A Dynamic Model for Cellulosic Biomass Hydrolysis: a Comprehensive Analysis and Validation of Hydrolysis and Product Inhibition Mechanisms

Abstract

The objective of this study is to perform a comprehensive enzyme kinetics analysis in view of validating and consolidating a semimechanistic kinetic model consisting of homogeneous and heterogeneous reactions for enzymatic hydrolysis of lignocellulosic biomass proposed by the U.S. National Renewable Energy Laboratory (Kadam et al., Biotechnol Prog 20(3):698-705, 2004) and its variations proposed in this work. A number of dedicated experiments were carried out under a range of initial conditions (Avicel® versus pretreated barley straw as substrate, different enzyme loadings and different product inhibitors such as glucose, cellobiose and xylose) to test the hydrolysis and product inhibition mechanisms of the model. A nonlinear least squares method was used to identify the model and estimate kinetic parameters based on the experimental data. The suitable mathematical model for industrial application was selected among the proposed models based on statistical information (weighted sum of square errors). The analysis showed that transglycosylation plays a key role at high glucose levels. It also showed that the values of parameters depend on the selected experimental data used for parameter estimation. Therefore, the parameter values are not universal and should be used with caution. The model proposed by Kadam et al. (Biotechnol Prog 20(3):698-705, 2004) failed to predict the hydrolysis phenomena at high glucose levels, but when combined with transglycosylation reaction(s), the prediction of cellulose hydrolysis behaviour over a broad range of substrate concentrations (50-150 g/L) and enzyme loadings (15.8-31.6 and 1-5.9 mg protein/g cellulose for Celluclast and Novozyme 188, respectively) was possible. This is the first study introducing transglycosylation into the semimechanistic model. As long as these type of models are used within the boundary of their validity (substrate type, enzyme source and substrate concentration), they can support process design and technology improvement efforts at pilot and full-scale studies.

A Weibull statistics‐based lignocellulose saccharification model and a built‐in parameter accurately predict lignocellulose hydrolysis performance

“…The WSSE of Weibull model for previously published saccharification reactions was calculated according to the following equation [14]: …”

“…A robust model is also critical to the industrial applications of the reaction because of its importance in engineering design. Various attempts have been made to model the kinetics of enzymatic lignocellulose degradation in the past [7][8][9][10][11][12][13][14]. However, the enzymatic lignocellulose degradation process involves in heterologous substrates, takes place on solid surface, is catalyzed by many synergistic enzymes and is affected by feedback inhibition.…”

“…niger ZLF cellulase cocktails; as well as the saccharification of DCCR using T. reesei T cel- Table S1). 76 more groups of data from saccharification experiments reported in literature were also tested for the validity of the Weibull model [14,18,[22][23][24][25][26][27][28]. These reactions were chosen because the numeric glucose production levels at each time point were reported so that mathematical fitting of the model was possible.…”

“…The high R 2 obtained with the fitting clearly showed that these saccharification data follow the Weibull distribution (Supporting information, Table S1, Fig. S1), demonstrating the validity of this kinetic model in enzymatic lignocellulose saccharification, regardless the type of substrates and enzymes.To further identify whether the Weibull model can describe the lignocellulose saccharification process, we compared this model with previously established and widely acknowledged saccharification models that were based on Langmuir-type enzyme adsorption and Michaelis-Menten kinetics, using previously published saccharification data [14]. It can be shown that a comparable WSSE is obtained using the Weibull model and these more analytical models (Supporting information, Table S2), which suggests the Weibull model has a comparable accuracy on describing lignocellulose degradation with previous models and further confirms the validity of the Weibull model.…”

Parameter determination and validation for a mechanistic model of the enzymatic saccharification of cellulose-Iβ

“…Several experimental studies have indicated that the overall rate of sugar liberation by enzymatic hydrolysis of cellulose and lignocellulosic biomass decreases significantly after an initial phase of rapid substrate depletion. A multitude of structural and physico‐chemical factors, e.g., substrate accessibility, crystallinity, substrate morphology, product inhibition, enzyme crowding, particle size, and mass‐transfer limitations due to insufficient mixing, have been previously reported to limit the rate of enzymatic hydrolysis. While the rate slow‐down is likely due to a combination of several factors, mechanistic models can be helpful in identifying the rate‐limiting phenomena.…”