Low-level endoglucanase contamination in a Trichoderma reesei cellobiohydrolase II preparation affects its enzymatic activity on β-glucan

Dynamic changes of substrate reactivity and enzyme adsorption on partially hydrolyzed cellulose

The enzymatic hydrolysis of cellulose is a thermodynamically challenging catalytic process that is influenced by both substrate-related and enzyme-related factors. In this study, a proteolysis approach was applied to recover and clean the partially converted cellulose at the different stages of enzymatic hydrolysis to monitor the hydrolysis rate as a function of substrate reactivity/accessibility and investigate surface characteristics of the partially converted cellulose. Enzyme-substrate interactions between individual key cellulase components from wild-type Trichoderma reesei and partially converted cellulose were followed and correlated to the enzyme adsorption capacity and dynamic sugar release. Results suggest that cellobiohydrolase CBH1 (Cel7A) and endoglucanases EG2 (Cel5A) adsorption capacities decreased as cellulose was progressively hydrolyzed, likely due to the "depletion" of binding sites. Furthermore, the degree of synergism between CBH1 and EG2 varied depending on the enzyme loading and the substrates. The results provide a better understanding of the relationship between dynamic change of substrate features and the functionality of various cellulase components during enzymatic hydrolysis. Biotechnol. Bioeng. 2017;114: 503-515. © 2016 Wiley Periodicals, Inc.

Directed evolution and structural prediction of cellobiohydrolase II from the thermophilic fungus Chaetomium thermophilum

Cellulases can be engineered with enhanced properties for broad use in scientific and industrial applications. In this study, the wild-type cbh2 gene of the thermophilic fungus Chaetomium thermophilum encoding cellobiohydrolase II (CBHII) was mutagenized through in vitro directed evolution. The resulting Pichia pastoris yeast library was screened, and two transformants were selected for enhanced CBHII activities that were not attributed to increased gene copy numbers. The optimum fermentation times of the two mutant transformants were shortened to 4-5 days after methanol induction compared to 6 days for the wild-type. The optimum reaction temperature (60 °C) and pH level (5 or 6) of the mutant CBHII proteins, designated CBHIIX16 and CBHIIX305, were higher than those of wild-type CBHII (50 °C and pH 4). Kept at 80 °C for 1 h, CBHIIX16 and CBHIIX305 retained >50% of their activities, while the wild-type CBHII lost all activity. Sequence analysis of CBHIIX16 and CBHIIX305 revealed that they contained five and six mutated amino acids, respectively. Structural modeling confirmed the presence of carbohydrate binding type-1 and catalytic domains, where the hydrogen bond numbers between the 227th and 203rd amino acids were increased, which perhaps contributed to the elevated enzyme stability. Therefore, the two CBHII mutants selected for increased enzymatic activities also demonstrated elevated optimum reaction temperature and pH levels and enhanced thermal stability. These properties may be beneficial in practical applications for CBHII.

Toward an aggregated understanding of enzymatic hydrolysis of cellulose: noncomplexed cellulase systems

Information pertaining to enzymatic hydrolysis of cellulose by noncomplexed cellulase enzyme systems is reviewed with a particular emphasis on development of aggregated understanding incorporating substrate features in addition to concentration and multiple cellulase components. Topics considered include properties of cellulose, adsorption, cellulose hydrolysis, and quantitative models. A classification scheme is proposed for quantitative models for enzymatic hydrolysis of cellulose based on the number of solubilizing activities and substrate state variables included. We suggest that it is timely to revisit and reinvigorate functional modeling of cellulose hydrolysis, and that this would be highly beneficial if not necessary in order to bring to bear the large volume of information available on cellulase components on the primary applications that motivate interest in the subject.

Kinetic parameters and mode of action of the cellobiohydrolases produced by Talaromyces emersonii

