Genetic and biochemical characterization of a highly thermostable alpha-L-arabinofuranosidase from Thermobacillus xylanilyticus

Biochemical characterization of bifunctional enzymatic activity of a recombinant protein (Bp0469) from Blautia producta ATCC 27340 and its role in the utilization of arabinogalactan oligosaccharides

Human consumption of larch arabinogalactan has a significant effect on enhancing probiotic microflora in the gut, and it also promotes the production of short-chain fatty acids. Bacterial members of Lachnospiraceae family are important and play significant roles in maintaining our gut health. However, it is less known about biochemistry of members of this family by which they utilize non-cellulosic fiber in the gut. For enhancing this understanding, we studied that B. producta ATCC 27340 grew on arabinogalactan oligosaccharides (AGOs) as compared to polysaccharide form of arabinogalactan. Recombinant protein (Bp0469) was heterologously expressed in Escherichia coli BL21 (DE3) and revealed the optimum pH and temperature at 7.4 in phosphate buffer and 45 °C, respectively. Catalytic efficiency of recombinant Bp0469 for p-nitrophenyl (pNP)-α-L-arabinofuranoside was about half of pNP-β-D-galactopyranoside. It also cleaved natural substrates (lactose, arabinobiose and 3-O-(β-d-galactopyranosyl)-d-galactopyranose) and characterized AGOs in this study. Based on genomic, structural models, and biochemical characteristics, identified Bp0469 is a peculiar enzyme with two distinct domains that cleave α1-5 linked arabinobiose and β-D-Galp-1-3/4 linkages. Overall, the study enhances the knowledge on nutritional perspective of B. producta ATCC 27340 for thriving on non-cellulosic biomass, and identified enzyme can also be used for producing industrial important AGOs.

Recombinant Production and Biochemical Characterization of Thermostable Arabinofuranosidase from Acidothermophilic Alicyclobacillus Acidocaldarius

The complete enzymatic degradation of lignocellulosic biomass requires the cooperative action of cellulosic, hemicellulosic, and lignolytic enzymes such as cellulase, xylanase, laccase, galactosidase, and arabinofuranosidase. Arabinofuranosidases (E.C 3.2.1.55), which belong to the glycoside hydrolase family of enzymes, hydrolyze the 1,3- and 1,5-α-arabinosyl bonds in L-arabinose- containing molecules. L-arabinoses are present in hemicellulosic part of lignocellulosic biomass. Arabinofuranosidases also play an important role in the complete hydrolysis of arabinoxylans. Analysis of the genome project and CAZY database revealed two putative arabinofuranosidase genes in the A. acidocaldarius genome. The aim of the study was cloning, heterologous expression, purification and biochemical characterization of the arabinofuranosidase enzyme encoded in A. acidocaldarius genome. For this purpose, the AbfA gene of the arabinofuranosidase protein was cloned into the pQE-40 vector, heterologously expressed in E. coli BL21 GOLD (DE3) and successfully purified using His-Tag. Biochemical characterization of the purified enzyme revealed that A. acidocaldarius arabinofuranosidase exhibited activity over a wide pH and temperature range with optimum activity at 45 ºC and pH 6.5 in phosphate buffer towards 4-nitrophenyl-α-L-arabinofuranoside as the substrate. In addition, the enzyme is highly stable over wide range of temperature and maintaining 60% of its activity after 90 min of incubation at 80 ºC. Through the bioinformatics studies, the homology model of A. acidocaldarius arabinofuranosidase was generated and the substrate binding site and residues located in this site were identified. Further molecular docking analysis revealed that the substrate located in the catalytically active pose and, residues N174, E175, and E294 have direct interaction with 4-nitrophenyl-α-L-arabinofuranoside. Moreover, based on phylogenetic analysis, A. acidocaldarius arabinofuranosidase exists in the sub-group of intracellular arabinofuranosidases, and G. stearothermophilus and B.subtilis arabinofuranosidases are close relatives of A. acidocaldarius arabinofuranosidase. This is the first study to report the gene cloning, recombinant expression and biochemical and bioinformatic characterization of an auxiliary GH51 arabinofuranosidase from an acidothermophilic bacterium A. acidocaldarius.

