GH10 and GH11 endoxylanases in Penicillium subrubescens: comparative characterization and synergy with GH51, GH54, GH62 α-L-arabinofuranosidases from the same fungus

High-Molecular-Weight Xylanase from B. pumilus US570 Strain: Purification, Characterization and Application in Banana and Orange Peels Hydrolysis and Breadmaking

New xylanase (XylUS570) was purified from the Bacillus pumilus US570 strain. It has a molecular mass of about 232 kDa. This is the first report on the highest molecular weight monomeric xylanase produced by bacteria. The optimum pH and temperature recorded for enzyme activity were 7 and 55 °C, respectively with a half-life time of 10 min at 60 °C. At 37 °C, the enzyme retains more than 50% of its activity at a pH ranging from 6 to 9.5 for 24 h. The XylUS570 exhibited a high activity on xylan, but no activity was detected for cellulosic substrates. The Vmax and Km values exhibited by the purified enzyme on beechwood xylan were 37.05 U mL-1 and 4.189 mg mL-1, respectively. The XylUS570 was used in banana and orange peels hydrolysis and showed potential efficiency to liberate reducing sugars. It could be a good candidate for bio-ethanol production from fruit waste. The purified enzyme was used also as an additive in breadmaking. A decrease in water absorption, an increase in dough rising and improvements in volume and specific volume of the bread were recorded.

Structure-dependent stimulation of gut bacteria by arabinoxylo-oligosaccharides (AXOS): a review

Arabinoxylo-oligosaccharides (AXOS) are non-digestible dietary fibers that potentially confer a health benefit by stimulating beneficial bacteria in the gut. Still, a detailed overview of the diversity of gut bacteria and their specificity to utilize structurally different AXOS has not been provided to date and was aimed for in this study. Moreover, we assessed the genetic information of summarized bacteria, and we extracted genes expected to encode for enzymes that are involved in AXOS hydrolysis (based on the CAZy database). The taxa involved in AXOS fermentation in the gut display a large variety of AXOS-active enzymes in their genome and consequently utilize AXOS to a highly different extent. Clostridia and Bacteroidales are generalists that consume many structurally diverse AXOS, whereas Bifidobacterium are specialists that specifically consume AXOS with a low degree of polymerization. Further complexity is evident from the fact that the exact bacterial species, and in some cases even the bacterial strains (e.g. in Bifidobacterium longum) that are stimulated, highly depend on the specific AXOS molecular structure. Furthermore, certain species in Bifidobacterium and Lactobacillaceae are active as cross-feeders and consume monosaccharides and unbranched short xylo-oligosaccharides released from AXOS. Our review highlights the possibility that (enzymatic) fine-tuning of specific AXOS structures leads to improved precision in targeting growth of specific beneficial bacterial species and strains in the gut.

Discovery of a bifunctional xylanolytic enzyme with arabinoxylan arabinofuranohydrolase-d3 and endo-xylanase activities and its application in the hydrolysis of cereal arabinoxylans

Xylanolytic enzymes, with both endo-xylanase and arabinoxylan arabinofuranohydrolase (AXH) activities, are attractive for the economically feasible conversion of recalcitrant arabinoxylan. However, their characterization and utilization of these enzymes in biotechnological applications have been limited. Here, we characterize a novel bifunctional enzyme, rAbf43A, cloned from a bacterial consortium that exhibits AXH and endo-xylanase activities. Hydrolytic pattern analyses revealed that the AXH activity belongs to AXHd3 because it attacked only the C(O)-3-linked arabinofuranosyl residues of double-substituted xylopyranosyl units of arabinoxylan and arabinoxylan-derived oligosaccharides, which are usually resistant to hydrolysis. The enzyme rAbf43A also liberated a series of xylo-oligosaccharides (XOSs) from beechwood xylan, xylohexaose and xylopentaose, indicating that rAbf43A exhibited endo-xylanase activity. Homology modelling based on AlphaFold2 and site-directed mutagenesis identified three non-catalytic residues (H161, A270 and L505) located in the substrate-binding pocket essential for its dual-functionality, while the mutation of A117 located in the -1 subsite to the proline residue only affected its endo-xylanase activity. Additionally, rAbf43A showed significant synergistic action with the bifunctional xylanase/feruloyl esterase rXyn10A/Fae1A from the same bacterial consortium on insoluble wheat arabinoxylan and de-starched wheat bran degradation. When rXyn10A/Fae1A was added to the rAbf43A pre-hydrolyzed reactions, the amount of released reducing sugars, xylose and ferulic acid increased by 9.43% and 25.16%, 189.37% and 93.54%, 31.39% and 32.30%, respectively, in comparison with the sum of hydrolysis products released by each enzyme alone. The unique characteristics of rAbf43A position it as a promising candidate not only for designing high-performance enzyme cocktails but also for investigating the structure-function relationship of GH43 multifunctional enzymes.

