Action of xylan deacetylating enzymes on monoacetyl derivatives of 4-nitrophenyl glycosides of β-D-xylopyranose and α-L-arabinofuranose

Characterization of a GDS(L)-like hydrolase from Pleurotus sapidus with an unusual SGNH motif

The GDS(L)-like lipase from the Basidiomycota Pleurotus sapidus (PSA_Lip) was heterologously expressed using Trichoderma reesei with an activity of 350 U L-1. The isoelectric point of 5.0 was determined by isoelectric focusing. The novel PSA_Lip showed only 23.8-25.1%, 25.5%, 26.6% and 28.4% identity to the previously characterized GDSL-like enzymes phospholipase, plant lipase, acetylcholinesterase and acetylxylan esterase, from the carbohydrate esterase family 16, respectively. Therefore, the enzyme was purified from the culture supernatant and the catalytic properties and the substrate specificity of the enzyme were investigated using different assays to reveal its potential function. While no phospholipase, acetylcholinesterase and acetylxylan esterase activities were detected, studies on the hydrolysis of ferulic acid methyl ester (~ 8.3%) and feruloylated carbohydrate 5-O-transferuloyl-arabino-furanose (~ 0.8%) showed low conversions of these substrates. By investigating the hydrolytic activity towards p-nitrophenyl-(pNP)-esters with various chain-lengths, the highest activity was determined for medium chain-length pNP-octanoate at 65 °C and a pH value of 8, while almost no activity was detected for pNP-hexanoate. The enzyme is highly stable when stored at pH 10 and 4 °C for at least 7 days. Moreover, using consensus sequence analysis and homology modeling, we could demonstrate that the PSA_Lip does not contain the usual SGNH residues in the actives site, which are usually present in GDS(L)-like enzymes.

Development of a Vector Set for High or Inducible Gene Expression and Protein Secretion in the Yeast Genus Blastobotrys

Converting lignocellulosic biomass into value-added products is one of the challenges in developing a sustainable economy. Attempts to engineer fermenting yeasts to recover plant waste are underway. Although intensive metabolic engineering has been conducted to obtain Saccharomyces cerevisiae strains capable of metabolising pentose sugars mainly found in hemicellulose, enzymatic hydrolysis after pretreatment is still required. Blastobotrys raffinosifermentans, which naturally assimilates xylose and arabinose and displays numerous glycoside hydrolases, is a good candidate for direct and efficient conversion of renewable biomass. However, a greater diversity of tools for genetic engineering is needed. Here, we report the characterisation of four new promising promoters, a new dominant marker, and two vectors for the secretion of epitope tagged proteins along with a straightforward transformation protocol. The TDH3 promoter is a constitutive promoter stronger than TEF1, and whose activity is maintained at high temperature or in the presence of ethanol. The regulated promoters respond to high temperature for HSP26, gluconeogenic sources for PCK1 or presence of xylose oligomers for XYL1. Two expression/secretion vectors were designed based on pTEF1 and pTDH3, two endogenous signal peptides from an α-arabinanase and an α-glucuronidase, and two epitopes. A heterologous α-arabinoxylan hydrolase from Apiotrichum siamense was efficiently secreted using these two vectors.

Challenges of Biomass Utilization for Bioenergy in a Climate Change Scenario

The climate changes expected for the next decades will expose plants to increasing occurrences of combined abiotic stresses, including drought, higher temperatures, and elevated CO2 atmospheric concentrations. These abiotic stresses have significant consequences on photosynthesis and other plants' physiological processes and can lead to tolerance mechanisms that impact metabolism dynamics and limit plant productivity. Furthermore, due to the high carbohydrate content on the cell wall, plants represent a an essential source of lignocellulosic biomass for biofuels production. Thus, it is necessary to estimate their potential as feedstock for renewable energy production in future climate conditions since the synthesis of cell wall components seems to be affected by abiotic stresses. This review provides a brief overview of plant responses and the tolerance mechanisms applied in climate change scenarios that could impact its use as lignocellulosic biomass for bioenergy purposes. Important steps of biofuel production, which might influence the effects of climate change, besides biomass pretreatments and enzymatic biochemical conversions, are also discussed. We believe that this study may improve our understanding of the plant biological adaptations to combined abiotic stress and assist in the decision-making for selecting key agronomic crops that can be efficiently adapted to climate changes and applied in bioenergy production.

