Swollenin aids in the amorphogenesis step during the enzymatic hydrolysis of pretreated biomass

Abstract

A key limitation in the overall hydrolysis process is the restricted access that the hydrolytic enzymes have due to the macro-and-micro structure of cellulose and its association with hemicellulose and lignin. Previous work has shown that several non-hydrolytic proteins can disrupt cellulose structure and boost the activity of hydrolytic enzymes when purer forms of cellulose are used. In the work reported here, Swollenin primarily disrupted the hemicellulosic fraction of pretreated corn stover, resulting in the solubilisation of monomeric and oligomeric sugars. Although Swollenin showed little synergism when combined with the cellulase monocomponents exoglucanase (CEL7A) and endoglucanase (CEL5A), it showed pronounced synergism with xylanase monocomponents Xylanase GH10 and Xylanase GH11, resulting in the release of significantly more xylose (>300%). It appears that Swollenin plays a role in amorphogenesis and that its primary action is enhancing access to the hemicellulose fraction that limits or masks accessibility to the cellulose component of lignocellulosic substrates.

The Use of Carbohydrate Binding Modules (CBMs) to Monitor Changes in Fragmentation and Cellulose Fiber Surface Morphology during Cellulase- and Swollenin-induced Deconstruction of Lignocellulosic Substrates

“…As previous work had suggested that Swollenin can weaken hydrogen bonding within amorphous regions of cellulose and hemicellulose (8,9,12), it is possible that Swollenin promotes fiber fragmentation by weakening hydrogen bonds and enhancing slippage between adjacent chains within fiber dislocations. This weakening of hydrogen bonding and chain slippage might be sufficient to cause fragmentation within certain severe dislocations, but not within the less severe dislocations.…”

“…Recent work has demonstrated that rapid fiber fragmentation at the very early stages of hydrolysis leads to a dramatic reduction in slurry viscosity, which alleviates some of the issues associated with high solids hydrolysis (2). It has been suggested that fiber fragmentation is primarily induced by the endoglucanase family of enzymes (3,4), although other proteins present in the cellulolytic mixture, such as the more recently characterized oxidative enzymes (5,6) and amorphogenesis-inducing proteins (7)(8)(9), have also been shown to contribute to the overall deconstruction process.…”

“…These disruptive proteins include examples from plants (Expansins) (13), bacteria (Expansin-like proteins (14,15) and some carbohydrate binding modules (CBMs) 2 (16)), and fungi * This work was supported by the Natural Sciences and Engineering Council (Swollenin (8,9,12), Loosenin (17), and CBMs (18)). These proteins have also been shown to promote a variety of disruptive effects on cellulosic biomass, including filter paper dispersion, crystallinity reduction, particle size reduction, swelling of cotton fibers, and roughening of cotton fiber surface (7).…”

“…Swollenin, in particular, has been shown to effectively swell cotton fibers (9), disperse filter paper squares (12), roughen filter paper fibers (12), reduce the particle size of a number of model cellulosic substrates (12), enhance the accessibility of cotton fibers (19), and enhance hydrolysis (8,20). Recent work has shown that Swollenin targets the less-ordered regions of the cellulose, rather than directly disrupting the more crystalline regions of the substrate (8,19), and enhances the solubilization of the hemicellulosic fraction of steam-pretreated corn stover when it synergistically interacted with family 10 and 11 xylanases, promoting a 3-fold increase in xylose released by these hydrolases (8).…”

“…Recent work has shown that Swollenin targets the less-ordered regions of the cellulose, rather than directly disrupting the more crystalline regions of the substrate (8,19), and enhances the solubilization of the hemicellulosic fraction of steam-pretreated corn stover when it synergistically interacted with family 10 and 11 xylanases, promoting a 3-fold increase in xylose released by these hydrolases (8). It appears that, although Swollenin does to some extent enhance the disruption of well structured components of cellulosic substrates, its primary role is facilitating the disruption of the less-ordered regions of pretreated lignocellulosic substrates (8,19).…”