Bioproduction of Benzylamine from Renewable Feedstocks via a Nine-Step Artificial Enzyme Cascade and Engineered Metabolic Pathways

Precision Engineering of the Co-immobilization of Enzymes for Cascade Biocatalysis

The design and orderly layered co-immobilization of multiple enzymes on resin particles remain challenging. In this study, the SpyTag/SpyCatcher binding pair was fused to the N-terminus of an alcohol dehydrogenase (ADH) and an aldo-keto reductase (AKR), respectively. A non-canonical amino acid (ncAA), p-azido-L-phenylalanine (p-AzF), as the anchor for covalent bonding enzymes, was genetically inserted into preselected sites in the AKR and ADH. Employing the two bioorthogonal counterparts of SpyTag/SpyCatcher and azide-alkyne cycloaddition for the immobilization of AKR and ADH enabled sequential dual-enzyme coating on porous microspheres. The ordered dual-enzyme reactor was subsequently used to synthesize (S)-1-(2-chlorophenyl)ethanol asymmetrically from the corresponding prochiral ketone, enabling the in situ regeneration of NADPH. The reactor exhibited a high catalytic conversion of 74 % and good reproducibility, retaining 80 % of its initial activity after six cycles. The product had 99.9 % ee, which that was maintained in each cycle. Additionally, the double-layer immobilization method significantly increased the enzyme loading capacity, which was approximately 1.7 times greater than that of traditional single-layer immobilization. More importantly, it simultaneously enabled both the purification and immobilization of multiple enzymes on carriers, thus providing a convenient approach to facilitate cascade biocatalysis.

Design of a cofactor self-sufficient whole-cell biocatalyst for enzymatic asymmetric reduction via engineered metabolic pathways and multi-enzyme cascade

NAD(P)H-dependent oxidoreductases are crucial biocatalysts for synthesizing chiral compounds. Yet, the industrial implementation of enzymatic redox reactions is often hampered by an insufficient supply of expensive nicotinamide cofactors. Here, a cofactor self-sufficient whole-cell biocatalyst was developed for the enzymatic asymmetric reduction of 2-oxo-4-[(hydroxy)(-methyl)phosphinyl] butyric acid (PPO) to L-phosphinothricin (L-PPT). The endogenous NADP+ pool was significantly enhanced by regulating Preiss-Handler pathway toward NAD(H) synthesis and, in the meantime, introducing NAD kinase to phosphorylate NAD(H) toward NADP+. The intracellular NADP(H) concentration displayed a 2.97-fold increase with the strategy compared with the wild-type strain. Furthermore, a recombinant multi-enzyme cascade biocatalytic system was constructed based on the Escherichia coli chassis. In order to balance multi-enzyme co-expression levels, the strategy of modulating rate-limiting enzyme PmGluDH by RBS strengths regulation successfully increased the catalytic efficiency of PPO conversion. Finally, the cofactor self-sufficient whole-cell biocatalyst effectively converted 300 mM PPO to L-PPT in 2 h without the need to add exogenous cofactors, resulting in a 2.3-fold increase in PPO conversion (%) from 43% to 100%, with a high space-time yield of 706.2 g L-1 d-1 and 99.9% ee. Overall, this work demonstrates a technological example for constructing a cofactor self-sufficient system for NADPH-dependent redox biocatalysis.

Production of Biobased Ethylbenzene by Cascade Biocatalysis with an Engineered Photodecarboxylase

Production of commodity chemicals, such as benzene, toluene, ethylbenzene, and xylenes (BTEX), from renewable resources is key for a sustainable society. Biocatalysis enables one-pot multistep transformation of bioresources under mild conditions, yet it is often limited to biochemicals. Herein, we developed a non-natural three-enzyme cascade for one-pot conversion of biobased l-phenylalanine into ethylbenzene. The key rate-limiting photodecarboxylase was subjected to structure-guided semirational engineering, and a triple mutant CvFAP(Y466T/P460A/G462I) was obtained with a 6.3-fold higher productivity. With this improved photodecarboxylase, an optimized two-cell sequential process was developed to convert l-phenylalanine into ethylbenzene with 82 % conversion. The cascade reaction was integrated with fermentation to achieve the one-pot bioproduction of ethylbenzene from biobased glycerol, demonstrating the potential of cascade biocatalysis plus enzyme engineering for the production of biobased commodity chemicals.