CE16 acetylesterases: in silico analysis, catalytic machinery prediction and comparison with related SGNH hydrolases

Bioinformatics analysis was focused on unique acetylesterases annotated in the CAZy database within the CE16 family and simultaneously belonging to the SGNH hydrolase superfamily. The CE16 acetylesterases were compared to structurally related SGNH hydrolases: (i) selected members of the CE2, CE3, CE6, CE12 and CE17 family of the CAZy database and (ii) structural representatives of the Lipase_GDSL and Lipase_GDSL_2 families according to the Pfam database. Sequence alignment based on four conserved sequence regions (CSRs) containing active-site residues was used to calculate sequence logos specific for each CE family and to construct a phylogenetic tree. In many members of the CE16 family, aspartic acid from the Ser-His-Asp catalytic triad has been replaced by asparagine, and based on structure-sequence comparison, an alternative catalytic dyad mechanism was predicted for these enzymes. In addition to four conserved regions, CSR-I, CSR-II, CSR-III and CSR-V, containing catalytic and oxyanion-hole residues, CSR-IV was found in the CE16 family as the only CAZy family. Tertiary structures of the characterized CE16 members prepared by homology modeling showed that the α/β/α sandwich fold as well as the topology of their active sites are preserved. The phylogenetic tree and sequence alignment indicate the existence of a subfamily in the CE16 family fully consistent with the known biochemical data. In addition, nonstandard CE16 members that differ from others were analyzed and their active-site residues were predicted. A better understanding of the structure-function relationship of acetylesterases can help in the targeted design of these enzymes for biotechnology.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-020-02575-w.

Hybrid Aspen Expressing a Carbohydrate Esterase Family 5 Acetyl Xylan Esterase Under Control of a Wood-Specific Promoter Shows Improved Saccharification

Fast-growing broad-leaf tree species can serve as feedstocks for production of bio-based chemicals and fuels through biochemical conversion of wood to monosaccharides. This conversion is hampered by the xylan acetylation pattern. To reduce xylan acetylation in the wood, the Hypocrea jecorina acetyl xylan esterase (HjAXE) from carbohydrate esterase (CE) family 5 was expressed in hybrid aspen under the control of the wood-specific PtGT43B promoter and targeted to the secretory pathway. The enzyme was predicted to deacetylate polymeric xylan in the vicinity of cellulose due to the presence of a cellulose-binding module. Cell-wall-bound protein fractions from developing wood of transgenic plants were capable of releasing acetyl from finely ground wood powder, indicative of active AXE present in cell walls of these plants, whereas no such activity was detected in wild-type plants. The transgenic lines grew in height and diameter as well as wild-type trees, whereas their internodes were slightly shorter, indicating higher leaf production. The average acetyl content in the wood of these lines was reduced by 13%, mainly due to reductions in di-acetylated xylose units, and in C-2 and C-3 mono-acetylated xylose units. Analysis of soluble cell wall polysaccharides revealed a 4% reduction in the fraction of xylose units and an 18% increase in the fraction of glucose units, whereas the contents of cellulose and lignin were not affected. Enzymatic saccharification of wood from transgenic plants resulted in 27% higher glucose yield than for wild-type plants. Brunauer-Emmett-Teller (BET) analysis and Simons' staining pointed toward larger surface area and improved cellulose accessibility for wood from transgenic plants compared to wood from wild-type plants, which could be achieved by HjAXE deacetylating xylan bound to cellulose. The results show that CE5 family can serve as a source of enzymes for in planta reduction of recalcitrance to saccharification.

Crystal structure and functional characterization of a cold-active acetyl xylan esterase (PbAcE) from psychrophilic soil microbe Paenibacillus sp

Cold-active acetyl xylan esterases allow for reduced bioreactor heating costs in bioenergy production. Here, we isolated and characterized a cold-active acetyl xylan esterase (PbAcE) from the psychrophilic soil microbe Paenibacillus sp. R4. The enzyme hydrolyzes glucose penta-acetate and xylan acetate, reversibly producing acetyl xylan from xylan, and it shows higher activity at 4°C than at 25°C. We solved the crystal structure of PbAcE at 2.1-Å resolution to investigate its active site and the reason for its low-temperature activity. Structural analysis showed that PbAcE forms a hexamer with a central substrate binding tunnel, and the inter-subunit interactions are relatively weak compared with those of its mesophilic and thermophilic homologs. PbAcE also has a shorter loop and different residue composition in the β4–α3 and β5–α4 regions near the substrate binding site. Flexible subunit movements and different active site loop conformations may enable the strong low-temperature activity and broad substrate specificity of PbAcE. In addition, PbAcE was found to have strong activity against antibiotic compound substrates, such as cefotaxime and 7-amino cephalosporanic acid (7-ACA). In conclusion, the PbAcE structure and our biochemical results provide the first example of a cold-active acetyl xylan esterase and a starting template for structure-based protein engineering.

Two-Dimensional High-Throughput Endo-Enzyme Screening Assays Based on Chromogenic Polysaccharide Hydrogel and Complex Biomass Substrates

In this chapter, we present a two-dimensional approach for high-throughput screening of endo-cellulases as well as other endo-acting enzymes. The method is based on chromogenic substrates, produced either from purified or complex material, providing valuable information about enzyme activity toward its target as well as that same target in a context of complex natural material normally encountered in bioindustrial settings. The enzymes that can be tested using this assay can be from virtually any source: in purified form, directly from microbial cultures or even from raw materials, enabling study of the interplay between enzyme mixtures such as synergistic or inhibitory effects. By using the method of analysis described in this chapter, enzymes can be screened and evaluated quickly and information pertinent to both the inherent properties of the enzyme itself as well as predictions about its performance on complex biomass samples can be obtained.

