(390b) A Detailed Kinetic Modeling Study of the Kinetic Slowdown of Enzymatic Hydrolysis of Cellulose

Currently the enzymatic hydrolysis of biomass is one of the slowest and most expensive processes involved in producing lignocellulosic biofuels. Improving enzymatic hydrolysis is critical for industrial success producing these second generation biofuels. Enzymatic hydrolysis of cellulose is characterized by a rapid initial degradation of the cellulose followed by a large slowdown in the rate of degradation. This kinetic slowdown, which usually occurs at low to moderate conversions, makes achieving high conversion hydrolysis of cellulosic substrates in reasonable reaction times difficult. While a large number of hypotheses as to the cause of this slowdown have been put forth in the literature ranging from product inhibition to substrate heterogeneity, the exact cause is still poorly understood. The complex nature of the hydrolysis system containing multiple enzymes and a solid substrate with a polymeric nature make the use of kinetic modeling a promising approach not just for predicting the system behavior but also for gaining insight into the mechanistic and kinetic causes of that behavior.

We have developed a detailed kinetic model for the hydrolysis of cellulose by cellulase enzymes. Our model tracks the action of multiple enzymes including endoglucanase, both reducing end and non-reducing end acting cellobiohydrolases, and beta-glucosidase. The cellulose substrate is characterized by accessible area, degree of polymerization, and concentration. A key feature of our model is the inclusion of enzyme adsorption to the solid surface reactions distinct from the reaction steps where adsorbed enzyme complexing with cellulose chains on the solid surface. The model utilized experimentally obtained parameters where available. The model was utilized to study the effects of important effects that can cause a kinetic slowdown including product inhibition, thermal enzyme deactivation, unproductive enzyme adsorption, and surface denaturation of the enzyme. The sensitivity of the model to the key parameters for these processes was looked at to determine how likely the process was to be important in causing the observed kinetic slowdown seen during enzymatic hydrolysis