Isolation and characterization of unhydrolyzed oligosaccharides from switchgrass (Panicum virgatum, L.) xylan after exhaustive enzymatic treatment with commercial enzyme preparations

Authors: Bowman, Michael; Dien, Bruce; Vermillion, Karl; Mertens, Jeffrey

Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2015
Publication Date: 4/30/2015
Publication URL: http://handle.nal.usda.gov/10113/60608
Citation: Bowman, M.J., Dien, B.S., Vermillion, K.E., Mertens, J.A. 2015. Isolation and characterization of unhydrolyzed oligosaccharides from switchgrass (Panicum virgatum, L.) xylan after exhaustive enzymatic treatment with commercial enzyme preparations. Carbohydrate Research. 407:42-50.

Interpretive Summary: Switchgrass is a perennial grass presently used as for forage and being developed as a bioenergy crop for conversion to biofuels. Up to 30% of switchgrass cell wall associated carbohydrates are xylan. Previously, switchgrass xylan structural features that have not been previously reported were identified in three cultivars. In the current study, these structures were not readily hydrolyzed by commercial enzymes demonstrating that the chemical bond contained in these structures is partially responsible for xylan recalcitrance. Identification of the presence of this variation is relevant to commercial enzyme producers interested in designing improved enzymatic combinations to more effectively hydrolyze xylan to improve bioconversion yields.

Technical Abstract: Switchgrass (Panicum virgatum, L.) is a potential renewable source of carbohydrates for use in microbial conversion to biofuels. Xylan comprises approximately 30% of the switchgrass cell wall. To understand the limitations of commercial enzyme mixtures, alkali-extracted, isolated switchgrass xylan was hydrolyzed by the action of two commercial enzyme cocktails, in the presence and absence of an additional alpha-arabinofuranosidase enzyme. The two most abundant enzymatic digestion products from each commercial enzyme treatment were separated and characterized by LC-MSn, linkage analysis, and NMR. The most abundant oligosaccharide from each commercial cocktail was susceptible to hydrolysis when supplemented with a GH62 alpha-arabinofuranosidase enzyme; further characterization confirmed the presence of (1->3)-alpha-arabinose linkages. These results demonstrate the lack of the required selectivity for arabinose-containing substrates in the commercial enzyme preparations tested. One product from each condition remained intact and was found to contain (1->2)-beta-xylose-(1->3)-alpha-arabinose side chains; this linkage acts as a source of oligosaccharide recalcitrance.