Structural and functional analyses of GH51 alpha-L-arabinofuranosidase of Geobacillus vulcani GS90 reveal crucial residues for catalytic activity and thermostability

Alpha-L-arabinofuranosidase (Abf) is of big interest in various industrial areas. Directed evolution is a powerful strategy to identify significant residues underlying Abf properties. Here, six active variants from GH51 Abf of Geobacillus vulcani GS90 (GvAbf) by directed evolution were overproduced, extracted, and analyzed at biochemical and structural levels. According to the activity and thermostability results, the most-active and the least-active variants were found as GvAbf51 and GvAbf52, respectively. GvAbf63 variant was more active than parent GvAbf by 20% and less active than GvAbf51. Also, the highest thermostability belonged to GvAbf52 with 80% residual activity after 1 h. Comparative sequence and structure analyses revealed that GvAbf51 possessed L307S displacement. Thus, this study suggested that L307 residue may be critical for GvAbf activity. GvAbf63 had H30D, Q90H, and L307S displacements, and H30 was covalently bound to E29 catalytic residue. Thus, H30D may decrease the positive effect of L307S on GvAbf63 activity, preventing E29 action. Besides, GvAbf52 possessed S215N, L307S, H473P, and G476C substitutions and S215 was close to E175 (acid-base residue). S215N may partially disrupt E175 action. Overall effect of all substitutions in GvAbf52 may result in the formation of the C-C bond between C171 and C213 by becoming closer to each other.

Control of phenotypic diversification based on serial cultivations on different carbon sources leads to improved bacterial xylanase production

Thermobacillus xylanilyticus is a thermophilic and hemicellulolytic bacterium of interest for the production of thermostable hemicellulases. Enzymes' production by this bacterium is challenging, because the proliferation of a cheating subpopulation of cells during exponential growth impairs the production of xylanase after serial cultivations. Accordingly, a strategy of successive cultivations with cells transfers in stationary phase and the use of wheat bran and wheat straw as carbon sources were tested. The ratio between subpopulations and their corresponding metabolic activities were studied by flow cytometry and the resulting hemicellulases production (xylanase, acetyl esterase and β-xylosidase) followed. During serial cultivations, the results pointed out an increase of the enzymatic activities. On xylan, compared to the first cultivation, the xylanase activity increases by 7.15-fold after only four cultivations. On the other hand, the debranching activities were increased by 5.88-fold and 57.2-fold on wheat straw and by 2.77-fold and 3.34-fold on wheat bran for β-xylosidase and acetyl esterase, respectively. The different enzymatic activities then stabilized, reached a plateau and further decreased. Study of the stability and reversibility of the enzyme production revealed cell-to-cell heterogeneities in metabolic activities which could be linked to the reversibility of enzymatic activity changes. Thus, the strategy of successive transfers during the stationary phase of growth, combined with the use of complex lignocellulosic substrates as carbon sources, is an efficient strategy to optimize the hemicellulases production by T. xylanilyticus, by preventing the selection of cheaters.

Xylanase production by Thermobacillus xylanilyticus is impaired by population diversification but can be mitigated based on the management of cheating behavior

Background

The microbial production of hemicellulasic cocktails is still a challenge for the biorefineries sector and agro-waste valorization. In this work, the production of hemicellulolytic enzymes by Thermobacillus xylanilyticus has been considered. This microorganism is of interest since it is able to produce an original set of thermostable hemicellulolytic enzymes, notably a xylanase GH11, Tx-xyn11. However, cell-to-cell heterogeneity impairs the production capability of the whole microbial population.

Results

Sequential cultivations of the strain on xylan as a carbon source has been considered in order to highlight and better understand this cell-to-cell heterogeneity. Successive cultivations pointed out a fast decrease of xylanase activity (loss of ~ 75%) and Tx-xyn11 gene expression after 23.5 generations. During serial cultivations on xylan, flow cytometry analyses pointed out that two subpopulations, differing at their light-scattering properties, were present. An increase of the recurrence of the subpopulation exhibiting low forward scatter (FSC) signal was correlated with a progressive loss of xylanase activity over several generations. Cell sorting and direct observation of the sorted subpopulations revealed that the low-FSC subpopulation was not sporulating, whereas the high-FSC subpopulation contained cells at the onset of the sporulation stage. The subpopulation differences (growth and xylanase activity) were assessed during independent growth. The low-FSC subpopulation exhibited a lag phase of 10 h of cultivation (and xylanase activities from 0.15 ± 0.21 to 3.89 ± 0.14 IU/mL along the cultivation) and the high-FSC subpopulation exhibited a lag phase of 5 h (and xylanase activities from 0.52 ± 0.00 to 4.43 ± 0.61 over subcultivations). Serial cultivations on glucose, followed by a switch to xylan led to a ~ 1.5-fold to ~ 15-fold improvement of xylanase activity, suggesting that alternating cultivation conditions could lead to an efficient population management strategy for the production of xylanase.

