Characterization of Feruloyl Esterase from Klebsiella oxytoca Z28 and Its Application in the Release of Ferulic Acid from De-Starching Wheat Bran

Feruloyl esterase (EC3.1.1.73; FAE) can degrade biomass to release ferulic acid (FA), which has a high application in bioprocessing, food, pharmaceutical, paper, feed, and other industrial fields. A strain of Klebsiella oxytoca Z28 with ferulic esterase activity was screened from Daqu. In addition, the FAE gene was expressed in Escherichia coli BL21 (DE3). The enzyme consists of 340 amino acids with a molecular mass of 37.7 kDa. The FAE enzyme activity was 463 U/L when the substrate was ethyl 4-hydroxy-3-methoxycinnamate and the optimum temperature and pH were 50 °C and 8.0, respectively. The enzyme had good stability at temperatures of 25-40 °C and a pH of 8.0. Ba²⁺, Cu²⁺, Mn²⁺, and Ca²⁺ had a strong inhibitory effect on the enzyme activity, and Na⁺ had a promotive effect on the enzyme activity. The de-starching wheat bran was degraded by KoFAE, and the FA release was up to 227.15 µg/g. This indicated that the heterologous expression of KoFAE from Klebsiella oxytoca Z28 in E. coli had a certain potential of biodegradation, which can be applied to the degradation of agricultural waste to obtain high value-added FA products.

Bioprospecting for Thermozymes and Characterization of a Novel Lipolytic Thermozyme Belonging to the SGNH/GDSL Family of Hydrolases

Functional screenings were conducted on two metagenomic libraries from hot springs in order to find novel thermozymes with potential biotechnological applications. These included enzymes acting on plant cell walls such as endoglucanases and exoglucanases, β-glucosidases, xylanases, and β-xylosidases, and broad application enzymes such as proteases and lipolytic hydrolases. Of all the enzymes found by this bioprospection, we selected a novel lipolytic enzyme for further characterization. The protein was found to belong to the SGNH/GDSL family of hydrolases. It was purified and its biochemical parameters determined. We found that the enzyme was most active at 60 °C and pH 9 using pNP-laurate as substrate and was highly thermostable. It also showed preference for short-chained substrates and activation with temperature and with certain detergents such as Tween 80. Proteins of this family of hydrolases are relevant for their broad substrate specificity, that coupled with this protein's high temperature optima, broad pH range, and thermostability further highlights its biotechnological potential.

Metagenomic Approaches as a Tool to Unravel Promising Biocatalysts from Natural Resources: Soil and Water

Natural resources are considered a promising source of microorganisms responsible for producing biocatalysts with great relevance in several industrial areas. However, a significant fraction of the environmental microorganisms remains unknown or unexploited due to the limitations associated with their cultivation in the laboratory through classical techniques. Metagenomics has emerged as an innovative and strategic approach to explore these unculturable microorganisms through the analysis of DNA extracted from environmental samples. In this review, a detailed discussion is presented on the application of metagenomics to unravel the biotechnological potential of natural resources for the discovery of promising biocatalysts. An extensive bibliographic survey was carried out between 2010 and 2021, covering diverse metagenomic studies using soil and/or water samples from different types and locations. The review comprises, for the first time, an overview of the worldwide metagenomic studies performed in soil and water and provides a complete and global vision of the enzyme diversity associated with each specific environment.

Efficient ferulic acid and xylo-oligosaccharides production by a novel multi-modular bifunctional xylanase/feruloyl esterase using agricultural residues as substrates

Liberating high value-added compounds ferulic acid (FA) and xylo-oligosaccharides (XOSs) from agricultural residues is a promising strategy for the utilization of lignocellulose. In this study, a bifunctional xylanase/feruloyl esterase from bacterial consortium EMSD5 was heterogeneously expressed in Escherichia coli. Depending on the inter-domain synergism of the recombinant enzyme rXyn10A/Fae1A, high yields of FA (2.78, 1.82, 1.15 and 7.31 mg/g substrate, respectively) were obtained from 20 mg in-soluble wheat arabinoxylan, de-starched wheat bran, ultrafine-grinding corn stover and steam-exploded corncob. Meanwhile, 3.210, 1.235, 1.215 and 0.823 mg xylose/XOSs were also released. For cost-saving enzyme production, we firstly constructed a recombinant E. coli, which could secrete the bifunctional xylanase/feruloyl esterase out of cells. When the recombinant E. coli was cultured in medium containing 200 mg de-starched wheat bran, 474 μg FA and 18.2 mg xylose/XOSs were also detected. Hence, rXyn10A/Fae1A and the recombinant strain showed great applied potential for FA and XOSs production.

