Enzymatic degradation of xyloglucans by Aspergillus species: a comparative view of this genus

A novel GH12 xyloglucanase from the white rot fungus Abortiporus biennis, synergistically enhances lignocellulose saccharification by commercial cellulases

Xyloglucan is a complex, highly substituted plant biomass polysaccharide, which is largely overlooked in the design of enzyme cocktails for lignocellulose saccharification, due to its presence in specific plant tissues only, and its low content. Thus, the microbial mechanisms for its degradation have not been thoroughly studied. However, in the frame of the biorefinery concept, xyloglucan monomers also have to be utilized for the design of efficient bioprocesses. Moreover, in plant tissues, xyloglucan often covers cellulose fibrils, impeding the access of cellulases. In order to shed light on the enzymatic degradation of xyloglucan, a novel GH12 family xyloglucanase was studied, from the basidiomycete Abortiporus biennis. The enzyme was heterologously produced in Pichia pastoris, purified and characterized. AbiXeg12a is a 28 kDa glycoprotein, with relatively strict substrate specificity, since it is only active in xyloglucan and β-glucan. The main hydrolysis products are the oligomers XXXG, XLXG/XXLG, XLLG and the optimum activity conditions are pH 4.5 and 55 °C. The enzyme contributes to the saccharification of corn bran and apple pulp by a commercial cellulase preparation, increasing the release of reducing sugars by up to 39 % and 18 %, respectively, while the addition of AbiXeg12a can minimize the enzyme load of the reaction, at least for apple pulp, without loss in reducing sugar yield. Overall, the importance of xyloglucanases on the saccharification of xyloglucan-containing substrates was demonstrated in this study. The results could contribute to the design of more efficient, tailor-made enzyme cocktails for the saccharification and subsequent valorization of lignocellulose.

Taxonomic, genomic, and ecological insights into a novel Flavobacteriaceae strain from coastal tidal flats

BACKGROUND

Tidal flats are vital coastal ecosystems that play a significant role in organic carbon accumulation and biogeochemical cycles. Members of the family Flavobacteriaceae is known for its ability to degrade complex organic matter, including polysaccharides. However, the ecological roles and metabolic capabilities of Flavobacteriaceae in tidal flat environments remain underexplored.

RESULTS

In this study, we isolated and characterized a novel bacterium, strain NBU2967T, from the tidal flats of Meishan Island in the East China Sea. Phylogenetic and genomic analyses identified this strain as a new genus and species within the family Flavobacteriaceae, for which we propose the name Meishania litoralis gen. nov., sp. nov. Comprehensive polyphasic characterization, including morphological, physiological, chemotaxonomic, and genomic analyses, confirmed its distinct taxonomic status. Genomic analysis revealed a diverse set of carbohydrate-active enzymes (CAZymes), along with multiple metabolic pathways associated with carbon and sulfur cycling, highlighting the strain's potential adaptation to organic-rich marine environments. Comparative genomic and pangenome analyses further demonstrated significant genetic divergence from related taxa. Environmental distribution data revealed that the newly proposed genus Meishania is widely distributed across global marine ecosystems.

CONCLUSIONS

We isolated and characterized a novel bacterium, designated NBU2967T (= KCTC 82912 T = MCCC 1K06391T), for which we propose the name Meishania litoralis gen. nov., sp. nov. This strain is classified as a new genus within the family Flavobacteriaceae. The strain's ability to process both carbon and sulfur compounds underscores its ecological significance in marine ecosystems. These findings provide novel insights into the ecological functions of the family Flavobacteriaceae in coastal tidal flats environments and enhance our understanding of microbial-mediated degradation and transformation of chemical compounds in dynamic coastal ecosystems.

