Rational engineering of p -hydroxybenzoate hydroxylase to enable efficient gallic acid synthesis via a novel artificial biosynthetic pathway

Hydroxybenzoic acids: Microbial metabolism, pathway engineering and products

Hydroxybenzoic acids (HBAs) are plant secondary metabolites exhibiting antioxidant, antiviral, anticancer and antibacterial activities. A high and constantly increasing demand for these compounds underlines the need for novel and efficient production methods, as commonly applied plant extraction and chemical synthesis approaches are susceptible to low yields and are environmentally hazardous. Switching to biotechnology and replacing petroleum-based chemicals has potential to improve eco-efficiency in sustainable bioeconomy. With the increased focus on the production of materials using renewable resources and bio-based feedstocks, microbial fermentation and engineering drives the development and optimization of sustainable bioproduction. This systematic review summarizes current knowledge of microbial HBAs metabolism and biosynthesis. Here, the existing challenges are highlighted and the potential strategies for improved microbial production of HBAs are identified. Key aspects of HBAs metabolism and complexity of the factors related to bacterial strain selection, titer, and bioprocess strategy are examined. The opportunities of HBAs bioproduction using engineered microbial cell factories are discussed in detail and insights for synthesis improvement are presented.

Efficient biosynthesis of gallic acid by a syntrophic Escherichia coli co-culture system

Gallic acid (GA), a natural phenolic acid antioxidant, has significant therapeutic and industrial applications. However, its traditional manufacturing approach, based on plant extraction, has been associated with risks of environmental pollution as well as a limited range of applications. Consequently, microbial-based production of GA, being more environmental-friendly, is viewed as a potential alternative. This study reports the efficient biosynthesis of GA from renewable glucose via a syntrophic Escherichia coli co-culture system. An effective GA biosynthesis pathway was first analyzed and determined. Then the rate-limiting step involving the hydroxylation of protocatechuic acid (PCA) to GA was removed by integrating multiple copies of the key gene pobAT294A/Y385F into the chromosome of a PCA-overproducing strain. The resulting strain GA10 produced 41.88 g/L GA with a yield of 0.185 mol/mol, but up to 9.54 g/L of the intermediate PCA accumulated in the fermentation broth. To overcome this issue, a catalytic strain COT03 was constructed by coupling the metabolism of excess intracellular NADPH supply with the NADPH-consuming reaction catalyzed by PobAT294A/Y385F. This yielded a syntrophic E. coli co-culture system that consisted of a GA-overproducing strain (GA10) and a growth-coupled biocatalytic strain (COT03). Following optimization of the culture conditions, the co-culture system produced 57.66 g/L GA from glucose within 75 h, with a yield of 0.233 mol/mol and an average productivity of 0.769 g/L/h. This study lays the foundation for the potential industrial biomanufacturing of GA from glucose.

Microbial engineering for monocyclic aromatic compounds production

Aromatic compounds serve pivotal roles in plant physiology and exhibit antioxidative and antimicrobial properties, leading to their widespread application, such as in food preservation and pharmaceuticals. However, direct plant extraction and petrochemical synthesis often struggle to meet current needs due to low yield or facing economic and environmental hurdles. In the past decades, systems metabolic engineering enabled eco-friendly production of various aromatic compounds, with some reaching industrial levels. In this review, we highlight monocyclic aromatic chemicals, which have relatively simple structures and are currently the primary focus of microbial synthesis research. We then discuss systems metabolic engineering at the enzyme, pathway, cellular, and bioprocess levels to improve the production of these chemicals. Finally, we overview the current limitations and potential resolution strategies, aiming to provide reference for future studies on the biosynthesis of aromatic products.

Gallic acid: a dietary metabolite's therapeutic potential in the management of atherosclerotic cardiovascular disease

Atherosclerotic cardiovascular disease (ASCVD) causes significant morbidity and mortality globally. Most of the chemicals specifically target certain pathways and minimally impact other diseases associated with ASCVD. Moreover, interactions of these drugs can cause toxic reactions. Consequently, the exploration of multi-targeted and safe medications for treating and preventing ASCVD has become an increasingly popular trend. Gallic acid (GA), a natural secondary metabolite found in various fruits, plants, and nuts, has demonstrated potentials in preventing and treating ASCVD, in addition to its known antioxidant and anti-inflammatory effects. It alleviates the entire process of atherosclerosis (AS) by reducing oxidative stress, improving endothelial dysfunction, and inhibiting platelet activation and aggregation. Additionally, GA can treat ASCVD-related diseases, such as coronary heart disease (CHD) and cerebral ischemia. However, the pharmacological actions of GA in the prevention and treatment of ASCVD have not been comprehensively reviewed, which limits its clinical development. This review primarily summarizes the in vitro and in vivo pharmacological actions of GA on the related risk factors of ASCVD, AS, and ASCVD. Additionally, it provides a comprehensive overview of the toxicity, extraction, synthesis, pharmacokinetics, and pharmaceutics of GA,aimed to enhance understanding of its clinical applications and further research and development.

Directed evolution and metabolic engineering generate an Escherichia coli cell factory for de novo production of 4-hydroxymandelate

4-hydroxymandelate is a high-value aromatic compound used in the medicine, cosmetics, food, and chemical industry. However, existing natural extraction and chemical synthesis methods are costly and lead to environmental pollution. This study employed metabolic engineering and directed evolution strategies for de novo 4-hydroxymandelate biosynthesis. Two key challenges were addressed: insufficient precursor supply and limited activity of crucial enzymes. Through gene overexpression and multi-level gene interference using CRISPRi, An Escherichia coli chassis capable of producing the key precursor 4-hydroxyphenylpyruvate and the titer reached 5.05 mM (0.91 g/L). A mutant clone was obtained, HmaSV152G, which showed a 5.13-fold improvement in the catalytic rate. During fermentation, a high production of 194.87 mM (32.768 g/L) 4-hydroxymandelate was achieved in 76 h with a batch supply of glucose in a 5-L bioreactor. This study demonstrated the great potential of biosensors in protein engineering and provides a reference for large-scale production of other high-value aromatic compounds.

Microbial synthesis of gallic acid and its glucoside β-glucogallin

Gallic acid (GA) and β-glucogallin (BGG) are natural products with diverse uses in pharmaceutical, food, chemical and cosmetic industries. They are valued for their wide-ranging properties such as antioxidant, antibacterial, antidiabetic, and anticancer properties. Despite their significant importance, microbial production of GA and BGG faces challenges such as limited titers and yields, along with the incomplete understanding of BGG biosynthesis pathways in microorganisms. To address these challenges, we developed a recombinant Escherichia coli strain capable of efficiently producing GA. Our approach involved screening efficient pathway enzymes, integrating biosynthetic pathway genes into the genome while balancing carbon flux via adjusting expression levels, and strengthening the shikimate pathway to remove bottlenecks. The resultant strain achieved impressive results, producing 51.57 g/L of GA with a carbon yield of 0.45 g/g glucose and a productivity of 1.07 g/L/h. Furthermore, we extended this microbial platform to biosynthesize BGG by screening GA 1-O-glucosyltransferase, leading to the de novo production of 92.42 mg/L of BGG. This work establishes an efficient chassis for producing GA at an industrial level and provides a microbial platform for generating GA derivatives.