Control of secondary cell wall patterning involves xylan deacetylation by a GDSL esterase

O-acetylation, a ubiquitous modification of cell wall polymers, has striking impacts on plant growth and biomass utilization and needs to be tightly controlled. However, the mechanisms that underpin the control of cell wall acetylation remain elusive. Here, we show a rice brittle leaf sheath1 (bs1) mutant, which contains a lesion in a Golgi-localized GDSL esterase that deacetylates the prominent hemicellulose xylan. Cell wall composition, detailed xylan structure characterization and enzyme kinetics and activity assays on acetylated sugars and xylooligosaccharides demonstrate that BS1 is an esterase that cleaves acetyl moieties from the xylan backbone at O-2 and O-3 positions of xylopyranosyl residues. BS1 thus plays an important role in the maintenance of proper acetylation level on the xylan backbone, which is crucial for secondary wall formation and patterning. Our findings outline a mechanism for how plants modulate wall acetylation and endow a plethora of uncharacterized GDSL esterases with surmisable activities.

In muro deacetylation of xylan affects lignin properties and improves saccharification of aspen wood

BACKGROUND

Lignocellulose from fast growing hardwood species is a preferred source of polysaccharides for advanced biofuels and "green" chemicals. However, the extensive acetylation of hardwood xylan hinders lignocellulose saccharification by obstructing enzymatic xylan hydrolysis and causing inhibitory acetic acid concentrations during microbial sugar fermentation. To optimize lignocellulose for cost-effective saccharification and biofuel production, an acetyl xylan esterase AnAXE1 from Aspergillus niger was introduced into aspen and targeted to cell walls.

RESULTS

AnAXE1-expressing plants exhibited reduced xylan acetylation and grew normally. Without pretreatment, their lignocellulose yielded over 25% more glucose per unit mass of wood (dry weight) than wild-type plants. Glucose yields were less improved (+7%) after acid pretreatment, which hydrolyses xylan. The results indicate that AnAXE1 expression also reduced the molecular weight of xylan, and xylan-lignin complexes and/or lignin co-extracted with xylan, increased cellulose crystallinity, altered the lignin composition, reducing its syringyl to guaiacyl ratio, and increased lignin solubility in dioxane and hot water. Lignin-associated carbohydrates became enriched in xylose residues, indicating a higher content of xylo-oligosaccharides.

CONCLUSIONS

This work revealed several changes in plant cell walls caused by deacetylation of xylan. We propose that deacetylated xylan is partially hydrolyzed in the cell walls, liberating xylo-oligosaccharides and their associated lignin oligomers from the cell wall network. Deacetylating xylan thus not only increases its susceptibility to hydrolytic enzymes during saccharification but also changes the cell wall architecture, increasing the extractability of lignin and xylan and facilitating saccharification.

Colorimetric Detection of Acetyl Xylan Esterase Activities

Colorimetric detection of reaction products is typically preferred for initial surveys of acetyl xylan esterase (AcXE) activity. This chapter will describe common colorimetric methods, and variations thereof, for measuring AcXE activities on commercial, synthesized, and natural substrates. Whereas assays using pNP-acetate, α-naphthyl acetate, and 4-methylumbelliferyl acetate (4MUA) are emphasized, common methods used to measure AcXE activity towards carbohydrate analogs (e.g., acetylated p-nitrophenyl β-D-xylopyranosides) and various acetylated xylans are also described. Strengths and limitations of the colorimetric assays are highlighted.

High-throughput Screening of Carbohydrate-degrading Enzymes Using Novel Insoluble Chromogenic Substrate Assay Kits

Carbohydrates active enzymes (CAZymes) have multiple roles in vivo and are widely used for industrial processing in the biofuel, textile, detergent, paper and food industries. A deeper understanding of CAZymes is important from both fundamental biology and industrial standpoints. Vast numbers of CAZymes exist in nature (especially in microorganisms) and hundreds of thousands have been cataloged and described in the carbohydrate active enzyme database (CAZy). However, the rate of discovery of putative enzymes has outstripped our ability to biochemically characterize their activities. One reason for this is that advances in genome and transcriptome sequencing, together with associated bioinformatics tools allow for rapid identification of candidate CAZymes, but technology for determining an enzyme's biochemical characteristics has advanced more slowly. To address this technology gap, a novel high-throughput assay kit based on insoluble chromogenic substrates is described here. Two distinct substrate types were produced: Chromogenic Polymer Hydrogel (CPH) substrates (made from purified polysaccharides and proteins) and Insoluble Chromogenic Biomass (ICB) substrates (made from complex biomass materials). Both CPH and ICB substrates are provided in a 96-well high-throughput assay system. The CPH substrates can be made in four different colors, enabling them to be mixed together and thus increasing assay throughput. The protocol describes a 96-well plate assay and illustrates how this assay can be used for screening the activities of enzymes, enzyme cocktails, and broths.