Optimization of multi-enzyme cascade process for the biosynthesis of benzylamine

Benzylamine is a valuable intermediate in the synthesis of organic compounds such as curing agents and antifungal drugs. To improve the efficiency of benzylamine biosynthesis, we identified the enzymes involved in the multi-enzyme cascade, regulated the expression strength by using RBS engineering in Escherichia coli, and established a regeneration-recycling system for alanine. This is a cosubstrate, coupled to cascade reactions, which resulted in E. coli RARE-TP and can synthesize benzylamine using phenylalanine as a precursor. By optimizing the supply of cosubstrates alanine and ammonia, the yield of benzylamine produced by whole-cell catalysis was increased by 1.5-fold and 2.7-fold, respectively, and the final concentration reached 6.21 mM. In conclusion, we achieved conversion from l-phenylalanine to benzylamine and increased the yield through enzyme screening, expression regulation, and whole-cell catalytic system optimization. This demonstrated a green and sustainable benzylamine synthesis method, which provides a reference and additional information for benzylamine biosynthesis research.

Synergistic improvement of cinnamylamine production by metabolic regulation

BACKGROUND

Aromatic primary amines (APAs) are key intermediates in the chemical industry with numerous applications. Efficient and mild biocatalytic synthesis is an excellent complement to traditional chemical synthesis. Our lab previously reported a whole-cell catalytic system for the synthesis of APAs catalyzed by carboxylic acid reductase from Neurospora crassa (ncCAR) and ω-transaminase from Ochrobactrum anthropi (OATA). However, the accumulation of toxic intermediates (aromatic aldehydes) during biocatalytic synthesis affected yields of APAs due to metabolic imbalance.

RESULTS

In this work, the biocatalytic synthesis of APAs (taking cinnamylamine as an example) was metabolically regulated by the overexpression or knockout of five native global transcription factors (TFs), the overexpression of eight native resistance genes, and optimization of promoters. Transcriptome analysis showed that knockout of the TF arcA increased the fluxes of NADPH and ATP in E. coli, while the rate of pyruvate metabolism was accelerated. In addition, the genes related to stress and detoxification were upregulated with the overexpression of resistance gene marA, which reduced the NADPH level in E. coli. Then, the expression level of soluble OATA increased by promoter optimization. Overall, arcA and marA could regulate the catalytic rate of NADPH- dependent ncCAR, while arcA and optimized promoter could regulate the catalytic rate of OATA. Lastly, the cinnamylamine yield of the best metabolically engineered strain S020 was increased to 90% (9 mM, 1.2 g/L), and the accumulation of cinnamaldehyde was below 0.9 mM. This work reported the highest production of cinnamylamine by biocatalytic synthesis.

CONCLUSION

This regulatory process provides a common strategy for regulating the biocatalytic synthesis of other APAs. Being entirely biocatalytic, our one-pot procedure provides considerable advantages in terms of environmental and safety impacts over reported chemical methods.

Prospective bacterial and fungal sources of hyaluronic acid: A review

The unique biological and rheological properties make hyaluronic acid a sought-after material for medicine and cosmetology. Due to very high purity requirements for hyaluronic acid in medical applications, the profitability of streptococcal fermentation is reduced. Production of hyaluronic acid by recombinant systems is considered a promising alternative. Variations in combinations of expressed genes and fermentation conditions alter the yield and molecular weight of produced hyaluronic acid. This review is devoted to the current state of hyaluronic acid production by recombinant bacterial and fungal organisms.

Bioproduction of Natural Phenethyl Acetate, Phenylacetic Acid, Ethyl Phenylacetate, and Phenethyl Phenylacetate from Renewable Feedstock

