Intracellular removal of acetyl, feruloyl and p-coumaroyl decorations on arabinoxylo-oligosaccharides imported from lignocellulosic biomass degradation by Ruminiclostridium cellulolyticum

BACKGROUND

Xylans are polysaccharides that are naturally abundant in agricultural by-products, such as cereal brans and straws. Microbial degradation of arabinoxylan is facilitated by extracellular esterases that remove acetyl, feruloyl, and p-coumaroyl decorations. The bacterium Ruminiclostridium cellulolyticum possesses the Xua (xylan utilization associated) system, which is responsible for importing and intracellularly degrading arabinoxylodextrins. This system includes an arabinoxylodextrins importer, four intracellular glycosyl hydrolases, and two intracellular esterases, XuaH and XuaJ which are encoded at the end of the gene cluster.

RESULTS

Genetic studies demonstrate that the genes xuaH and xuaJ are part of the xua operon, which covers xuaABCDD'EFGHIJ. This operon forms a functional unit regulated by the two-component system XuaSR. The esterases encoded at the end of the cluster have been further characterized: XuaJ is an acetyl esterase active on model substrates, while XuaH is a xylan feruloyl- and p-coumaryl-esterase. This latter is active on oligosaccharides derived from wheat bran and wheat straw. Modelling studies indicate that XuaH has the potential to interact with arabinoxylobiose acylated with mono- or diferulate. The intracellular esterases XuaH and XuaJ are believed to allow the cell to fully utilize the complex acylated arabinoxylo-dextrins imported into the cytoplasm during growth on wheat bran or straw.

CONCLUSIONS

This study reports for the first time that a cytosolic feruloyl esterase is part of an intracellular arabinoxylo-dextrin import and degradation system, completing its cytosolic enzymatic arsenal. This system represents a new pathway for processing highly-decorated arabinoxylo-dextrins, which could provide a competitive advantage to the cell and may have interesting biotechnological applications.

A New Phenolic Acid Decarboxylase from the Brown-Rot Fungus Neolentinus lepideus Natively Decarboxylates Biosourced Sinapic Acid into Canolol, a Bioactive Phenolic Compound

Rapeseed meal (RSM) is a cheap, abundant and renewable feedstock, whose biorefinery is a current challenge for the sustainability of the oilseed sector. RSM is rich in sinapic acid (SA), a p-hydroxycinnamic acid that can be decarboxylated into canolol (2,6-dimethoxy-4-vinylphenol), a valuable bioactive compound. Microbial phenolic acid decarboxylases (PADs), mainly described for the non-oxidative decarboxylation of ferulic and p-coumaric acids, remain very poorly documented to date, for SA decarboxylation. The species Neolentinus lepideus has previously been shown to biotransform SA into canolol in vivo, but the enzyme responsible for bioconversion of the acid has never been characterized. In this study, we purified and characterized a new PAD from the canolol-overproducing strain N. lepideus BRFM15. Proteomic analysis highlighted a sole PAD-type protein sequence in the intracellular proteome of the strain. The native enzyme (NlePAD) displayed an unusual outstanding activity for decarboxylating SA (Vmax of 600 U.mg-1, kcat of 6.3 s-1 and kcat/KM of 1.6 s-1.mM-1). We showed that NlePAD (a homodimer of 2 × 22 kDa) is fully active in a pH range of 5.5-7.5 and a temperature range of 30-55 °C, with optima of pH 6-6.5 and 37-45 °C, and is highly stable at 4 °C and pH 6-8. Relative ratios of specific activities on ferulic, sinapic, p-coumaric and caffeic acids, respectively, were 100:24.9:13.4:3.9. The enzyme demonstrated in vitro effectiveness as a biocatalyst for the synthesis of canolol in aqueous medium from commercial SA, with a molar yield of 92%. Then, we developed processes to biotransform naturally-occurring SA from RSM into canolol by combining the complementary potentialities of an Aspergillus niger feruloyl esterase type-A, which is able to release free SA from the raw meal by hydrolyzing its conjugated forms, and NlePAD, in aqueous medium and mild conditions. NlePAD decarboxylation of biobased SA led to an overall yield of 1.6-3.8 mg canolol per gram of initial meal. Besides being the first characterization of a fungal PAD able to decarboxylate SA, this report shows that NlePAD is very promising as new biotechnological tool to generate biobased vinylphenols of industrial interest (especially canolol) as valuable platform chemicals for health, nutrition, cosmetics and green chemistry.

Challenges and advances in biotechnological approaches for the synthesis of canolol and other vinylphenols from biobased p-hydroxycinnamic acids: a review

