High-yield production of vanillin from ferulic acid by a coenzyme-independent decarboxylase/oxygenase two-stage process

Harnessing nature's catalysts: Advances in enzymatic alkene cleavage

Double bonds are prevalent in various substrates and renewable feedstocks, and their cleavage typically necessitates harsh reaction conditions involving high temperatures, organic solvents, and hazardous catalysts such as heavy metals or ozone. This review explores the sustainable enzymatic alternatives developed by nature for alkene cleavage. It provides a comprehensive overview of alkene-cleaving enzymes, detailing their mechanisms, substrate specificities, and applications. The enzymes discussed include those acting on aliphatic, cyclic, and activated aromatic systems. Emphasizing the significance of these biocatalysts in green chemistry and biocatalysis, this review highlights their potential to replace traditional chemical oxidants with safer, cost-effective, and environmentally friendly options. Future research directions include expanding enzyme substrate scopes, enhancing their operational stability and activity, and integrating them into scalable processes for broader application in the pharmaceutical, flavor, and fragrance industries.

Genetic Engineering Approaches for the Microbial Production of Vanillin

Vanilla flavour is widely used in various industries and is the most broadly used flavouring agent in the food industry. The demand for this flavour is, therefore, extremely high, yet vanilla bean extracts can only meet about 1% of the overall demand. Vanillin, the main constituent of vanilla flavour, can easily be obtained through chemical synthesis. Nonetheless, consumer demands for natural products and environmentally friendly industrial processes drive the development of biotechnological approaches for its production. Some microorganisms can naturally produce vanillin when fed with various substrates, including eugenol, isoeugenol, and ferulic acid. The characterisation of the genes and enzymes involved in these bioconversion pathways, as well as progress in the understanding of vanillin biosynthesis in Vanilla orchids, allowed the development of genetic engineering and synthetic biology approaches to increase vanillin production in naturally vanillin-producing microorganisms, or to implement novel vanillin biosynthetic pathways in microbial chassis. This review summarises and discusses these genetic engineering and synthetic biology approaches for the microbial production of vanillin.

Distal mutations enhance efficiency of free and immobilized NOV1 dioxygenase for vanillin synthesis

Protein engineering is crucial to improve enzymes' efficiency and robustness for industrial biocatalysis. NOV1 is a bacterial dioxygenase that holds biotechnological potential by catalyzing the one-step oxidation of the lignin-derived isoeugenol into vanillin, a popular flavoring agent used in food, cleaning products, cosmetics and pharmaceuticals. This study aims to enhance NOV1 activity and operational stability through the identification of distal hotspots, located at more than 9 Å from the active site using Zymspot, a tool that predicts advantageous distant mutations, streamlining protein engineering. A total of 41 variants were constructed using site-directed mutagenesis and the six most active enzyme variants were then recombined. Two variants, with two and three mutations, showed nearly a 10-fold increase in activity and up to 40-fold higher operational stability than the wild-type. Furthermore, these variants show 90-100 % immobilization efficiency in metal affinity resins, compared to approximately 60 % for the wild-type. In bioconversions where 50 mM of isoeugenol was added stepwise over 24-h cycles, the 1D2 variant produced approximately 144 mM of vanillin after six reaction cycles, corresponding to around 22 mg, indicating a 35 % molar conversion yield. This output was around 2.5 times higher than that obtained using the wild-type. Our findings highlight the efficacy of distal protein engineering in enhancing enzyme functions like activity, stability, and metal binding selectivity, thereby fulfilling the criteria for industrial biocatalysts. This study provides a novel approach to enzyme optimization that could have significant implications for various biotechnological applications.

Engineering a coenzyme-independent dioxygenase for one-step production of vanillin from ferulic acid

Vanillin is one of the world's most important flavor and fragrance compounds used in foods and cosmetics. In plants, vanillin is reportedly biosynthesized from ferulic acid via the hydratase/lyase-type enzyme VpVAN. However, in biotechnological and biocatalytic applications, the use of VpVAN limits the production of vanillin. Although microbial enzymes are helpful as substitutes for plant enzymes, synthesizing vanillin from ferulic acid in one step using microbial enzymes remains a challenge. Here, we developed a single enzyme that catalyzes vanillin production from ferulic acid in a coenzyme-independent manner via the rational design of a microbial dioxygenase in the carotenoid cleavage oxygenase family using computational simulations. This enzyme acquired catalytic activity toward ferulic acid by introducing mutations into the active center to increase its affinity for ferulic acid. We found that the single enzyme can catalyze not only the production of vanillin from ferulic acid but also the synthesis of other aldehydes from p-coumaric acid, sinapinic acid, and coniferyl alcohol. These results indicate that the approach used in this study can greatly expand the range of substrates available for the dioxygenase family of enzymes. The engineered enzyme enables efficient production of vanillin and other value-added aldehydes from renewable lignin-derived compounds.