Natural phenethyl acetate (PEA), phenylacetic acid (PAA), ethyl phenylacetate (Et-PA), and phenethyl phenylacetate (PE-PA) are highly desirable aroma chemicals, but with limited availability and high price. Here, green, sustainable, and efficient bioproduction of these chemicals as natural products from renewable feedstocks was developed. PEA and PAA were synthesized from l-phenylalanine (l-Phe) via novel six- and five-enzyme cascades, respectively. Whole-cell-based cascade biotransformation of 100 mm l-Phe in a two-phase system (aqueous/organic: 1 : 0.5 v/v) containing ethyl oleate or biodiesel as green solvent gave 13.6 g L^-1 PEA (83.1 % conv.) and 11.6 g L^-1 PAA (87.1 % conv.), respectively. Coupled fermentation and biotransformation approach produced 10.4 g L^-1 PEA and 9.2 g L^-1 PAA from glucose or glycerol, respectively. The biosynthesized PAA was converted to natural Et-PA and PE-PA by esterification using lipases with ethanol or 2-phenylethanol derived from sugar, affording 2.7 g L^-1 Et-PA (83.1 % conv.) and 4.6 g L^-1 PE-PA (96.3 % conv.), respectively.

Metabolic engineering of Pseudomonas putida for production of vanillylamine from lignin-derived substrates

Whole-cell bioconversion of technical lignins using Pseudomonas putida strains overexpressing amine transaminases (ATAs) has the potential to become an eco-efficient route to produce phenolic amines. Here, a novel cell growth-based screening method to evaluate the in vivo activity of recombinant ATAs towards vanillylamine in P. putida KT2440 was developed. It allowed the identification of the native enzyme Pp-SpuC-II and ATA from Chromobacterium violaceum (Cv-ATA) as highly active towards vanillylamine in vivo. Overexpression of Pp-SpuC-II and Cv-ATA in the strain GN442ΔPP_2426, previously engineered for reduced vanillin assimilation, resulted in 94- and 92-fold increased specific transaminase activity, respectively. Whole-cell bioconversion of vanillin yielded 0.70 ± 0.20 mM and 0.92 ± 0.30 mM vanillylamine, for Pp-SpuC-II and Cv-ATA, respectively. Still, amine production was limited by a substantial re-assimilation of the product and formation of the by-products vanillic acid and vanillyl alcohol. Concomitant overexpression of Cv-ATA and alanine dehydrogenase from Bacillus subtilis increased the production of vanillylamine with ammonium as the only nitrogen source and a reduction in the amount of amine product re-assimilation. Identification and deletion of additional native genes encoding oxidoreductases acting on vanillin are crucial engineering targets for further improvement.

Recent advances in (chemo)enzymatic cascades for upgrading bio-based resources

Developing (chemo)enzymatic cascades is very attractive for green synthesis, because they streamline multistep synthetic processes. In this Feature Article, we have summarized the recent advances in in vitro or whole-cell cascade reactions with a focus on the use of renewable bio-based resources as starting materials. This includes the synthesis of rare sugars (such as ketoses, L-ribulose, D-tagatose, myo-inositol or aminosugars) from readily available carbohydrate sources (cellulose, hemi-cellulose, starch), in vitro enzyme pathways to convert glucose to various biochemicals, cascades to convert 5-hydroxymethylfurfural and furfural obtained from lignin or xylose into novel precursors for polymer synthesis, the syntheses of phenolic compounds, cascade syntheses of aliphatic and highly reduced chemicals from plant oils and fatty acids, upgrading of glycerol or ethanol as well as cascades to transform natural L-amino acids into high-value (chiral) compounds. In several examples these processes have demonstrated their efficiency with respect to high space-time yields and low E-factors enabling mature green chemistry processes. Also, the strengths and limitations are discussed and an outlook is provided for improving the existing and developing new cascades.

Four-Step Pathway from Phenylpyruvate to Benzylamine, an Intermediate to the High-Energy Propellant CL-20

Benzylamine is a commodity chemical used in the synthesis of motion-sickness treatments and anticonvulsants, in dyeing textiles, and as a precursor to the high-energy propellant CL-20. Because chemical production generates toxic waste streams, biosynthetic alternatives have been explored, recently resulting in a functional nine-step pathway from central metabolism (phenylalanine) in E. coli. We report a novel four-step pathway for benzylamine production, which generates the product from cellular phenylpyruvate using enzymes from different sources: a mandelate synthase (Amycolatopsis orientalis), a mandelate oxidase (Streptomyces coelicolor), a benzoylformate decarboxylase (Pseudomonas putida), and an aminotransferase (Salicibacter pomeroyi). This pathway produces benzylamine at 24 mg/L in 15 h (4.5% yield) in cultures of unoptimized cells supplemented with phenylpyruvate. Because the yield is low, supplementation with pathway intermediates is used to troubleshoot the design. This identifies conversion inefficiencies in the mandelate synthase-mediated synthesis of (S)-mandelic acid, and subsequent genome mining identifies a new mandelate synthase (Streptomyces sp. 1114.5) with improved yield. Supplementation experiments also reveal native redirection of ambient phenylpyruvate away from the pathway to phenylalanine. Overall, this work illustrates how retrosynthetic design can dramatically reduce the number of enzymes in a pathway, potentially reducing its draw on cellular resources. However, it also shows that such benefits can be abrogated by inefficiencies of individual conversions. Addressing these barriers can provide an alternative approach to green production of benzylamine, eliminating upstream dependence on chlorination chemistry.