p-Hydroxycinnamic acids, such as sinapic, ferulic, p-coumaric and caffeic acids, are among the most abundant phenolic compounds found in plant biomass and agro-industrial by-products (e.g. cereal brans, sugar-beet and coffee pulps, oilseed meals). These p-hydroxycinnamic acids, and their resulting decarboxylation products named vinylphenols (canolol, 4-vinylguaiacol, 4-vinylphenol, 4-vinylcatechol), are bioactive molecules with many properties including antioxidant, anti-inflammatory and antimicrobial activities, and potential applications in food, cosmetic or pharmaceutical industries. They were also shown to be suitable precursors of new sustainable polymers and biobased substitutes for fine chemicals such as bisphenol A diglycidyl ethers. Non-oxidative microbial decarboxylation of p-hydroxycinnamic acids into vinylphenols involves cofactor-free and metal-independent phenolic acid decarboxylases (EC 4.1.1 carboxyl lyase family). Historically purified from bacteria (Bacillus, Lactobacillus, Pseudomonas, Enterobacter genera) and some yeasts (e.g. Brettanomyces or Candida), these enzymes were described for the decarboxylation of ferulic and p-coumaric acids into 4-vinylguaiacol and 4-vinylphenol, respectively. The catalytic mechanism comprised a first step involving p-hydroxycinnamic acid conversion into a semi-quinone that then decarboxylated spontaneously into the corresponding vinyl compound, in a second step. Bioconversion processes for synthesizing 4-vinylguaiacol and 4-vinylphenol by microbial decarboxylation of ferulic and p-coumaric acids historically attracted the most research using bacterial recombinant phenolic acid decarboxylases (especially Bacillus enzymes) and the processes developed to date included mono- or biphasic systems, and the use of free- or immobilized cells. More recently, filamentous fungi of the Neolentinus lepideus species were shown to natively produce a more versatile phenolic acid decarboxylase with high activity on sinapic acid in addition to the others p-hydroxycinnamic acids, opening the way to the production of canolol by biotechnological processes applied to rapeseed meal. Few studies have described the further microbial/enzymatic bioconversion of these vinylphenols into valuable compounds: (i) synthesis of flavours such as vanillin, 4-ethylguaiacol and 4-ethylphenol from 4-vinylguaiacol and 4-vinylphenol, (ii) laccase-mediated polymer synthesis from canolol, 4-vinylguaiacol and 4-vinylphenol.

Lavender- and lavandin-distilled straws: an untapped feedstock with great potential for the production of high-added value compounds and fungal enzymes

BACKGROUND

Lavender (Lavandula angustifolia) and lavandin (a sterile hybrid of L. angustifolia × L. latifolia) essential oils are among those most commonly used in the world for various industrial purposes, including perfumes, pharmaceuticals and cosmetics. The solid residues from aromatic plant distillation such as lavender- and lavandin-distilled straws are generally considered as wastes, and consequently either left in the fields or burnt. However, lavender- and lavandin-distilled straws are a potentially renewable plant biomass as they are cheap, non-food materials that can be used as raw feedstocks for green chemistry industry. The objective of this work was to assess different pathways of valorization of these straws as bio-based platform chemicals and fungal enzymes of interest in biorefinery.

RESULTS

Sugar and lignin composition analyses and saccharification potential of the straw fractions revealed that these industrial by-products could be suitable for second-generation bioethanol prospective. The solvent extraction processes, developed specifically for these straws, released terpene derivatives (e.g. τ-cadinol, β-caryophyllene), lactones (e.g. coumarin, herniarin) and phenolic compounds of industrial interest, including rosmarinic acid which contributed to the high antioxidant activity of the straw extracts. Lavender and lavandin straws were also suitable inducers for the secretion of a wide panel of lignocellulose-acting enzymes (cellulases, hemicellulases and oxido-reductases) from the white-rot model fungus Pycnoporus cinnabarinus. Interestingly, high amounts of laccase and several lytic polysaccharide monooxygenases were identified in the lavender and lavandin straw secretomes using proteomics.

CONCLUSIONS

The present study demonstrated that the distilled straws of lavender and lavandin are lignocellulosic-rich materials that can be used as raw feedstocks for producing high-added value compounds (antioxidants, aroma) and fungal oxidative enzymes, which represent opportunities to improve the decomposition of recalcitrant lignocellulose into biofuel. Hence, the structure and the physico-chemical properties of these straws clearly open new perspectives for use in biotechnological processes involving especially filamentous fungi. These approaches represent sustainable strategies to foster the development of a local circular bioeconomy.

A Two-Step Bioconversion Process for Canolol Production from Rapeseed Meal Combining an Aspergillus niger Feruloyl Esterase and the Fungus Neolentinus lepideus

Rapeseed meal is a cheap and abundant raw material, particularly rich in phenolic compounds of biotechnological interest. In this study, we developed a two-step bioconversion process of naturally occurring sinapic acid (4-hydroxy-3,5-dimethoxycinnamic acid) from rapeseed meal into canolol by combining the complementary potentialities of two filamentous fungi, the micromycete Aspergillus niger and the basidiomycete Neolentinus lepideus. Canolol could display numerous industrial applications because of its high antioxidant, antimutagenic and anticarcinogenic properties. In the first step of the process, the use of the enzyme feruloyl esterase type-A (named AnFaeA) produced with the recombinant strain A. niger BRFM451 made it possible to release free sinapic acid from the raw meal by hydrolysing the conjugated forms of sinapic acid in the meal (mainly sinapine and glucopyranosyl sinapate). An amount of 39 nkat AnFaeA per gram of raw meal, at 55 °C and pH 5, led to the recovery of 6.6 to 7.4 mg of free sinapic acid per gram raw meal, which corresponded to a global hydrolysis yield of 68 to 76% and a 100% hydrolysis of sinapine. Then, the XAD2 adsorbent (a styrene and divinylbenzene copolymer resin), used at pH 4, enabled the efficient recovery of the released sinapic acid, and its concentration after elution with ethanol. In the second step, 3-day-old submerged cultures of the strain N. lepideus BRFM15 were supplied with the recovered sinapic acid as the substrate of bioconversion into canolol by a non-oxidative decarboxylation pathway. Canolol production reached 1.3 g/L with a molar yield of bioconversion of 80% and a productivity of 100 mg/L day. The same XAD2 resin, when used at pH 7, allowed the recovery and purification of canolol from the culture broth of N. lepideus. The two-step process used mild conditions compatible with green chemistry.