The key role that cellulose accessibility plays in restricting enzyme-mediated hydrolysis of cellulose

Although cellulose can be found in nature in an unassociated form (e.g., cotton, microbially derived cellulose, etc.), it is typically associated with other polymers such as lignin and hemicellulose. However, even "pure" cellulose has proven difficult to hydrolyze, primarily due to the lack of enzyme accessibility to the glycan chains. Thus, typically, much higher protein/enzyme concentrations and longer incubation times are needed as compared to hydrolyzing starch, a related glucose polymer. The "crystalline" structure of most of the cellulose and its close association with other lignocellulosic components (e.g., lignin, etc.) restrict the enzyme accessibility of the cellulase enzyme "cocktail". Consequently, some form of pretreatment plus the addition of accessory enzymes are typically needed to enhance cellulose hydrolysis. Although biomass-derived sugars can be readily detected and quantified, assessing cellulose accessibility by methods such as pore-volume, Simon's stain, cellulose binding domain (CBM) adsorption, etc., has proven problematic. Effective pretreatment, which maximizes the recovery of biomass components and increases cellulose accessibility, is typically required to achieve high glucose yields from biomass feedstocks. In addition, an optimized "cellulase cocktail," which further improves accessibility and is more resistant to factors such as end-product inhibition, is usually necessary to reach efficient hydrolysis. The influence of these and other issues are discussed below.