The effect of an oligosaccharide reducing-end xylanase, BhRex8A, on the synergistic degradation of xylan backbones by an optimised xylanolytic enzyme cocktail

• Published in: Enzyme and microbial technology 2019-03, Vol.122, p.74-81
• Main Authors: Malgas, Samkelo, Pletschke, Brett I.
• Format: Article
• Language: English
• Subjects: Biofuels; Glycoside hydrolase; Glycoside Hydrolases - metabolism; Hydrolysis; Oligosaccharide reducing-end xylanase; Oligosaccharides - metabolism; Substrate Specificity; Synergy; Xylan degradation; Xylans - metabolism; Xylose - metabolism; Xylosidases - metabolism; β-Xylanases
• ISSN: 0141-0229; 1879-0909
• DOI: 10.1016/j.enzmictec.2018.12.010

[Display omitted] •Rex8As did not show high activity on xylanase and xylosidase indicator substrates.•GH10 to GH11 xylanase synergism was established during xylan degradation.•Rex8A performance was enzyme specific and substrate dependent.•Rex8As released smaller XOS from xylans compared to xylanases.•Rex8A’s presence in the xylanolytic cocktail improves xylose release from xylans. Xylan, the most abundant hemicellulose in lignocellulosic biomass, requires a consortium of xylanolytic enzymes to achieve its complete de-polymerisation. As global interest in using xylan-containing lignocellulosic feedstocks for biofuel production increases, an accompanying knowledge on how to efficiently depolymerise these feedstocks into fermentable sugars is required. Since it has been observed that the same enzyme [i.e. an enzyme with the same EC (Enzyme Commission) classification] from different GH families can display different substrate specificities and properties, we evaluated GH10 (XT6) and 11 (Xyn2A) xylanase performance alone, and in combination, during xylan depolymerisation. Synergistic enhancement with respect to reducing sugar release was observed when Xyn2A at 75% loading was supplemented with 25% loading of XT6 for both beechwood glucuronoxylan (1.14-fold improvement) and wheat arabinoxylan (1.1-fold improvement) degradation. Following this, the optimised xylanase mixture was dosed with an oligosaccharide reducing-end xylanase (Rex8A) from either Bifidobacterium adolescentis or Bacillus halodurans for further synergistic enhancement. Dosing 75% of the xylanase mixture (Xyn2A:XT6 at 75:25%) with 25% loading of Rex8A led to an enhancement of reducing sugar (up to an 1.1-fold improvement) and xylose release (up to an 1.5-fold improvement); however, this effect was both xylan and Rex8A specific. Using thin layer chromatography, synergism appeared to be a result of the GH10 and 11 xylanases liberating xylo-oligomers that are preferred substrates of the processive Rex8As. Rex8As then hydrolysed xylo-oligomers to xylose - and xylobiose which was the preferred substrate for xylosidase, SXA. This likely explains why there was a significant improvement in xylose release in the presence of Rex8As. Here, it was shown that Rex8As are key enzymes in the efficient saccharification of hetero-xylan into xylose, a major component of lignocellulosic substrates.