Towards enzymatic breakdown of complex plant xylan structures: State of the art

Significant progress over the past few years has been achieved in the enzymology of microbial degradation and saccharification of plant xylan, after cellulose being the most abundant natural renewable polysaccharide. Several new types of xylan depolymerizing and debranching enzymes have been described in microorganisms. Despite the increasing variety of known glycoside hydrolases and carbohydrate esterases, some xylan structures still appear quite recalcitrant. This review focuses on the mode of action of different types of depolymerizing endoxylanases and their cooperation with β-xylosidase and accessory enzymes in breakdown of complex highly branched xylan structures. Emphasis is placed on the enzymatic hydrolysis of alkali-extracted deesterified polysaccharide as well as acetylated xylan isolated from plant cell walls under non-alkaline conditions. It is also shown how the combination of selected endoxylanases and debranching enzymes can determine the nature of prebiotic xylooligosaccharides or lead to complete hydrolysis of the polysaccharide. The article also highlights the possibility for discovery of novel xylanolytic enzymes, construction of multifunctional chimeric enzymes and xylanosomes in parallel with increasing knowledge on the fine structure of the polysaccharide.

A new generation of versatile chromogenic substrates for high-throughput analysis of biomass-degrading enzymes

BACKGROUND

Enzymes that degrade or modify polysaccharides are widespread in pro- and eukaryotes and have multiple biological roles and biotechnological applications. Recent advances in genome and secretome sequencing, together with associated bioinformatic tools, have enabled large numbers of carbohydrate-acting enzymes to be putatively identified. However, there is a paucity of methods for rapidly screening the biochemical activities of these enzymes, and this is a serious bottleneck in the development of enzyme-reliant bio-refining processes.

RESULTS

We have developed a new generation of multi-coloured chromogenic polysaccharide and protein substrates that can be used in cheap, convenient and high-throughput multiplexed assays. In addition, we have produced substrates of biomass materials in which the complexity of plant cell walls is partially maintained.

CONCLUSIONS

We show that these substrates can be used to screen the activities of glycosyl hydrolases, lytic polysaccharide monooxygenases and proteases and provide insight into substrate availability within biomass. We envisage that the assays we have developed will be used primarily for first-level screening of large numbers of putative carbohydrate-acting enzymes, and the assays have the potential to be incorporated into fully or semi-automated robotic enzyme screening systems.

Positional preferences of acetyl esterases from different CE families towards acetylated 4-O-methyl glucuronic acid-substituted xylo-oligosaccharides

BACKGROUND

Acetylation of the xylan backbone restricts the hydrolysis of plant poly- and oligosaccharides by hemicellulolytic enzyme preparations to constituent monosaccharides. The positional preferences and deacetylation efficiencies of acetyl esterases from seven different carbohydrate esterase (CE) families towards different acetylated xylopyranosyl units (Xylp) - as present in 4-O-methyl-glucuronic acid (MeGlcA)-substituted xylo-oligosaccharides (AcUXOS) derived from Eucalyptus globulus - were monitored by (1)H NMR, using common conditions for biofuel production (pH 5.0, 50°C).

RESULTS

Differences were observed regarding the hydrolysis of 2-O, 3-O, and 2,3-di-O acetylated Xylp and 3-O acetylated Xylp 2-O substituted with MeGlcA. The acetyl esterases tested could be categorized in three groups having activities towards (i) 2-O and 3-O acetylated Xylp, (ii) 2-O, 3-O, and 2,3-di-O acetylated Xylp, and (iii) 2-O, 3-O, and 2,3-di-O acetylated Xylp, as well as 3-O acetylated Xylp 2-O substituted with MeGlcA at the non-reducing end. A high deacetylation efficiency of up to 83% was observed for CE5 and CE1 acetyl esterases. Positional preferences were observed towards 2,3-di-O acetylated Xylp (TeCE1, AnCE5, and OsCE6) or 3-O acetylated Xylp (CtCE4).

CONCLUSIONS

Different positional preferences, deacetylation efficiencies, and initial deacetylation rates towards 2-O, 3-O, and 2,3-di-O acetylated Xylp and 3-O acetylated Xylp 2-O substituted with MeGlcA were demonstrated for acetyl esterases from different CE families at pH 5.0 and 50°C. The data allow the design of optimal, deacetylating hemicellulolytic enzyme mixtures for the hydrolysis of non-alkaline-pretreated bioenergy feedstocks.

