Observing and modeling BMCC degradation by commercial cellulase cocktails with fluorescently labeled Trichoderma reseii Cel7A through confocal microscopy

Abstract

Understanding the depolymerization mechanisms of cellulosic substrates by cellulase cocktails is a critical step towards optimizing the production of monosaccharides from biomass. The Spezyme CP cellulase cocktail combined with the Novo 188 β-glucosidase blend was used to depolymerize bacterial microcrystalline cellulose (BMCC), which was immobilized on a glass surface. The enzyme mixture was supplemented with a small fraction of fluorescently labeled Trichoderma reseii Cel7A, which served as a reporter to track cellulase binding onto the physical structure of the cellulosic substrate. Both micro-scale imaging and bulk experiments were conducted. All reported experiments were conducted at 50 °C, the optimal temperature for maximum hydrolytic activity of the enzyme cocktail. BMCC structure was observed throughout degradation by labeling it with a fluorescent dye. This method allowed us to measure the binding of cellulases in situ and follow the temporal morphological changes of cellulose during its depolymerization by a commercial cellulase mixture. Three kinetic models were developed and fitted to fluorescence intensity data obtained through confocal microscopy: irreversible and reversible binding models, and an instantaneous binding model. The models were successfully used to predict the soluble sugar concentrations that were liberated from BMCC in bulk experiments. Comparing binding and kinetic parameters from models with different assumptions to previously reported constants in the literature led us to conclude that exposing new binding sites is an important rate-limiting step in the hydrolysis of crystalline cellulose.