Abstract
BackgroundGlycoside hydrolases (GHs) and accessory proteins are key components for efficient and cost-effective enzymatic hydrolysis of polysaccharides in modern, biochemically based biorefineries. Currently, commercialized GHs and accessory proteins are produced by ascomycetes. However, the role of wood decay basidiomycetes proteins in biomass saccharification has not been extensively pursued. Wood decay fungi degrade polysaccharides in highly lignified tissues in natural environments, and are a promising enzyme source for improving enzymatic cocktails that are designed for in vitro lignocellulose conversion.ResultsGHs and accessory proteins were produced by representative brown- and white-rot fungi, Laetiporus sulphureus and Pleurotus ostreatus, respectively. Concentrated protein extracts were then used to amend commercial enzymatic cocktails for saccharification of alkaline-sulfite pretreated sugarcane bagasse. The main enzymatic activities found in the wood decay fungal protein extracts were attributed to endoglucanases, xylanases and β-glucosidases. Cellobiohydrolase (CBH) activities in the L. sulphureus and P. ostreatus extracts were low and nonexistent, respectively. The initial glucan conversion rates were boosted when the wood decay fungal proteins were used to replace half of the enzymes from the commercial cocktails. L. sulphureus proteins increased the glucan conversion levels, with values above those observed for the full load of commercial enzymes. Wood decay fungal proteins also enhanced the xylan conversion efficiency due to their high xylanase activities. Proteomic studies revealed 104 and 45 different proteins in the P. ostreatus and L. sulphureus extracts, respectively. The enhancement of the saccharification of alkaline-pretreated substrates by the modified enzymatic cocktails was attributed to the following protein families: GH5- and GH45-endoglucanases, GH3-β-glucosidases, and GH10-xylanases.ConclusionsThe extracellular proteins produced by wood decay fungi provide useful tools to improve commercial enzyme cocktails that are currently used for the saccharification of alkaline-pretreated lignocellulosic substrates. The relevant proteins encompass multiple glycoside hydrolase families, including the GH5- and GH45-endoglucanases, GH3-β-glucosidases, and GH10-xylanases.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0525-y) contains supplementary material, which is available to authorized users.
Highlights
Glycoside hydrolases (GHs) and accessory proteins are key components for efficient and cost-effective enzymatic hydrolysis of polysaccharides in modern, biochemically based biorefineries
Endoglucanase activities in Pleurotus ostreatus and Laetiporus sulphureus cultures Both fungal species were grown in submerged cultures containing different carbon sources, and a comparative evaluation revealed that high endoglucanase (EG) activities were evident in the media with sugarcane bagasse and carboxymethyl cellulose (CMC) (Table 1)
Despite the studied basidiomycetes presented low levels of extracellular proteins and endoglucanase activities, the proteins produced by these fungal species could present high diversity, requiring additional characterization to explore their potential as supplementary proteins for biomass hydrolysis
Summary
Glycoside hydrolases (GHs) and accessory proteins are key components for efficient and cost-effective enzymatic hydrolysis of polysaccharides in modern, biochemically based biorefineries. Wood decay fungi degrade polysaccharides in highly lignified tissues in natural environments, and are a promising enzyme source for improving enzymatic cocktails that are designed for in vitro lignocellulose conversion. The synergistic actions of these enzymes depend on different, highly specific ratios between the individual proteins [4]. Several of these enzymes contain a cellulose binding module (CBM), which contributes to the adsorption of the enzyme to the substrate, enhancing the enzyme performance [6, 7]. CDHs can reduce Fe3+ to Fe2+ and O2 to H2O2, which generates Fenton reagents during the in vivo degradation of lignocellulose [14]
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