Biodegradation: the best solution to the world problem of discarded polymers

The widespread use of polymers has made our lives increasingly convenient by offering a more convenient and dependable material. However, the challenge of efficiently decomposing these materials has resulted in a surge of polymer waste, posing environment and health risk. Currently, landfill and incineration treatment approaches have notable shortcomings, prompting a shift towards more eco-friendly and sustainable biodegradation approaches. Biodegradation primarily relies on microorganisms, with research focusing on both solitary bacterial strain and multi-strain communities for polymer biodegradation. Furthermore, directed evolution and rational design of enzyme have significantly contributed to the polymer biodegradation process. However, previous reviews often undervaluing the role of multi-strain communities. In this review, we assess the current state of these three significant fields of research, provide practical solutions to issues with polymer biodegradation, and outline potential future directions for the subject. Ultimately, biodegradation, whether facilitated by single bacteria, multi-strain communities, or engineered enzymes, now represents the most effective method for managing waste polymers.

Application of modern synthetic biology technology in aromatic amino acids and derived compounds biosynthesis

Aromatic amino acids (AAA) and derived compounds have enormous commercial value with extensive applications in the food, chemical and pharmaceutical fields. Microbial production of AAA and derived compounds is a promising prospect for its environmental friendliness and sustainability. However, low yield and production efficiency remain major challenges for realizing industrial production. With the advancement of synthetic biology, microbial production of AAA and derived compounds has been significantly facilitated. In this review, a comprehensive overview on the current progresses, challenges and corresponding solutions for AAA and derived compounds biosynthesis is provided. The most cutting-edge developments of synthetic biology technology in AAA and derived compounds biosynthesis, including CRISPR-based system, genetically encoded biosensors and synthetic genetic circuits, were highlighted. Finally, future prospects of modern strategies conducive to the biosynthesis of AAA and derived compounds are discussed. This review offers guidance on constructing microbial cell factory for aromatic compound using synthetic biology technology.

Structure, dimeric conformation, and coenzyme versatility of p-hydroxybenzoate hydroxylase from Arthrobacter sp. PAMC25564

p-Hydroxybenzoate hydroxylase (PHBH) catalyzes the ortho-hydroxylation of 4-hydroxybenzoate (4-HB) to protocatechuate (PCA). PHBHs are commonly known as homodimers, and the prediction of pyridine nucleotide binding and specificity remains an ongoing focus in this field. Therefore, our study aimed to determine the dimerization interface in AspPHBH from Arthrobacter sp. PAMC25564 and identify the canonical pyridine nucleotide-binding residues, along with coenzyme specificity, through site-directed mutagenesis. The results confirm a functional dimeric assembly from a tetramer that appeared in the crystallographic asymmetric unit identical to that established in previous studies. Furthermore, AspPHBH exhibits coenzyme versatility, utilizing both NADH and NADPH, with a preference for NADH. Rational engineering experiments demonstrated that targeted mutations in coenzyme surrounding residues profoundly impact NADPH binding, leading to nearly abrogated enzymatic activity compared to that of NADH. R50, R273, and S166 emerged as significant residues for NAD(P)H binding, having a near-fatal impact on NADPH binding compared to NADH. Likewise, the E44 residue plays a critical role in determining coenzyme specificity. Overall, our findings contribute to the fundamental understanding of the determinants of PHBH's active dimeric conformation, coenzyme binding and specificity holding promise for biotechnological advancements.

Engineering cascade biocatalysis in whole cells for syringic acid bioproduction

BACKGROUND

Syringic acid (SA) is a high-value natural compound with diverse biological activities and wide applications, commonly found in fruits, vegetables, and herbs. SA is primarily produced through chemical synthesis, nonetheless, these chemical methods have many drawbacks, such as considerable equipment requirements, harsh reaction conditions, expensive catalysts, and numerous by-products. Therefore, in this study, a novel biotransformation route for SA production was designed and developed by using engineered whole cells.

RESULTS

An O-methyltransferase from Desulfuromonas acetoxidans (DesAOMT), which preferentially catalyzes a methyl transfer reaction on the meta-hydroxyl group of catechol analogues, was identified. The whole cells expressing DesAOMT can transform gallic acid (GA) into SA when S-adenosyl methionine (SAM) is used as a methyl donor. We constructed a multi-enzyme cascade reaction in Escherichia coli, containing an endogenous shikimate kinase (AroL) and a chorismate lyase (UbiC), along with a p-hydroxybenzoate hydroxylase mutant (PobA**) from Pseudomonas fluorescens, and DesAOMT; SA was biosynthesized from shikimic acid (SHA) by using whole cells catalysis. The metabolic system of chassis cells also affected the efficiency of SA biosynthesis, blocking the chorismate metabolism pathway improved SA production. When the supply of the cofactor NADPH was optimized, the titer of SA reached 133 μM (26.2 mg/L).

CONCLUSION

Overall, we designed a multi-enzyme cascade in E. coli for SA biosynthesis by using resting or growing whole cells. This work identified an O-methyltransferase (DesAOMT), which can catalyze the methylation of GA to produce SA. The multi-enzyme cascade containing four enzymes expressed in an engineered E. coli for synthesizing of SA from SHA. The metabolic system of the strain and biotransformation conditions influenced catalytic efficiency. This study provides a new green route for SA biosynthesis.

Gallic acid fermentation by metabolically engineered Escherichia coli producing p-hydroxybenzoate hydroxylase from Hylemonella gracilis NS1

Plant-derived phenolic gallic acid (GA) is an important raw material for antioxidants and food additives. Efforts to ferment GA using microbial processes have aimed at minimizing production costs and environmental load using enzymes that hydroxylate p-hydroxybenzoate and protocatechuate (PCA). Here, we found a p-hydroxybenzoate hydroxylase (PobA) in the bacterium Hylemonella gracilis NS1 (HgPobA) with 1.5-fold more hydroxylation activity than that from Pseudomonas aeruginosa PAO1 and thus converted PCA to GA more efficiently. The PCA hydroxylation activity of HgPobA was improved by introducing the amino acid substitutions L207V/Y393F or T302A/Y393F. These mutants had 2.9- and 3.7-fold lower Kmapp for PCA than wild-type HgPobA. An Escherichia coli strain that reinforces shikimate pathway metabolism and produces HgPobA when cultured for 60 h generated 0.27 g L-1 of GA. This is the first report of fermenting glucose to generate GA using a natural enzyme from the PobA family. The E. coli strain harboring the HgPobA L207V/Y393F mutant increased GA production to 0.56 g L-1. During the early stages of culture, GA was fermented at a 10-fold higher rate by a strain producing either HgPobA L207V/Y393F or T302A/Y393F compared with wild-type HgPobA, which agreed with the high kcatapp/Kmapp PCA values of this mutant. We enhanced a PobA isozyme and its PCA hydroxylating function to efficiently and cost-effectively ferment GA.

