Abstract
BackgroundAlthough α-linked xylose is a major constituent of the hemicelluloses of land plants, few secreted α-xylosidases have been described from fungi or bacteria. AxlA of Aspergillus niger is a secreted α-xylosidase that was earlier shown to promote the release of free glucose (Glc) and xylose (Xyl) from substrates containing α-linked xylose, including isoprimeverose (IP), the heptasaccharide subunit of pea xyloglucan (XG), and tamarind XG.ResultsThe utility of AxlA for enhancing release of free Glc and Xyl in combination with commercial enzyme cocktails from dicotyledonous and monocotyledonous plants was examined. Without AxlA supplementation, a mixture of CTec2 and HTec2 (both of which are derived from T. reesei) did not release significant levels of Glc from pea XG or tamarind XG. This is consistent with their lack of detectable α-xylosidase activity using model substrates. On alkaline hydrogen peroxide-pretreated corn stover, supplementation of CTec2/HTec2 (at a loading of 2.5 mg/g glucan) with AxlA (at a loading of 8 mg/g glucan) increased Glc yields from 82% to 88% of the total available Glc and increased Xyl yields from 55% to 60%. AxlA supplementation also improved Glc yields from corn stover treated with the commercial cellulase Accellerase 1000. The AxlA enhancement was not a general protein effect because bovine serum albumin or bovine gamma-globulin at similar concentrations did not enhance Glc yields from corn stover in response to CTec2/HTec2. Supplementation of CTec2/HTec2 with AxlA did not enhance Glc release from pretreated green or etiolated pea tissue. However, AxlA did enhance Glc and Xyl yields compared to CTec2/HTec2 alone from another dicotyledonous herbaceous plant, Chenopodium album (lamb’s quarters).ConclusionSupplementation of commercial cellulase cocktails with AxlA enhances yields of Glc and Xyl from some biomass substrates under some conditions, and may prove useful in industrial lignocellulose conversion.
Highlights
Α-linked xylose is a major constituent of the hemicelluloses of land plants, few secreted α-xylosidases have been described from fungi or bacteria
Xyloglucans (XG), which are present in the primary walls of terrestrial plants, comprise a β1,4linked glucan backbone substituted with α-linked xylose, and in some plants with galactose (Gal) and fucose (Fuc) [2,3,4]
Commercial cellulases do not degrade xyloglucan because they lack α-xylosidase In mixtures of pure enzymes (i.e., β-glucosidase, βgalactosidase, and xyloglucanase), AxlA was required for release of free Glc and Xyl from isolated pea XG fragments and from tamarind XG [6]
Summary
Α-linked xylose is a major constituent of the hemicelluloses of land plants, few secreted α-xylosidases have been described from fungi or bacteria. AxlA of Aspergillus niger is a secreted α-xylosidase that was earlier shown to promote the release of free glucose (Glc) and xylose (Xyl) from substrates containing α-linked xylose, including isoprimeverose (IP), the heptasaccharide subunit of pea xyloglucan (XG), and tamarind XG. The fermentative production of biofuels from lignocellulosic materials requires the efficient deconstruction of plant polysaccharides to free sugars. This process can be catalyzed by complex mixtures of enzymes containing cellulases, xylanases, and other glycosidases. The soft rot fungus Aspergillus niger secretes an α-xylosidase, AxlA, which is active against p-nitrophenylα-xyloside (pNPαX), isoprimeverose (IP), and an XGderived heptasaccharide isolated from pea walls [6]. Because the action of AxlA releases both Glc and Xyl from XG, inclusion of AxlA might have utility in improving the efficiency of commercial enzyme mixtures on realistic biomass substrates, including corn stover
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