Three forms of cellobiohydrolase (EC 3.2.1.91), CBH IA, CBH IB and CBH II, were isolated to apparent homogeneity from culture filtrates of the aerobic fungus Talaromyces emersonii. The three enzymes are single sub-unit glycoproteins, and unlike most other fungal cellobiohydrolases are characterised by noteworthy thermostability. The kinetic properties and mode of action of each enzyme against polymeric and small soluble oligomeric substrates were investigated in detail. CBH IA, CBH IB and CBH II catalyse the hydrolysis of microcrystalline cellulose, albeit to varying extents. Hydrolysis of a soluble cellulose derivative (CMC) and barley 1,3;1,4-beta-D-glucan was not observed. Cellobiose (G2) is the main reaction product released by CBH IA, CBH IB, and CBH II from microcrystalline cellulose. All three CBHs are competitively inhibited by G2; inhibition constant values (K(i)) of 2.5 and 0.18 mM were obtained for CBH IA and CBH IB, respectively (4-nitrophenyl-beta-cellobioside as substrate), while a K(i) of 0.16 mM was determined for CBH II (2-chloro-4-nitrophenyl-beta-cellotrioside as substrate). Bond cleavage patterns were determined for each CBH on 4-methylumbelliferyl derivatives of beta-cellobioside and beta-cellotrioside (MeUmbG(n)). While the Tal. emersonii CBHs share certain properties with their counterparts from Trichoderma reesei, Humicola insolens and other fungal sources, distinct differences were noted.

Homologous expression and characterization of Cel61A (EG IV) of Trichoderma reesei

There are currently four proteins in family 61 of the glycoside hydrolases, from Trichoderma reesei, Agaricus bisporus, Cryptococcus neoformans and Neurospora crassa. The enzymatic activity of these proteins has not been studied thoroughly. We report here the homologous expression and purification of T. reesei Cel61A [previously named endoglucanase (EG) IV]. The enzyme was expressed in high amounts with a histidine tag on the C-terminus and purified by metal affinity chromatography. This is the first time that a histidine tag has been used as a purification aid in the T. reesei expression system. The enzyme activity was studied on a series of carbohydrate polymers. The only activity exhibited by Cel61A was an endoglucanase activity observed on substrates containing beta-1,4 glycosidic bonds, e.g. carboxymethylcellulose (CMC), hydroxyethylcellulose (HEC) and beta-glucan. The endoglucanase activity on CMC and beta-glucan was determined by viscosity analysis, by measuring the production of reducing ends and by following the degradation of the polymer on a size exclusion chromatography system. The formation of soluble sugars by Cel61A from microcrystalline cellulose (Avicel; Merck), phosphoric acid swollen cellulose (PASC), and CMC were analysed on a HPLC system. Cel61A produced small amounts of oligosaccharides from these substrates. Furthermore, Cel61A showed activity against cellotetraose and cellopentaose. The activity of Cel61A was several orders of magnitude lower compared to Cel7B (previously EG I) of T. reesei on all substrates. One significant difference between Cel61A and Cel7B was that cellotriose was a poor substrate for Cel61A but was readily hydrolysed by Cel7B. The enzyme activity for Cel61A was further studied on a large number of carbohydrate substrates but the enzyme showed no activity towards any of these substrates.

Enzymatic Treatments on Tencel in Water and Microemulsion

Tencel is a relatively new fabric, obtained from wood pulp, that looks like natural cotton. In order to be suitable for commercial purposes, Tencel must be processed to improve its qualities. In this paper we report our studies on the enzymatic defibrillation of Tencel, in which we checked the different behavior of the same set of enzymes dispersed in pure water and in a microemulsion system. Surface properties, such as scanning electron microscopy, contact angle, porosimetry, breaking load, and thickness, were determined, in order to monitor the surface modification of the fabric upon enzymatic defibrillation, and indicate that the process is more efficient and less damaging when carried out in the microemulsion medium. Furthermore, we chemically modified Tencel by attaching fluorinated chains to the fabric surface. Surface properties show that fluorination of Tencel leads to a high degree of water- and oil-repellency in the fabric. Copyright 2001 Academic Press.

A comparative study of two retaining enzymes of Trichoderma reesei: transglycosylation of oligosaccharides catalysed by the cellobiohydrolase I, Cel7A, and the beta-mannanase, Man5A

HPLC, MALDI-TOF MS and NMR spectroscopy were used to investigate the hydrolysis of cello- and mannooligosaccharides by Cel7A and Man5A from Trichoderma reesei. The experimental progress curves were analysed by fitting the numerically integrated kinetic equations, which provided cleavage patterns for oligosaccharides. This data evaluation procedure accounts for product inhibition and avoids the initial slope approximation. In addition, a transglycosylation step had to be included in the model to reproduce the experimental progress curves. For the hydrolysis of manno-oligosaccharides, Man4-6, by Man5A no mannose was detected at the beginning of the reaction showing that only the internal linkages are hydrolysed. For cellotriose and cellotetraose hydrolysis by Cel7A, the main product is cellobiose and glucose is released from the non-reducing end of the substrate. Intermediary products longer than the substrates were detected by MALDI-TOF MS when oligosaccharides (Glc4-6 or Man4-6) were hydrolysed by either Cel7A or Man5A. Interestingly, two distinct transglycosylation pathways could be observed. Cel7A produced intermediates that are one unit longer than the substrate, whereas Man5A produced intermediates that are two units longer than the substrate.

