Abstract
Cellulosic materials constitute most of the biomass on earth, and can be converted into biofuel or bio-based materials if fermentable sugars can be released using cellulose-related enzymes. Acremonium cellulolyticus is a mesophilic fungus which produces a high amount of cellulose-related enzymes. In the genome sequence data of A. cellulolyticus, ORFs showing homology to GH10 and GH11 xylanases were found. The xylanases of A. cellulolyticus play an important role in cellulolytic biomass degradation. Search of a draft genome sequence of A. cellulolyticus for xylanase coding regions identified seven ORFs showing homology to GH 11 xylanase genes (xylA, xylB, xylC, xylD, xylE, xylF and xylG). These genes were cloned and their enzymes were prepared with a homologous expression system under the control of a glucoamylase promoter. Six of the seven recombinant enzymes were successfully expressed, prepared, and characterized. These enzymes exhibited optimal xylanase activity at pH 4.0 – 4.5. But this time, we found that only XylC had enormously higher relative activity (2947 U•mg −1) than the other xylanases at optimum pH. This result is surprising because XylC does not retain a carbohydrate-binding module 1 (CBM-1) that is necessary to bind tightly own substrate such as xylan. In this study, we discuss the relationship between activity, pH and sequence of seven xylanases in A. cellulolyticus.
Highlights
Lignocellulosic biomass can be converted into biofuel or bio-based materials (Deutschmann and Dekker 2012; Kumar et al 2008) but must be converted into fermentable sugars by saccharification
N-terminal sequence analysis showed that each signal peptide is cleaved by an aminopeptidase N-terminal to the arginine residue in this homologous expression system (Inoue et al 2013)
Blast alignments indicated that only XylB and XylF have a carbohydratebinding module (CBM-1) at their C-terminus, suggesting that XylB and XylF bind more strongly to their substrate than the other xylanases
Summary
Lignocellulosic biomass can be converted into biofuel or bio-based materials (Deutschmann and Dekker 2012; Kumar et al 2008) but must be converted into fermentable sugars by saccharification. Xylanases (endo-1,4-β-xylanases; EC 3.2.1.8) catalyze the hydrolysis of the β-1,4 bonds of xylan and are important enzymes for the degradation of hemicellulosic polysaccharides (Collins et al 2005; Prade 1996). Based on their amino acid sequence similarities, xylanases are mainly classified into families 10 and 11 of the glycoside hydrolases (GH; http://www.cazy.org/Glycoside-Hydrolases.html; (Coutinho and Henrissat 1999).
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