Unravelling and reconstructing the biosynthetic pathway of bergenin

Bergenin, a rare C-glycoside of 4-O-methyl gallic acid with pharmacological properties of antitussive and expectorant, is widely used in clinics to treat chronic tracheitis in China. However, its low abundance in nature and structural specificity hampers the accessibility through traditional crop-based manufacturing or chemical synthesis. In the present work, we elucidate the biosynthetic pathway of bergenin in Ardisia japonica by identifying the highly regio- and/or stereoselective 2-C-glycosyltransferases and 4-O-methyltransferases. Then, in Escherichia coli, we reconstruct the de novo biosynthetic pathway of 4-O-methyl gallic acid 2-C-β-D-glycoside, which is the direct precursor of bergenin and is conveniently esterified into bergenin by in situ acid treatment. Moreover, further metabolic engineering improves the production of bergenin to 1.41 g L-1 in a 3-L bioreactor. Our work provides a foundation for sustainable supply of bergenin and alleviates its resource shortage via a synthetic biology approach.

Exploring the role of flavin-dependent monooxygenases in the biosynthesis of aromatic compounds

Hydroxylated aromatic compounds exhibit exceptional biological activities. In the biosynthesis of these compounds, three types of hydroxylases are commonly employed: cytochrome P450 (CYP450), pterin-dependent monooxygenase (PDM), and flavin-dependent monooxygenase (FDM). Among these, FDM is a preferred choice due to its small molecular weight, stable expression in both prokaryotic and eukaryotic fermentation systems, and a relatively high concentration of necessary cofactors. However, the catalytic efficiency of many FDMs falls short of meeting the demands of large-scale production. Additionally, challenges arise from the limited availability of cofactors and compatibility issues among enzyme components. Recently, significant progress has been achieved in improving its catalytic efficiency, but have not yet detailed and informative viewed so far. Therefore, this review emphasizes the advancements in FDMs for the biosynthesis of hydroxylated aromatic compounds and presents a summary of three strategies aimed at enhancing their catalytic efficiency: (a) Developing efficient enzyme mutants through protein engineering; (b) enhancing the supply and rapid circulation of critical cofactors; (c) facilitating cofactors delivery for enhancing FDMs catalytic efficiency. Furthermore, the current challenges and further perspectives on improving catalytic efficiency of FDMs are also discussed.

Recent Advances in Microbial Metabolic Engineering for Production of Natural Phenolic Acids

Phenolic acids are important natural bioactive compounds with varied physiological functions. They are extensively used in food, pharmaceutical, cosmetic, and other chemical industries and have attractive market prospects. Compared to plant extraction and chemical synthesis, microbial fermentation for phenolic acid production from renewable carbon sources has significant advantages. This review focuses on the structural information, physiological functions, current applications, and biosynthesis pathways of phenolic acids, especially advances in the development of metabolically engineered microbes for the production of phenolic acids. This review provides useful insights concerning phenolic acid production through metabolic engineering of microbial cell factories.

Biosynthetic pathway of prescription bergenin from Bergenia purpurascens and Ardisia japonica

Bergenin is a typical carbon glycoside and the primary active ingredient in antitussive drugs widely prescribed for central cough inhibition in China. The bergenin extraction industry relies on the medicinal plant species Bergenia purpurascens and Ardisia japonica as their resources. However, the bergenin biosynthetic pathway in plants remains elusive. In this study, we functionally characterized a shikimate dehydrogenase (SDH), two O-methyltransferases (OMTs), and a C-glycosyltransferase (CGT) involved in bergenin synthesis through bioinformatics analysis, heterologous expression, and enzymatic characterization. We found that BpSDH2 catalyzes the two-step dehydrogenation process of shikimic acid to form gallic acid (GA). BpOMT1 and AjOMT1 facilitate the methylation reaction at the 4-OH position of GA, resulting in the formation of 4-O-methyl gallic acid (4-O-Me-GA). AjCGT1 transfers a glucose moiety to C-2 to generate 2-Glucosyl-4-O-methyl gallic acid (2-Glucosyl-4-O-Me-GA). Bergenin production ultimately occurs in acidic conditions or via dehydration catalyzed by plant dehydratases following a ring-closure reaction. This study for the first time uncovered the biosynthetic pathway of bergenin, paving the way to rational production of bergenin in cell factories via synthetic biology strategies.

Protocatechuate hydroxylase is a novel group A flavoprotein monooxygenase with a unique substrate recognition mechanism

Para-hydroxybenzoate hydroxylase (PHBH) is a group A flavoprotein monooxygenase that hydroxylates p-hydroxybenzoate to protocatechuate (PCA). Despite intensive studies of Pseudomonas aeruginosa p-hydroxybenzoate hydroxylase (PaPobA), the catalytic reactions of extremely diverse putative PHBH isozymes remain unresolved. We analyzed the phylogenetic relationships of known and predicted PHBHs and identified eight divergent clades. Clade F contains a protein that lacks the critical amino acid residues required for PaPobA to generate PHBH activity. Among proteins in this clade, Xylophilus ampelinus PobA (XaPobA) preferred PCA as a substrate and is the first known natural PCA 5-hydroxylase (PCAH). Crystal structures and kinetic properties revealed similar mechanisms of substrate carboxy group recognition between XaPobA and PaPobA. The unique Ile75, Met72, Val199, Trp201, and Phe385 residues of XaPobA form the bottom of a hydrophobic cavity with a shape that complements the 3-and 4-hydroxy groups of PCA and its binding site configuration. An interaction between the δ-sulfur atom of Met210 and the aromatic ring of PCA is likely to stabilize XaPobA-PCA complexes. The 4-hydroxy group of PCA forms a hydrogen bond with the main chain carbonyl of Thr294. These modes of binding constitute a novel substrate recognition mechanism that PaPobA lacks. This mechanism characterizes XaPobA and sheds light on the diversity of catalytic mechanisms of PobA-type PHBHs and group A flavoprotein monooxygenases.