Production of (R)-mandelic acid from styrene, L-phenylalanine, glycerol, or glucose via cascade biotransformations

(R)-mandelic acid is an industrially important chemical, especially used for producing antibiotics. Its chemical synthesis often uses highly toxic cyanide to produce its racemic form, followed by kinetic resolution with 50% maximum yield. Here we report a green and sustainable biocatalytic method for producing (R)-mandelic acid from easily available styrene, biobased L-phenylalanine, and renewable feedstocks such as glycerol and glucose, respectively. An epoxidation-hydrolysis-double oxidation artificial enzyme cascade was developed to produce (R)-mandelic acid at 1.52 g/L from styrene with > 99% ee. Incorporation of deamination and decarboxylation into the above cascade enables direct conversion of L-phenylalanine to (R)-mandelic acid at 913 mg/L and > 99% ee. Expressing the five-enzyme cascade in an L-phenylalanine-overproducing E. coli NST74 strain led to the direct synthesis of (R)-mandelic acid from glycerol or glucose, affording 228 or 152 mg/L product via fermentation. Moreover, coupling of E. coli cells expressing L-phenylalanine biosynthesis pathway with E. coli cells expressing the artificial enzyme cascade enabled the production of 760 or 455 mg/L (R)-mandelic acid from glycerol or glucose. These simple, safe, and green methods show great potential in producing (R)-mandelic acid from renewable feedstocks.

Recent advances in artificial enzyme cascades for the production of value-added chemicals

Enzyme cascades are efficient tools to perform multi-step synthesis in one-pot in a green and sustainable manner, enabling non-natural synthesis of valuable chemicals from easily available substrates by artificially combining two or more enzymes. Bioproduction of many high-value chemicals such as chiral and highly functionalised molecules have been achieved by developing new enzyme cascades. This review summarizes recent advances on engineering and application of enzyme cascades to produce high-value chemicals (alcohols, aldehydes, ketones, amines, carboxylic acids, etc) from simple starting materials. While 2-step enzyme cascades are developed for versatile enantioselective synthesis, multi-step enzyme cascades are engineered to functionalise basic chemicals, such as styrenes, cyclic alkanes, and aromatic compounds. New cascade reactions have also been developed for producing valuable chemicals from bio-based substrates, such as ʟ-phenylalanine, and renewable feedstocks such as glucose and glycerol. The challenges in current process and future outlooks in the development of enzyme cascades are also addressed.

Obesity of mice lacking VAP-1/SSAO by Aoc3 gene deletion is reproduced in mice expressing a mutated vascular adhesion protein-1 (VAP-1) devoid of amine oxidase activity

The product of Aoc3 gene is known as vascular adhesion protein-1 (VAP-1), a glycoprotein contributing to leukocyte extravasation and exhibiting semicarbazide-sensitive amine oxidase activity (SSAO). Regarding the immune functions of VAP-1/SSAO, it is known that mice bearing Aoc3 gene knock-out (AOC3KO) exhibit defects in leukocyte migration similar to those of mice expressing a mutated VAP-1 lacking functional SSAO activity (knock-in, AOC3KI). However, it has not been reported whether these models differ regarding other disturbances. Thus, we further compared endocrine-metabolic phenotypes of AOC3KO and AOC3KI mice to their respective control. Special attention was paid on adiposity, glucose and lipid handling, since VAP-1/SSAO is highly expressed in adipose tissue (AT). In both mouse lines, no tissue SSAO activity was found, while Aoc3 mRNA was absent in AOC3KO only. Although food consumption was unchanged, both AOC3KO and AOC3KI mice were heavier and fatter than their respective controls. Other alterations commonly found in adipocytes from both lines were loss of benzylamine insulin-like action with unchanged insulin lipogenic responsiveness and adiponectin expression. A similar downregulation of inflammatory markers (CD45, IL6) was found in AT. Glucose handling and liver mass remained unchanged, while circulating lipid profile was distinctly altered, with increased cholesterol in AOC3KO only. These results suggest that the lack of oxidase activity found in AOC3KI is sufficient to reproduce the metabolic disturbances observed in AOC3KO mice, save those related with cholesterol transport. Modulation of SSAO activity therefore constitutes a potential target for the treatment of cardiometabolic diseases, especially obesity when complicated by low-grade inflammation.