Structural insights into substrate recognition of tri-modular xyloglucanase from Aspergillus oryzae

Xeg5A from Aspergillus oryzae belongs to glycoside hydrolase family 5 subfamily 4. This enzyme has been characterized as a xyloglucan-specific endo-β-1,4-glucanase (xyloglucanase) that cleaves the main chain of xyloglucan at both unbranched and xylosylated glucosyl residues in an endo-processive mode of action. X-ray crystallography revealed that Xeg5A is a tri-modular enzyme composed of a catalytic, an Ig-like, and a C-terminal CBM46-like domains. Xeg5A structures complexed with branched xyloglucan oligosaccharides at subsite -4 to +4 showed that the recognition of xyloglucan side-chain moieties is important for Xeg5A activity. The crystal structure also provided structural insights into the role of the CBM46-like domain in contributing to regiospecificity and, possibly, processivity.

Effects of N-linked glycosylation on the enzymatic properties of GH12 bifunctional enzymes from Aspergillus terreus expressed in Pichia pastoris

In industry, bioenergy, food process, and feed application, endoglucanases are highly valuable for lignocellulose degradation with high catalytic activity under high temperatures. The glycoside hydrolase family 12 endoglucanase (AtEglD) from Aspergillus terreus can efficiently hydrolyze both β-glucan and xyloglucan of barley with an optimal temperature of 55 °C under pH 5.0. To enhance the industrial potential of AtEglD, the rational design of its N-glycosylation sites is imperative. The genes encoding AtEglD (N-glycosylation site at Asn65), along with two mutants: D168S (N-glycosylation site at Asn166) and N65Q (which lacks an N-glycosylation site) were successfully expressed and characterized. AtEglD exhibits reduced activity at 60 °C whereas, the N65Q mutant exhibited enhanced activity, maintaining substantial activity even after 90 min incubation. In barley-β-glucan, its specific activity reached 3204.27 U·mg-1, representing 2.73 times increase compared to AtEglD (1175.35 U·mg-1), while the catalytic efficiency was measured at 779.00 S-1·mM-1, indicating a 74.4 % enhancement relative to AtEglD (447.34 S-1·mM-1). For xyloglucan, N65Q demonstrated a significantly greater affinity compared to AtEglD, with 36.0 % increase in catalytic efficiency. Intriguingly, the D168S mutant exhibited a marked reduction in both specific activity and catalytic efficiency across both substrates. The structure analysis of AtEglD revealed that the N65 residues are far away from the catalytic domain, while the N166 residues are close to the catalytic site. It is implied that N-glycosylation proximal to the catalytic site maybe constrict the substrate-binding channel, thereby diminishing substrate recognition. These findings underscore the pivotal role of N-glycosylation site variations of GH12 endoglucanase in modulating enzyme characteristics.

Production of Isoprimeverose from Xyloglucan Using Aspergillus oryzae

Isoprimeverose [α-D-xylopyranosyl-(1→6)-D-glucose] is produced from xyloglucan using the cooperative action of glycoside hydrolases including isoprimeverose-producing oligoxyloglucan hydrolase and β-galactosidase in Aspergillus oryzae. This study investigated A. oryzae strains and culture conditions suitable for isoprimeverose production from xyloglucan. Each strain of A. oryzae had a different ability to degrade xyloglucans. When an A. oryzae strain with high xyloglucan-degradation activity was cultured in a medium containing partially degraded xyloglucan as the carbon source, the production of glycoside hydrolases that degrade xyloglucan into isoprimeverose was highly induced. Our procedure efficiently produced isoprimeverose from xyloglucan without any genetically modified microorganisms or purification of enzymes.

