Driving Transamination Irreversible by Decomposing Byproduct α-Ketoglutarate into Ethylene Using Ethylene-Forming Enzyme

Identification of a novel thermostable transaminase and its application in L-phosphinothricin biosynthesis

Transaminase (TA) is a crucial biocatalyst for enantioselective production of the herbicide L-phosphinothricin (L-PPT). The use of enzymatic cascades has been shown to effectively overcome the unfavorable thermodynamic equilibrium of TA-catalyzed transamination reaction, also increasing demand for TA stability. In this work, a novel thermostable transaminase (PtTA) from Pseudomonas thermotolerans was mined and characterized. The PtTA showed a high specific activity (28.63 U/mg) towards 2-oxo-4-[(hydroxy)(methyl)phosphinoyl]butyric acid (PPO), with excellent thermostability and substrate tolerance. Two cascade systems driven by PtTA were developed for L-PPT biosynthesis, including asymmetric synthesis of L-PPT from PPO and deracemization of D, L-PPT. For the asymmetric synthesis of L-PPT from PPO, a three-enzyme cascade was constructed as a recombinant Escherichia coli (E. coli G), by co-expressing PtTA, glutamate dehydrogenase (GluDH) and D-glucose dehydrogenase (GDH). Complete conversion of 400 mM PPO was achieved using only 40 mM amino donor L-glutamate. Furthermore, by coupling D-amino acid aminotransferase (Ym DAAT) from Bacillus sp. YM-1 and PtTA, a two-transaminase cascade was developed for the one-pot deracemization of D, L-PPT. Under the highest reported substrate concentration (800 mM D, L-PPT), a 90.43% L-PPT yield was realized. The superior catalytic performance of the PtTA-driven cascade demonstrated that the thermodynamic limitation was overcome, highlighting its application prospect for L-PPT biosynthesis. KEY POINTS: • A novel thermostable transaminase was mined for L-phosphinothricin biosynthesis. • The asymmetric synthesis of L-phosphinothricin was achieved via a three-enzyme cascade. • Development of a two-transaminase cascade for D, L-phosphinothricin deracemization.

Enhanced catalytic activity of recombinant transaminase by molecular modification to improve L-phosphinothricin production

Transaminases catalyze the transfer of an amino group from a donor to a keto group of an acceptor substrate and are applicable to the asymmetric synthesis of herbicide L-phosphinothricin (L-PPT). Here, the important residue sites (C390, I22, V52, R141, Y138 and D239) of transaminase from Salmonella enterica (SeTA) were modified at the adjacency of the substrate-binding pocket to improve the enzyme activity. Among the constructed mutant library, the SeTA-Y138F mutant displayed higher activity than the wild-type enzyme. Compared to the wild-type, SeTA-Y138F showed improved catalytic efficiency with a 4.36-fold increase. The K_m and k_cat of SeTA -Y138F toward 4-(hydroxy(methyl) phosphoryl)-2-oxobutanoic acid (PPO) were 26.39 mM and 34.28 s^-1, respectively. Subsequently, the three-enzyme co-expression system of E. coli BL21 (DE3)/pACYCDuet-SeTA-Y138F/pETDuet-AlaDH-BsGDH was developed by combining a alanine dehydrogenase (AlaDH) to recycle the byproduct of amino donor, a glucose dehydrogenase (BsGDH) for cofactor recycling. Under the optimized conditions, an excellent L-PPT yield of 90.8% was achieved by the whole-cell biotransformation with 500 mM PPO. It exhibited the tri-enzymatic coupling system was potential for effective production of target L-PPT.

Effects of His-tag on Catalytic Activity and Enantioselectivity of Recombinant Transaminases

Recombinant proteins were often expressed with His-tag to simplify the purification process. Among them, transaminase was mostly expressed with fusion tags and widely used in the production of numerous amino moieties. However, the existence of the His-tag has been reported to affect various properties of different recombinant enzymes, while the effect on transaminase was rarely studied. In this paper, we investigated the effect of His-tag on transaminase based on the various activities of 4-aminobutyrate-2-oxoglutarate transaminase (GabT) when it was expressed in vector pETDuet-1. We found that His-tag did not affect the enantioselectivity, but decreased the catalytic activity to different extents according to its existence and location. Native GabT maintained the highest catalytic activity; GabT with C-terminal His-tag showed slightly lower activity than native GabT but about 2.2-fold higher than GabT with N-terminal His-tag. Besides, other fusion tags like T7-tag and S-tag inserted between N-His-tag and GabT can relieve the decreasing effect of His-tag on GabT activity. Furthermore, whole cell catalytic activity of several transaminases was improved by deleting the N-terminal His-tag. This study provided a strategy for the efficient expression of recombinant transaminase with improved catalytic activity and might attract attention to the effect of His-tag on other enzymatic properties.