Access to cellulose limits the efficiency of enzymatic hydrolysis: the role of amorphogenesis

Abstract

The efficient enzymatic saccharification of cellulose at low cellulase (protein) loadings continues to be a challenge for commercialization of a process for bioconversion of lignocellulose to ethanol. Currently, effective pretreatment followed by high enzyme loading is needed to overcome several substrate and enzyme factors that limit rapid and complete hydrolysis of the cellulosic fraction of biomass substrates. One of the major barriers faced by cellulase enzymes is their limited access to much of the cellulose that is buried within the highly ordered and tightly packed fibrillar architecture of the cellulose microfibrils. Rather than a sequential 'shaving' or 'planing' of the cellulose fibrils from the outside, it has been suggested that these inaccessible regions are disrupted or loosened by non-hydrolytic proteins, thereby increasing the cellulose surface area and making it more accessible to the cellulase enzyme complex. This initial stage in enzymatic saccharification of cellulose has been termed amorphogenesis. In this review, we describe the various amorphogenesis-inducing agents that have been suggested, and their possible role in enhancing the enzymatic hydrolysis of cellulose.

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

“…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).…”

“…However, it is now largely recognized that, to achieve effective lignocellulose deconstruction of more realistic biomass substrates, the enzyme mixture must also contain accessory enzymes/proteins such as hemicellulases, lytic polysaccharide monooxygenases, and non-hydrolytic/non-oxidative proteins that have been shown to facilitate biomass degradation through the opening up of the lignocellulosic matrix (5,7,10,11).…”

“…The non-hydrolytic non-oxidative proteins (also known as "disruptive" or "amorphogenesis-inducing" proteins) are of interest (7,9,12) due to their apparent ability to disrupt the structural matrix of biomass, consequently facilitating the subsequent depolymerization of the carbohydrate polymers by hydrolytic and oxidative enzymes (7). 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)).…”

Enhanced Bioethanol Production from Industrial Xylose Residue Using Efficient Delignification

“…The lignin sheath limits the accessibility of the enzymes to the cellulose. For lignin, its physicochemical features of low molecular weight and hydrophobicity restrict the swelling of cellulose and, therefore, negatively affect the accessible surface area (Donohoe et al 2008;Arantes and Saddler 2010). Additionally, cellulase can be irreversibly adsorbed on lignin via hydrophobic interactions, ionic bond interactions, and hydrogen bond interactions, consequently reducing the efficiency of saccharification.…”