Lignosulfonate-mediated cellulase adsorption: enhanced enzymatic saccharification of lignocellulose through weakening nonproductive binding to lignin

Abstract

BackgroundThermochemical pretreatment of lignocellulose is crucial to bioconversion in the fields of biorefinery and biofuels. However, the enzyme inhibitors in pretreatment hydrolysate make solid substrate washing and hydrolysate detoxification indispensable prior to enzymatic hydrolysis. Sulfite pretreatment to overcome recalcitrance of lignocelluloses (SPORL) is a relatively new process, but has demonstrated robust performance for sugar and biofuel production from woody biomass in terms of yield and energy efficiency. This study demonstrated the advantage of SPORL pretreatment whereby the presentation of lignosulfonate (LS) renders the hydrolysate non-inhibitory to cellulase (Cel) due to the formation of lignosulfonate-cellulase complexes (LCCs) which can mediate the Cel adsorption between lignin and cellulose, contrary to the conventional belief that pretreatment hydrolysate inhibits the enzymatic hydrolysis unless detoxified.ResultsParticular emphasis was made on the formation mechanisms and stability phase of LCCs, the electrostatic interaction between LCCs and lignin, and the redistributed Cel adsorption between lignin and cellulose. The study found that LS, the byproduct of SPORL pretreatment, behaves as a polyelectrolyte to form LCCs with Cel by associating to the oppositely charged groups of protein. Compared to Cel, the zeta potential of LCCs is more negative and adjustable by altering the molar ratio of LS to Cel, and thereby LCCs have the ability to mitigate the nonproductive binding of Cel to lignin because of the enlarged electrostatic repulsion. Experimental results showed that the benefit from the reduced nonproductive binding outweighed the detrimental effects from the inhibitors in pretreatment hydrolysate. Specifically, the glucan conversions of solid substrate from poplar and lodgepole pine were greatly elevated by 25.9% and 31.8%, respectively, with the complete addition of the corresponding hydrolysate. This contradicts the well-acknowledged concept in the fields of biofuels and biorefinery that the pretreatment hydrolysate is inhibitory to enzymes.ConclusionsThe results reported in this study also suggest significant advantages of SPORL pretreatment in terms of water consumption and process integration, that is, it should abolish the steps of solid substrate washing and pretreatment hydrolysate detoxification for direct simultaneous saccharification and combined fermentation (SSCombF) of enzymatic and pretreatment hydrolysate, thereby facilitating bioprocess consolidation. Furthermore, this study not only has practical significance to biorefinery and bioenergy, but it also provides scientific importance to the molecular design of composite enzyme-polyelectrolyte systems, such as immobilized enzymes and enzyme activators, as well as to the design of enzyme separation processes using water-soluble polyelectrolytes.

Effects of pre-treatment technologies on dark fermentative biohydrogen production: A review

“…Prior to enhanced hydrolysis, pre-treatment technologies assist in cell wall lysis and delignification of lignocellulosic materials like agricultural residues and biomass, sludge and municipal solid wastes (Hsu et al, 1980;Mood et al, 2013;Appels et al, 2008;Sun and Cheng, 2002). As a result, cellulose molecules, previously shielded from enzymatic hydrolysis by the lignin seal, are liberated and released in solution (Hsu et al, 1980;Zheng et al, 2009;Wang et al, 2013) as depicted in Fig. 3 (Mood et al, 2013).…”

Current Understanding of the Correlation of Lignin Structure with Biomass Recalcitrance

“…However, cellulolytic lignin residues from corn stover and wheat straw had minimal impact on the hydrolyzability of pretreated biomass (Barsberg et al, 2013). Surprisingly, recent studies reported certain technical lignins with promoting effects, e.g., lignosulfonate (Wang et al, 2013; Zhou et al, 2013), organosolv lignin (Lai et al, 2014), Kraft lignin (Wang et al, 2015), and alkali lignin (Li Y. et al, 2016). These authors attributed the positive effect of lignin to the alleviation of non-productive binding via lignosulfonate-enzyme complex or surfactant protection.…”

Conversion of technical lignins to functional materials with retained polymeric properties

“…Lignosulfonates facilitate the enzymatic saccharification of lignocellulosic materials by the formation of lignosulfonate-cellulase complexes that can mediate the cellulase adsorption between lignin and cellulose [243]. Lignosulfonate fractions with a low molecular weight and a high degree of sulfonation can enhance enzymatic cellulose saccharification.…”