Self-assembly systems to troubleshoot metabolic engineering challenges

Enzyme self-assembly is a technology in which enzyme units can aggregate into ordered macromolecules, assisted by scaffolds. In metabolic engineering, self-assembly strategies have been explored for aggregating multiple enzymes in the same pathway to improve sequential catalytic efficiency, which in turn enables high-level production. The performance of the scaffolds is critical to the formation of an efficient and stable assembly system. This review comprehensively analyzes these scaffolds by exploring how they assemble, and it illustrates how to apply self-assembly strategies for different modules in metabolic engineering. Functional modifications to scaffolds will further promote efficient strategies for production.

Transcriptomic analysis approach towards an improved tolerance of Escherichia coli to gallic acid stress

As a natural green additive, gallic acid has been widely used in food production. However, it can inhibit the physiological metabolism of Escherichia coli, which severely limits the ability and efficiency of gallic acid production. To explore the adaptation mechanism of E. coli under gallic acid stress and further explore the target of genetic modification, the effects of gallic acid stress on the fermentation characteristics of E. coli W3110 ATCC (82057) were investigated by cell biomass and cell morphometry. Moreover, transcriptome analysis was used to analyze the gene transcription level of E. coli W3110 ATCC (82057) to explore effects of gallic acid stress on important essential physiological processes. The results showed that under high concentration of gallic acid, the biomass of E. coli W3110 ATCC (82057) decreased significantly and the cells showed irregular morphology. Transcriptome analysis showed that E. coli W3110 ATCC (82057) improved its adaptive capacity through three strategies. First, genes of bamD, ompC, and ompF encoding outer membrane protein BamD, OmpC, and OmpC were decreased 5-, 31.1- and 8.1-fold, respectively, under gallic acid stress compared to the control, leading to the reduction of gallic acid absorption. Moreover, genes (mdtA, mdtB, mdtC, mdtD, mdtE, and mdtF) related to MdtABC multidrug efflux system and multidrug efflux pump MdtEF were up-regulated by1.0-53.0 folds, respectively, and genes (aaeA, aaeB, and aaeX) related to AaeAB efflux system were up-regulated by 8.0-13.3 folds, respectively, which contributed to the excretion of gallic acid. In addition, genes of acid fitness island also were up-regulated by different degrees under the stress of an acidic environment to maintain the stability of the intracellular environment. In conclusion, E. coli W3110 ATCC (82057) would enhance its tolerance to gallic acid by reducing absorption, increasing excretion, and maintaining intracellular environment stability. This study provides research ideas for the construction of engineered strains with high gallic acid yield.

Valorization of lignin-derived compounds into poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by engineered Halomonas sp. Y3

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a biopolyester with great potential, but its high production cost via the propionate-dependent pathway has hindered its development. Herein, we engineer Halomonas sp. Y3 to achieve efficient conversion of various LDCs into PHBV without propionate supplement. Initially, we successfully achieve PHBV production without propionate supplement by overexpressing threonine synthesis. The resulting biopolyester exhibits a 3 HV proportion of up to 7.89 mol%, comparable to commercial PHBV (8 mol%) available from Sigma Aldrich (403105). To further enhance PHBV production, we rationally design the reconstruction of aromatic compound catabolism. The engineered strain Y3_18 efficiently assimilates all LDCs containing syringyl (S), guaiacyl (G), and p-hydroxyphenyl-type (H) units. From 1 g/L of S-, G-, and H-type LDCs, Y3_18 produces PHBV at levels of 449 mg/L, 488 mg/L, and 716 mg/L, respectively, with yields of 44.9 % (g/g), 48.8 % (g/g), and 71.6 % (g/g). Moreover, to improve PHBV yield from lignin, we integrate laccase-secretion and PHBV production modules. This integration leads to the accumulation of 425.84 mg/L of PHBV with a yield of 21.29 % (g/g) and a 3 HV proportion of 6.38 mol%. By harnessing the capabilities of Halomonas sp. Y3, we demonstrate an efficient and sustainable approach for PHBV production from a variety of LDCs.

A 4-hydroxybenzoate 3-hydroxylase mutant enables 4-amino-3-hydroxybenzoic acid production from glucose in Corynebacterium glutamicum

BACKGROUND

Microbial production of aromatic chemicals is an attractive method for obtaining high-performance materials from biomass resources. A non-proteinogenic amino acid, 4-amino-3-hydroxybenzoic acid (4,3-AHBA), is expected to be a precursor of highly functional polybenzoxazole polymers; however, methods for its microbial production have not been reported. In this study, we attempted to produce 4,3-AHBA from glucose by introducing 3-hydroxylation of 4-aminobenzoic acid (4-ABA) into the metabolic pathway of an industrially relevant bacterium, Corynebacterium glutamicum.

RESULTS

Six different 4-hydroxybenzoate 3-hydroxylases (PHBHs) were heterologously expressed in C. glutamicum strains, which were then screened for the production of 4,3-AHBA by culturing with glucose as a carbon source. The highest concentration of 4,3-AHBA was detected in the strain expressing PHBH from Caulobacter vibrioides (CvPHBH). A combination of site-directed mutagenesis in the active site and random mutagenesis via laccase-mediated colorimetric assay allowed us to obtain CvPHBH mutants that enhanced 4,3-AHBA productivity under deep-well plate culture conditions. The recombinant C. glutamicum strain expressing CvPHBHM106A/T294S and having an enhanced 4-ABA biosynthetic pathway produced 13.5 g/L (88 mM) 4,3-AHBA and 0.059 g/L (0.43 mM) precursor 4-ABA in fed-batch culture using a nutrient-rich medium. The culture of this strain in the chemically defined CGXII medium yielded 9.8 C-mol% of 4,3-AHBA from glucose, corresponding to 12.8% of the theoretical maximum yield (76.8 C-mol%) calculated using a genome-scale metabolic model of C. glutamicum.

CONCLUSIONS

Identification of PHBH mutants that could efficiently catalyze the 3-hydroxylation of 4-ABA in C. glutamicum allowed us to construct an artificial biosynthetic pathway capable of producing 4,3-AHBA on a gram-scale using glucose as the carbon source. These findings will contribute to a better understanding of enzyme-catalyzed regioselective hydroxylation of aromatic chemicals and to the diversification of biomass-derived precursors for high-performance materials.