IMPORTANCE

The final step of vanillin biosynthesis in plants is reportedly catalyzed by the enzyme VpVAN. Prior to our study, VpVAN was the only reported enzyme that directly converts ferulic acid to vanillin. However, as many characteristics of VpVAN remain unknown, this enzyme is not yet suitable for biocatalytic applications. We show that an enzyme that converts ferulic acid to vanillin in one step could be constructed by modifying a microbial dioxygenase-type enzyme. The engineered enzyme is of biotechnological importance as a tool for the production of vanillin and related compounds via biocatalytic processes and metabolic engineering. The results of this study may also provide useful insights for understanding vanillin biosynthesis in plants.

Engineering a Carotenoid Cleavage Oxygenase for Coenzyme-Free Synthesis of Vanillin from Ferulic Acid

One-pot biosynthesis of vanillin from ferulic acid without providing energy and cofactors adds significant value to lignin waste streams. However, naturally evolved carotenoid cleavage oxygenase (CCO) with extreme catalytic conditions greatly limited the above pathway for vanillin bioproduction. Herein, CCO from Thermothelomyces thermophilus (TtCCO) was rationally engineered for achieving high catalytic activity under neutral pH conditions and was further utilized for constructing a one-pot synthesis system of vanillin with Bacillus pumilus ferulic acid decarboxylase. TtCCO with the K192N-V310G-A311T-R404N-D407F-N556A mutation (TtCCOM3) was gradually obtained using substrate access channel engineering, catalytic pocket engineering, and pocket charge engineering. Molecular dynamics simulations revealed that reducing the site-blocking effect in the substrate access channel, enhancing affinity for substrates in the catalytic pocket, and eliminating the pocket's alkaline charge contributed to the high catalytic activity of TtCCOM3 under neutral pH conditions. Finally, the one-pot synthesis of vanillin in our study could achieve a maximum rate of up to 6.89 ± 0.3 mM h-1. Therefore, our study paves the way for a one-pot biosynthetic process of transforming renewable lignin-related aromatics into valuable chemicals.

A comprehensive review of eclectic approaches to the biological synthesis of vanillin and their application towards the food sector

Vanillin, a highly regarded flavor compound, has earned widespread recognition for its natural and aromatic qualities, piquing substantial interest in the scientific community. This comprehensive review delves deeply into the intricate world of vanillin synthesis, encompassing a wide spectrum of methodologies, including enzymatic, microbial, and immobilized systems. This investigation provides a thorough analysis of the precursors of vanillin and also offers a comprehensive overview of its transformation through these diverse processes, making it an invaluable resource for researchers and enthusiasts alike. The elucidation of different substrates such as ferulic acid, eugenol, veratraldehyde, vanillic acid, glucovanillin, and C6-C3 phenylpropanoids adds a layer of depth and insight to the understanding of vanillin synthesis. Moreover, this comprehensive review explores the multifaceted applications of vanillin within the food industry. While commonly known as a flavoring agent, vanillin transcends this role by finding extensive use in food preservation and food packaging. The review meticulously examines the remarkable preservative properties of vanillin, providing a profound understanding of its crucial role in the culinary and food science sectors, thus making it an indispensable reference for professionals and researchers in these domains.

Graphical abstract

Nature stays natural: two novel chemo-enzymatic one-pot cascades for the synthesis of fragrance and flavor aldehydes

Novel synthetic strategies for the production of high-value chemicals based on the 12 principles of green chemistry are highly desired. Herein, we present a proof of concept for two novel chemo-enzymatic one-pot cascades allowing for the production of valuable fragrance and flavor aldehydes. We utilized renewable phenylpropenes, such as eugenol from cloves or estragole from estragon, as starting materials. For the first strategy, Pd-catalyzed isomerization of the allylic double bond and subsequent enzyme-mediated (aromatic dioxygenase, ADO) alkene cleavage were performed to obtain the desired aldehydes. In the second route, the double bond was oxidized to the corresponding ketone via a copper-free Wacker oxidation protocol followed by enzymatic Baeyer-Villiger oxidation (phenylacetone monooxygenase from Thermobifida fusca), esterase-mediated (esterase from Pseudomonas fluorescens, PfeI) hydrolysis and subsequent oxidation of the primary alcohol (alcohol dehydrogenase from Pseudomonas putida, AlkJ) to the respective aldehyde products. Eight different phenylpropene derivatives were subjected to these reaction sequences, allowing for the synthesis of seven aldehydes in up to 55% yield after 4 reaction steps (86% for each step).

Strategies for improving the production of bio-based vanillin

Vanillin (4-hydroxy-3-methoxybenzaldehyde) is one of the most popular flavors with wide applications in food, fragrance, and pharmaceutical industries. However, the high cost and limited yield of plant extraction failed to meet the vast market demand of natural vanillin. Vanillin biotechnology has emerged as a sustainable and cost-effective alternative to supply vanillin. In this review, we explored recent advances in vanillin biosynthesis and highlighted the potential of vanillin biotechnology. In particular, we addressed key challenges in using microorganisms and provided promising approaches for improving vanillin production with a special focus on chassis development, pathway construction and process optimization. Future directions of vanillin biosynthesis using inexpensive precursors are also thoroughly discussed.