Redistribution of acetyl groups on the non-reducing end xylopyranosyl residues and their removal by xylan deacetylases

BACKGROUND

Monoacetylated xylosyl residues of the main hardwood hemicellulose acetylglucuronoxylan undergo acetyl group migration between positions 2 and 3, and predominantly to position 4 of the non-reducing end xylopyranosyl (NRE-Xylp) residues which are amplified by saccharifying enzymes. On monoacetylated non-reducing end xylopyranosyl (NRE-Xylp) residues of xylooligosaccharides the acetyl group migrates predominantly to position 4 and hinders their hydrolysis by β-xylosidase.

METHODS

Acetyl migration on the NRE-Xylp residues and their enzymatic deacetylation by various xylan deacetylases was followed by (1)H-NMR spectroscopy and TLC.

RESULTS

A 5-min heat treatment of 4-nitrophenyl 3-O-acetyl-β-D-xylopyranoside was sufficient to establish equilibrium between monoacetate derivatives acetylated at positions 2, 3 and 4. Rapid acetyl migration along the NRE-Xylp ring at elevated temperature was confirmed in derivatives of methyl β-1,4-xylotrioside (Xyl3Me) monoacetylated solely on the NRE-Xylp residue, the analogues of naturally occurring acetylated xylooligosaccharides. The Xyl3Me monoacetates were used as substrates to study regioselectivity of the NRE-Xylp residue deacetylation by various acetylxylan esterases (AcXEs) of distinct carbohydrate esterase (CE) families. CE1, CE4 and CE6 AcXEs hydrolyzed considerably faster the 2″-O-acetyl derivative than the 3″-O-acetyl derivative. In contrast, the CE16 acetyl esterase preferred to attack the ester bond at position 3 followed by position 4.

CONCLUSIONS

Redistribution of acetyl group on the NRE-Xylp residues is extremely rapid at elevated temperature and includes the formation of 4-acetate. Regioselectivity of AcXEs and CE16 acetyl esterase on these monoacetates is complementary.

GENERAL SIGNIFICANCE

The formation of all isomers of acetylated xylosyl residues must be taken into account after a long-term incubation of acetylxylan and acetylated xylooligosaccharides solutions or upon their treatment at elevated temperatures. This phenomenon emphasizes requirement of both types of xylan deacetylases to enable a rapid saccharification of xylooligosaccharides by glycoside hydrolases.

Comparative analysis of the Geobacillus hemicellulose utilization locus reveals a highly variable target for improved hemicellulolysis

Background

Members of the thermophilic genus Geobacillus can grow at high temperatures and produce a battery of thermostable hemicellulose hydrolytic enzymes, making them ideal candidates for the bioconversion of biomass to value-added products. To date the molecular determinants for hemicellulose degradation and utilization have only been identified and partially characterized in one strain, namely Geobacillus stearothermophilus T-6, where they are clustered in a single genetic locus.

Results

Using the G. stearothermophilus T-6 hemicellulose utilization locus as genetic marker, orthologous hemicellulose utilization (HUS) loci were identified in the complete and partial genomes of 17/24 Geobacillus strains. These HUS loci are localized on a common genomic island. Comparative analyses of these loci revealed extensive variability among the Geobacillus hemicellulose utilization systems, with only seven out of 41–68 proteins encoded on these loci conserved among the HUS⁺ strains. This translates into extensive differences in the hydrolytic enzymes, transport systems and metabolic pathways employed by Geobacillus spp. to degrade and utilize hemicellulose polymers.

Conclusions

The genetic variability among the Geobacillus HUS loci implies that they have variable capacities to degrade hemicellulose polymers, or that they may degrade distinct polymers, as are found in different plant species and tissues. The data from this study can serve as a basis for the genetic engineering of a Geobacillus strain(s) with an improved capacity to degrade and utilize hemicellulose.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-836) contains supplementary material, which is available to authorized users.

Preliminary crystallographic analysis of a double mutant of the acetyl xylo-oligosaccharide esterase Axe2 in its dimeric form

Xylans are polymeric sugars constituting a significant part of the plant cell wall. They are usually substituted with acetyl side groups attached at positions 2 or 3 of the xylose backbone units. Acetylxylan esterases are part of the hemicellulolytic system of many microorganisms which utilize plant biomass for growth. These enzymes hydrolyze the ester linkages of the xylan acetyl groups and thus improve the accessibility of main-chain-hydrolyzing enzymes and their ability to break down the sugar backbone units. The acetylxylan esterases are therefore critically important for those microorganisms and as such could be used for a wide range of biotechnological applications. The structure of an acetylxylan esterase (Axe2) isolated from the thermophilic bacterium Geobacillus stearothermophilus T6 has been determined, and it has been demonstrated that the wild-type enzyme is present as a unique torus-shaped octamer in the crystal and in solution. In order to understand the functional origin of this unique oligomeric structure, a series of rational noncatalytic, site-specific mutations have been made on Axe2. Some of these mutations led to a different dimeric form of the protein, which showed a significant reduction in catalytic activity. One of these double mutants, Axe2-Y184F-W190P, has recently been overexpressed, purified and crystallized. The best crystals obtained belonged to the orthorhombic space group P212121, with unit-cell parameters a = 71.1, b = 106.0, c = 378.6 Å. A full diffraction data set to 2.3 Å resolution has been collected from a flash-cooled crystal of this type at 100 K using synchrotron radiation. This data set is currently being used for the three-dimensional structure analysis of the Axe2-Y184F-W190P mutant in its dimeric form.

