Telescopic one-pot condensation-hydroamination strategy for the synthesis of optically pure L-phenylalanines from benzaldehydes

Phenylalanine ammonia-lyases: combining protein engineering and natural diversity

In this study, rational design and saturation mutagenesis efforts for engineering phenylalanine ammonia-lyase from Petroselinum crispum (PcPAL) provided tailored PALs active towards challenging, highly valuable di-substituted substrates, such as the L-DOPA precursor 3,4-dimethoxy-L-phenylalanine or the 3-bromo-4-methoxy-phenylalanine. The rational design approach and saturation mutagenesis strategy unveiled identical PcPAL variants of improved activity, highlighting the limited mutational variety of the substrate specificity-modulator residues, L134, F137, I460 of PcPAL. Due to the restricted catalytic efficiency of the best performing L134A/I460V and F137V/I460V PcPAL variants, we imprinted these beneficial mutations to PALs of different origins. The variants of PALs from Arabidopsis thaliana (AtPAL) and Anabaena variabilis (AvPAL) showed higher catalytic efficiency than their PcPAL homologues. Further, the engineered PALs were also compared in terms of catalytic efficiency with a novel aromatic ammonia-lyase from Loktanella atrilutea (LaAAL), close relative of the metagenome-derived aromatic ammonia-lyase AL-11, reported recently to possess atypically high activity towards substrates with electron-donor aromatic substituents. Indeed, LaAAL outperformed the engineered Pc/At/AvPALs in the production of 3,4-dimethoxy-L-phenylalanine; however, in case of 3-bromo-4-methoxy derivatives it showed no activity, with computational results supporting the occurrence of steric hindrance. Transferring the unique array of selectivity modulator residues from LaAAL to the well-characterized PALs did not enhance their activity towards the targeted substrates. Moreover, applying the rational design strategy valid for these well-characterized PALs to LaAAL decreased its activity. These results suggest that distinct tailoring rationale is required for LaAAL/AL-11-like aromatic ammonia-lyases, which might represent a distinct PAL subclass, with natural reaction and substrate scope modified through evolutionary processes. KEY POINTS: • PAL-activity for challenging substrates generated by protein engineering • Rational/semi-rational protein engineering reveals constrained mutational variability • Engineered PALs are outperformed by novel ALs of distinct catalytic site signature.

Biosynthesis of Raffinose and Stachyose from Sucrose via an In Vitro Multienzyme System

Herein, we present a biocatalytic method to produce raffinose and stachyose using sucrose as the substrate. An in vitro multienzyme system was developed using five enzymes, namely, sucrose synthase (SUS), UDP-glucose 4-epimerase (GalE), galactinol synthase (GS), raffinose synthase (RS), and stachyose synthase (STS), and two intermedia, namely, UDP and inositol, which can be recycled. This reaction system produced 11.1 mM raffinose using purified enzymes under optimal reaction conditions and substrate concentrations. Thereafter, a stepwise cascade reaction strategy was employed to circumvent the instability of RS and STS in this system, and a 4.2-fold increase in raffinose production was observed. The enzymatic cascade reactions were then conducted using cell extracts to avoid the need for enzyme purification and supplementation with UDP. Such modification further increased raffinose production to 86.6 mM and enabled the synthesis of 61.1 mM stachyose. The UDP turnover number reached 337. Finally, inositol in the reaction system was recycled five times, and 255.8 mM raffinose (128.9 g/liter) was obtained.IMPORTANCE Soybean oligosaccharides (SBOS) have elicited considerable attention because of their potential applications in the pharmaceutical, cosmetics, and food industries. This study demonstrates an alternative method to produce raffinose and stachyose, which are the major bioactive components of SBOS, from sucrose via an in vitro enzyme system. High concentrations of galactinol, raffinose, and stachyose were synthesized with the aid of a stepwise cascade reaction process, which can successfully address the issue of mismatched enzyme characteristics of an in vitro metabolic engineering platform. The biocatalytic approach presented in this work may enable the synthesis of other valuable galactosyl oligosaccharides, such as verbascose and higher homologs, which are difficult to obtain through plant extraction.