Rationally Guided Improvement of NOV1 Dioxygenase for the Conversion of Lignin-Derived Isoeugenol to Vanillin

Biocatalysis is a key tool in both green chemistry and biorefinery fields. NOV1 is a dioxygenase that catalyzes the one-step, coenzyme-free oxidation of isoeugenol into vanillin and holds enormous biotechnological potential for the complete valorization of lignin as a sustainable starting material for biobased chemicals, polymers, and materials. This study integrates computational, kinetic, structural, and biophysical approaches to characterize a new NOV1 variant featuring improved activity and stability compared to those of the wild type. The S283F replacement results in a 2-fold increased turnover rate (k_cat) for isoeugenol and a 4-fold higher catalytic efficiency (k_cat/K_m) for molecular oxygen compared to those of the wild type. Furthermore, the variant exhibits a half-life that is 20-fold higher than that of the wild type, which most likely relates to the enhanced stabilization of the iron cofactor in the active site. Molecular dynamics supports this view, revealing that the S283F replacement decreases the optimal pK_a and favors conformations of the iron-coordinating histidines compatible with an increased level of binding to iron. Importantly, whole cells containing the S283F variant catalyze the conversion of ≤100 mM isoeugenol to vanillin, yielding >99% molar conversion yields within 24 h. This integrative strategy provided a new enzyme for biotechnological applications and mechanistic insights that will facilitate the future design of robust and efficient biocatalysts.

Developing efficient vanillin biosynthesis system by regulating feruloyl-CoA synthetase and enoyl-CoA hydratase enzymes

Vanillin is one of the most commonly used natural-occurring flavors in the world. This study successfully constructed an efficient whole-cell catalytic system for vanillin biosynthesis from ferulic acid by regulating feruloyl-CoA synthetase (FCS) and enoyl-CoA hydratase (ECH). First, we constructed an efficient cell-free catalytic system with FCS-Str (fcs from Streptomyces sp. V-1) and ECH-Str (ech from Streptomyces sp. V-1) combination at 1:1. The efficient cell-free catalytic system provided necessary strategies for optimizing the whole-cell catalytic system. Then, we constructed the recombinant Escherichia coli by heterologously expressing the fcs-Str and ech-Str combination. Moreover, E. coli JM109 was a better recombinant Escherichia coli than E. coli BL21 with higher vanillin production. Finally, we first adjusted the ratio of FCS and ECH in E. coli JM109 to 1:1 using two copies of fcs-Str. For higher vanillin production, we further optimized the induction conditions of E. coli JM109 to increase the amount of FCS and ECH. The optimized E. coli JM109-FE-F constructed in this study has the highest vanillin synthesis rate of converting 20 mM ferulic acid to 15 mM vanillin in 6 h among all of the E. coli catalytic systems. Our study made a significant contribution to the construction of the vanillin biosynthesis system and provided a valuable strategy for increasing vanillin production. KEY POINTS: • The efficient cell-free vanillin biosynthesis system was constructed by FCS-Str and ECH-Str combination at 1:1. • Escherichia coli JM109 was determined as a better recombinant Escherichia coli than E. coli BL21 with higher vanillin production. • Escherichia coli JM109-FE-F with two copies of fcs-Str and one copy of ech-Str has the highest catalytic efficiency for vanillin production.

Significant production of vanillin and in vitro amplification of ech gene in local bacterial isolates

Vanillin is the major component which is responsible for flavor and aroma of vanilla extract and is produced by 3 ways: natural extraction from vanilla plant, chemical synthesis and from microbial transformation. Current research was aimed to study bacterial production of vanillin from native natural sources including sewage and soil from industrial areas. The main objective was vanillin bio-production by isolating bacteria from these native sources. Also to adapt methodologies to improve vanillin production by optimized fermentation media and growth conditions. 47 soil and 13 sewage samples were collected from different industrial regions of Lahore, Gujranwala, Faisalabad and Kasur. 67.7% bacterial isolates produced vanillin and 32.3% were non-producers. From these 279 producers, 4 bacterial isolates selected as significant producers were; A3, A4, A7 and A10. These isolates were identified by ribotyping as A3 Pseudomonas fluorescence (KF408302), A4 Enterococcus faecium (KT356807), A7 Alcaligenes faecalis (MW422815) and A10 Bacillus subtilis (KT962919). Vanillin producers were further tested for improved production of vanillin and were grown in different fermentation media under optimized growth conditions for enhanced production of vanillin. The fermentation media (FM) were; clove oil based, rice bran waste (residues oil) based, wheat bran based and modified isoeugenol based. In FM5, FM21, FM22, FM23, FM24, FM30, FM31, FM32, FM34, FM35, FM36, and FM37, the selected 4 bacterial strains produced significant amounts of vanillin. A10 B. subtilis produced maximum amount of vanillin. This strain produced 17.3 g/L vanillin in FM36. Cost of this fermentation medium 36 was 131.5 rupees/L. This fermentation medium was modified isoeugenol based medium with 1% of isoeugenol and 2.5 g/L soybean meal. ech gene was amplified in A3 P. fluorescence using ech specific primers. As vanillin use as flavor has increased tremendously, the bioproduction of vanillin must be focused.