Distinct roles of carbohydrate esterase family CE16 acetyl esterases and polymer-acting acetyl xylan esterases in xylan deacetylation

Mass spectrometric analysis was used to compare the roles of two acetyl esterases (AE, carbohydrate esterase family CE16) and three acetyl xylan esterases (AXE, families CE1 and CE5) in deacetylation of natural substrates, neutral (linear) and 4-O-methyl glucuronic acid (MeGlcA) substituted xylooligosaccharides (XOS). AEs were similarly restricted in their action and apparently removed in most cases only one acetyl group from the non-reducing end of XOS, acting as exo-deacetylases. In contrast, AXEs completely deacetylated longer neutral XOS but had difficulties with the shorter ones. Complete deacetylation of neutral XOS was obtained after the combined action of AEs and AXEs. MeGlcA substituents partially restricted the action of both types of esterases and the remaining acidic XOS were mainly substituted with one MeGlcA and one acetyl group, supposedly on the same xylopyranosyl residue. These resisting structures were degraded to great extent only after inclusion of α-glucuronidase, which acted with the esterases in a synergistic manner. When used together with xylan backbone degrading endoxylanase and β-xylosidase, both AE and AXE enhanced the hydrolysis of complex XOS equally.

Trichoderma reesei CE16 acetyl esterase and its role in enzymatic degradation of acetylated hemicellulose

BACKGROUND

Trichoderma reesei CE16 acetyl esterase (AcE) is a component of the plant cell wall degrading system of the fungus. The enzyme behaves as an exo-acting deacetylase removing acetyl groups from non-reducing end sugar residues.

METHODS

In this work we demonstrate this exo-deacetylating activity on natural acetylated xylooligosaccharides using MALDI ToF MS.

RESULTS

The combined action of GH10 xylanase and acetylxylan esterases (AcXEs) leads to formation of neutral and acidic xylooligosaccharides with a few resistant acetyl groups mainly at their non-reducing ends. We show here that these acetyl groups serve as targets for TrCE16 AcE. The most prominent target is the 3-O-acetyl group at the non-reducing terminal Xylp residues of linear neutral xylooligosaccharides or on aldouronic acids carrying MeGlcA at the non-reducing terminus. Deacetylation of the non-reducing end sugar may involve migration of acetyl groups to position 4, which also serves as substrate of the TrCE16 esterase.

CONCLUSION

Concerted action of CtGH10 xylanase, an AcXE and TrCE16 AcE resulted in close to complete deacetylation of neutral xylooligosaccharides, whereas substitution with MeGlcA prevents removal of acetyl groups from only a small fraction of the aldouronic acids. Experiments with diacetyl derivatives of methyl β-d-xylopyranoside confirmed that the best substrate of TrCE16 AcE is 3-O-acetylated Xylp residue followed by 4-O-acetylated Xylp residue with a free vicinal hydroxyl group.

GENERAL SIGNIFICANCE

This study shows that CE16 acetyl esterases are crucial enzymes to achieve complete deacetylation and, consequently, complete the saccharification of acetylated xylans by xylanases, which is an important task of current biotechnology.

Mode of action of acetylxylan esterases on acetyl glucuronoxylan and acetylated oligosaccharides generated by a GH10 endoxylanase

BACKGROUND

Substitutions on the xylan main chain are widely accepted to limit plant cell wall degradability and acetylations are considered as one of the most important obstacles. Hence, understanding the modes of action of a range of acetylxylan esterases (AcXEs) is of ample importance not only to increase the understanding of the enzymology of plant decay/bioremediation but also to enable efficient bioconversion of plant biomass.

METHODS

In this study, the modes of action of acetylxylan esterases (AcXEs) belonging to carbohydrate esterase (CE) families 1, 4, 5 and 6 on xylooligosaccharides generated from hardwood acetyl glucuronoxylan were compared using MALDI ToF MS. Supporting data were obtained by following enzymatic deacetylation by (1)H NMR spectroscopy.

CONCLUSIONS

None of the used enzymes were capable of complete deacetylation, except from linear xylooligosaccharides which were completely deacetylated by some of the esterases in the presence of endoxylanase. A clear difference was observed between the performance of the serine-type esterases of CE families 1, 5 and 6, and the aspartate-metalloesterases of family CE4. The difference is mainly due to the inability of CE4 AcXEs to catalyze deacetylation of 2,3-di-O-acetylated xylopyranosyl residues. Complete deacetylation of a hardwood acetyl glucuronoxylan requires additional deacetylating enzyme(s).