Conclusions

Taken altogether, the data from this study point out that a cheating behavior is responsible for the progressive reduction in xylanase activity during serial cultivations of T. xylanilyticus. Alternating cultivation conditions between glucose and xylan could be used as an efficient strategy for promoting population stability and higher enzymatic productivity from this bacterium.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01762-z.

Draft Genome Sequence of the Lignocellulolytic and Thermophilic Bacterium Thermobacillus xylanilyticus XE

Thermobacillus xylanilyticus is a thermophilic and hemicellulolytic bacterium able to use several lignocelluloses as its main carbon source. This draft genome sequence gives insight into the genomic potential of this bacterium and provides new resources to understand the enzymatic mechanisms used by the bacterium during lignocellulose degradation and will allow the identification of robust lignocellulolytic enzymes.

Synthesis of α-l-Araf and β-d-Galf series furanobiosides using mutants of a GH51 α-l-arabinofuranosidase

The GH-51 α-l-arabinofuranosidase from Thermobacillus xylanilyticus (TxAbf) possesses versatile catalytic properties, displaying not only the ability to hydrolyze glycosidic linkages but also to synthesize furanobiosides in α-l-Araf and β-d-Galf series. Herein, mutants are investigated to evaluate their ability to perform self-condensation, assessing both yield improvements and changes in regioselectivity. Overall yields of oligo-α-l-arabino- and oligo-β-d-galactofuranosides were increased up to 4.8-fold compared to the wild-type enzyme. In depth characterization revealed that the mutants exhibit increased transfer rates and thus a hydrolysis/self-condensation ratio in favor of synthesis. The consequence of the substitution N216W is the creation of an additional binding subsite that provides the basis for an alternative acceptor substrate binding mode. As a result, mutants bearing N216W synthesize not only (1,2)-linked furanobiosides, but also (1,3)- and even (1,5)-linked furanobiosides. Since the self-condensation is under kinetic control, the yield of homo-disaccharides was maximized using higher substrate concentrations. In this way, the mutant R69H-N216W produced oligo-β-d-galactofuranosides in > 70% yield. Overall, this study further demonstrates the potential usefulness of TxAbf mutants for glycosynthesis and shows how these might be used to synthesize biologically-relevant glycoconjugates.

Rational Enzyme Design without Structural Knowledge: A Sequence-Based Approach for Efficient Generation of Transglycosylases

Glycobiology is dogged by the relative scarcity of synthetic, defined oligosaccharides. Enzyme-catalysed glycosylation using glycoside hydrolases is feasible but is hampered by the innate hydrolytic activity of these enzymes. Protein engineering is useful to remedy this, but it usually requires prior structural knowledge of the target enzyme, and/or relies on extensive, time-consuming screening and analysis. Here, a straightforward strategy that involves rational rapid in silico analysis of protein sequences is described. The method pinpoints 6-12 single-mutant candidates to improve transglycosylation yields. Requiring very little prior knowledge of the target enzyme other than its sequence, the method is generic and procures catalysts for the formation of glycosidic bonds involving various d/l-, α/β-pyranosides or furanosides, and exo or endo action. Moreover, mutations validated in one enzyme can be transposed to others, even distantly related enzymes.

Arabinoxylo- and Arabino-Oligosaccharides-Specific α-L-Arabinofuranosidase GH51 Isozymes from the Amylolytic Yeast Saccharomycopsis fibuligera

Two genes encoding probable α-L-arabinofuranosidase (E.C. 3.2.1.55) isozymes (ABFs) with 92.3% amino acid sequence identity, ABF51A and ABF51B, were found from chromosomes 3 and 5 of Saccharomycopsis fibuligera KJJ81, an amylolytic yeast isolated from Korean wheat-based nuruk, respectively. Each open reading frame consists of 1,551 nucleotides and encodes a protein of 517 amino acids with the molecular mass of approximately 59 kDa. These isozymes share approximately 49% amino acid sequence identity with eukaryotic ABFs from filamentous fungi. The corresponding genes were cloned, functionally expressed, and purified from Escherichia coli. SfABF51_A and SfABF51_B showed the highest activities on p-nitrophenyl arabinofuranoside at 40~45°C and pH 7.0 in sodium phosphate buffer and at 50°C and pH 6.0 in sodium acetate buffer, respectively. These exo-acting enzymes belonging to the glycoside hydrolase (GH) family 51 could hydrolyze arabinoxylo-oligosaccharides (AXOS) and arabino-oligosaccharides (AOS) to produce only L-arabinose, whereas they could hardly degrade any polymeric substrates including arabinans and arabinoxylans. The detailed product analyses revealed that both SfABF51 isozymes can catalyze the versatile hydrolysis of α-(1,2)-and α-(1,3)-L-arabinofuranosidic linkages of AXOS, and α-(1,2)-, α-(1,3)-, and α-(1,5)-linkages of linear and branched AOS. On the contrary, they have much lower activity against the α-(1,2)-and α-(1,3)-double-substituted substrates than the single-substituted ones. These hydrolases could potentially play important roles in the degradation and utilization of hemicellulosic biomass by S. fibuligera.