A Novel VIII Carboxylesterase with High Hydrolytic Activity Against Ampicillin from a Soil Metagenomic Library

A novel carboxylesterase gene, named dlfae4, was discovered and sequenced from a soil metagenomic library. The dlfae4 gene was composed of 1017 base pairs encoding 338 amino acid residues with a predicted molecular mass of 37.2 kDa. DLFae4 exhibited strong hydrolytic activity towards methyl ferulate under optimum pH and temperature conditions (pH 8.6, 50 °C) and displayed remarkable thermostability, with residual activity as high as 50% after incubation for 3 h at 60 °C. A family VIII esterase DLFae4 was found to contain a typical serine residue within the S-X-X-K motif, which serves as a catalytic nucleophile in class C β-lactamases and family VIII esterases. As a consequence of its high sequence similarity with β-lactamases, DLFae4 exhibited significant hydrolytic activity towards ampicillin. In addition, DLFae4 was found to be the first known member of family VIII carboxylesterases with phthalate-degrading ability. Site-directed mutagenesis studies revealed that Ser11, Lys14, and Tyr121 residues play an essential catalytic role in DLFae4. These new findings, which are of great importance for further in-depth research and engineering development of carboxylesterases, should advance the implementation of biotechnological applications.

Molecular cloning, expression and characterization of a novel feruloyl esterase from a soil metagenomic library with phthalate-degrading activity

OBJECTIVES

To discover novel feruloyl esterases (FAEs) by the function-driven screening procedure from soil metagenome.

RESULTS

A novel FAE gene bds4 was isolated from a soil metagenomic library and over-expressed in Escherichia coli. The recombinant enzyme BDS4 was purified to homogeneity with a predicted molecular weight of 38.8 kDa. BDS4 exhibited strong activity (57.05 U/mg) toward methyl ferulate under the optimum pH and temperature of 8.0 and 37°C. Based on its amino acid sequence and model substrates specificity, BDS4 was classified as a type-C FAE. The quantity of the releasing ferulic acid can be enhanced significantly in the presence of xylanase compared with BDS4 alone from de-starched wheat bran. In addition, BDS4 can also hydrolyze several phthalates such as diethyl phthalate, dimethyl phthalate and dibutyl phthalate.

CONCLUSION

The current investigation discovered a novel FAE with phthalate-degrading activity and highlighted the usefulness of metagenomic approaches as a powerful tool for discovery of novel FAEs.

Feruloyl esterases: Biocatalysts to overcome biomass recalcitrance and for the production of bioactive compounds

Ferulic acid and its hydroxycinnamate derivatives represent one of the most abundant forms of low molecular weight phenolic compounds in plant biomass. Feruloyl esterases are part of a microorganism's plant cell wall-degrading enzymatic arsenal responsible for cleaving insoluble wall-bound hydroxycinnamates and soluble cytosolic conjugates. Stimulated by industrial requirements, accelerating scientific discoveries and knowledge transfer, continuous improvement efforts have been made to identify, create and repurposed biocatalysts dedicated to plant biomass conversion and biosynthesis of high-added value molecules. Here we review the basic knowledge and recent advances in biotechnological characteristics and the gene content encoding for feruloyl esterases. Information about several enzymes is systematically organized according to their function, biochemical properties, substrate specificity, and biotechnological applications. This review contributes to further structural, functional, and biotechnological R&D both for obtaining hydroxycinnamates from agricultural by-products as well as for lignocellulose biomass treatments aiming for production of bioethanol and other derivatives of industrial interest.

