BackgroundIn the bioconversion of lignocellulosic substrates, the adsorption behavior of cellulase onto lignin has a negative effect on enzymatic hydrolysis of cellulose, decreasing glucose production during enzymatic hydrolysis, thus decreasing the yield of fermentation and the production of useful products. Understanding the interaction between lignin and cellulase is necessary to optimize the components of cellulase mixture, genetically engineer high-efficiency cellulase, and reduce cost of bioconversion. Most lignin is not removed during liquid hot water (LHW) pretreatment, and the characteristics of lignin in solid substrate are also changed. To understand the interactions between cellulase and lignin, this study investigated the change in the characteristics of lignin obtained from corn stover, as well as the behavior of cellulase adsorption onto lignin, under various severities of LHW pretreatment.ResultsLHW pretreatment removed most hemicellulose and some lignin in corn stover, as well as improved enzymatic digestibility of corn stover. After LHW pretreatment, the molecular weight of lignin obviously increased, whereas its polydispersity decreased and became more negative. The hydrophobicity and functional groups in lignin also changed. Adsorption of cellulase from Penicillium oxalicum onto lignin isolated from corn stover was enhanced after LHW pretreatment, and increased under increasing pretreatment severity. Different adsorption behaviors were observed in different lignin samples and components of cellulase mixtures, even in different cellobiohydrolases (CBHs), endo-beta-1, 4-glucanases (EGs). The greatest reduction in enzyme activity caused by lignin was observed in CBH, followed by that in xylanase and then in EG and β-Glucosidase (BGL). The adsorption behavior exerted different effects on subsequent enzymatic hydrolysis of various biomass substrates. Hydrophobic and electrostatic interactions may be important factors affecting different adsorption behaviors between lignin and cellulase.ConclusionsLHW pretreatment changed the characteristics of the remaining lignin in corn stover, thus affected the adsorption behavior of lignin toward cellulase. For different protein components in cellulase solution from P. oxalicum, electrostatic action was a main factor influencing the adsorption of EG and xylanase onto lignin in corn stover, while hydrophobicity affected the adsorption of CBH and BGL onto lignin.

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Using fermentable sugars from lignocelluloses to produce chemicals and fuels is a way to resolve the energy crisis and environmental problems, but nonproductive adsorption of cellulase onto lignin is big challenge for bioconversion of cellulose to fermentable sugars. As cellobiohydrolase (CBH) is the main enzyme in cellulase mixture, understanding the adsorption mechanism of CBH onto lignin is important to enhance CBH catalytic efficiency by avoiding the nonproductive adsorption. This study suggested that CBH displayed high adsorption affinity to lignin, especially for the lignin isolated from liquid-hot-water pretreated substrate, and a stable combination of lignin-CBH was formed. The CBH adsorbed to lignin still had the catalytic ability on Avicel, although the ability was decreased. The possible reason, by molecular docking, was that the catalytic domain of CBH was irreversibly adsorbed onto lignin mainly through a hydrophobic cleft located oppositely to catalytic tunnel, not influencing the catalytic process of the enzyme except for the protein movement. Electrostatic interaction played a significant role in the adsorption of carbohydrate binding domain (CBM) of CBH to lignin. Altering the charges from positive to negative of amino acid on the CBM of CBH by engineering enzyme may be a feasible way to reduce the adsorption affinity of CBH onto lignin.

Elephant grass (Pennisetum purpureum Schum) is an important forage, biofuels and industrial plant widely distributed in tropical and subtropical areas globally. It is characterized with robust growth and high biomass. We sequenced its allopolyploid genome and assembled 2.07 Gb into A' and B subgenomes of 14 chromosomes with scaffold N50 of 8.47 Mb, yielding a total of 77,139 genes. The allotetraploid speciation occurred approximately 15 Ma after the divergence between Setaria italica

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