Probing the determinants of the transglycosylation/hydrolysis partition in a retaining α-l-arabinofuranosidase

The use of retaining glycoside hydrolases as synthetic tools for glycochemistry is highly topical and the focus of considerable research. However, due to the incomplete identification of the molecular determinants of the transglycosylation/hydrolysis partition (t/h), rational engineering of retaining glycoside hydrolases to create transglycosylases remains challenging. Therefore, to understand better the factors that underpin transglycosylation in a GH51 retaining α-l-arabinofuranosidase from Thermobacillus xylanilyticus, the investigation of this enzyme's active site was pursued. Specifically, the properties of two mutants, F26L and L352M, located in the vicinity of the active site are described, using kinetic and 3D structural analyses and molecular dynamics simulations. The results reveal that the presence of L352M in the context of a triple mutant (also containing R69H and N216W) generates changes both in the donor and acceptor subsites, the latter being the result of a domino-like effect. Overall, the mutant R69H-N216W-L352M displays excellent transglycosylation activity (70 % yield, 78 % transfer rate and reduced secondary hydrolysis of the product). In the course of this study, the central role played by the conserved R69 residue was also reaffirmed. The mutation R69H affects both the catalytic nucleophile and the acid/base, including their flexibility, and has a determinant effect on the t/h partition. Finally, the results reveal that increased loop flexibility in the acceptor subsites creates new interactions with the acceptor, in particular with a hydrophobic binding platform composed of N216W, W248 and W302.

Characterization and functional analysis of two novel thermotolerant α-L-arabinofuranosidases belonging to glycoside hydrolase family 51 from Thielavia terrestris and family 62 from Eupenicillium parvum

Abstract

Arabinofuranose substitutions on xylan are known to interfere with enzymatic hydrolysis of this primary hemicellulose. In this work, two novel α-l-arabinofuranosidases (ABFs), TtABF51A from Thielavia terrestris and EpABF62C from Eupenicillium parvum, were characterized and functionally analyzed. From sequences analyses, TtABF51A and EpABF62C belong to glycoside hydrolase (GH) families 51 and 62, respectively. Recombinant TtABF51A showed high activity on 4-nitrophenyl-α-l-arabinofuranoside (83.39 U/mg), low-viscosity wheat arabinoxylan (WAX, 39.66 U/mg), high-viscosity rye arabinoxylan (RAX, 32.24 U/mg), and sugarbeet arabinan (25.69 U/mg), while EpABF62C preferred to degrade arabinoxylan. For EpABF62C, the rate of hydrolysis of RAX (94.10 U/mg) was 2.1 times that of WAX (45.46 U/mg). The optimal pH and reaction temperature for the two enzymes was between 4.0 and 4.5 and 65 °C, respectively. Calcium played an important role in the thermal stability of EpABF62C. TtABF51A and EpABF62C showed the highest thermal stabilities at pH 4.5 or 5.0, respectively. At their optimal pHs, TtABF51A and EpABF62C retained greater than 80% of their initial activities after incubation at 55 °C for 96 h or 144 h, respectively. ¹H NMR analysis indicated that the two enzymes selectively removed arabinose linked to C-3 of mono-substituted xylose residues in WAX. Compared with the singular application of the GH10 xylanase EpXYN1 from E. parvum, co-digestions of WAX including TtABF51A and/or EpABF62C released 2.49, 3.38, and 4.81 times xylose or 3.38, 1.65, and 2.57 times of xylobiose, respectively. Meanwhile, the amount of arabinose released from WAX by TtABF51A with EpXYN1 was 2.11 times the amount with TtABF51A alone.

Key points

• Two novel α-l-arabinofuranosidases (ABFs) displayed high thermal stability.

• The thermal stability of GH62 family EpABF62C was dependent on calcium.

• Buffer pH affects the thermal stability of the two ABFs.

• Both ABFs enhance the hydrolysis of WAX by a GH10 xylanase.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10867-7) contains supplementary material, which is available to authorized users.

