Abstract
BackgroundXylan is the most abundant hemicellulose polysaccharide in nature, which can be converted into high value-added products. However, its recalcitrance to breakdown requires the synergistic action of multiple enzymes. Aspergillus niger, possessing numerous xylan degrading isozyme-encoding genes, are highly effective xylan degraders in xylan-rich habitats. Therefore, it is necessary to explore gene transcription, the mode of action and cooperation mechanism of different xylanase isozymes to further understand the efficient xylan-degradation by A. niger.ResultsAspergillus niger An76 encoded a comprehensive set of xylan-degrading enzymes, including five endo-xylanases (one GH10 and four GH11). Quantitative transcriptional analysis showed that three xylanase genes (xynA, xynB and xynC) were up-regulated by xylan substrates, and the order and amount of enzyme secretion differed. Specifically, GH11 xylanases XynA and XynB were initially secreted successively, followed by GH10 xylanase XynC. Biochemical analyses displayed that three GH11 xylanases (XynA, XynB and XynD) showed differences in catalytic performance and product profiles, possibly because of intricate hydrogen bonding between substrates and functional residues in the active site architectures impacted their binding capacity. Among these, XynB had the best performance in the degradation of xylan and XynE had no catalytic activity. Furthermore, XynA and XynB showed synergistic effects during xylan degradation.ConclusionsThe sequential secretion and different action modes of GH11 xylanases were essential for the efficient xylan degradation by A. niger An76. The elucidation of the degradation mechanisms of these xylanase isozymes further improved our understanding of GH-encoding genes amplification in filamentous fungi and may guide the design of the optimal enzyme cocktails in industrial applications.
Highlights
Xylan is the most abundant hemicellulose polysaccharide in nature, which can be converted into high value-added products
Structural bioinformatic analysis was carried out to distinguish the degradation patterns of three GH11 xylanases. These findings provide novel mechanistic insights into the synergistic degradation of xylan by A. niger An76 GH11 xylanases and facilitate the optimization of glycoside hydrolase cocktails for biomass conversion
Quantitative transcriptional analysis of five xylanase‐encoding genes cultured with different substrates According to genome analysis, A. niger An76 possesses one GH10 xylanase (XynC) and four GH11 xylanases (XynA, XynB, XynD, and XynE; Additional file 1: Table S1)
Summary
Xylan is the most abundant hemicellulose polysaccharide in nature, which can be converted into high value-added products. Endo-β-1,4-xylanases are the most critical enzymes in the degradation of xylan backbone, breaking the β-1,4 glycosidic bond and producing different types of oligosaccharides [8], most of which belong to glycoside hydrolase (GH) 10 and 11 families. They differ greatly in their structures, substrate specificity and catalytic mechanism. GH11 xylanases can access complex biomass due to their low molecular weight and high catalytic activity [13] These xylanases have been widely studied for their potential applications in various industries, including biofuel, textiles, pulp and paper technologies, food and medical industries [14,15,16]. The catalytic activity and mode of action of these commercial xylanases are closely related to industrial conversion and cost
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