Designing artificial pathways for improving chemical production

Metabolic engineering exploits manipulation of catalytic and regulatory elements to improve a specific function of the host cell, often the synthesis of interesting chemicals. Although naturally occurring pathways are significant resources for metabolic engineering, these pathways are frequently inefficient and suffer from a series of inherent drawbacks. Designing artificial pathways in a rational manner provides a promising alternative for chemicals production. However, the entry barrier of designing artificial pathway is relatively high, which requires researchers a comprehensive and deep understanding of physical, chemical and biological principles. On the other hand, the designed artificial pathways frequently suffer from low efficiencies, which impair their further applications in host cells. Here, we illustrate the concept and basic workflow of retrobiosynthesis in designing artificial pathways, as well as the most currently used methods including the knowledge- and computer-based approaches. Then, we discuss how to obtain desired enzymes for novel biochemistries, and how to trim the initially designed artificial pathways for further improving their functionalities. Finally, we summarize the current applications of artificial pathways from feedstocks utilization to various products synthesis, as well as our future perspectives on designing artificial pathways.

Development and Application of Whole-Cell Biosensors for the Detection of Gallic Acid

Gallic acid is a prevalent secondary plant metabolite distinguished as one of the most effective free-radical scavengers among phenolic acids. This compound is also known for its cytotoxic, anti-inflammatory, and antimicrobial activities. Bulk quantities of gallic acid are conventionally produced by acid hydrolysis of tannins, a costly and environmentally hazardous process. With the aim to develop more sustainable approaches, microbial bioproduction strategies have been attempted recently. To advance synthetic biology and metabolic engineering of microorganisms for gallic acid production, we characterize here a transcription factor-based inducible system PpGalR/P_{PP_RS13150} that responds to the extracellular gallic acid in a dose-dependent manner in Pseudomonas putida KT2440. Surprisingly, this compound does not mediate induction when PpGalR/P_{PP_RS13150} is used in non-native host background. We show that the activation of the inducible system requires gallate dioxygenase activity encoded by galA gene. The 4-oxalomesaconic acid, an intermediate of gallic acid-metabolism, is identified as the effector molecule that interacts with the transcription factor GalR mediating activation of gene expression. Introduction of galA gene along galR enables development of biosensors suitable for detection and monitoring of gallic acid extracellularly using non-native hosts such as E. coli and C. necator. Moreover, the P. putida-based biosensor's applicability is demonstrated by detecting and measuring gallic acid in extracts of Camellia sinensis leaves. This study reports the strategy, which can be applied for developing gallic acid biosensors using bacterial species outside Pseudomonas genus.

Creative biological lignin conversion routes toward lignin valorization

Lignin, the largest renewable aromatic resource, is a promising alternative feedstock for the sustainable production of various chemicals, fuels, and materials. Despite this potential, lignin is characterized by heterogeneous and macromolecular structures that must be addressed. In this review, we present biological lignin conversion routes (BLCRs) that offer opportunities for overcoming these challenges, making lignin valorization feasible. Funneling heterogeneous aromatics via a 'biological funnel' offers a high-specificity bioconversion route for aromatic platform chemicals. The inherent aromaticity of lignin drives atom-economic functionalization routes toward aromatic natural product generation. By harnessing the ligninolytic capacities of specific microbial systems, powerful aromatic ring-opening routes can be developed to generate various value-added products. Thus, BLCRs hold the promise to make lignin valorization feasible and enable a lignocellulose-based bioeconomy.

A high-throughput visual screening method for p-hydroxybenzoate hydroxylase to increase phenolic compounds biosynthesis

Background

Gallic acid (GA) and pyrogallol are phenolic hydroxyl compounds and have diverse biological activities. Microbial-based biosynthesis, as an ecofriendly method, has been used for GA and pyrogallol production. In GA and pyrogallol biosynthetic pathways, the low hydroxylation activity of p-hydroxybenzoate hydroxylase (PobA) towards 3,4-dihydroxybenzoic acid (3,4-DHBA) limited the high-level biosynthesis of GA and pyrogallol.

Results

This work reported a high activity PobA mutant (Y385F/T294A/V349A PobA) towards 3,4-DHBA. This mutant was screened out from a PobA random mutagenesis library through a novel naked eye visual screening method. In vitro enzyme assay showed this mutant has a k_cat/K_m of 0.059 μM⁻¹ s⁻¹ towards 3,4-DHBA, which was 4.92-fold higher than the reported mutant (Y385F/T294A PobA). Molecular docking simulation provided the possible catalytic mechanism explanation of the high activity mutant. Expression of this mutant in E. coli BW25113 (Fʹ) can generate 840 ± 23 mg/L GA from 1000 mg/L 3,4-DHBA. After that, this mutant was assembled into a de novo GA biosynthetic pathway. Subsequently, this pathway was introduced into a 3,4-DHBA-producing strain (E. coli BW25113 (Fʹ)ΔaroE) to achieve 301 ± 15 mg/L GA production from simple carbon sources. Similarly, assembling this mutant into a de novo pyrogallol biosynthetic pathway enabled 129 ± 15 mg/L pyrogallol production.

Conclusions

This work established an efficient screening method and generated a high activity PobA mutant. Assembling this mutant into de novo GA and pyrogallol biosynthetic pathways achieved the production of these two compounds from glucose. Besides, this mutant has great potential for the production of GA or pyrogallol derivatives. The screening method could be used for other GA biosynthesis-related enzymes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02142-w.

Advances in biosynthesis of scopoletin

Scopoletin is a typical example of coumarins, which can be produced in plants. Scopoletin acts as a precursor for pharmaceutical and health care products, and also possesses promising biological properties, including antibacterial, anti-tubercular, anti-hypertensive, anti-inflammatory, anti-diabetic, and anti-hyperuricemic activity. Despite the potential benefits, the production of scopoletin using traditional extraction processes from plants is unsatisfactory. In recent years, synthetic biology has developed rapidly and enabled the effective construction of microbial cell factories for production of high value-added chemicals. Herein, this review summarizes the progress of scopoletin biosynthesis in artificial microbial cell factories. The two main pathways of scopoletin biosynthesis are summarized firstly. Then, synthetic microbial cell factories are reviewed as an attractive improvement strategy for biosynthesis. Emerging techniques in synthetic biology and metabolic engineering are introduced as innovative tools for the efficient synthesis of scopoletin. This review showcases the potential of biosynthesis of scopoletin in artificial microbial cell factories.

Metabolic Engineering of Shikimic Acid Biosynthesis Pathway for the Production of Shikimic Acid and Its Branched Products in Microorganisms: Advances and Prospects

The shikimate pathway is a necessary pathway for the synthesis of aromatic compounds. The intermediate products of the shikimate pathway and its branching pathway have promising properties in many fields, especially in the pharmaceutical industry. Many important compounds, such as shikimic acid, quinic acid, chlorogenic acid, gallic acid, pyrogallol, catechol and so on, can be synthesized by the shikimate pathway. Among them, shikimic acid is the key raw material for the synthesis of GS4104 (Tamiflu^®), an inhibitor of neuraminidase against avian influenza virus. Quininic acid is an important intermediate for synthesis of a variety of raw chemical materials and drugs. Gallic acid and catechol receive widespread attention as pharmaceutical intermediates. It is one of the hotspots to accumulate many kinds of target products by rationally modifying the shikimate pathway and its branches in recombinant strains by means of metabolic engineering. This review considers the effects of classical metabolic engineering methods, such as central carbon metabolism (CCM) pathway modification, key enzyme gene modification, blocking the downstream pathway on the shikimate pathway, as well as several expansion pathways and metabolic engineering strategies of the shikimate pathway, and expounds the synthetic biology in recent years in the application of the shikimate pathway and the future development direction.