Arabinofuranosidases: Characteristics, microbial production, and potential in waste valorization and industrial applications

Alpha-L-arabinofuranoside arabinofuranohydrolase (ARA), more commonly known as alpha-L-arabinofuranosidase (E.C. number 3.2.1.55), is a hydrolytic enzyme, catalyzing the cleavage of alpha-L-arabinose by acting on the non-reducing ends of alpha-L-arabinofuranosides, alpha-L-arabinans containing (1,3)- and/or (1,5)-linked arabinoxylans and arabinogalactans. ARA functions as debranching enzyme removing arabinose substituents from arabinoxylan and arabinoxylooligomers, thereby, boosting the hydrolysis of arabinoxylan fraction of hemicellulose and improving bioconversion of lignocellulosic biomass. Previously, comprehensive information on this enzyme has not been reviewed thoroughly. Therefore, the main aim of this review is to highlight the important properties of this interesting enzyme, microorganisms used for its production, and enhanced production using genetic engineering approach. An account on synergism with other biomass hydrolyzing enzymes and various industrial applications of this enzyme has also been provided along with an outlook on further research and development.

Screening of a Novel Glycoside Hydrolase Family 51 α-L-Arabinofuranosidase from Paenibacillus polymyxa KF-1: Cloning, Expression, and Characterization

Paenibacillus polymyxa exhibits remarkable hemicellulolytic activity. In the present study, 13 hemicellulose-degrading enzymes were identified from the secreted proteome of P. polymyxa KF-1 by liquid chromatography-tandem mass spectrometry analysis. α-L-arabinofuranosidase is an important member of hemicellulose-degrading enzymes. A novel α-L-arabinofuranosidase (PpAbf51b), belonging to glycoside hydrolase family 51, was identified from P. polymyxa. Recombinant PpAbf51b was produced in Escherichia coli BL21 (DE3) and was found to be a tetramer using gel filtration chromatography. PpAbf51b hydrolyzed neutral arabinose-containing polysaccharides, including sugar beet arabinan, linear-1,5-α-L-arabinan, and wheat arabinoxylan, with L-arabinose as the main product. The products from hydrolysis indicate that PpAbf51b functions as an exo-α-L-arabinofuranosidase. Combining PpAbf51b and Trichoderma longibrachiatum endo-1,4-xylanase produced significant synergistic effects for the degradation of wheat arabinoxylan. The α-L-arabinofuranosidase identified from the secretome of P. polymyxa KF-1 is potentially suitable for application in biotechnological industries.

Improved activity of α-L-arabinofuranosidase from Geobacillus vulcani GS90 by directed evolution: Investigation on thermal and alkaline stability

α-L-Arabinofuranosidase (Abf) is a potential enzyme because of its synergistic effect with other hemicellulases in agro-industrial field. In this study, directed evolution was applied to Abf from Geobacillus vulcani GS90 (GvAbf) using one round error-prone PCR and constructed a library of 73 enzyme variants of GvAbf. The activity screening of the enzyme variants was performed on soluble protein extracts using p-nitrophenyl α-L-arabinofuranoside as substrate. Two high activity displaying variants (GvAbf L307S and GvAbf Q90H/L307S) were selected, purified, partially characterized, and structurally analyzed. The specific activities of both variants were almost 2.5-fold more than that of GvAbf. Both GvAbf variants also exhibited higher thermal stability but lower alkaline stability in reference to GvAbf. The structural analysis of GvAbf model indicated that two mutation sites Q90H and L307S in both GvAbf variants are located in TIM barrel domain, responsible for catalytic action in many Glycoside Hydrolase Families including GH51. The structure of GvAbf model displayed that the position of L307S mutation is closer to the catalytic residues of GvAbf compared with Q90H mutation and also L307S mutation is conserved in both variants of GvAbf. Therefore, it was hypothesized that L307S amino acid substitution may play a critical role in catalytic activity of GvAbf.

The mechanism by which a distinguishing arabinofuranosidase can cope with internal di-substitutions in arabinoxylans

BACKGROUND

Arabinoxylan is an abundant polysaccharide in industrially relevant biomasses such as sugarcane, corn stover and grasses. However, the arabinofuranosyl di-substitutions that decorate the xylan backbone are recalcitrant to most known arabinofuranosidases (Abfs).