Enhanced Thermostability of Pseudomonas nitroreducens Isoeugenol Monooxygenase by the Combinatorial Strategy of Surface Residue Replacement and Consensus Mutagenesis

Vanillin has many applications in industries. Isoeugenol monooxygenase (IEM) can catalyze the oxidation of isoeugenol to vanillin in the presence of oxygen under mild conditions. However, the low thermal stability of IEM limits its practical application in the biosynthesis of natural vanillin. Herein, two rational strategies were combined to improve the thermostability of IEM from Pseudomonas nitroreducens Jin1. Two variants (K83R and K95R) with better thermostability and one mutant (G398A) with higher activity were identified from twenty candidates based on the Surface Residue Replacement method. According to the Consensus Mutagenesis method, one mutant (I352R) with better thermostability and another mutant (L273F) with higher activity were also identified from nine candidates. After combinatorial mutation, a triple mutant K83R/K95R/L273F with the best thermostability and catalytic efficiency was generated. Compared with the wild-type IEM, the thermal inactivation half-lives (t1/2) of K83R/K95R/L273F at 25 °C, 30 °C, and 35 °C increased 2.9-fold, 11.9-fold, and 24.7-fold, respectively. Simultaneously, it also exhibited a 4.8-fold increase in kcat, leading to a 1.2-fold increase in catalytic efficiency (kcat/Km). When the whole cell of K83R/K95R/L273F was applied to the biotransformation of isoeugenol on preparative scale, the vanillin concentration reached 240.1 mM with space-time yield of 109.6 g/L/d, and vanillin was achieved in 77.6% isolated yield and >99% purity.

Novel Cold-Adapted Recombinant Laccase KbLcc1 from Kabatiella bupleuri G3 IBMiP as a Green Catalyst in Biotransformation

Cold-adapted enzymes are useful tools in the organic syntheses conducted in mixed aqueous-organic or non-aqueous solvents due to their molecular flexibility that stabilizes the proteins in low water activity environments. A novel psychrophilic laccase gene from Kabatiella bupleuri, G3 IBMiP, was spliced by Overlap-Extension PCR (OE-PCR) and expressed in Pichia pastoris. Purified recombinant KbLcc1 laccase has an optimal temperature of 30 °C and pH of 3.5, 5.5, 6.0, and 7.0 in the reaction with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), guaiacol, sinapic acid, and syringaldazine, respectively. Moreover, laccase KbLcc1 is highly thermolabile, as it loses 40% of activity after 30 min at 40 °C and is inactivated at 50 °C after the same period of incubation. The new enzyme remained active with 1 mM of Ni2+, Cu2+, Mn2+, and Zn2+ and with 2 mM of Co2+, Ca2+, and Mg2+, but Fe2+ greatly inhibited the laccase activity. Moreover, 1% ethanol had no impact on KbLcc1, although acetone and ethyl acetate decreased the laccase activity. The presence of hexane (40%, v/v) caused a 58% increase in activity. Laccase KbLcc1 could be applied in the decolorization of synthetic dyes and in the biotransformation of ferulic acid to vanillin. After 5 days of reaction at 20 °C, pH 3.5, with 1 mM ABTS as a mediator, the vanillin concentration was 21.9 mg/L and the molar yield of transformation reached 14.39%.

Aspergillus niger uses the peroxisomal CoA-dependent β-oxidative genes to degrade the hydroxycinnamic acids caffeic acid, ferulic acid, and p-coumaric acid

