Engineering a Synthetic Pathway for Tyrosol Synthesis in Escherichia coli

Multienzymatic Cascade for Synthesis of Hydroxytyrosol via Two-Stage Biocatalysis

Hydroxytyrosol, a naturally occurring compound with antioxidant and antiviral activity, is widely applied in the cosmetic, food, and nutraceutical industries. The development of a biocatalytic approach for producing hydroxytyrosol from simple and readily accessible substrates remains a challenge. Here, we designed and implemented an effective biocatalytic cascade to obtain hydroxytyrosol from 3,4-dihydroxybenzaldehyde and l-threonine via a four-step enzymatic cascade composed of seven enzymes. To prevent cross-reactions and protein expression burden caused by multiple enzymes expressed in a single cell, the designed enzymatic cascade was divided into two modules and catalyzed in a stepwise manner. The first module (FM) assisted the assembly of 3,4-dihydroxybenzaldehyde and l-threonine into (2S,3R)-2-amino-3-(3,4-dihydroxyphenyl)-3-hydroxypropanoic acid, and the second module (SM) entailed converting (2S,3R)-2-amino-3-(3,4-dihydroxyphenyl)-3-hydroxypropanoic acid into hydroxytyrosol. Each module was cloned into Escherichia coli BL21 (DE3) and engineered in parallel by fine-tuning enzyme expression, resulting in two engineered whole-cell catalyst modules, BL21(FM01) and BL21(SM13), capable of converting 30 mM 3,4-dihydroxybenzaldehyde to 28.7 mM hydroxytyrosol with a high space-time yield (0.88 g/L/h). To summarize, the current study proposes a simple and effective approach for biosynthesizing hydroxytyrosol from low-cost substrates and thus has great potential for industrial applications.

Efficient Biosynthesis of Salidroside via Artificial in Vivo enhanced UDP-Glucose System Using Cheap Sucrose as Substrate

Salidroside, a valuable phenylethanoid glycoside, is obtained from plants belonging to the Rhodiola genus, known for its diverse biological properties. At present, salidroside is still far from large-scale industrial production due to its lower titer and higher process cost. In this study, we have for the first time increased salidroside production by enhancing UDP-glucose supply in situ. We constructed an in vivo UDP-glucose regeneration system that works in conjunction with UDP-glucose transferase from Rhodiola innovatively to improve UDP-glucose availability. And a coculture was formed in order to enable de novo salidroside synthesis. Confronted with the influence of tyrosol on strain growth, an adaptive laboratory evolution strategy was implemented to enhance the strain's tolerance. Similarly, salidroside production was optimized through refinement of the fermentation medium, the inoculation ratio of the two microbes, and the inoculation size. The final salidroside titer reached 3.8 g/L. This was the highest titer achieved at the shake flask level in the existing reports. And this marked the first successful synthesis of salidroside in an in situ enhanced UDP-glucose system using sucrose. The cost was reduced by 93% due to the use of inexpensive substrates. This accomplishment laid a robust foundation for further investigations into the synthesis of other notable glycosides and natural compounds.

Optimization of tyrosol-producing pathway with tyrosine decarboxylase and tyramine oxidase in high-tyrosine-producing Escherichia coli

Tyrosol (4-hydroxyphenylethanol) is a phenolic compound used in the pharmaceutical and chemical industries. However, current supply methods, such as extraction from natural resources and chemical synthesis, have disadvantages from the viewpoint of cost and environmental protection. Here, we developed a tyrosol-producing Escherichia coli cell factory from a high-tyrosine-producing strain by expressing selected tyrosine decarboxylase-, tyramine oxidase (TYO)-, and medium-chain dehydrogenase/reductase (YahK)-encoding genes. The genes were controlled by the strong T7 promoter and integrated into the chromosome because of the advantages over plasmid-based systems. The strain produced a melanin-like pigment as a by-product, which is suggested to be formed from 4-hydroxyphenylacetaldehyde (a TYO product/YahK substrate). By using a culture medium containing a high concentration of glycerol, which was reported to enhance NADH supply required for YahK activity, the final titer of tyrosol reached 2.42 g/L in test tube-scale cultivation with a concomitant decrease in the amount of pigment. These results indicate that chromosomally integrated and T7 promoter-controlled gene expression system in E. coli is useful for high production of heterologous enzymes and might be applied for industrial production of useful compounds including tyrosine and tyrosol.