Design, Synthesis, Anticancer Evaluation and Docking Studies of Novel Heterocyclic Derivatives Obtained via Reactions Involving Curcumin

Curcumin, a widely utilized flavor and coloring agent in food, has been shown to demonstrate powerful antioxidant, antitumor promoting and anti-inflammatory properties in vitro and in vivo. In the present work, synthesis of new heterocyclic derivatives based on Curcumin was studied. Compound 3 was synthesized via the reaction of furochromone carbaldehyde (1) with Curcumin (2) using pipredine as catalyst. Also, novel, 4,9-dimethoxy-5H-furo [3, 2-g] chromen-5-one derivatives 4a–d, 6a–d, 7, 8a–d, 9 and 10 were synthesized by the reactions of furochromone carbaldehyde (1) with different reagents (namely: appropriate amine 3a–d, appropriate hydrazine 5a–d, hydroxylamine hydrochloride, urea/thiourea, malononitrile, malononitrile with hydrazine hydrate). The structure of the synthesized products had been confirmed from their spectroscopic data (IR, ¹H-NMR, ¹³C-NMR and mass spectra). In the present investigation, the newly synthesized products were screened using the MTT colorimetric assay for their in vitro inhibition capacity in two human cancer cell lines (hepatocellular carcinoma (HEPG2) and breast cancer (MCF-7) as well as the normal cell line (human normal melanocyte, HFB4) in comparison to the known anticancer drugs: 5-flurouracil and doxorubicin. The anticancer activity results indicated that the synthesized products 4c and 8b showed growth inhibition activity against HEPG2 cell line and synthesized products 4b and 8a showed growth inhibition activity against MCF-7, but with varying intensities in comparison to the known anticancer drugs, 5-flurouracil and doxorubicin. Cyclin dependent kinase 2 (CDK2), a major cell cycle protein, was identified as a potential molecular target of Curcumin. Furthermore, Curcumin induced G1 cell cycle arrest, which is regulated by CDK2 in cancer cells. Therefore, we used molecular modelling to study in silico the possible inhibitory effect of CDK2 by Curcumin derivatives as a possible mechanism of these compounds as anticancer agents. The molecular docking study revealed that compounds 4b, 8a and 8b were the most effective compounds in inhibiting CDk2, and, this result was in agreement with cytotoxicity assay.

One-Pot Enzymatic Synthesis of D-Arylalanines Using Phenylalanine Ammonia Lyase and L-Amino Acid Deaminase

The phenylalanine ammonia-lyase (AvPAL) from Anabaena variabilis catalyzes the amination of substituent trans-cinnamic acid (t-CA) to produce racemic D,L-enantiomer arylalanine mixture owing to its low stereoselectivity. To produce high optically pure D-arylalanine, a modified AvPAL with high D-selectivity is expected. Based on the analyses of catalytic mechanism and structure, the Asn347 residue in the active site was proposed to control stereoselectivity. Therefore, Asn347 was mutated to construct mutant AvPAL-N347A, the stereoselectivity of AvPAL-N347A for D-enantiomer arylalanine was 2.3-fold higher than that of wild-type AvPAL (WtPAL). Furthermore, the residual L-enantiomer product in reaction solution could be converted into the D-enantiomer product through stereoselective oxidation by PmLAAD and nonselective reduction by reducing agent NH₃BH₃. At optimal conditions, the conversion rate of t-CA and optical purity (enantiomeric excess (ee_D)) of D-phenylalanine reached 82% and exceeded 99%, respectively. The two enzymes displayed activity toward a broad range of substrate and could be used to efficiently synthesize D-arylalanine with different groups on the phenyl ring. Among these D-arylalanines, the yield of m-nitro-D-phenylalanine was highest and reached 96%, and the ee_D exceeded 99%. This one-pot synthesis using AvPAL and PmLAAD has prospects for industrial application.

Bioamination of alkane with ammonium by an artificially designed multienzyme cascade

Biocatalytic C-H amination is one of the most challenging tasks. C-H amination reaction can hardly be driven efficiently by solely one enzyme so far. Thus, enzymatic synergy represents an alternative strategy. Herein, we report an "Artificially Bioamination Pathway" for C-H amination of cyclohexane as a model substrate. Three enzymes, a monooxygenase P450_BM3 mutant, an alcohol dehydrogenase ScCR from Streptomyces coelicolor and an amine dehydrogenase EsLeuDH from Exiguobacterium sibiricum, constituted a clean cascade reaction system with easy product isolation. Two independent cofactor regeneration systems were optimized to avoid interference from the endogenous NADH oxidases in the host E. coli cells. Based on a stepwise pH adjustment and ammonium supplement strategy, and using an in vitro mixture of cell-free extracts of the three enzymes, cyclohexylamine was produced in a titer of 14.9 mM, with a product content of up to 92.5%. Furthermore, designer cells coexpressing the three required enzymes were constructed and their capability of alkane bio-amination was examined. This artificially designed bioamination paves an attractive approach for enzymatic synthesis of amines from accessible and cheap alkanes.

Enzymatic asymmetric synthesis of chiral amino acids

This review summarizes the progress achieved in the enzymatic asymmetric synthesis of chiral amino acids from prochiral substrates.

Intensified biocatalytic production of enantiomerically pure halophenylalanines from acrylic acids using ammonium carbamate as the ammonia source

An industrial-scale method employing a phenylalanine ammonia lyase enzyme.