De Novo Biosynthesis of (S)- and (R)-Phenylethanediol in Yeast via Artificial Enzyme Cascades

Due to oil depletion and global climate change, sustainable manufacturing of fine chemicals from renewable feedstocks has gained increasing attention in the scientific community. In the present study, we attempted to engineer Saccharomyces cerevisiae toward de novo synthesis of (S)- or (R)-phenylethanediol, an important pharmaceutical intermediate. More specifically, the biocatalytic cascades contain the following: l-phenylalanine undergoes deamination/decarboxylation to styrene by using phenylalanine ammonia lyase (PAL) and ferulic acid decarboxylase (FDC), followed by S-selective epoxidation of styrene to give (S)-styrene oxide with styrene monooxygenase (SMO); regioselective hydrolysis of (S)-styrene oxide with epoxide hydrolase from Sphingomonas HXN-200 (SpEH) or from potato (StEH) gives rise to (S)- or (R)-phenylethanediol. In this work, we found that the artificial enzyme cascades could be functionally expressed in the heterologous host of S. cerevisiae. Small-scale shake flask studies revealed that the engineered yeast cell factories produced approximately 100-120 mg/L of (S)- or (R)-phenylethanediol after 96 h cultivation. To the best of our knowledge, this is the first attempt to explore an artificial route with styrene as an intermediate for producing phenylethanediol in S. cerevisiae. We envision that our engineering strategy will open a new research field for synthesizing other vicinal diol derived chemicals in yeast.

One-pot, three-step cascade synthesis of D-danshensu using engineered Escherichia coli whole cells

D-danshensu (D-DSS), extracted from the plant Salvia miltiorrhiza (Danshen), is widely used to treat cardiovascular and cerebrovascular diseases. Here we engineered Escherichia coli strains to produce D-DSS from catechol, pyruvate and ammonia by one-pot biotransformation. Tyrosin-phenol lyase (TPL), L-amino acid deaminase (aadL), D-lactate dehydrogenase (ldhD) and glucose dehydrogenase (gdh) genes were overexpressed in Escherichia coli strain. First, the expression of genes was regulated by different copy number plasmids combination, the result of E. coli TALG6, with strong overexpression of TPL, aadL, ldhD and moderate overexpression of gdh, exhibited 253.7% increase D-DSS production compared to E. coli TALG1. Second, the optimum concentration of catechol was found to be 50 mM. Finally, a fed-batch biotransformation strategy was proposed, namely the amount of catechol was added to 50 mM every 2 h. The total production of D-DSS reached 55.35 mM within 14 h, which was 1.7 times that without feeding.

Biocatalytic selective functionalisation of alkenes via single-step and one-pot multi-step reactions

Alkenes are excellent starting materials for organic synthesis due to the versatile reactivity of C[double bond, length as m-dash]C bonds and the easy availability of many unfunctionalised alkenes. Direct regio- and/or enantioselective conversion of alkenes into functionalised (chiral) compounds has enormous potential for industrial applications, and thus has attracted the attention of researchers for extensive development using chemo-catalysis over the past few years. On the other hand, many enzymes have also been employed for conversion of alkenes in a highly selective and much greener manner to offer valuable products. Herein, we review recent advances in seven well-known types of biocatalytic conversion of alkenes. Remarkably, recent mechanism-guided directed evolution and enzyme cascades have enabled the development of seven novel types of single-step and one-pot multi-step functionalisation of alkenes, some of which are even unattainable via chemo-catalysis. These new reactions are particularly highlighted in this feature article. Overall, we present an ever-expanding enzyme toolbox for various alkene functionalisations inspiring further research in this fast-developing theme.