RESULTS

In this work, we identified a novel GH51 Abf (XacAbf51) that forms trimers in solution and can cope efficiently with both mono- and di-substitutions at terminal or internal xylopyranosyl units of arabinoxylan. Using mass spectrometry, the kinetic parameters of the hydrolysis of 33-α-l-arabinofuranosyl-xylotetraose and 23,33-di-α-l-arabinofuranosyl-xylotetraose by XacAbf51 were determined, demonstrating the capacity of this enzyme to cleave arabinofuranosyl linkages of internal mono- and di-substituted xylopyranosyl units. Complementation studies of fungal enzyme cocktails with XacAbf51 revealed an increase of up to 20% in the release of reducing sugars from pretreated sugarcane bagasse, showing the biotechnological potential of a generalist GH51 in biomass saccharification. To elucidate the structural basis for the recognition of internal di-substitutions, the crystal structure of XacAbf51 was determined unveiling the existence of a pocket strategically arranged near to the - 1 subsite that can accommodate a second arabinofuranosyl decoration, a feature not described for any other GH51 Abf structurally characterized so far.

CONCLUSIONS

In summary, this study reports the first kinetic characterization of internal di-substitution release by a GH51 Abf, provides the structural basis for this activity and reveals a promising candidate for industrial processes involving plant cell wall depolymerization.

Enzymatic Mechanism for Arabinan Degradation and Transport in the Thermophilic Bacterium Caldanaerobius polysaccharolyticus

The plant cell wall polysaccharide arabinan provides an important supply of arabinose, and unraveling arabinan-degrading strategies by microbes is important for understanding its use as a source of energy. Here, we explored the arabinan-degrading enzymes in the thermophilic bacterium Caldanaerobius polysaccharolyticus and identified a gene cluster encoding two glycoside hydrolase (GH) family 51 α-l-arabinofuranosidases (CpAbf51A, CpAbf51B), a GH43 endoarabinanase (CpAbn43A), a GH27 β-l-arabinopyranosidase (CpAbp27A), and two GH127 β-l-arabinofuranosidases (CpAbf127A, CpAbf127B). The genes were expressed as recombinant proteins, and the functions of the purified proteins were determined with para-nitrophenyl (pNP)-linked sugars and naturally occurring pectin structural elements as the substrates. The results demonstrated that CpAbn43A is an endoarabinanase while CpAbf51A and CpAbf51B are α-l-arabinofuranosidases that exhibit diverse substrate specificities, cleaving α-1,2, α-1,3, and α-1,5 linkages of purified arabinan-oligosaccharides. Furthermore, both CpAbf127A and CpAbf127B cleaved β-arabinofuranose residues in complex arabinan side chains, thus providing evidence of the function of this family of enzymes on such polysaccharides. The optimal temperatures of the enzymes ranged between 60°C and 75°C, and CpAbf43A and CpAbf51A worked synergistically to release arabinose from branched and debranched arabinan. Furthermore, the hydrolytic activity on branched arabinan oligosaccharides and degradation of pectic substrates by the endoarabinanase and l-arabinofuranosidases suggested a microbe equipped with diverse activities to degrade complex arabinan in the environment. Based on our functional analyses of the genes in the arabinan degradation cluster and the substrate-binding studies on a component of the cognate transporter system, we propose a model for arabinan degradation and transport by C. polysaccharolyticusIMPORTANCE Genomic DNA sequencing and bioinformatic analysis allowed the identification of a gene cluster encoding several proteins predicted to function in arabinan degradation and transport in C. polysaccharolyticus The analysis of the recombinant proteins yielded detailed insights into the putative arabinan metabolism of this thermophilic bacterium. The use of various branched arabinan oligosaccharides provided a detailed understanding of the substrate specificities of the enzymes and allowed assignment of two new GH127 polypeptides as β-l-arabinofuranosidases able to degrade pectic substrates, thus expanding our knowledge of this rare group of glycoside hydrolases. In addition, the enzymes showed synergistic effects for the degradation of arabinans at elevated temperatures. The enzymes characterized from the gene cluster are, therefore, of utility for arabinose production in both the biofuel and food industries.

Characterization of a Novel α-l-Arabinofuranosidase from Ruminococcus albus 7 and Rational Design for Its Thermostability

An α-l-arabinofuranosidase (Abf) encoding gene was obtained via genomic mining from a Ruminococcus albus strain. The specific activity of this GH 51 Abf was 73.3 U/mg at pH 6.0 and 50 °C. The modification of Abf, aimed at improving thermostability, was performed through different strategies. Structure-based rational design using the PoPMuSiC and the Enzyme Thermal Stability System (ETSS) predicted thermal stability of Abf and enhanced the half-life of thermal inactivation (t_1/2) at 50 °C for K208W more than 11.1 times versus the wild-type (WT). Sequence-based rational design was also conducted by substituting histidine with lysine at various sites. Among eight mutants, the t_1/2 at 50 °C of H337K was prolonged by 5.0-fold, and the specific activity of this mutant was increased to 121.8 U/mg. In addition, the mutant H337K was utilized with some enzymes to extract pectin from apple pomace. The enzymatically produced pectin got less moisture and ash, milder pH, and higher viscosity than its acid-extracted counterpart, indicating that Abf has an application prospect in pectin production.

