzyme to create amine dehydrogenase (AmDH) variants that possess a new hydrophobic cavity in their active site such that aromatic ketones can bind and be converted into α-chiral amines with excellent enantioselectivity. We also recently observed that LysEDH was capable of reducing aromatic aldehydes into primary alcohols. Herein, we harnessed the promiscuous alcohol dehydrogenase (ADH) activity of LysEDH to create new variants that exhibited enhanced catalytic activity for the reduction of substituted benzaldehydes and arylaliphatic aldehydes to primary alcohols. Notably, these novel engineered dehydrogenases also catalyzed the reductive amination of a variety of aldehydes and ketones with excellent enantioselectivity, thus exhibiting a dual AmDH/ADH activity. We envisioned that the catalytic bi-functionality of these enzymes could be applied for the direct conversion of alcohols into amines. As a proof-of-principle, we performed an unprecedented one-pot "hydrogen-borrowing" cascade to convert benzyl alcohol to benzylamine using a single enzyme. Conducting the same biocatalytic cascade in the presence of cofactor recycling enzymes (i.e., NADH-oxidase and formate dehydrogenase) increased the reaction yields. In summary, this work provides the first examples of enzymes showing "alcohol aminase" activity.

Kinetic Resolution of Racemic Primary Amines Using Geobacillus stearothermophilus Amine Dehydrogenase Variant

A NADH-dependent engineered amine dehydrogenase from Geobacillus stearothermophilus (LE-AmDH-v1) was applied together with a NADH-oxidase from Streptococcus mutans (NOx) for the kinetic resolution of pharmaceutically relevant racemic α-chiral primary amines. The reaction conditions (e. g., pH, temperature, type of buffer) were optimised to yield S-configured amines with up to >99 % ee.

Generation of amine dehydrogenases with increased catalytic performance and substrate scope from ε-deaminating L-Lysine dehydrogenase

Amine dehydrogenases (AmDHs) catalyse the conversion of ketones into enantiomerically pure amines at the sole expense of ammonia and hydride source. Guided by structural information from computational models, we create AmDHs that can convert pharmaceutically relevant aromatic ketones with conversions up to quantitative and perfect chemical and optical purities. These AmDHs are created from an unconventional enzyme scaffold that apparently does not operate any asymmetric transformation in its natural reaction. Additionally, the best variant (LE-AmDH-v1) displays a unique substrate-dependent switch of enantioselectivity, affording S- or R-configured amine products with up to >99.9% enantiomeric excess. These findings are explained by in silico studies. LE-AmDH-v1 is highly thermostable (Tm of 69 °C), retains almost entirely its catalytic activity upon incubation up to 50 °C for several days, and operates preferentially at 50 °C and pH 9.0. This study also demonstrates that product inhibition can be a critical factor in AmDH-catalysed reductive amination.

Mechanistic Insight into the Catalytic Promiscuity of Amine Dehydrogenases: Asymmetric Synthesis of Secondary and Primary Amines

Biocatalytic asymmetric amination of ketones, by using amine dehydrogenases (AmDHs) or transaminases, is an efficient method for the synthesis of α-chiral primary amines. A major challenge is to extend amination to the synthesis of secondary and tertiary amines. Herein, for the first time, it is shown that AmDHs are capable of accepting other amine donors, thus giving access to enantioenriched secondary amines with conversions up to 43 %. Surprisingly, in several cases, the promiscuous formation of enantiopure primary amines, along with the expected secondary amines, was observed. By conducting practical laboratory experiments and computational experiments, it is proposed that the promiscuous formation of primary amines along with secondary amines is due to an unprecedented nicotinamide (NAD)-dependent formal transamination catalysed by AmDHs. In nature, this type of mechanism is commonly performed by pyridoxal 5'-phosphate aminotransferase and not by dehydrogenases. Finally, a catalytic pathway that rationalises the promiscuous NAD-dependent formal transamination activity and explains the formation of the observed mixture of products is proposed. This work increases the understanding of the catalytic mechanism of NAD-dependent aminating enzymes, such as AmDHs, and will aid further research into the rational engineering of oxidoreductases for the synthesis of α-chiral secondary and tertiary amines.