Abstract

Aromatic compounds are important molecules which are widely applied in many industries and are mainly produced from nonrenewable sources. Renewable sources such as plant biomass are interesting alternatives for the production of aromatic compounds. Ferulic acid and p-coumaric acid, a precursor for vanillin and p-vinyl phenol, respectively, can be released from plant biomass by the fungus Aspergillus niger. The degradation of hydroxycinnamic acids such as caffeic acid, ferulic acid, and p-coumaric acid has been observed in many fungi. In A. niger, multiple metabolic pathways were suggested for the degradation of hydroxycinnamic acids. However, no genes were identified for these hydroxycinnamic acid metabolic pathways. In this study, several pathway genes were identified using whole-genome transcriptomic data of A. niger grown on different hydroxycinnamic acids. The genes are involved in the CoA-dependent β-oxidative pathway in fungi. This pathway is well known for the degradation of fatty acids, but not for hydroxycinnamic acids. However, in plants, it has been shown that hydroxycinnamic acids are degraded through this pathway. We identified genes encoding hydroxycinnamate-CoA synthase (hcsA), multifunctional β-oxidation hydratase/dehydrogenase (foxA), 3-ketoacyl CoA thiolase (katA), and four thioesterases (theA-D) of A. niger, which were highly induced by all three tested hydroxycinnamic acids. Deletion mutants revealed that these genes were indeed involved in the degradation of several hydroxycinnamic acids. In addition, foxA and theB are also involved in the degradation of fatty acids. HcsA, FoxA, and KatA contained a peroxisomal targeting signal and are therefore predicted to be localized in peroxisomes.

Key points

• Metabolism of hydroxycinnamic acid was investigated in Aspergillus niger

• Using transcriptome data, multiple CoA-dependent β-oxidative genes were identified.

• Both foxA and theB are involved in hydroxycinnamate but also fatty acid metabolism.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11311-0.

Synthetic Enzyme-Catalyzed CO2 Fixation Reactions

In recent years, (de)carboxylases that catalyze reversible (de)carboxylation have been targeted for application as carboxylation catalysts. This has led to the development of proof-of-concept (bio)synthetic CO2 fixation routes for chemical production. However, further progress towards industrial application has been hampered by the thermodynamic constraint that accompanies fixing CO2 to organic molecules. In this Review, biocatalytic carboxylation methods are discussed with emphases on the diverse strategies devised to alleviate the inherent thermodynamic constraints and their application in synthetic CO2 -fixation cascades.

2G waste lignin to fuel and high value-added chemicals: Approaches, challenges and future outlook for sustainable development

Lignin is produced as a byproduct in cellulosic biorefinery as well in pulp and paper industries and has the potential for the synthesis of a variety of phenolics chemicals, biodegradable polymers, and high value-added chemicals surrogate to conventional petro-based fuels. Therefore, in this critical review, we emphasize the possible scenario for lignin isolation, transformation into value addition chemicals/materials for the economic viability of current biorefineries. Additionally, this review covers the chemical structure of lignocellulosic biomass/lignin, worldwide availability of lignin and describe various thermochemical (homogeneous/heterogeneous base/acid-catalyzed depolymerization, oxidative, hydrogenolysis etc.) and biotechnological developments for the production of bio-based low molecular weight phenolics, i.e. polyhydroxyalkanoates, vanillin, adipic acid, lipids etc. Besides, some functional chemicals applications, lignin-formaldehyde ion exchange resin, electrochemical and production of few targeted chemicals are also elaborated. Finally, we examine the challenges, opportunities and prospects way forward related to lignin valorization.

Catabolic Machinery of the Human Gut Microbes Bestow Resilience Against Vanillin Antimicrobial Nature

Vanillin is a phenolic food additive commonly used for flavor, antimicrobial, and antioxidant properties. Though it is one of the widely used food additives, strategies of the human gut microbes to evade its antimicrobial activity await extensive elucidation. The current study explores the human gut microbiome with a multi-omics approach to elucidate its composition and metabolic machinery to counter vanillin bioactivity. A combination of SSU rRNA gene diversity, metagenomic RNA features diversity, phylogenetic affiliation of metagenome encoded proteins, uniformly (R = 0.99) indicates the abundance of Bacteroidetes followed by Firmicutes and Proteobacteria. Manual curation of metagenomic dataset identified gene clusters specific for the vanillin metabolism (ligV, ligK, and vanK) and intermediary metabolic pathways (pca and cat operon). Metagenomic dataset comparison identified the omnipresence of vanillin catabolic features across diverse populations. The metabolomic analysis brings forth the functionality of the vanillin catabolic pathway through the Protocatechuate branch of the beta-ketoadipate pathway. These results highlight the human gut microbial features and metabolic bioprocess involved in vanillin catabolism to overcome its antimicrobial activity. The current study advances our understanding of the human gut microbiome adaption toward changing dietary habits.

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.

Bioprocess Optimization for the Production of Aromatic Compounds With Metabolically Engineered Hosts: Recent Developments and Future Challenges

The most common route to produce aromatic chemicals - organic compounds containing at least one benzene ring in their structure - is chemical synthesis. These processes, usually starting from an extracted fossil oil molecule such as benzene, toluene, or xylene, are highly environmentally unfriendly due to the use of non-renewable raw materials, high energy consumption and the usual production of toxic by-products. An alternative way to produce aromatic compounds is extraction from plants. These extractions typically have a low yield and a high purification cost. This motivates the search for alternative platforms to produce aromatic compounds through low-cost and environmentally friendly processes. Microorganisms are able to synthesize aromatic amino acids through the shikimate pathway. The construction of microbial cell factories able to produce the desired molecule from renewable feedstock becomes a promising alternative. This review article focuses on the recent advances in microbial production of aromatic products, with a special emphasis on metabolic engineering strategies, as well as bioprocess optimization. The recent combination of these two techniques has resulted in the development of several alternative processes to produce phenylpropanoids, aromatic alcohols, phenolic aldehydes, and others. Chemical species that were unavailable for human consumption due to the high cost and/or high environmental impact of their production, have now become accessible.