Origins of xyloglucan-degrading enzymes in fungi

The origin story of land plants - the pivotal evolutionary event that paved the way for terrestrial ecosystems of today to flourish - lies within their closest living relatives: the streptophyte algae. Streptophyte cell wall composition has evolved such that profiles of cell wall polysaccharides can be used as taxonomic markers. Since xyloglucan is restricted to the streptophyte lineage, we hypothesized that fungal enzymes evolved in response to xyloglucan availability in streptophyte algal or land plant cell walls. The record of the origins of these enzymes is embedded in fungal genomes, and comparing genomes of fungi that share an ancient common ancestor can provide insights into fungal interactions with early plants. This Viewpoint contributes a review of evidence underlying current assumptions about the distribution of xyloglucan in plant and algal cell walls. We evaluate evolutionary scenarios that may have given rise to the observed distribution of putative xyloglucanases in fungi and discuss possible biological contexts in which these enzymes could have evolved. Our findings suggest that fungal xyloglucanase evolution was more likely driven by land plant diversification and biomass accumulation than by the first origins of xyloglucan in streptophyte algal cell walls.

Bioinformatics-based identification of GH12 endoxyloglucanases in citrus-pathogenic Penicillium spp

Millions of tons of citrus peel waste are produced every year as a byproduct of the juice industry. Citrus peel is rich in pectin and xyloglucan, but while the pectin is extracted for use in the food industry, the xyloglucan is currently not valorized. To target hydrolytic degradation of citrus peel xyloglucan into oligosaccharides, we have used bioinformatics to identify three glycoside hydrolase 12 (GH12) endoxyloglucanases (EC 3.2.1.151) from the citrus fruit pathogens Penicillium italicum GL-Gan1 and Penicillium digitatum Pd1 and characterized them on xyloglucan obtained by alkaline extraction from citrus peel. The enzymes displayed pH-temperature optima of pH 4.6-5.3 and 35-37°C. PdGH12 from P. digitatum and PiGH12A from P. italicum share 84% sequence identity and displayed similar kinetics, although kcat was highest for PdGH12. In contrast, PiGH12B from P. italicum, which has the otherwise conserved Trp in subsite -4 replaced with a Tyr, displayed a 3 times higher KM and a 4 times lower kcat/KM than PiGH12A, but was the most thermostable enzyme of the three Penicillium-derived endoxyloglucanases. The benchmark enzyme AnGH12 from Aspergillus nidulans was more thermally stable and had a higher pH-temperature optimum than the enzymes from Penicillum spp. The difference in structure of the xyloglucan oligosaccharides extracted from citrus peel xyloglucan and tamarind xyloglucan by the new endoxyloglucanases was determined by LC-MS. The inclusion of citrus peel xyloglucan demonstrated that the endoxyloglucanases liberated fucosylated xyloglucan oligomers, implying that these enzymes have the potential to upgrade citrus peel residues to produce oligomers useful as intermediates or bioactive compounds.

Screening, identification, and mechanism analysis of starch-degrading bacteria during curing process in tobacco leaf

Tobacco, a vital economic crop, had its quality post-curing significantly influenced by starch content. Nonetheless, the existing process parameters during curing were inadequate to satisfy the starch degradation requirements. Microorganisms exhibit inherent advantages in starch degradation, offering significant potential in the tobacco curing process. Our study concentrated on the microbial populations on the surface of tobacco leaves and in the rhizosphere soil. A strain capable of starch degradation, designated as BS3, was successfully isolated and identified as Bacillus subtilis by phylogenetic tree analysis based on 16SrDNA sequence. The application of BS3 on tobacco significantly enhanced enzyme activity and accelerated starch degradation during the curing process. Furthermore, analyses of the metagenome, transcriptome, and metabolome indicated that the BS3 strain facilitated starch degradation by regulating surface microbiota composition and affecting genes related to starch hydrolyzed protein and key metabolites in tobacco leaves. This study offered new strategies for efficiently improving the quality of tobacco leaves.