A biocatalytic method for the chemoselective aerobic oxidation of aldehydes to carboxylic acids

Herein, we present a study on the oxidation of aldehydes to carboxylic acids using three recombinant aldehyde dehydrogenases (ALDHs). The ALDHs were used in purified form with a nicotinamide oxidase (NOx), which recycles the catalytic NAD+ at the expense of dioxygen (air at atmospheric pressure). The reaction was studied also with lyophilised whole cell as well as resting cell biocatalysts for more convenient practical application. The optimised biocatalytic oxidation runs in phosphate buffer at pH 8.5 and at 40 °C. From a set of sixty-one aliphatic, aryl-aliphatic, benzylic, hetero-aromatic and bicyclic aldehydes, fifty were converted with elevated yield (up to >99%). The exceptions were a few ortho-substituted benzaldehydes, bicyclic heteroaromatic aldehydes and 2-phenylpropanal. In all cases, the expected carboxylic acid was shown to be the only product (>99% chemoselectivity). Other oxidisable functionalities within the same molecule (e.g. hydroxyl, alkene, and heteroaromatic nitrogen or sulphur atoms) remained untouched. The reaction was scaled for the oxidation of 5-(hydroxymethyl)furfural (2 g), a bio-based starting material, to afford 5-(hydroxymethyl)furoic acid in 61% isolated yield. The new biocatalytic method avoids the use of toxic or unsafe oxidants, strong acids or bases, or undesired solvents. It shows applicability across a wide range of substrates, and retains perfect chemoselectivity. Alternative oxidisable groups were not converted, and other classical side-reactions (e.g. halogenation of unsaturated functionalities, Dakin-type oxidation) did not occur. In comparison to other established enzymatic methods such as the use of oxidases (where the concomitant oxidation of alcohols and aldehydes is common), ALDHs offer greatly improved selectivity.

Molecular characterization of pyrethroid resistance in the olive fruit fly Bactrocera oleae

Α reduction of pyrethroid efficacy has been recently recorded in Bactrocera oleae, the most destructive insect of olives. The resistance levels of field populations collected from Crete-Greece scaled up to 22-folds, compared to reference laboratory strains. Sequence analysis of the IIS4-IIS6 region of para sodium channel gene in a large number of resistant flies indicated that resistance may not be associated with target site mutations, in line with previous studies in other Tephritidae species. We analyzed the transcriptomic differences between two resistant populations versus an almost susceptible field population and two laboratory strains. A large number of genes was found to be significantly differentially transcribed across the pairwise comparisons. Interestingly, gene set analysis revealed that genes of the 'electron carrier activity' GO group were enriched in one specific comparison, which might suggest a P450-mediated resistance mechanism. The up-regulation of several transcripts encoding detoxification enzymes was qPCR validated, focusing on transcripts coding for P450s. Of note, the expression of contig00436 and contig02103, encoding CYP6 P450s, was significantly higher in all resistant populations, compared to susceptible ones. These results suggest that an increase in the amount of the CYP6 P450s might be an important mechanism of pyrethroid resistance in B. oleae.

Functional characterization of glutathione S-transferases associated with insecticide resistance in Tetranychus urticae

The two-spotted spider mite Tetranychus urticae is one of the most important agricultural pests world-wide. It is extremely polyphagous and develops resistance to acaricides. The overexpression of several glutathione S-transferases (GSTs) has been associated with insecticide resistance. Here, we functionally expressed and characterized three GSTs, two of the delta class (TuGSTd10, TuGSTd14) and one of the mu class (TuGSTm09), which had been previously associated with striking resistance phenotypes against abamectin and other acaricides/insecticides, by transcriptional studies. Functional analysis showed that all three GSTs were capable of catalyzing the conjugation of both 1-chloro-2,4 dinitrobenzene (CDNB) and 1,2-dichloro-4-nitrobenzene(DCNB) to glutathione (GSH), as well as exhibiting GSH-dependent peroxidase activity toward Cumene hydroperoxide (CumOOH). The steady-state kinetics of the T. urticae GSTs for the GSH/CDNB conjugation reaction were determined and compared with other GSTs. The interaction of the three recombinant proteins with several acaricides and insecticides was also investigated. TuGSTd14 showed the highest affinity toward abamectin and a competitive type of inhibition, which suggests that the insecticide may bind to the H-site of the enzyme. The three-dimensional structure of the TuGSTd14 was predicted based on X-ray structures of delta class GSTs using molecular modeling. Structural analysis was used to identify key structural characteristics and to provide insights into the substrate specificity and the catalytic mechanism of TuGSTd14.