A functionally based model for hydrolysis of cellulose by fungal cellulase

Abstract

A new functionally based kinetic model for enzymatic hydrolysis of pure cellulose by the Trichoderma cellulase system is presented. The model represents the actions of cellobiohydrolases I, cellobiohydrolase II, and endoglucanase I; and incorporates two measurable and physically interpretable substrate parameters: the degree of polymerization (DP) and the fraction of b-glucosidic bonds accessible to cellulase, F a (Zhang and Lynd, 2004). Initial enzyme-limited reaction rates simulated by the model are consistent with several important behaviors reported in the literature, including the effects of substrate characteristics on exoglucanase and endoglucanase activities; the degree of endo/exoglucanase synergy; the endoglucanase partition coefficient on hydrolysis rates; and enzyme loading on relative reaction rates for different substrates. This is the first cellulase kinetic model involving a single set of kinetic parameters that is successfully applied to a variety of cellulosic substrates, and the first that describes more than one behavior associated with enzymatic hydrolysis. The model has potential utility for data accommodation and design of industrial processes, structuring, testing, and extending understanding of cellulase enzyme systems when experimental date are available, and providing guidance for functional design of cellulase systems at a molecular scale. Opportunities to further refine cellulase kinetic models are discussed, including parameters that would benefit from further study.

Comparative study of corn stover pretreated by dilute acid and cellulose solvent‐based lignocellulose fractionation: Enzymatic hydrolysis, supramolecular structure, and substrate accessibility

“…2B). The CAC of the COSLIF-pretreated corn stover was nearly double that of DA-pretreated biomass, consistent with the hypothesis that CAC was one of the most important (rate-limiting) factors influencing enzymatic hydrolysis (Zhang and Lynd, 2006). However, while changes in CAC appear to be a major causative factor for the changes in hydrolysis rates (especially initial rates), additional factors involving feedstock structure and composition undoubtedly contribute to the increased glucan digestibility (enzymatic hydrolysis yields) observed in COSLIF-pretreated as compared to DA-pretreated samples.…”

“…This new approach more accurately assesses substrate characteristic related to enzymatic cellulose hydrolysis than traditional methods such as nitrogen adsorption-based Brunauer-EmmettTeller (BET), size exclusion, and small angle X-ray scattering (Hong et al, 2007;Zhang and Lynd, 2004). Regenerated amorphous cellulose (RAC) that is prepared from microcrystalline cellulose (Avicel) has $20-fold higher CAC (Hong et al, 2007(Hong et al, , 2008b and exhibits much faster enzymatic hydrolysis rates than microcrystalline cellulose (Zhang et al, 2006a), which is in agreement with the model prediction that increasing CAC is more important for increasing hydrolysis rates than decreasing DP (Zhang and Lynd, 2006). The CAC value of Avicel (m 2 per gram of Avicel) based on the TGC adsorption was only one-tenth of that based on the BET method (Marshall and Sixsmith, 1974), implying that about 90% gross surface area measure based on nitrogen adsorption cannot be accessible to largesize cellulase protein molecules, at least initially.…”

“…A functionally based mathematical model of fungal enzyme-based enzymatic cellulose hydrolysis has been developed, accounting for cellulose characteristics (DP and substrate accessibility) and different modes of action for endoglucanase and cellobiohydrolase enzyme system components (Zhang and Lynd, 2006). This model not only correlates disparate phenomena reported in the literature but also clearly suggests that low cellulose accessibility is the most important substrate characteristic limiting enzymatic hydrolysis rates (Zhang and Lynd, 2006).…”

“…A functionally based mathematical model of fungal enzyme-based enzymatic cellulose hydrolysis has been developed, accounting for cellulose characteristics (DP and substrate accessibility) and different modes of action for endoglucanase and cellobiohydrolase enzyme system components (Zhang and Lynd, 2006). This model not only correlates disparate phenomena reported in the literature but also clearly suggests that low cellulose accessibility is the most important substrate characteristic limiting enzymatic hydrolysis rates (Zhang and Lynd, 2006). More recently, a quantitative assay for determining cellulose accessibility to cellulase (CAC) has been established based on adsorption of a non-hydrolytic fusion protein (TGC) containing a cellulose-binding module and a green fluorescence protein (Hong et al, 2007).…”

Potencial da palha de cana-de-açúcar para produção de etanol

“…40 Diversos autores têm atribuído a recalcitrância da biomassa, principalmente, à cristalinidade, ao grau de polimerização e à baixa acessibilidade das enzimas celulolíticas ao substrato. [41][42][43][44] Devido a essa dificuldade na estrutura da biomassa, muitos processos de produção têm sido desenvolvidos no intuito de converter os carboidratos presentes na biomassa em açúcares fermentescíveis, buscando por melhores rendimentos e menores custos de processamento. 2,4,45 A produção de etanol a partir de celulose exige várias etapas que envolvem basicamente pré-tratamento, hidrólise e fermentação.…”