Genetic redundancy of 4-hydroxybenzoate 3-hydroxylase genes ensures the catabolic safety of Pigmentiphaga sp. H8 in 3-bromo-4-hydroxybenzoate-contaminated habitats

Genetic redundancy is prevalent in organisms and plays important roles in the evolution of biodiversity and adaptation to environmental perturbation. However, selective advantages of genetic redundancy in overcoming metabolic disturbance due to structural analogues have received little attention. Here, functional divergence of the three 4-hydroxybenzoate 3-hydroxylase (PHBH) genes (phbh1~3) was found in Pigmentiphaga sp. strain H8. The genes phbh1/phbh2 were responsible for 3-bromo-4-hydroxybenzoate (3-Br-4-HB, an anthropogenic pollutant) catabolism, whereas phbh3 was primarily responsible for 4-hydroxybenzoate (4-HB, a natural intermediate of lignin) catabolism. 3-Br-4-HB inhibited 4-HB catabolism by competitively binding PHBH3, and was toxic to strain H8 cells especially at high concentrations. The existence of phbh1/phbh2 not only enabled strain H8 to utilize 3-Br-4-HB, but also ensured the catabolic safety of 4-HB. Molecular docking and site-directed mutagenesis analyses revealed that Val199 and Phe384 of PHBH1/PHBH2 were required for the hydroxylation activity towards 3-Br-4-HB. Phylogenetic analysis indicated that phbh1 and phbh2 originated from a common ancestor and evolved specifically in strain H8 to adapt to 3-Br-4-HB-contaminated habitats, whereas phbh3 evolved independently. This study deepens our understanding of selective advantages of genetic redundancy in prokaryote's metabolic robustness and reveals the factors driving the divergent evolution of redundant genes in adaptation to environmental perturbation. This article is protected by copyright. All rights reserved.

Toxicity and inhibition mechanism of gallic acid on physiology and fermentation performance of Escherichia coli

Gallic acid is a natural phenolic acid that has a stress inhibition effect on Escherichia coli. This study by integrates fermentation characteristics and transcriptional analyses to elucidate the physiological mechanism of E. coli 3110 response to gallic acid. Compared with the control (without stress), the cell growth was severely retarded, and irregular cell morphology appeared in the case of high levels of gallic acid stress. The glucose consumption of E. coli was reduced successively with the increase of gallic acid content in the fermentation medium. After 20 h of gallic acid stress, cofactor levels (ATP, NAD+ and NADH) of E. coli 3110 were similarly decreased, indicating a more potent inhibitory effect of gallic acid on E. coli. The transcriptional analysis revealed that gallic acid altered the gene expression profiles related to five notable differentially regulated pathways. The genes related to the two-component system were up-regulated, while the genes associated with ABC-transporter, energy metabolism, carbon metabolism, and fatty acid biosynthesis were down-regulated. This is the first report to comprehensively assess the toxicity of gallic acid on E. coli. This study has implications for the efficient production of phenolic compounds by E. coli and provides new ideas for the study of microbial tolerance to environmental stress and the identification of associated tolerance targets.

Current Advances in the Biodegradation and Bioconversion of Polyethylene Terephthalate

Polyethylene terephthalate (PET) is a widely used plastic that is polymerized by terephthalic acid (TPA) and ethylene glycol (EG). In recent years, PET biodegradation and bioconversion have become important in solving environmental plastic pollution. More and more PET hydrolases have been discovered and modified, which mainly act on and degrade the ester bond of PET. The monomers, TPA and EG, can be further utilized by microorganisms, entering the tricarboxylic acid cycle (TCA cycle) or being converted into high value chemicals, and finally realizing the biodegradation and bioconversion of PET. Based on synthetic biology and metabolic engineering strategies, this review summarizes the current advances in the modified PET hydrolases, engineered microbial chassis in degrading PET, bioconversion pathways of PET monomers, and artificial microbial consortia in PET biodegradation and bioconversion. Artificial microbial consortium provides novel ideas for the biodegradation and bioconversion of PET or other complex polymers. It is helpful to realize the one-step bioconversion of PET into high value chemicals.

In Vitro Biosynthesis of Isobutyraldehyde Through the Establishment of a One-Step Self-Assembly-Based Immobilization Strategy

The in vitro biosynthesis of high-value compounds has become popular and attractive. The convenient and simple strategy of enzyme immobilization has been significant for continuous and efficient in vitro biosynthesis. On the basis of that, this work established a one-step self-assembly-based immobilization strategy to efficiently biosynthesize isobutyraldehyde in vitro. Isobutyraldehyde is a crucial precursor for the synthesis of foods and spices. The established CipA scaffold-based strategy can express and immobilize enzymes at the same time, and purification requires only one centrifugation step. Structural simulations indicated that this scaffold-dependent self-assembly did not influence the structure or catalytic mechanisms of the isobutyraldehyde production-related enzymes leucine dehydrogenase (LeuDH) and ketoisovalerate decarboxylase (Kivd). Immobilized LeuDH and Kivd displayed a higher conversion capacity and thermal stability than the free enzymes. Batch conversion experiments demonstrated that the recovered immobilized LeuDH and Kivd have similar conversion capacities to the enzymes used in the first round of reaction. The continuous production of isobutyraldehyde was achieved by filling the immobilized enzymes into the column of a constructed device. This study not only expands the application range of self-assembly systems but also provides guidance for the in vitro production of value-added compounds.

One-Pot Bioconversion of Lignin-Derived Substrates into Gallic Acid

Lignin is regarded as the most abundant renewable aromatic compound on earth. In this study, we established Escherichia coli-based whole-cell biocatalytic systems to efficiently convert two lignin-derived substrates (ferulic acid and p-coumaric acid) to gallic acid. For the synthesis of gallic acid from ferulic acid, we used the recombinant E. coli expressing feruloyl-CoA synthetase and enoyl-CoA hydratase/aldolase from Pseudomonas putida, aldehyde dehydrogenase (HFD1) from Saccharomyces cerevisiae, vanillic acid O-demethylase (VanAB) from P. putida, and a mutant version of p-hydroxybenzoate hydroxylase (PobA^Y385F) from P. putida. Under the fed-batch mode, 19.57 mM gallic acid was obtained from 20 mM ferulic acid with a conversion rate of 97.9%. To achieve gallic acid synthesis from p-coumaric acid, we replaced VanAB with the two-component flavin-dependent monooxygenase (HpaBC) from E. coli. Under optimal conditions, 20 mM p-coumaric acid afforded the production of 19.96 mM gallic acid with near 100% conversion. To the best of our knowledge, our work represented the first study to develop E. coli-based whole-cell biocatalysts for the eco-friendly synthesis of gallic acid from lignin-derived renewable feedstocks.

