Restructuring the Crystalline Cellulose Hydrogen Bond Network Enhances Its Depolymerization Rate

Abstract

Conversion of lignocellulose to biofuels is partly inefficient due to the deleterious impact of cellulose crystallinity on enzymatic saccharification. We demonstrate how the synergistic activity of cellulases was enhanced by altering the hydrogen bond network within crystalline cellulose fibrils. We provide a molecular-scale explanation of these phenomena through molecular dynamics (MD) simulations and enzymatic assays. Ammonia transformed the naturally occurring crystalline allomorph I(β) to III(I), which led to a decrease in the number of cellulose intrasheet hydrogen bonds and an increase in the number of intersheet hydrogen bonds. This rearrangement of the hydrogen bond network within cellulose III(I), which increased the number of solvent-exposed glucan chain hydrogen bonds with water by ~50%, was accompanied by enhanced saccharification rates by up to 5-fold (closest to amorphous cellulose) and 60-70% lower maximum surface-bound cellulase capacity. The enhancement in apparent cellulase activity was attributed to the "amorphous-like" nature of the cellulose III(I) fibril surface that facilitated easier glucan chain extraction. Unrestricted substrate accessibility to active-site clefts of certain endocellulase families further accelerated deconstruction of cellulose III(I). Structural and dynamical features of cellulose III(I), revealed by MD simulations, gave additional insights into the role of cellulose crystal structure on fibril surface hydration that influences interfacial enzyme binding. Subtle alterations within the cellulose hydrogen bond network provide an attractive way to enhance its deconstruction and offer unique insight into the nature of cellulose recalcitrance. This approach can lead to unconventional pathways for development of novel pretreatments and engineered cellulases for cost-effective biofuels production.

Shotgun Approach to Increasing Enzymatic Saccharification Yields of Ammonia Fiber Expansion Pretreated Cellulosic Biomass

“…There are a number of significant correlations including those between total CBH and both glucan (r = 0.914, p < 0.01) and xylan (r = 0.551, p < 0.01) slopes, between total EG and both glucan (r = 0.888, p < 0.01) and xylan (r = 0.485, p < 0.01) slopes, between total CBH + EG and both glucan (r = 0.926, p < 0.05) and xylan (r = 0.545, p < 0.05) slopes, and between total CBH + EG + BG and both glucan (r = 0.919, p < 0.05) and xylan (r = 0.578, p < 0.05) slopes. These results validate previous findings that glucan/xylan yields are highly correlated and enzymes that facilitate glucan hydrolysis (CBH, EG, and BG) also facilitate increased digestibility of xylans by hemicellulases or multifunctional glycosyl hydrolases like EG I (Gao et al, 2010a;Chundawat et al, 2011b).…”

Trade-off between Processivity and Hydrolytic Velocity of Cellobiohydrolases at the Surface of Crystalline Cellulose

“…38,39 Allthough we are unable to distinguish decrystallization of the substrate and the sliding movement of CD, because the sliding movement should promote decrystallization of cellulose, while the decrystallization should cause movement vice versa, we speculate that the rate-limiting step of crystalline cellulose degradation is the process rather than the catalysis as discussed from "bulk" experiments. 38,39 Although a direct comparison between the results of singlemolecule observation and biochemical activity measurement is often difficult, the order of hydrolytic activity on PASC was the same as the order of the velocity of processive movement and k off . This result implies that the rate-limiting step of PASC hydrolysis is the catalytic process or the off-rate from the substrate, but not decided by processivity.…”

Spherical nanocomposite particles prepared from mixed cellulose–chitosan solutions

“…In preparing novel cellulose-chitosan biocomposites, we need usually to disassemble the molecular networks, either directly or after chemical modification (Chundawat et al 2011) and to mix the two polymers. Among the different ways of disassembly, dissolution enables to separate the polymer chains from each other and produce molecular ''bricks'' for construction of novel materials (Trygg 2015).…”