Enhancing the Thermostability of Feruloyl Esterase EstF27 by Directed Evolution and the Underlying Structural Basis

To improve the thermostability of EstF27, two rounds of random mutagenesis were performed. A thermostable mutant, M6, with six amino acid substitutions was obtained. The half-life of M6 at 55 °C is 1680 h, while that of EstF27 is 0.5 h. The Kcat/Km value of M6 is 1.9-fold higher than that of EstF27. The concentrations of ferulic acid released from destarched wheat bran by EstF27 and M6 at their respective optimal temperatures were 223.2 ± 6.8 and 464.8 ± 11.9 μM, respectively. To further understand the structural basis of the enhanced thermostability, the crystal structure of M6 is determined at 2.0 Å. Structural analysis shows that a new disulfide bond and hydrophobic interactions formed by the mutations may play an important role in stabilizing the protein. This study not only provides us with a robust catalyst, but also enriches our knowledge about the structure-function relationship of feruloyl esterase.

Metagenomics for the development of new biocatalysts to advance lignocellulose saccharification for bioeconomic development

The world economy is moving toward the use of renewable and nonedible lignocellulosic biomasses as substitutes for fossil sources in order to decrease the environmental impact of manufacturing processes and overcome the conflict with food production. Enzymatic hydrolysis of the feedstock is a key technology for bio-based chemical production, and the identification of novel, less expensive and more efficient biocatalysts is one of the main challenges. As the genomic era has shown that only a few microorganisms can be cultured under standard laboratory conditions, the extraction and analysis of genetic material directly from environmental samples, termed metagenomics, is a promising way to overcome this bottleneck. Two screening methodologies can be used on metagenomic material: the function-driven approach of expression libraries and sequence-driven analysis based on gene homology. Both techniques have been shown to be useful for the discovery of novel biocatalysts for lignocellulose conversion, and they enabled identification of several (hemi)cellulases and accessory enzymes involved in (hemi)cellulose hydrolysis. This review summarizes the latest progress in metagenomics aimed at discovering new enzymes for lignocellulose saccharification.

Identification and characterization of a novel salt-tolerant esterase from a Tibetan glacier metagenomic library

A salt-tolerant esterase, designated H9Est, was identified from a metagenomic library of the Karuola glacier. H9Est gene comprised 1071 bp and encoded a polypeptide of 357 amino acids with a molecular mass of 40 kDa. Sequence analysis revealed that H9Est belonged to the family IV of bacterial lypolitic enzyme. H9Est was overexpressed in Escherichia coli and the purified enzyme showed hydrolytic activity towards p-nitrophenyl esters with carbon chain from 2 to 8. The optimal esterase activity was at 40°C and pH 8.0 and the enzyme retained its activity towards some miscible organic solvents such as polyethylene glycol. A three-dimensional model of H9Est revealed that S200, D294, and H324 formed the H9Est catalytic triad. Circular Dichroism spectra and molecular dynamic simulation indicated that the esterase had a wide denaturation temperature range and flexible loops that would be beneficial for H9Est performance at low temperatures while retaining heat-resistant features.

A unique mono- and diacylglycerol lipase from Penicillium cyclopium: heterologous expression, biochemical characterization and molecular basis for its substrate selectivity

A cDNA gene encoding a mature peptide of the mono- and diacylglycerol lipase (abbreviated to PcMdl) from Penicillium cyclopium PG37 was cloned and expressed in Pichia pastoris GS115. The recombinant PcMdl (rePcMdl) with an apparent molecular weight of 39 kDa showed the highest activity (40.5 U/mL of culture supernatant) on 1,2-dibutyrin substrate at temperature 35°C and pH 7.5. The rePcMdl was stable at a pH range of 6.5–9.5 and temperatures below 35°C. The activity of rePcMdl was inhibited by Hg²⁺ and Fe³⁺, but not significantly affected by EDTA or the other metal ions such as Na⁺, K⁺, Li⁺, Mg²⁺, Zn²⁺, Ca²⁺, Mn²⁺, Cu²⁺, and Fe²⁺. PcMdl was identified to be strictly specific to mono- and diacylglycerol, but not triacylglycerol. Stereographic view of PcMdl docked with substrate (tri- or diacylglycerol) analogue indicated that the residue Phe²⁵⁶ plays an important role in conferring the substrate selectivity. Phe²⁵⁶ projects its side chain towards the substrate binding groove and makes the sn-1 moiety difficult to insert in. Furthermore, sn-1 moiety prevents the phosphorus atom (substitution of carboxyl carbon) from getting to the O_γ of Ser¹⁴⁵, which results in the failure of triacylglycerol hydrolysis. These results should provide a basis for molecular engineering of PcMdl and expand its applications in industries.