Maximizing the Efficiency of Vanillin Production by Biocatalyst Enhancement and Process Optimization

The rising demand of bio-vanillin and the possibility to use microbial biotransformation to produce this compound from agroindustrial byproducts are economically attractive. However, there are still several bottlenecks, including substrate and product toxicity, formation of undesired products and genetic stability of the recombinant strains, that impede an efficient use of recombinant Escherichia coli strains to make the whole process cost effective. To overcome these problems, we developed a new E. coli strain, named FR13, carrying the Pseudomonas genes encoding feruloyl-CoA synthetase and feruloyl-CoA hydratase/aldolase integrated into the chromosome and, using resting cells, we demonstrated that the vanillin yield and selectivity were strongly affected by the physiological state of the cells, the temperature used for the growth and the recovery of the biomass and the composition and pH of the bioconversion buffer. The substrate consumption rate and the vanillin yield increased using a sodium/potassium phosphate buffer at pH 9.0 as bioconversion medium. Optimization of the bioprocess variables, using response surface methodology, together with the use of a two-phase (solid-liquid) system for the controlled release of ferulic acid allowed us to increase the vanillin yield up to 28.10 ± 0.05 mM. These findings showed that recombinant plasmid-free E. coli strains are promising candidates for the production of vanillin at industrial scale and that a reduction of the cost of the bioconversion process requires approaches that minimize the toxicity of both ferulic acid and vanillin.

Expression and Characterization of Carotenoid Cleavage Oxygenases From Herbaspirillum seropedicae and Rhodobacteraceae bacterium Capable of Biotransforming Isoeugenol and 4-Vinylguaiacol to Vanillin

HsCCO and RbCCO from Herbaspirillum seropedicae and Rhodobacteraceae bacterium were selected and characterized from five putative bacterial carotenoid cleavage oxygenase gene sequences, due to merits in expression solubility and catalytic properties. Both enzymes can convert 4-vinylguaiacol and isoeugenol to vanillin. HsCCO showed maximum activity at 40°C and pH 7.0 and was stable at pH 6.5-10 and temperature around 25°C, retaining over 90 and 80% of initial activity, respectively. RbCCO showed maximum activity at 35°C and pH 9.0 and was stable at pH 6-11 and temperatures of 25-30°C, retaining over 80% of initial activity. The kinetic constants K _m of HsCCO for isoeugenol and 4-vinylguaiacol were 1.55 and 1.65 mM and V _max were 74.09 and 27.91 nmol min^-1 mg^-1, respectively. The kinetic constants K _m of RbCCO for isoeugenol and 4-vinylguaiacol were 2.24 and 0.85 mM and V _max were 76.48 and 19.96 nmol min^-1 mg^-1, respectively. The transformed Escherichia coli cells harboring HsCCO converted isoeugenol and 4-vinylguaiacol at molar conversion yields of 80 and 55% and the maximum vanillin concentrations were up to 1.22 and 0.84 g L^-1, respectively. Comparably, the molar conversion yields of the transformed E. coli cells harboring RbCCO against isoeugenol 4-vinylguaiacol were 75 and 58%, and the maximum vanillin yields were up to 1.14 and 0.88 g L^-1, respectively.

Proteomic Profiling, Transcription Factor Modeling, and Genomics of Evolved Tolerant Strains Elucidate Mechanisms of Vanillin Toxicity in Escherichia coli