Valorization of lignin components into gallate by integrated biological hydroxylation, O-demethylation, and aryl side-chain oxidation

Converting lignin components into a single product is a promising way to upgrade lignin. Here, an efficient biocatalyst was developed to selectively produce gallate from lignin components by integrating three main reactions: hydroxylation, O-demethylation, and aryl side-chain oxidation. A rationally designed hydroxylase system was first introduced into a gallate biodegradation pathway–blocked Rhodococcus opacus mutant so that gallate accumulated from protocatechuate and compounds in its upper pathways. Native and heterologous O-demethylation systems were then used, leading to multiple lignin-derived methoxy aromatics being converted to gallate. Furthermore, an aryl side-chain oxidase was engaged to broaden the substrate spectrum. Consequently, the developed biocatalyst showed that gallate yields as high as 0.407 and 0.630 g of gallate per gram of lignin when alkaline-pretreated lignin and base-depolymerized ammonia fiber explosion lignin were applied as substrates, respectively. These results suggested that this rationally developed biocatalyst enabled the lignin valorization process to be simple and efficient.

The beauty of biocatalysis: sustainable synthesis of ingredients in cosmetics

Covering: 2015 up to July 2021The market for cosmetics is consumer driven and the desire for green, sustainable and natural ingredients is increasing. The use of isolated enzymes and whole-cell organisms to synthesise these products is congruent with these values, especially when combined with the use of renewable, recyclable or waste feedstocks. The literature of biocatalysis for the synthesis of ingredients in cosmetics in the past five years is herein reviewed.

Flavoprotein monooxygenases: Versatile biocatalysts

Flavoprotein monooxygenases (FPMOs) are single- or two-component enzymes that catalyze a diverse set of chemo-, regio- and enantioselective oxyfunctionalization reactions. In this review, we describe how FPMOs have evolved from model enzymes in mechanistic flavoprotein research to biotechnologically relevant catalysts that can be applied for the sustainable production of valuable chemicals. After a historical account of the development of the FPMO field, we explain the FPMO classification system, which is primarily based on protein structural properties and electron donor specificities. We then summarize the most appealing reactions catalyzed by each group with a focus on the different types of oxygenation chemistries. Wherever relevant, we report engineering strategies that have been used to improve the robustness and applicability of FPMOs.

Engineering a Synthetic Pathway for Gentisate in Pseudomonas Chlororaphis P3

Pseudomonas chlororaphis P3 has been well-engineered as a platform organism for biologicals production due to enhanced shikimate pathway and excellent physiological and genetic characteristics. Gentisate displays high antiradical and antioxidant activities and is an important intermediate that can be used as a precursor for drugs. Herein, a plasmid-free biosynthetic pathway of gentisate was constructed by connecting the endogenous degradation pathway from 3-hydroxybenzoate in Pseudomonas for the first time. As a result, the production of gentisate reached 365 mg/L from 3-HBA via blocking gentisate conversion and enhancing the gentisate precursors supply through the overexpression of the rate-limiting step. With a close-up at the future perspectives, a series of bioactive compounds could be achieved by constructing synthetic pathways in conventional Pseudomonas to establish a cell factory.

Phenolic hydroxylases

Many flavin-dependent phenolic hydroxylases (monooxygenases) have been extensively investigated. Their crystal structures and reaction mechanisms are well understood. These enzymes belong to groups A and D of the flavin-dependent monooxygenases and can be classified as single-component and two-component flavin-dependent monooxygenases. The insertion of molecular oxygen into the substrates catalyzed by these enzymes is beneficial for modifying the biological properties of phenolic compounds and their derivatives. This chapter provides an in-depth discussion of the structural features of single-component and two-component flavin-dependent phenolic hydroxylases. The reaction mechanisms of selected enzymes, including 3-hydroxy-benzoate 4-hydroxylase (PHBH) and 3-hydroxy-benzoate 6-hydroxylase as representatives of single-component enzymes and 3-hydroxyphenylacetate 4-hydroxylase (HPAH) as a representative of two-component enzymes, are discussed in detail. This chapter comprises the following four main parts: general reaction, structures, reaction mechanisms, and enzyme engineering for biocatalytic applications. Enzymes belonging to the same group catalyze similar reactions but have different unique structural features to control their reactivity to substrates and the formation and stabilization of C4a-hydroperoxyflavin. Protein engineering has been employed to improve the ability to use these enzymes to synthesize valuable compounds. A thorough understanding of the structural and mechanistic features controlling enzyme reactivity is useful for enzyme redesign and enzyme engineering for future biocatalytic applications.

Protocatechuate overproduction by Corynebacterium glutamicum via simultaneous engineering of native and heterologous biosynthetic pathways

Protocatechuic acid (3, 4-dihydroxybenzoic acid, PCA) is a natural bioactive phenolic acid potentially valuable as a pharmaceutical raw material owing to its diverse pharmacological activities. Corynebacterium glutamicum forms PCA as a key intermediate in a native pathway to assimilate shikimate/quinate through direct conversion of the shikimate pathway intermediate 3-dehydroshikimate (DHS), which is catalyzed by qsuB-encoded DHS dehydratase (the DHS pathway). PCA can also be formed via an alternate pathway extending from chorismate by introducing heterologous chorismate pyruvate lyase that converts chorismate into 4-hydroxybenzoate (4-HBA), which is then converted into PCA catalyzed by endogenous 4-HBA 3-hydroxylase (the 4-HBA pathway). In this study, we generated three plasmid-free C. glutamicum strains overproducing PCA based on the markerless chromosomal recombination by engineering each or both of the above mentioned two PCA-biosynthetic pathways combined with engineering of the host metabolism to enhance the shikimate pathway flux and to block PCA consumption. Aerobic growth-arrested cell reactions were performed using the resulting engineered strains, which revealed that strains dependent on either the DHS or 4-HBA pathway as the sole PCA-biosynthetic route produced 43.8 and 26.2 g/L of PCA from glucose with a yield of 35.3% and 10.0% (mol/mol), respectively, indicating that PCA production through the DHS pathway is significantly efficient compared to that produced through the 4-HBA pathway. Remarkably, a strain simultaneously using both DHS and 4-HBA pathways achieved the highest reported PCA productivity of 82.7 g/L with a yield of 32.8% (mol/mol) from glucose in growth-arrested cell reaction. These results indicated that simultaneous engineering of both DHS and 4-HBA pathways is an efficient method for PCA production. The generated PCA-overproducing strain is plasmid-free and does not require supplementation of aromatic amino acids and vitamins due to the intact shikimate pathway, thereby representing a promising platform for the industrial bioproduction of PCA and derived chemicals from renewable sugars.

