(258b) Kinetic Modeling of Cellulose Hydrolysis with First Order Inactivation of Adsorbed Cellulases

The rate of glucose release during enzymatic hydrolysis of cellulose slows down significantly as the reaction proceeds, and the mechanism is not yet clear. It is, therefore, desirable to develop a kinetic model that accurately describes the enzymatic hydrolysis reaction. In this work, a model based on Langmuir binding kinetics that also incorporates first order inactivation of adsorbed cellulase was developed that accurately predicts rates and yields during the hydrolysis reaction for two different cellulose substrates. Substrate concentrations of 20g/L and less were tested in order to remove any potential effects due to product inhibition or mass transfer limitations.  A key result of the modeling suggests that Vmax decreases over time due to deactivation of the adsorbed enzyme.  Rate constants associated with product formation and enzyme inactivation from our model yield are comparable to reported values elsewhere. The model predicted results within 10% of experimental results for both Solka Floc and Sigmacell substrates. Key model parameters were similar for the two substrates: the apparent hydrolysis rate (k₂) and inactivation rate (k_f) were 33 min^-1 and 0.38 h^-1, respectively, comparable to values reported elsewhere. The rate constants were within 5% for the two different substrates, further strengthening the value of the model and implying that a common but yet undefined rate-limiting step associated with loss of enzyme activity likely exists in the pathway of cellulose hydrolysis.  It is proposed that this rate-limiting step may be either dissociation or isomerization of the enzyme.