Polystyrene Sulfonate is Effective for Enhancing Biomass Enzymatic Saccharification Under Green Liquor Pretreatment in Bioenergy Poplar

Abstract

Background: Water-soluble lignin (particularly lignosulfonate, LS) has been well documented for its significance on enzymatic saccharification of lignocellulose, though the promotion mechanism has not been fully understood. Much attention has been paid to natural lignin or its derivatives. The disadvantage of using natural lignin-based polymers as promoting agents lies in the difficulty in tailor-incorporating functional groups due to their complex 3D structures. To further improve our understanding on the promotion mechanism of water-soluble lignin in the bio-conversion of lignocellulose and to pursue better alternatives with different skeleton structures other than natural lignin, herein we reported a synthetic soluble linear aromatic polymer, sodium polystyrene sulfonate (PSS), to mimic LS for enhancing the efficiency of enzymatic saccharification. Results: The role of PSS played in enzymatic saccharification of pure cellulose and green liquor pretreated poplar (GL-P) was explored by analyzing substrate enzymatic digestibility (SED) under different addition dosages and various pH media, along with LS for comparison. At the cellulase loading of 13.3 FPU/g-glucan, the glucose yield of GL-P increased from 53% for the control to 81.5% with PSS addition of 0.1 g/g-substrate. It outperformed LS with the addition of 0.2 g/g-substrate by 6.3%. In the pH range from 4.5 to 6, PSS showed a positive effect on lignocellulose saccharification with the optimum pH at 4.8, where the most pronounced SED of GL-P was achieved. The underlying mechanism was unveiled by measuring zeta potential and using Quartz Crystal Microbalance (QCM) and Multi-parametric Surface Plasmon Resonance (MP-SPR). The results confirmed that the complexes of cellulase and PSS were conjugated and the negatively superchanged complexes reduced non-productive binding effectively along with the improved saccharification efficiency. The thickness of PSS required to block the binding sites of cellulase film was less than half of that of LS, and the PSS adlayer on cellulase film is also more hydrated and with a much lower shear modulus than LS adlayer. Conclusions: PSS as LS analogue is effective for enhancing the biomass enzymatic saccharification of GL-pretreated poplar. PSS exhibited a severer inhibition on the enzymatic saccharification of pure cellulose, whilst a more positive effect on bioconversion of lignocellulose (GL-P) than LS. In addition, a much lower dosage is required by PSS. The dynamic enzymatic hydrolysis indicated PSS could prolong the processive activity of cellulase. The valid data stemmed from QCM and SPR expressed that PSS bound to cellulases and the as-formed complexes reduced the nonproductive adsorption of cellulase onto substrate lignin more efficiently than LS due to its flexible skeleton and highly hydrated structure. Therefore, PSS is a promising alternative promoting agent for lignocellulose saccharification. From another perspective, the synthetic lignin mimics with controllable structures enable us to reach an in-depth understanding of the promotion mechanism of soluble lignins on enzymatic saccharification.

Overexpression of PtoMYB115 improves lignocellulose recalcitrance to enhance biomass digestibility and bioethanol yield by specifically regulating lignin biosynthesis in transgenic poplar

“…The level of soluble cellulase is positively correlated with the yield of hexose released by enzymatic hydrolysis, but lignin contents were negatively correlated with the level of soluble protein (Fig. 5 ), indicating that lignin may be a specific substance for enzyme adsorption as reported by previous studies [ 7 , 8 ]. The DP of cellulose was negatively correlated with the biomass enzymatic saccharification, but the cellulose accessibility and CBHI activity were positively correlated with biomass enzymatic saccharification (Fig.…”

“…Since the composition and basic features of plant cell wall determine the biomass saccharification [3,[7][8][9][10][11][12][13][14][15]37], our study examined the hexoses yields released from enzymatic hydrolysis after various chemical pretreatments, and the bioethanol yield by yeast fermentation using hexoses released from enzymatic hydrolysis. In general, the PtoMYB115-OE lines produced much higher hexoses and bioethanol yields, but the PtoMYB115-KO lines had reduced hexoses and bioethanol yields (Fig.…”

“…As lignin has been considered as one of the crucial factors negatively affecting biomass digestibility [3,[7][8][9], we next measured the hexoses yields released from enzymatic hydrolysis in WT and transgenic plants. Using our previously established approaches [48], three mild acid and alkali pretreatments (4% H 2 SO 4 /120 ºC/20 min; 4% NaOH/50 ºC/2 h; and 10% CaO/50 ºC/2 h) were used in this study.…”

“…Since lignin content and structure were defined to account for cellulase enzyme loading and adsorption access to cellulose microfibrils [7][8][9][10][11][12][13], we further examined the soluble protein contents in the supernatants of enzymatic hydrolyses with the pretreated biomass residues (Fig. 5).…”

“…[ 5 , 6 ]. In the process of enzymatic saccharification of lignocellulose, lignin does not only act as a physical barrier to hinder the accessibility of enzymes to carbohydrates, but also restricts enzymatic hydrolysis through non-productive adsorption of enzymes [ 7 , 8 ]. The lignin-based derivatives and lignin macromolecule also has negative influence on subsequent enzymatic hydrolysis [ 9 ].…”