Abstract
BackgroundRegarding plant cell wall polysaccharides degradation, multimodular glycoside hydrolases (GHs) with two catalytic domains separated by one or multiple carbohydrate-binding domains are rare in nature. This special mode of domain organization endows the Caldicellulosiruptor bescii CelA (GH9-CBM3c-CBM3b-CBM3b-GH48) remarkably high efficiency in hydrolyzing cellulose. CbXyn10C/Cel48B from the same bacterium is also such an enzyme which has, however, evolved to target both xylan and cellulose. Intriguingly, the GH10 endoxylanase and GH48 cellobiohydrolase domains are both dual functional, raising the question if they can act synergistically in hydrolyzing cellulose and xylan, the two major components of plant cell wall.ResultsIn this study, we discovered that CbXyn10C and CbCel48B, which stood for the N- and C-terminal catalytic domains, respectively, cooperatively released much more cellobiose and cellotriose from cellulose. In addition, they displayed intramolecular synergy but only at the early stage of xylan hydrolysis by generating higher amounts of xylooligosaccharides including xylotriose, xylotetraose, and xylobiose. When complex lignocellulose corn straw was used as the substrate, the synergy was found only for cellulose but not xylan hydrolysis.ConclusionThis is the first report to reveal the synergy between a GH10 and a GH48 domain. The synergy discovered in this study is helpful for understanding how C. bescii captures energy from these recalcitrant plant cell wall polysaccharides. The insight also sheds light on designing robust and multi-functional enzymes for plant cell wall polysaccharides degradation.
Highlights
Regarding plant cell wall polysaccharides degradation, multimodular glycoside hydrolases (GHs) with two catalytic domains separated by one or multiple carbohydrate-binding domains are rare in nature
Multimodular glycoside hydrolases with two catalytic domains separated by one or multiple carbohydrate-binding modules (CBMs) are rare in nature and appear to be an intermediate paradigm existing between free enzymes and cellulosomes [9,10,11]
Expression of three truncation mutants of CbXyn10C/ Cel48B bearing one or both of the GH10/GH48 catalytic domains Caldicellulosiruptor bescii uses predominantly multimodular enzymes to degrade cellulose, which is prototyped by components of multimodular glycoside hydrolases (CelA) with one GH9 and one GH48 catalytic domain tethered in a single polypeptide
Summary
Regarding plant cell wall polysaccharides degradation, multimodular glycoside hydrolases (GHs) with two catalytic domains separated by one or multiple carbohydrate-binding domains are rare in nature This special mode of domain organization endows the Caldicellulosiruptor bescii CelA (GH9-CBM3c-CBM3b-CBM3b-GH48) remarkably high efficiency in hydrolyzing cellulose. CelA [12] (or CbCel9A/Cel48A [13]) from a thermophilic bacterium Caldicellulosiruptor bescii is such an enzyme with this special mode of domain organization by having N-terminal GH9 and C-terminal GH48 catalytic domains separated by three family 3 cellulose-binding CBMs (GH9-CBM3c-CBM3bCBM3b-GH48). This enzyme displays a high efficiency in hydrolyzing crystalline cellulose [12, 14, 15]. In this bacterium, there are five more multimodular GHs all sharing a similar domain architecture, majorly differing in their N- and C-terminal catalytic domains [16]
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