Novel α-L-arabinofuranosidase from Cellulomonas fimi ATCC 484 and its substrate-specificity analysis with the aid of computer

In the process of gene mining for novel α-L-arabinofuranosidases (AFs), the gene Celf_3321 from Cellulomonas fimi ATCC 484 encodes an AF, termed as AbfCelf, with potent activity, 19.4 U/mg under the optimum condition, pH 6.0 and 40 °C. AbfCelf can hydrolyze α-1,5-linked oligosaccharides, sugar beet arabinan, linear 1,5-α-arabinan, and wheat flour arabinoxylan, which is partly different from some previously well-characterized GH 51 AFs. The traditional substrate-specificity analysis for AFs is labor-consuming and money costing, because the substrates include over 30 kinds of various 4-nitrophenol (PNP)-glycosides, oligosaccharides, and polysaccharides. Hence, a preliminary structure and mechanism based method was applied for substrate-specificity analysis. The binding energy (ΔG, kcal/mol) obtained by docking suggested the reaction possibility and coincided with the experimental results. AbfA crystal 1QW9 was used to test the rationality of docking method in simulating the interaction between enzyme and substrate, as well the credibility of the substrate-specificity analysis method in silico.

Dynamic study of how the bacterial breakdown of plant cell walls allows the reconstitution of efficient hemicellulasic cocktails

Designing more efficient mixtures of enzymes is necessary to produce molecules of interest from biomass lignocellulosic fractionation. The present study aims to investigate the strategies used by the thermophilic and hemicellulolytic bacterium Thermobacillus xylanilyticus to fractionate wheat bran and wheat straw during its growth. Results demonstrated ratios and levels of hemicellulases produced varied during growth on both biomasses. Xylanase activity was mainly produced during stationary stages of growth whereas esterase and arabinosidase activities were detected earlier. This enzymatic profile is correlated with the expression pattern of genes encoding four hemicellulases (two xylanases, one arabinosidase and one esterase) produced by T. xylanilyticus during growth. Based on identification of the bacterial strategy, the synergistic efficiency of the four hemicellulases during the hydrolysis of both substrates was evaluated. The four hemicellulases worked together with high degree of synergy and released high amounts of xylose, arabinose and phenolic acids from wheat bran and wheat straw.

β-xylosidases and α-L-arabinofuranosidases: accessory enzymes for arabinoxylan degradation

Arabinoxylan (AX) is among the most abundant hemicelluloses on earth and one of the major components of feedstocks that are currently investigated as a source for advanced biofuels. As global research into these sustainable biofuels is increasing, scientific knowledge about the enzymatic breakdown of AX advanced significantly over the last decade. This review focuses on the exo-acting AX hydrolases, such as α-arabinofuranosidases and β-xylosidases. It aims to provide a comprehensive overview of the diverse substrate specificities and corresponding structural features found in the different glycoside hydrolase families. A careful review of the available literature reveals a marked difference in activity between synthetically labeled and naturally occurring substrates, often leading to erroneous enzymatic annotations. Therefore, special attention is given to enzymes with experimental evidence on the hydrolysis of natural polymers.

Assessing the xylanolytic bacterial diversity during the malting process

The presence of microorganisms producing cell wall hydrolyzing enzymes such as xylanases during malting can improve mash filtration behavior and consequently have potential for more efficient wort production. In this study, the xylanolytic bacterial community during malting was assessed by isolation and cultivation on growth media containing arabinoxylan, and identification by 16S rRNA gene sequencing. A total of 33 species-level operational taxonomic units (OTUs) were found, taking into account a 3% sequence dissimilarity cut-off, belonging to four phyla (Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria) and 25 genera. Predominant OTUs represented xylanolytic bacteria identified as Sphingobacterium multivorum, Stenotrophomonas maltophilia, Aeromonas hydrophila and Pseudomonas fulva. DNA fingerprinting of all xylanolytic isolates belonging to S. multivorum obtained in this study revealed shifts in S. multivorum populations during the process. Xylanase activity was determined for a selection of isolates, with Cellulomonas flavigena showing the highest activity. The xylanase of this species was isolated and purified 23.2-fold by ultrafiltration, 40% ammonium sulfate precipitation and DEAE-FF ion-exchange chromatography and appeared relatively thermostable. This study will enhance our understanding of the role of microorganisms in the barley germination process. In addition, this study may provide a basis for microflora management during malting.