GENERAL SIGNIFICANCE

The results contribute to the understanding of microbial degradation of plant biomass and outline the way to achieve complete saccharification of plant hemicelluloses which did not undergo alkaline pretreatment.

Crystallization and preliminary crystallographic analysis of Axe2, an acetylxylan esterase from Geobacillus stearothermophilus

Acetylxylan esterases are part of the hemi-cellulolytic system of many microorganisms which utilize plant biomass for growth. Xylans, which are polymeric sugars that constitute a significant part of the plant biomass, are usually substituted with acetyl side groups attached at position 2 or 3 of the xylose backbone units. Acetylxylan esterases hydrolyse the ester linkages of the xylan acetyl groups and thus improve the ability of main-chain hydrolysing enzymes to break down the sugar backbone units. As such, these enzymes play an important part in the hemi-cellulolytic utilization system of many microorganisms that use plant biomass for growth. Interest in the biochemical characterization and structural analysis of these enzymes stems from their numerous potential biotechnological applications. An acetylxylan esterase (Axe2) of this type from Geobacillus stearothermophilus T-6 has recently been cloned, overexpressed, purified, biochemically characterized and crystallized. One of the crystal forms obtained (RB1) belonged to the tetragonal space group I422, with unit-cell parameters a = b = 110.2, c = 213.1 Å. A full diffraction data set was collected to 1.85 Å resolution from flash-cooled crystals of the wild-type enzyme at 100 K using synchrotron radiation. A selenomethionine derivative of Axe2 has also been prepared and crystallized for single-wavelength anomalous diffraction experiments. The crystals of the selenomethionine-derivatized Axe2 appeared to be isomorphous to those of the wild-type enzyme and enabled the measurement of a full 1.85 Å resolution diffraction data set at the selenium absorption edge and a full 1.70 Å resolution data set at a remote wavelength. These data are currently being used for three-dimensional structure determination of the Axe2 protein.

Novel enzymes for the degradation of cellulose

The bulk terrestrial biomass resource in a future bio-economy will be lignocellulosic biomass, which is recalcitrant and challenging to process. Enzymatic conversion of polysaccharides in the lignocellulosic biomass will be a key technology in future biorefineries and this technology is currently the subject of intensive research. We describe recent developments in enzyme technology for conversion of cellulose, the most abundant, homogeneous and recalcitrant polysaccharide in lignocellulosic biomass. In particular, we focus on a recently discovered new type of enzymes currently classified as CBM33 and GH61 that catalyze oxidative cleavage of polysaccharides. These enzymes promote the efficiency of classical hydrolytic enzymes (cellulases) by acting on the surfaces of the insoluble substrate, where they introduce chain breaks in the polysaccharide chains, without the need of first "extracting" these chains from their crystalline matrix.

Microbial carbohydrate esterases deacetylating plant polysaccharides

Several plant polysaccharides are partially esterified with acetic acid. One of the roles of this modification is protection of plant cell walls against invading microorganisms. Acetylation of glycosyl residues of polysaccharides prevents hydrolysis of their glycosidic linkages by the corresponding glycoside hydrolases. In this way the acetylation also represents an obstacle of enzymatic saccharification of plant hemicelluloses to fermentable sugars which appears to be a hot topic of current research. We can eliminate this obstacle by alkaline extraction or pretreatment leading to saponification of ester linkages. However, this task has been accomplished in a different way in the nature. The acetyl groups became targets of microbial carbohydrate esterases that evolved to overcome the complexity of the plant cell walls and that cooperate with glycoside hydrolases in plant polysaccharide degradation. This article concentrates on enzymes deacetylating plant hemicelluloses excluding pectin. They are currently grouped in at least 8 families, specifically in CE families 1-7 and 16, originally assigned as acetylxylan esterases, the enzymes acting on hardwood acetyl glucuronoxylan and its fragments generated by endo-β-1,4-xylanases. There are esterases deacetylating softwood galactoglucomannan, but they have not been classified yet. The enzymes present in CE families 1-7 differ in structure and substrate and positional specificity. There are families behaving as endo-type and exo-type deacetylates, i.e. esterases deacetylating internal sugar residues of partially acetylated polysaccharides and also esterases deacetylating non-reducing end sugar residues in oligosaccharides. With one exception, the enzymes of all mentioned CE families belong to serine type esterases. CE family 4 harbors enzymes that are metal-dependent aspartic esterases. Three-dimensional structures have been solved for members of the first seven CE families, however, there is still insufficient knowledge about their substrate specificity and real physiological role. Current knowledge on catalytic properties of the selected families of CEs is summarized in this review. Some of the families are emerging also as new biocatalysts for regioselective acylation and deacylation of carbohydrates.