Vanillin (4-hydroxy-3-methoxybenzaldehyde) is an economically important flavor compound that can be made in bacterial cell factories, but toxicity is a major problem for cells producing this aromatic aldehyde. Using (i) a global proteomic analysis supported by multiple physiological experiments, mutant analyses, and inferred transcription factor modeling and (ii) adaptive laboratory evolution (ALE) of vanillin tolerance combined with genome-wide analysis of the underlying mutations, mechanisms of vanillin toxicity in Escherichia coli have been elucidated. We identified 147 proteins that exhibited a significant change in abundance in response to vanillin, giving the first detailed insight into the cellular response to this aldehyde. Vanillin caused accumulation of reactive oxygen species invoking adaptations coordinated by a MarA, OxyR, and SoxS regulatory network and increased RpoS/DksA-dependent gene expression. Differential fumarase C upregulation was found to prevent oxidative damage to FumA and FumB during growth with vanillin. Surprisingly, vanillin-dependent reduction pf copper (II) to copper (I) led to upregulation of the copA gene and growth in the presence of vanillin was shown to be hypersensitive to inhibition by copper ions. AcrD and AaeAB were identified as potential vanillin efflux systems. Vanillin-tolerant strains isolated by ALE had distinct nonsynonymous single nucleotide polymorphisms (SNPs) in gltA that led to increased citrate synthase activity. Strain-specific mutations in cpdA, rob, and marC were also present. One strain had a large (∼10-kb) deletion that included the marRAB region. Our data provide new understanding of bacterial vanillin toxicity and identify novel gene targets for future engineering of vanillin-tolerant strains of E. coli IMPORTANCE A particular problem for the biotechnological production of many of the valuable chemicals that we are now able to manufacture in bacterial cells is that these products often poison the cells producing them. Solutions to improve product yields or alleviate such toxicity using the techniques of modern molecular biology first require a detailed understanding of the mechanisms of product toxicity. Here we have studied the economically important flavor compound vanillin, an aromatic aldehyde that exerts significant toxic effects on bacterial cells. We used high-resolution protein abundance analysis as a starting point to determine which proteins are upregulated and which are downregulated by growth with vanillin, followed by gene expression and mutant studies to understand the mechanism of the response. In a second approach, we evolved bacterial strains with higher vanillin tolerance. Their genome sequences have yielded novel insights into vanillin tolerance that are complementary to the proteomics data set.

Isolation and characterization of Gram-negative and Gram-positive bacteria capable of producing piceatannol from resveratrol

Piceatannol is a valuable natural polyphenol with therapeutic potential in cardiovascular and metabolic disease treatment. In this study, we screened for microorganisms capable of producing piceatannol from resveratrol via regioselective hydroxylation. In the first screening, we isolated microorganisms utilizing resveratrol, phenol, or 4-hydroxyphenylacetic acid as a carbon source for growth. In the second screening, we assayed the isolated microorganisms for hydroxylation of resveratrol. Using this screening procedure, a variety of resveratrol-converting microorganisms were obtained. One Gram-negative bacterium, Ensifer sp. KSH1, and one Gram-positive bacterium, Arthrobacter sp. KSH3, utilized 4-hydroxyphenylacetic acid as a carbon source for growth and efficiently hydroxylated resveratrol to piceatannol without producing any detectable by-products. The hydroxylation activity of strains KSH1 and KSH3 was strongly induced by cultivation with 4-hydroxyphenylacetic acid as a carbon source during stationary growth phase. Using the 4-hydroxyphenylacetic acid-induced cells as a biocatalyst under optimal conditions, production of piceatannol by strains KSH1 and KSH3 reached 3.6 mM (0.88 g/L) and 2.6 mM (0.64 g/L), respectively. We also cloned genes homologous to the monooxygenase gene hpaBC from strains KSH1 and KSH3. Introduction of either hpaBC homolog into Escherichia coli endowed the host with resveratrol-hydroxylating activity.

Metabolic engineering of microorganisms for production of aromatic compounds

Metabolic engineering has been enabling development of high performance microbial strains for the efficient production of natural and non-natural compounds from renewable non-food biomass. Even though microbial production of various chemicals has successfully been conducted and commercialized, there are still numerous chemicals and materials that await their efficient bio-based production. Aromatic chemicals, which are typically derived from benzene, toluene and xylene in petroleum industry, have been used in large amounts in various industries. Over the last three decades, many metabolically engineered microorganisms have been developed for the bio-based production of aromatic chemicals, many of which are derived from aromatic amino acid pathways. This review highlights the latest metabolic engineering strategies and tools applied to the biosynthesis of aromatic chemicals, many derived from shikimate and aromatic amino acids, including L-phenylalanine, L-tyrosine and L-tryptophan. It is expected that more and more engineered microorganisms capable of efficiently producing aromatic chemicals will be developed toward their industrial-scale production from renewable biomass.

Vanillin biotechnology: the perspectives and future

The biotechnological production of fragrances is a recent trend that has expanded rapidly in the last two decades. Vanillin is the second most popular flavoring agent after saffron and is extensively used in various applications, e.g., as a food additive in food and beverages and as a masking agent in various pharmaceutical formulations. It is also considered a valuable product for other applications, such as metal plating and the production of other flavoring agents, herbicides, ripening agents, antifoaming agents, and personal and home-use products (such as in deodorants, air fresheners, and floor-polishing agents). In general, three types of vanillin, namely natural, biotechnological, and chemical/synthetic, are available on the market. However, only natural and nature-identical (biotechnologically produced from ferulic acid only) vanillins are considered as food-grade additives by most food-safety control authorities worldwide. In the present review, we summarize recent trends in fermentation technology for vanillin production and discuss the importance of the choice of raw materials for the economically viable production of vanillin. We also describe the key enzymes used in the biotechnological production of vanillin as well as their underlying genes. Research to advance our understanding of the molecular regulation of different pathways involved in vanillin production from ferulic acid is still ongoing. The enhanced knowledge is expected to offer new opportunities for the application of metabolic engineering to optimize the production of nature-identical vanillin. © 2018 Society of Chemical Industry.