Cloning, expression of a feruloyl esterase from Aspergillus usamii E001 and its applicability in generating ferulic acid from wheat bran

A cDNA gene (AufaeA), which encodes a mature polypeptide of the type-A feruloyl esterase from Aspergillus usamii E001 (abbreviated to AuFaeA), was cloned and heterologously expressed in Pichia pastoris GS115. One transformant, labeled as P. pastoris GSFaeA4-8, expressing the highest recombinant AuFaeA (reAuFaeA) activity of 10.76 U/ml was selected by the flask expression test. The expressed reAuFaeA was purified to homogeneity with an apparent molecular weight of 36.0 kDa by SDS-PAGE analysis, and characterized using the model substrate of methyl ferulate (MFA). The purified reAuFaeA was optimally active at pH 5.0 and 45 °C, and highly stable at pH 4.0–6.5 and 45 °C or below. Its activity was not significantly affected by metal ions tested and EDTA. The K  m and V  max of reAuFaeA towards MFA were 4.64 mM and 115.5 U/mg, respectively. High-performance liquid chromatography analysis showed that only 9.7 % of total alkali-extractable ferulic acid (FA) was released from destarched wheat bran by reAuFaeA alone. The released FA increased to 36.5 % when reAuFaeA was used together with a recombinant Aspergillus usamii GH family 11 xylanase A, indicating a synergistic interaction between them.

Isolation and characterization of a thermostable esterase from a metagenomic library

A novel esterase gene was isolated by functional screening of a metagenomic library prepared from an activated sludge sample. The gene (est-XG2) consists of 1,506 bp with GC content of 74.8 %, and encodes a protein of 501 amino acids with a molecular mass of 53 kDa. Sequence alignment revealed that Est-XG2 shows a maximum amino acid identity (47 %) with the carboxylesterase from Thermaerobacter marianensis DSM 12885 (YP_004101478). The catalytic triad of Est-XG2 was predicted to be Ser₁₉₂-Glu₃₁₃-His₄₁₂ with Ser₉₂ in a conserved pentapeptide (GXSXG), and further confirmed by site-directed mutagenesis. Phylogenetic analysis suggested Est-XG2 belongs to the bacterial lipase/esterase family VII. The recombinant Est-XG2, expressed and purified from Escherichia coli, preferred to hydrolyze short and medium length p-nitrophenyl esters with the best substrate being p-nitrophenyl acetate (K(m) and k(cat) of 0.33 mM and 36.21 s⁻¹, respectively). The purified enzyme also had the ability to cleave sterically hindered esters of tertiary alcohols. Biochemical characterization of Est-XG2 revealed that it is a thermophilic esterase that exhibits optimum activity at pH 8.5 and 70 °C. Est-XG2 had moderate tolerance to organic solvents and surfactants. The unique properties of Est-XG2, high thermostability and stability in the presence of organic solvents, may render it a potential candidate for industrial applications.

Crystallization and preliminary X-ray analysis of a novel halotolerant feruloyl esterase identified from a soil metagenomic library

Feruloyl esterase cleaves the ester linkage formed between ferulic acid and polysaccharides in plant cell walls and thus has wide potential industrial applications. A novel feruloyl esterase (EstF27) identified from a soil metagenomic library was crystallized and a complete data set was collected from a single cooled crystal using an in-house X-ray source. The crystal diffracted to 2.9 Å resolution and belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 94.35, b = 106.19, c = 188.51 Å, α = β = γ = 90.00°. A Matthews coefficient of 2.55 Å(3) Da(-1), with a corresponding solvent content of 51.84%, suggested the presence of ten protein subunits in the asymmetric unit.