Enzyme synergy for plant cell wall polysaccharide degradation

Valorizing plant cell wall, marine and algal polysaccharides is of utmost importance for the development of the circular bioeconomy. This is because polysaccharides are by far the most abundant organic molecules found in nature with complex chemical structures that require a large set of enzymes for their degradation. Microorganisms produce polysaccharide-specific enzymes that act in synergy when performing hydrolysis. Although discovered since decades enzyme synergy is still poorly understood at the molecular level and thus it is difficult to harness and optimize. In the last few years, more attention has been given to improve and characterize enzyme synergy for polysaccharide valorization. In this review, we summarize literature to provide an overview of the different type of synergy involving carbohydrate modifying enzymes and the recent advances in the field exemplified by plant cell-wall degradation.

Enzymatic debranching is a key determinant of the xylan-degrading activity of family AA9 lytic polysaccharide monooxygenases

BACKGROUND

Previous studies have revealed that some Auxiliary Activity family 9 (AA9) lytic polysaccharide monooxygenases (LPMOs) oxidize and degrade certain types of xylans when incubated with mixtures of xylan and cellulose. Here, we demonstrate that the xylanolytic activities of two xylan-active LPMOs, TtLPMO9E and TtLPMO9G from Thermothielavioides terrestris, strongly depend on the presence of xylan substitutions.

RESULTS

Using mixtures of phosphoric acid-swollen cellulose (PASC) and wheat arabinoxylan (WAX), we show that removal of arabinosyl substitutions with a GH62 arabinofuranosidase resulted in better adsorption of xylan to cellulose, and enabled LPMO-catalyzed cleavage of this xylan. Furthermore, experiments with mixtures of PASC and arabinoglucuronoxylan from spruce showed that debranching of xylan with the GH62 arabinofuranosidase and a GH115 glucuronidase promoted LPMO activity. Analyses of mixtures with PASC and (non-arabinosylated) beechwood glucuronoxylan showed that GH115 action promoted LPMO activity also on this xylan. Remarkably, when WAX was incubated with Avicel instead of PASC in the presence of the GH62, both xylan and cellulose degradation by the LPMO9 were impaired, showing that the formation of cellulose-xylan complexes and their susceptibility to LPMO action also depend on the properties of the cellulose. These debranching effects not only relate to modulation of the cellulose-xylan interaction, which influences the conformation and rigidity of the xylan, but likely also affect the LPMO-xylan interaction, because debranching changes the architecture of the xylan surface.

CONCLUSIONS

Our results shed new light on xylanolytic LPMO9 activity and on the functional interplay and possible synergies between the members of complex lignocellulolytic enzyme cocktails. These findings will be relevant for the development of future lignocellulolytic cocktails and biomaterials.

Two Subgroups within the GH43_36 α-l-Arabinofuranosidase Subfamily Hydrolyze Arabinosyl from Either Mono-or Disubstituted Xylosyl Units in Wheat Arabinoxylan

Fungal arabinofuranosidases (ABFs) catalyze the hydrolysis of arabinosyl substituents (Ara) and are key in the interplay with other glycosyl hydrolases to saccharify arabinoxylans (AXs). Most characterized ABFs belong to GH51 and GH62 and are known to hydrolyze the linkage of α-(1→2)-Ara and α-(1→3)-Ara in monosubstituted xylosyl residues (Xyl) (ABF-m2,3). Nevertheless, in AX a substantial number of Xyls have two Aras (i.e., disubstituted), which are unaffected by ABFs from GH51 and GH62. To date, only two fungal enzymes have been identified (in GH43_36) that specifically release the α-(1→3)-Ara from disubstituted Xyls (ABF-d3). In our research, phylogenetic analysis of available GH43_36 sequences revealed two major clades (GH43_36a and GH43_36b) with an expected substrate specificity difference. The characterized fungal ABF-d3 enzymes aligned with GH43_36a, including the GH43_36 from Humicola insolens (HiABF43_36a). Hereto, the first fungal GH43_36b (from Talaromyces pinophilus) was cloned, purified, and characterized (TpABF43_36b). Surprisingly, TpABF43_36b was found to be active as ABF-m2,3, albeit with a relatively low rate compared to other ABFs tested, and showed minor xylanase activity. Novel specificities were also discovered for the HiABF43_36a, as it also released α-(1→2)-Ara from a disubstitution on the non-reducing end of an arabinoxylooligosaccharide (AXOS), and it was active to a lesser extent as an ABF-m2,3 towards AXOS when the Ara was on the second xylosyl from the non-reducing end. In essence, this work adds new insights into the biorefinery of agricultural residues.