Microbial hydrolysis of polysaccharides

Microorganisms are efficient degraders of starch, chitin, and the polysaccharides in plant cell walls. Attempts to purify hydrolases led to the realization that a microorganism may produce a multiplicity of enzymes, referred to as a system, for the efficient utilization of a polysaccharide. In order to fully characterize a particular enzyme, it must be obtained free of the other components of a system. Quite often, this proves to be very difficult because of the complexity of a system. This realization led to the cloning of the genes encoding them as an approach to eliminating other components. More than 400 such genes have been cloned and sequenced, and the enzymes they encode have been grouped into more than 50 families of related amino acid sequences. The enzyme systems revealed in this manner are complex on two quite different levels. First, many of the individual enzymes are complex, as they are modular proteins comprising one or more catalytic domains linked to ancillary domains that often include one or more substrate-binding domains. Second, the systems are complex, comprising from a few to 20 or more enzymes, all of which hydrolyze a particular substrate. Systems for the hydrolysis of plant cell walls usually contain more components than systems for the hydrolysis of starch and chitin because the cell walls contain several polysaccharides. In general, the systems produced by different microorganisms for the hydrolysis of a particular polysaccharide comprise similar enzymes from the same families.

Cello-oligosaccharide hydrolysis by cellobiohydrolase II from Trichoderma reesei. Association and rate constants derived from an analysis of progress curves

The hydrolysis of soluble cello-oligosaccharides, with a degree of polymerisation of 4-6, catalysed by cellobiohydrolase II from Trichoderma reesei was studied using 1H-NMR spectroscopy and HPLC. The experimental progress curves were analysed by fitting numerically integrated kinetic equations, which provided cleavage patterns and kinetic constants for each oligosaccharide. This analysis procedure accounts for product inhibition and avoids the initial slope approximation. No glucose was detected at the beginning of the reaction indicating that only the internal glycosidic linkages are attacked. For cellotetraose only the second glycosidic linkage was cleaved. For cellopentaose and cellohexaose the second and the third glycosidic linkage from the non-reducing end were cleaved with approximately equal probability. The degradation rates of these cello-oligosaccharides, 1-12 s-1 at 27 degrees C, are about 10-100 times faster than for the 4-methylumbelliferyl substituted analogs or for collotriose. No intermediate products larger than cellotriose were released. The degradation rate for cellotetraose were higher than its off-rate, which accounts for the processive degradation of cellohexaose. A high cellohexaose/enzyme ratio caused slow reversible inactivation of the enzyme.

The Cellulases Endoglucanase I and Cellobiohydrolase II of Trichoderma reesei Act Synergistically To Solubilize Native Cotton Cellulose but Not To Decrease Its Molecular Size

Degradation of cotton cellulose by Trichoderma reesei endoglucanase I (EGI) and cellobiohydrolase II (CBHII) was investigated by analyzing the insoluble cellulose fragments remaining after enzymatic hydrolysis. Changes in the molecular-size distribution of cellulose after attack by EGI, alone and in combination with CBHII, were determined by size exclusion chromatography of the tricarbanilate derivatives. Cotton cellulose incubated with EGI exhibited a single major peak, which with time shifted to progressively lower degrees of polymerization (DP; number of glucosyl residues per cellulose chain). In the later stages of degradation (8 days), this peak was eventually centered over a DP of 200 to 300 and was accompanied by a second peak (DP, (apprx=)15); a final weight loss of 34% was observed. Although CBHII solubilized approximately 40% of bacterial microcrystalline cellulose, the cellobiohydrolase did not depolymerize or significantly hydrolyze native cotton cellulose. Furthermore, molecular-size distributions of cellulose incubated with EGI together with CBHII did not differ from those attacked solely by EGI. However, a synergistic effect was observed in the reducing-sugar production by the cellulase mixture. From these results we conclude that EGI of T. reesei degrades cotton cellulose by selectively cleaving through the microfibrils at the amorphous sites, whereas CBHII releases soluble sugars from the EGI-degraded cotton cellulose and from the more crystalline bacterial microcrystalline cellulose.