The hemicellulolytic enzyme arsenal of Thermobacillus xylanilyticus depends on the composition of biomass used for growth

Background

Thermobacillus xylanilyticus is a thermophilic and highly xylanolytic bacterium. It produces robust and stable enzymes, including glycoside hydrolases and esterases, which are of special interest for the development of integrated biorefineries. To investigate the strategies used by T. xylanilyticus to fractionate plant cell walls, two agricultural by-products, wheat bran and straw (which differ in their chemical composition and tissue organization), were used in this study and compared with glucose and xylans. The ability of T. xylanilyticus to grow on these substrates was studied. When the bacteria used lignocellulosic biomass, the production of enzymes was evaluated and correlated with the initial composition of the biomass, as well as with the evolution of any residues during growth.

Results

Our results showed that T. xylanilyticus is not only able to use glucose and xylans as primary carbon sources but can also use wheat bran and straw. The chemical compositions of both lignocellulosic substrates were modified by T. xylanilyticus after growth. The bacteria were able to consume 49% and 20% of the total carbohydrates in bran and straw, respectively, after 24 h of growth. The phenolic and acetyl ester contents of these substrates were also altered. Bacterial growth on both lignocellulosic biomasses induced hemicellulolytic enzyme production, and xylanase was the primary enzyme secreted. Debranching activities were differentially produced, as esterase activities were more important to bacterial cultures grown on wheat straw; arabinofuranosidase production was significantly higher in bacterial cultures grown on wheat bran.

Conclusion

This study provides insight into the ability of T. xylanilyticus to grow on abundant agricultural by-products, which are inexpensive carbon sources for enzyme production. The composition of the biomass upon which the bacteria grew influenced their growth, and differences in the biomass provided resulted in dissimilar enzyme production profiles. These results indicate the importance of using different biomass sources to encourage the production of specific enzymes.

Thermostable enzymes as biocatalysts in the biofuel industry

Lignocellulose is the most abundant carbohydrate source in nature and represents an ideal renewable energy source. Thermostable enzymes that hydrolyze lignocellulose to its component sugars have significant advantages for improving the conversion rate of biomass over their mesophilic counterparts. We review here the recent literature on the development and use of thermostable enzymes for the depolymerization of lignocellulosic feedstocks for biofuel production. Furthermore, we discuss the protein structure, mechanisms of thermostability, and specific strategies that can be used to improve the thermal stability of lignocellulosic biocatalysts.

Two distinct arabinofuranosidases contribute to arabino-oligosaccharide degradation in Bacillus subtilis

Bacillus subtilis produces alpha-l-arabinofuranosidases (EC 3.2.1.55; AFs) capable of releasing arabinosyl oligomers and l-arabinose from plant cell walls. Here, we show by insertion-deletion mutational analysis that genes abfA and xsa(asd), herein renamed abf2, encode AFs responsible for the majority of the intracellular AF activity in B. subtilis. Both enzyme activities were shown to be cytosolic and functional studies indicated that arabino-oligomers are natural substrates for the AFs. The products of the two genes were overproduced in Escherichia coli, purified and characterized. The molecular mass of the purified AbfA and Abf2 was about 58 kDa and 57 kDa, respectively. However, native PAGE gradient gel analysis and cross-linking assays detected higher-order structures (>250 kDa), suggesting a multimeric organization of both enzymes. Kinetic experiments at 37 degrees C, with p-nitrophenyl-alpha-l-arabinofuranoside as substrate, gave an apparent K(m) of 0.498 mM and 0.421 mM, and V(max) of 317 U mg(-1) and 311 U mg(-1) for AbfA and Abf2, respectively. The two enzymes displayed maximum activity at 50 degrees C and 60 degrees C, respectively, and both proteins were most active at pH 8.0. AbfA and Abf2 both belong to family 51 of the glycoside hydrolases but have different substrate specificity. AbfA acts preferentially on (1-->5) linkages of linear alpha-1,5-l-arabinan and alpha-1,5-linked arabino-oligomers, and is much less effective on branched sugar beet arabinan and arabinoxylan and arabinogalactan. In contrast, Abf2 is most active on (1-->2) and (1-->3) linkages of branched arabinan and arabinoxylan, suggesting a concerted contribution of these enzymes to optimal utilization of arabinose-containing polysaccharides by B. subtilis.