Expression and characterization of a 9-cis-epoxycarotenoid dioxygenase from Serratia sp. ATCC 39006 capable of biotransforming isoeugenol and 4-vinylguaiacol to vanillin

A 9-cis-epoxycarotenoid dioxygenase gene from Serratia sp. ATCC 39,006 (SeNCED) was overexpressed in soluble form in E.coli. SeNCED showed the maximum activity at 30 °C and pH 8.0, and it was stable relatively at range of pH 5-10 and temperature of 20 °C to 30 °C. SeNCED effectively catalyzes the side chain double bond cleavage of isoeugenol and 4-vinylguaiacol to vanillin. The kinetic constant Km values toward isoeugenol and 4-vinylguaiacol were 18.92 mM and 6.31 mM and Vmax values were 50.73 IU/g and 4.77 IU/g, respectively. Moreover, the SeNCED exhibited an excellent organic solvent tolerance and the enzyme activity was substantially improved at presence of 10% of trichloromethane. The produced vanillin was achieved at an around 0.53 g/L (3.47 mM) and 0.33 g/L (2.17 mM) after 8 h reaction at 4 mM of isoeugenol and 4-vinylguaiacol, respectively, using transformed Escherichia coli cells harboring SeNCED in the presence of trichloromethane.

Biotechnological production of vanillin using immobilized enzymes

Vanillin is an important and popular plant flavor, but the amount of this compound available from plant sources is very limited. Biotechnological methods have high potential for vanillin production as an alternative to extraction from plant sources. Here, we report a new approach using immobilized enzymes for the production of vanillin. The recently discovered oxygenase Cso2 has coenzyme-independent catalytic activity for the conversion of isoeugenol and 4-vinylguaiacol to vanillin. Immobilization of Cso2 on Sepabeads EC-EA anion-exchange carrier conferred enhanced operational stability enabling repetitive use. This immobilized Cso2 catalyst allowed 6.8mg yield of vanillin from isoeugenol through ten reaction cycles at a 1mL scale. The coenzyme-independent decarboxylase Fdc, which has catalytic activity for the conversion of ferulic acid to 4-vinylguaiacol, was also immobilized on Sepabeads EC-EA. We demonstrated that the immobilized Fdc and Cso2 enabled the cascade synthesis of vanillin from ferulic acid via 4-vinylguaiacol with repetitive use of the catalysts. This study is the first example of biotechnological production of vanillin using immobilized enzymes, a process that provides new possibilities for vanillin production.

Metabolic engineering of E. coli top 10 for production of vanillin through FA catabolic pathway and bioprocess optimization using RSM

Metabolic engineering and construction of recombinant Escherichia coli strains carrying feruloyl-CoA synthetase and enoyl-CoA hydratase genes for the bioconversion of ferulic acid to vanillin offers an alternative way to produce vanillin. Isolation and designing of fcs and ech genes was carried out using computer assisted protocol and the designed vanillin biosynthetic gene cassette was cloned in pCCIBAC expression vector for introduction in E. coli top 10. Recombinant strain was implemented for the statistical optimization of process parameters influencing F A to vanillin biotransformation. CCD matrix constituted of process variables like FA concentration, time, temperature and biomass with intracellular, extracellular and total vanillin productions as responses. Production was scaled up and 68 mg/L of vanillin was recovered from 10 mg/L of FA using cell extracts from 1 mg biomass within 30 min. Kinetic activity of enzymes were characterized. From LCMS-ESI analysis a metabolic pathway of FA degradation and vanillin production was predicted.

Measurement of oxygen transfer from air into organic solvents

BACKGROUND

The use of non-aqueous organic media is becoming increasingly important in many biotechnological applications in order to achieve process intensification. Such media can be used, for example, to directly extract poorly water-soluble toxic products from fermentations. Likewise many biological reactions require the supply of oxygen, most normally from air. However, reliable online measurements of oxygen concentration in organic solvents (and hence oxygen transfer rates from air to the solvent) has to date proven impossible due to limitations in the current analytical methods.

RESULTS

For the first time, online oxygen measurements in non-aqueous media using a novel optical sensor are demonstrated. The sensor was used to measure oxygen concentration in various organic solvents including toluene, THF, isooctane, DMF, heptane and hexane (which have all been shown suitable for several biological applications). Subsequently, the oxygen transfer rates from air into these organic solvents were measured.

CONCLUSION

The measurement of oxygen transfer rates from air into organic solvents using the dynamic method was established using the solvent resistant optical sensor. The feasibility of online oxygen measurements in organic solvents has also been demonstrated, paving the way for new opportunities in process control. © 2015 The Authors. Journal of Chemical Technology & Biotechnology published by JohnWiley & Sons Ltd on behalf of Society of Chemical Industry.