6-08: Real-time nanoscale imaging provides new insights into improvement of biomass pretreatment and enzyme hydrolysis

Greater understanding of the mechanisms contributing to chemical and enzymatic solubilization of plant cell walls is critical for enabling cost-effective industrial conversion of cellulosic biomass to biofuels. We recently reported important, new insights into this process using label-free, non-destructive, multi-modal imaging of cell walls under digestion at near-physiological conditions (Ding et al., 2012, Science). We found that plant cell wall nanoscale architecture strongly correlates with enzymatic digestibility. High-resolution measurement of the microfibrillar structure of cell walls suggests that digestion is primarily facilitated by enabling enzyme access to the hydrophobic cellulose face. Two enzyme systems were studied at the cellular and molecular levels of resolution: multi-enzyme cellulosome complexes, which are found to bind to cell wall surfaces and degrade individual microfibrils; and smaller separate fungal cellulases, which penetrate inside microfibril networks for higher accessibility and faster digestion. Both enzyme systems can only digest non-lignified walls in untreated biomass; chemical delocalization of lignins exposes cellulose planar face in lignified secondary walls resulting complete digestion. From real-time visualization, we proposed a new architectural model of the plant cell wall, which correlates digestibility with accessibility to enzymes at the nanoscale. The data support the conclusion that ideal pretreatments should maximize lignin removal and minimize polysaccharide modification, thereby retaining the essentially native microfibrillar structure.