Microbial Upcycling of Depolymerized Lignin into Value-Added Chemicals

Lignin is one of the most widespread organic compounds found on earth, boasting a wealth of aromatic molecules. The use of lignin feedstock for biochemical productions is of great importance for achieving "carbon neutrality." In recent years, a strategy for lignin valorization known as the "bio-funnel" has been proposed as a means to generate a variety of commercially valuable chemicals from lignin-derived compounds. The implementation of biocatalysis and metabolic engineering techniques has substantially advanced the biotransformation of depolymerized lignin into chemicals and materials within the supply chain. In this review, we present an overview of the latest advancements in microbial upcycling of depolymerized lignin into value-added chemicals. Besides, the review provides insights into the problems facing current biological lignin valorization while proposing further research directions to improve these technologies for the extensive accomplishment of the lignin upcycling.

Construction of an Escherichia coli cell factory for de novo synthesis of tyrosol through semi-rational design based on phenylpyruvate decarboxylase ARO10 engineering

Tyrosol (2-(4-hydroxyphenyl) ethanol) is extensively used in the pharmaceutical industry as an important natural product from plants. In previous research, we constructed a recombinant Escherichia coli strain capable of de novo synthesis of tyrosol by integrating the phenylpyruvate decarboxylase ARO10 derived from Saccharomyces cerevisiae. Nevertheless, the insufficient catalytic efficiency of ARO10 required the insertion of multiple gene copies into the genome to attain enhanced tyrosol production. In this study, we constructed a mutation library of ARO10 based on a computer-aided semi-rational design strategy and developed a high-throughput screening method for selecting high-yield tyrosol mutants by introducing the heterologous hydroxylase complex HpaBC. Through multiple rounds of screening and site-saturation mutagenesis, we ultimately identified the two optimal ARO10 mutants, ARO10D331V and ARO10D331C, which respectively achieved a tyrosol titer of 2.02 g/L and 2.04 g/L in shake flasks, both representing more than 50 % improvement compared to the wild-type. Our study demonstrates the great potential of computer-based semi-rational enzyme design strategy in metabolic engineering. The high-throughput screening method for target compound derivative possesses a certain level of generality. Ultimately, we obtained promising mutants capable of achieving industrial-scale production of tyrosol, which also lays a solid foundation for the efficient synthesis of tyrosol derivatives.

Engineering a Pseudomonas taiwanensis 4-coumarate platform for production of para-hydroxy aromatics with high yield and specificity

Aromatics are valuable bulk or fine chemicals with a myriad of important applications. Currently, their vast majority is produced from petroleum associated with many negative aspects. The bio-based synthesis of aromatics contributes to the much-required shift towards a sustainable economy. To this end, microbial whole-cell catalysis is a promising strategy allowing the valorization of abundant feedstocks derived from biomass to yield de novo-synthesized aromatics. Here, we engineered tyrosine-overproducing derivatives of the streamlined chassis strain Pseudomonas taiwanensis GRC3 for efficient and specific production of 4-coumarate and derived aromatics. This required pathway optimization to avoid the accumulation of tyrosine or trans-cinnamate as byproducts. Although application of tyrosine-specific ammonia-lyases prevented the formation of trans-cinnamate, they did not completely convert tyrosine to 4-coumarate, thereby displaying a significant bottleneck. The use of a fast but unspecific phenylalanine/tyrosine ammonia-lyase from Rhodosporidium toruloides (RtPAL) alleviated this bottleneck, but caused phenylalanine conversion to trans-cinnamate. This byproduct formation was greatly reduced through the reverse engineering of a point mutation in prephenate dehydratase domain-encoding pheA. This upstream pathway engineering enabled efficient 4-coumarate production with a specificity of >95% despite using an unspecific ammonia-lyase, without creating an auxotrophy. In shake flask batch cultivations, 4-coumarate yields of up to 21.5% (Cmol/Cmol) from glucose and 32.4% (Cmol/Cmol) from glycerol were achieved. Additionally, the product spectrum was diversified by extending the 4-coumarate biosynthetic pathway to enable the production of 4-vinylphenol, 4-hydroxyphenylacetate, and 4-hydroxybenzoate with yields of 32.0, 23.0, and 34.8% (Cmol/Cmol) from glycerol, respectively.

Styrene Oxide Isomerase-Catalyzed Meinwald Rearrangement in Cascade Biotransformations: Synthesis of Chiral and/or Natural Chemicals

Styrene oxide isomerase (SOI) catalyzes the Meinwald rearrangement of aryl epoxides to carbonyl compounds via a 1,2-trans-shift in a stereospecific manner. A number of cascade biotransformations with SOI-catalyzed epoxide isomerization as a key step have been developed to convert readily available substrates into valuable chiral chemicals. Cascade conversion of terminal or internal alkenes into chiral acids, alcohols or amines was achieved, which involved SOI-catalyzed enantio-retentive isomerization of terminal epoxides via 1,2-H shift, or internal epoxides via 1,2-methyl shift. SOI-involved cascades were also developed to convert racemic epoxides into chiral acids or amines via dynamic kinetic resolution. Additionally, combining SOI-catalyzed isomerization with enantioselective C-C bond forming enzymes enabled the synthesis of chiral amino acids or amino alcohols from racemic epoxides. Finally, integration of SOI-involved cascades with biosynthesis pathways allowed for the direct utilization of renewable substrates for the sustainable synthesis of high-value natural chemicals such as alcohols, acids, and esters.

