A structured understanding of cellobiohydrolase I binding to poplar lignin fractions after dilute acid pretreatment

Lan Yao; Chang Geun Yoo; Xianzhi Meng; Mi Li; Yunqiao Pu; Arthur J Ragauskas; Haitao Yang

doi:10.1186/s13068-018-1087-y

A structured understanding of cellobiohydrolase I binding to poplar lignin fractions after dilute acid pretreatment

Biotechnol Biofuels. 2018 Apr 4:11:96. doi: 10.1186/s13068-018-1087-y. eCollection 2018.

Authors

Lan Yao^{1

2

3}, Chang Geun Yoo⁴, Xianzhi Meng³, Mi Li⁴, Yunqiao Pu⁴, Arthur J Ragauskas^{4

3

5}, Haitao Yang^{1

2}

Affiliations

¹ 1School of Pulp & Paper Engineering, Hubei University of Technology, Wuhan, 430068 China.
² 2Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan, 430068 China.
³ 4Department of Chemical and Biomolecular Engineering, The University of Tennessee, Knoxville, TN 37996-2200 USA.
⁴ 3Joint Institute for Biological Sciences, Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
⁵ 5Department of Forestry, Wildlife and Fisheries, Center for Renewable Carbon, Institute of Agriculture, The University of Tennessee, Knoxville, TN 37996-2200 USA.

Abstract

Background: Cellulase adsorption to lignin is considered a cost barrier for bioethanol production; however, its detailed association mechanism is still not fully understood. In this study, two natural poplar variants with high and low sugar release performance were selected as the low and high recalcitrant raw materials (named L and H, respectively). Three different lignin fractions were extracted using ethanol, followed by p-dioxane and then cellulase treatment from the dilute acid pretreated poplar solids (fraction 1, 2, and 3, respectively).

Results: Each lignin fraction had different physicochemical properties. Ethanol-extracted lignin had the lowest weight average molecular weight, while the molecular weights for the other two lignin fractions were similar. ³¹P NMR analysis revealed that lignin fraction with higher molecular weight contained more aliphatic hydroxyl groups and less phenolic hydroxyl groups. Semi-quantitative analysis by 2D HSQC NMR indicated that the lignin fractions isolated from the natural variants had different contents of syringyl (S), guaiacyl (G) and interunit linkages. Lignin extracted by ethanol contained the largest amount of S units, the smallest amounts of G and p-hydroxybenzoate (PB) subunits, while the contents of these lignin subunits in the other two lignin fractions were similar. The lignin fraction obtained after cellulase treatment was primarily comprised of β-O-4 linkages with small amounts of β-5 and β-β linkages. The binding strength of these three lignin fractions obtained by Langmuir equations were in the order of L₁ > L₃ > L₂ for the low recalcitrance poplar and H₁ > H₂ > H₃ for the high recalcitrance poplar.

Conclusions: Overall, adsorption ability of lignin was correlated with the sugar release of poplar. Structural features of lignin were associated with its binding to CBH. For natural poplar variants, lignin fractions with lower molecular weight and polydispersity index (PDI) exhibited more CBH adsorption ability. Lignins with more phenolic hydroxyl groups had higher CBH binding strength. It was also found that lignin fractions with more condensed aromatics adsorbed more CBH likely attributed to stronger hydrophobic interactions.

Keywords: Cellobiohydrolase I; Dilute acid pretreatment; Enzyme binding; Lignin; Poplar.