Metabolic engineering of E. coli for producing phloroglucinol from acetate

Phloroglucinol is a three-hydroxyl phenolic compound and has diverse physiological and pharmacological activities such as antivirus and anti-inflammatory activities. Chemical synthesis of phloroglucinol suffered from many drawbacks such as high cost and environmental pollution. To avoid the above issues, microbial phloroglucinol biosynthesis was successfully accomplished in this study, while the abundant and low-cost acetate was used as the main carbon source. Firstly, the toxicity of phloroglucinol was tested, and E. coli BL21(DE3) could tolerate 5 g/L phloroglucinol. The ability of phloroglucinol synthase (PhlD) for catalyzing malonyl-CoA to phloroglucinol was confirmed, and E. coli BL21(DE3) expressing PhlD and acetyl-CoA carboxylase (ACCase) could produce 1107 ± 12 mg/L phloroglucinol from glucose. Then, E. coli BL21(DE3) was engineered to utilize acetate to produce 228 ± 15 mg/L phloroglucinol. Then, the endogenous citrate synthase (GltA) which could catalyze oxaloacetate and acetyl-CoA generated from acetate to citrate was knocked down by CRISPRi system in order to enhance the carbon flux for phloroglucinol production, and the titer was improved to 284 ± 8 mg/L. This work demonstrated that acetate could be used as low-cost substrate to achieve the biosynthesis of phloroglucinol and provided an example of effective utilization of acetate.

Metabolic engineering of Escherichia coli for production of chemicals derived from the shikimate pathway

The shikimate pathway is indispensable for the biosynthesis of natural products with aromatic moieties. These products have wide current and potential applications in food, cosmetics and medicine, and consequently have great commercial value. However, compounds extracted from various plants or synthesized from petrochemicals no longer satisfy the requirements of contemporary industries. As a result, an increasing number of studies has focused on this pathway to enable the biotechnological manufacture of natural products, especially in E. coli. Furthermore, the development of synthetic biology, systems metabolic engineering and high flux screening techniques has also contributed to improving the biosynthesis of high-value compounds based on the shikimate pathway. Here, we review approaches based on a combination of traditional and new metabolic engineering strategies to increase the metabolic flux of the shikimate pathway. In addition, applications of this optimized pathway to produce aromatic amino acids and a range of natural products is also elaborated. Finally, this review sums up the opportunities and challenges facing this field.

A Growth-Based, High-Throughput Selection Platform Enables Remodeling of 4-Hydroxybenzoate Hydroxylase Active Site

We report an aerobic, growth-based selection platform founded on NADP(H) redox balance restoration in Escherichia coli, and we demonstrate its application in the high-throughput evolution of an oxygenase. A single round of selection followed by a facile growth assay enabled Pseudomonas aeruginosa 4-hydroxybenzoate hydroxylase (PobA) to efficiently hydroxylate both 4-hydroxybenzoic acid (4-HBA) and 3,4-dihydroxybenzoic acid (3,4-DHBA), two consecutive steps in gallic acid biosynthesis. Structural modeling suggests precise reorganization of active site hydrogen bond network, which is difficult to obtain without deep navigation of combinatorial sequence space. We envision universal application of this selection platform in engineering NADPH-dependent oxidoreductases.

Advances and Prospects of Phenolic Acids Production, Biorefinery and Analysis

Biotechnological production of phenolic acids is attracting increased interest due to their superior antioxidant activity, as well as other antimicrobial, dietary, and health benefits. As secondary metabolites, primarily found in plants and fungi, they are effective free radical scavengers due to the phenolic group available in their structure. Therefore, phenolic acids are widely utilised by pharmaceutical, food, cosmetic, and chemical industries. A demand for phenolic acids is mostly satisfied by utilising chemically synthesised compounds, with only a low quantity obtained from natural sources. As an alternative to chemical synthesis, environmentally friendly bio-based technologies are necessary for development in large-scale production. One of the most promising sustainable technologies is the utilisation of microbial cell factories for biosynthesis of phenolic acids. In this paper, we perform a systematic comparison of the best known natural sources of phenolic acids. The advances and prospects in the development of microbial cell factories for biosynthesis of these bioactive compounds are discussed in more detail. A special consideration is given to the modern production methods and analytics of phenolic acids.

Strategies for the production of biochemicals in bioenergy crops

Industrial crops are grown to produce goods for manufacturing. Rather than food and feed, they supply raw materials for making biofuels, pharmaceuticals, and specialty chemicals, as well as feedstocks for fabricating fiber, biopolymer, and construction materials. Therefore, such crops offer the potential to reduce our dependency on petrochemicals that currently serve as building blocks for manufacturing the majority of our industrial and consumer products. In this review, we are providing examples of metabolites synthesized in plants that can be used as bio-based platform chemicals for partial replacement of their petroleum-derived counterparts. Plant metabolic engineering approaches aiming at increasing the content of these metabolites in biomass are presented. In particular, we emphasize on recent advances in the manipulation of the shikimate and isoprenoid biosynthetic pathways, both of which being the source of multiple valuable compounds. Implementing and optimizing engineered metabolic pathways for accumulation of coproducts in bioenergy crops may represent a valuable option for enhancing the commercial value of biomass and attaining sustainable lignocellulosic biorefineries.

Structural Insights into Catalytic Versatility of the Flavin-dependent Hydroxylase (HpaB) from Escherichia coli

4-Hydroxyphenylacetate 3-hydroxylase (EcHpaB) from Escherichia coli is capable of efficient ortho-hydroxylation of a wide range of phenolic compounds and demonstrates great potential for broad chemoenzymatic applications. To understand the structural and mechanistic basis of its catalytic versatility, we elucidated the crystal structure of EcHpaB by X-ray crystallography, which revealed a unique loop structure covering the active site. We further performed mutagenesis studies of this loop to probe its role in substrate specificity and catalytic activity. Our results not only showed the loop has great plasticity and strong tolerance towards extensive mutagenesis, but also suggested a flexible loop that enables the entrance and stable binding of substrates into the active site is the key factor to the enzyme catalytic versatility. These findings lay the groundwork for editing the loop sequence and structure for generation of EcHpaB mutants with improved performance for broader laboratory and industrial use.