De novo Synthesis of 2-phenylethanol from Glucose by Metabolically Engineered Escherichia coli

2-Phenylethanol (2- PE) is an aromatic alcohol with wide applications, but there is still no efficient microbial cell factory for 2-PE based on Escherichia coli. In this study, we constructed a metabolically engineered E. coli capable of de novo synthesis of 2-PE from glucose. Firstly, the heterologous styrene-derived and Ehrlich pathways were individually constructed in an L-Phe producer. The results showed that the Ehrlich pathway was better suited to the host than the styrene-derived pathway, resulting in a higher 2-PE titer of ∼0.76 ± 0.02 g/L after 72 h of shake flask fermentation. Furthermore, the phenylacetic acid synthase encoded by feaB was deleted to decrease the consumption of 2-phenylacetaldehyde, and the 2-PE titer increased to 1.75 ± 0.08 g/L. As phosphoenolpyruvate (PEP) is an important precursor for L-Phe synthesis, both the crr and pykF genes were knocked out, leading to ∼35% increase of the 2-PE titer, which reached 2.36 ± 0.06 g/L. Finally, a plasmid-free engineered strain was constructed based on the Ehrlich pathway by integrating multiple ARO10 cassettes (encoding phenylpyruvate decarboxylases) and overexpressing the yjgB gene. The engineered strain produced 2.28 ± 0.20 g/L of 2-PE with a yield of 0.076 g/g glucose and productivity of 0.048 g/L/h. To our best knowledge, this is the highest titer and productivity ever reported for the de novo synthesis of 2-PE in E. coli. In a 5-L fermenter, the 2-PE titer reached 2.15 g/L after 32 h of fermentation, suggesting that the strain has the potential to efficiently produce higher 2-PE titers following further fermentation optimization.

Efficient chemoenzymatic synthesis of α-aryl aldehydes as intermediates in C-C bond forming biocatalytic cascades

Multi-enzyme biocatalytic cascades are emerging as practical routes for the synthesis of complex bioactive molecules. However, the relative sparsity of water-stable carbon electrophiles limits the synthetic complexity of molecules made from such cascades. Here, we develop a chemoenzymatic platform that leverages styrene oxide isomerase (SOI) to covert readily accessible aryl epoxides into α-aryl aldehydes through a Meinwald rearrangement. These unstable aldehyde intermediates are then intercepted with a C-C bond forming enzyme, ObiH, that catalyzes a transaldolase reaction with l-threonine to yield synthetically challenging β-hydroxy-α-amino acids. Co-expression of both enzymes in E. coli yields a whole cell biocatalyst capable of synthesizing a variety of stereopure non-standard amino acids (nsAA) and can be produced on gram-scale. We used isotopically labelled substrates to probe the mechanism of SOI, which we show catalyzes a concerted isomerization featuring a stereospecific 1,2-hydride shift. The viability of in situ generated α-aryl aldehydes was further established by intercepting them with a recently engineered decarboxylative aldolase to yield γ-hydroxy nsAAs. Together, these data establish a versatile method of producing α-aryl aldehydes in simple, whole cell conditions and show that these intermediates are useful synthons in C‒C bond forming cascades.

Synergistic Improvement of 5-Aminolevulinic Acid Production with Synthetic Scaffolds and System Pathway Engineering

Engineered synthetic scaffolds to organize metabolic pathway enzymes and system pathway engineering to fine-tune metabolic fluxes play essential roles in microbial production. Here, we first obtained the most favorable combination of key enzymes for 5-aminolevulinic acid (5-ALA) synthesis through the C5 pathway by screening enzymes from different sources and optimizing their combination in different pathways. Second, we successfully constructed a multienzyme complex assembly system with PduA*, which spatially recruits the above three key enzymes for 5-ALA synthesis in a designable manner. By further optimizing the ratio of these key enzymes in synthetic scaffolds, the efficiency of 5-ALA synthesis through the C5 pathway was significantly improved. Then, the competitive metabolism pathway was fine-tuned by rationally designing different antisense RNAs, further significantly increasing 5-ALA titers. Furthermore, for efficient 5-ALA synthesis, obstacles of NADH and NADPH imbalances and feedback inhibition of the synthesis pathway were also overcome through engineering the NADPH regeneration pathway and transport pathway, respectively. Finally, combining these strategies with further fermentation optimization, we achieved a final 5-ALA titer of 11.4 g/L. These results highlight the importance of synthetic scaffolds and system pathway engineering to improve the microbial cell factory production performance.