Plant polysaccharide degradation-related enzymes in Aspergillus oryzae

Plants synthesize large amounts of stored and structural polysaccharides. Aspergillus oryzae is used in traditional Japanese fermentation and produces many types of plant polysaccharide degradation-related enzymes. The carbohydrate-active enzymes of A. oryzae are important in the fermentation process and biotechnological applications. Because plant polysaccharides have a complex structure, cooperative and synergistic actions of enzymes are crucial for the degradation of plant polysaccharides. For example, the cooperative action of isoprimeverose-producing oligoxyloglucan hydrolase, β-galactosidase, and α-xylosidase is important for the degradation of xyloglucan, and A. oryzae coordinates these enzymes at the expression level. In this review, I focus on the plant polysaccharide degradation-related enzymes identified in A. oryzae.

The Glycoside Hydrolase Family 35 β-galactosidase from Trichoderma reesei debranches xyloglucan oligosaccharides from tamarind and jatobá

Trichoderma reesei (anamorph Hypocrea jecorina) produces an extracellular beta-galactosidase from Glycoside Hydrolase Family 35 (TrBga1). Hydrolysis of xyloglucan oligosaccharides (XGOs) by TrBga1 has been studied by hydrolysis profile analysis of both tamarind (Tamarindus indica) and jatobá (Hymenaea courbaril) seed storage xyloglucans using PACE and MALDI-ToF-MS for separation, quantification and identification of the hydrolysis products. The TrBga1 substrate preference for galactosylated oligosaccharides from both the XXXG- and XXXXG-series of jatobá xyloglucan showed that the doubly galactosylated oligosaccharides were the first to be hydrolyzed. Furthermore, the TrBga1 showed more efficient hydrolysis against non-reducing end dexylosylated oligosaccharides (GLXG/GXLG and GLLG). This preference may play a key role in xyloglucan degradation, since galactosyl removal alleviates steric hindrance for other enzymes in the xyloglucanolytic complex resulting in complete xyloglucan mobilization. Indeed, mixtures of TrBga1 with the α-xylosidase from Escherichia coli (YicI), which shows a preference towards non-galactosylated xyloglucan oligosaccharides, reveals efficient depolymerization when either enzyme is applied first. This understanding of the synergistic depolymerization contributes to the knowledge of plant cell wall structure, and reveals possible evolutionary mechanisms directing the preferences of debranching enzymes acting on xyloglucan oligosaccharides.

Characterization of an extracellular α-xylosidase involved in xyloglucan degradation in Aspergillus oryzae

α-Xylosidases release the α-D-xylopyranosyl side chain from di- and oligosaccharides derived from xyloglucans and are involved in xyloglucan degradation. In this study, an extracellular α-xylosidase, named AxyB, is identified and characterized in Aspergillus oryzae. AxyB belongs to the glycoside hydrolase family 31 and releases D-xylose from isoprimeverose (α-D-xylopyranosyl-(1 → 6)-D-glucopyranose) and xyloglucan oligosaccharides. In the hydrolysis of xyloglucan oligosaccharides (XLLG, Glc₄Xyl₃Gal₂ nonasaccharide; XLXG/XXLG, Glc₄Xyl₃Gal₁ octasaccharide; and XXXG, Glc₄Xyl₃ heptasaccharide), AxyB releases one molecule of the xylopyranosyl side chain attached to the non-reducing end of the β-1,4-glucan main chain of these xyloglucan oligosaccharides to yield GLLG (Glc₄Xyl₂Gal₂), GLXG/GXLG (Glc₄Xyl₂Gal₁), and GXXG (Glc₄Xyl₂). A. oryzae has both extracellular and intracellular α-xylosidase, suggesting that xyloglucan oligosaccharides are degraded by a combination of isoprimeverose-producing oligoxyloglucan hydrolase and intracellular α-xylosidase and a combination of extracellular α-xylosidase and β-glucosidase(s) in A. oryzae. KEY POINTS: • An extracellular α-xylosidase, AxyB, is identified in Aspergillus oryzae. • AxyB releases the xylopyranosyl side chain from xyloglucan oligosaccharides. • Different sets of glycosidases degrade xyloglucan oligosaccharides in A. oryzae.