The role of acetyl xylan esterase in the solubilization of xylan and enzymatic hydrolysis of wheat straw and giant reed

BACKGROUND

Due to the complexity of lignocellulosic materials, a complete enzymatic hydrolysis into fermentable sugars requires a variety of cellulolytic and xylanolytic enzymes. Addition of xylanases has been shown to significantly improve the performance of cellulases and to increase cellulose hydrolysis by solubilizing xylans in lignocellulosic materials. The goal of this work was to investigate the effect of acetyl xylan esterase (AXE) originating from Trichoderma reesei on xylan solubilization and enzymatic hydrolysis of cellulose.

RESULTS

The solubilization of xylan in pretreated wheat straw and giant reed (Arundo donax) by xylanolytic enzymes and the impact of the sequential or simultaneous solubilization of xylan on the hydrolysis of cellulose by purified enzymes were investigated. The results showed that the removal of acetyl groups in xylan by AXE increased the accessibility of xylan to xylanase and improved the hydrolysis of xylan in pretreated wheat straw and giant reed. Solubilization of xylan led to an increased accessibility of cellulose to cellulases and thereby increased the hydrolysis extent of cellulose. A clear synergistic effect between cellulases and xylanolytic enzymes was observed. The highest hydrolysis yield of cellulose was obtained with a simultaneous use of cellulases, xylanase and AXE, indicating the presence of acetylated xylan within the cellulose matrix. Acetylated xylobiose and acetylated xylotriose were produced from xylan without AXE, as confirmed by atmospheric pressure matrix-assisted laser desorption/ionization ion trap mass spectrometry.

CONCLUSIONS

The results in this paper demonstrate that supplementation of xylanase with AXE enhances the solubilization of xylan to some extent and, consequently, increases the subsequent hydrolysis of cellulose. The highest hydrolysis yield was, however, obtained by simultaneous hydrolysis of xylan and cellulose, indicating a layered structure of cellulose and xylan chains in the cell wall substrate. AXE has an important role in the hydrolysis of lignocellulosic materials containing acetylated xylan.

A new family of carbohydrate esterases is represented by a GDSL hydrolase/acetylxylan esterase from Geobacillus stearothermophilus

Acetylxylan esterases hydrolyze the ester linkages of acetyl groups at positions 2 and/or 3 of the xylose moieties in xylan and play an important role in enhancing the accessibility of xylanases to the xylan backbone. The hemicellulolytic system of the thermophilic bacterium Geobacillus stearothermophilus T-6 comprises a putative acetylxylan esterase gene, axe2. The gene product belongs to the GDSL hydrolase family and does not share sequence homology with any of the carbohydrate esterases in the CAZy Database. The axe2 gene is induced by xylose, and the purified gene product completely deacetylates xylobiose peracetate (fully acetylated) and hydrolyzes the synthetic substrates 2-naphthyl acetate, 4-nitrophenyl acetate, 4-methylumbelliferyl acetate, and phenyl acetate. The pH profiles for k(cat) and k(cat)/K(m) suggest the existence of two ionizable groups affecting the binding of the substrate to the enzyme. Using NMR spectroscopy, the regioselectivity of Axe2 was directly determined with the aid of one-dimensional selective total correlation spectroscopy. Methyl 2,3,4-tri-O-acetyl-β-d-xylopyranoside was rapidly deacetylated at position 2 or at positions 3 and 4 to give either diacetyl or monoacetyl intermediates, respectively; methyl 2,3,4,6-tetra-O-acetyl-β-d-glucopyranoside was initially deacetylated at position 6. In both cases, the complete hydrolysis of the intermediates occurred at a much slower rate, suggesting that the preferred substrate is the peracetate sugar form. Site-directed mutagenesis of Ser-15, His-194, and Asp-191 resulted in complete inactivation of the enzyme, consistent with their role as the catalytic triad. Overall, our results show that Axe2 is a serine acetylxylan esterase representing a new carbohydrate esterase family.

Fungal enzyme sets for plant polysaccharide degradation

Enzymatic degradation of plant polysaccharides has many industrial applications, such as within the paper, food, and feed industry and for sustainable production of fuels and chemicals. Cellulose, hemicelluloses, and pectins are the main components of plant cell wall polysaccharides. These polysaccharides are often tightly packed, contain many different sugar residues, and are branched with a diversity of structures. To enable efficient degradation of these polysaccharides, fungi produce an extensive set of carbohydrate-active enzymes. The variety of the enzyme set differs between fungi and often corresponds to the requirements of its habitat. Carbohydrate-active enzymes can be organized in different families based on the amino acid sequence of the structurally related catalytic modules. Fungal enzymes involved in plant polysaccharide degradation are assigned to at least 35 glycoside hydrolase families, three carbohydrate esterase families and six polysaccharide lyase families. This mini-review will discuss the enzymes needed for complete degradation of plant polysaccharides and will give an overview of the latest developments concerning fungal carbohydrate-active enzymes and their corresponding families.