Characteristics of the binding of a bacterial expansin (BsEXLX1) to microcrystalline cellulose

Boosting enzymatic degradation of cellulose using a fungal expansin: Structural insight into the pretreatment mechanism

The recalcitrance of cellulosic biomass greatly hinders its enzymatic degradation. Expansins induce cell wall loosening and promote efficient cellulose utilization; however, the molecular mechanism underlying their action is not well understood. In this study, TlEXLX1, a fungal expansin from Talaromyces leycettanus JCM12802, was characterized in terms of phylogeny, synergy, structure, and mechanism of action. TlEXLX1 displayed varying degrees of synergism with commercial cellulase in the pretreatment of corn straw and filter paper. TlEXLX1 binds to cellulose via domain 2, mediated by CH-π interactions with residues Tyr291, Trp292, and Tyr327. Residues Asp237, Glu238, and Asp248 in domain 1 form hydrogen bonds with glucose units and break the inherent hydrogen bonding within the cellulose matrix. This study identified the expansin amino acid residues crucial for cellulose binding, and elucidated the structure and function of expansins in cell wall networks; this has potential applications in biomass utilization.

Enhanced enzymatic hydrolysis of cellulose by endoglucanase via expansin pretreatment and the addition of zinc ions

One of the major limitations of lignocellulose conversion is the relatively low efficiency of cellulases. Expansins can act as an accessory protein to loosen the rigid cellulose structure and promote cellulose hydrolysis. However, the synergistic action of expansin is not well understood. In this study, we employed quartz crystal microbalance with dissipation to real-time monitor the adsorption of Bacillus subtilis expansin (BsEXLX1) and endoglucanase I (Cel7B) and the hydrolysis of cellulose. The effects of pH, temperature, and zinc ions on the initial adsorption rate and adsorption capacity of BsEXLX1 were examined. When 36.5 mM of zinc ions was added, the irreversible adsorption ratio of BsEXLX1 further increased to 4.63 times the value in the absence of zinc ions, whereas the initial adsorption rate and the hydrolysis rate constants of Cel7B could reach 2.16 times and 2.05 times the values in the absence of zinc ions, respectively.

Analysis of the Binding of Expansin Exl1, from Pectobacterium carotovorum, to Plant Xylem and Comparison to EXLX1 from Bacillus subtilis

The plant xylem is a preferred niche for some important bacterial phytopathogens, some of them encoding expansin proteins, which bind plant cell walls. Yet, the identity of the substrate for bacterial expansins within the plant cell wall and the nature of its interaction with it are poorly known. Here, we determined the localization of two bacterial expansins with differing isoelectric points (and with differing binding patterns to cell wall extracts) on plant tissue through in vitro fluorophore labeling and confocal imaging. Differential localization was observed, in which Exl1 from Pectobacterium carotovorum located into the intercellular spaces between xylem vessels and adjacent cells of the plant xylem, whereas EXLX1 from Bacillus subtilis bound cell walls of most cell types. In isolated vascular tissue, however, both PcExl1 and BsEXLX1 preferentially bound to tracheary elements over the xylem fibers, even though both are composed of secondary cell walls. Fluorescence correlation spectroscopy, employed to analyze the interaction of expansins with isolated xylem, indicates that binding is governed by more than one factor, which could include interaction with more than one type of polymer in the fibers, such as cellulose and hemicellulose or pectin. Binding to different polysaccharides could explain the observed reduction of cellulolytic and xylanolytic activities in the presence of expansin, possibly because of competition for the substrate. Our findings are relevant for the comprehensive understanding of the pathogenesis by P. carotovorum during xylem invasion, a process in which Exl1 might be involved.

Enhanced features of Dictyoglomus turgidum Cellulase A engineered with carbohydrate binding module 11 from Clostridium thermocellum

Lignocellulosic biomass (LCB) is a low-cost and abundant source of fermentable sugars. Enzymatic hydrolysis is one of the main ways to obtain sugars from biomass, but most of the polysaccharide-degrading enzymes are poorly efficient on LCB and cellulases with higher performances are required. In this study, we designed a chimeric protein by adding the carbohydrate binding module (CBM) of the cellulosomal enzyme CtLic26A-Cel5E (endoglucanase H or CelH) from Clostridium (Ruminiclostridium) thermocellum to the C-terminus of Dtur CelA, an interesting hyperthermostable endoglucanase from Dictyoglomus turgidum. The activity and binding rate of both native and chimeric enzyme were evaluated on soluble and insoluble polysaccharides. The addition of a CBM resulted in a cellulase with enhanced stability at extreme pHs, higher affinity and activity on insoluble cellulose.

Functional characterization of the native swollenin from Trichoderma reesei: study of its possible role as C1 factor of enzymatic lignocellulose conversion

BACKGROUND

Through binding to cellulose, expansin-like proteins are thought to loosen the structural order of crystalline surface material, thus making it more accessible for degradation by hydrolytic enzymes. Swollenin SWO1 is the major expansin-like protein from the fungus Trichoderma reesei. Here, we have performed a detailed characterization of a recombinant native form of SWO1 with respect to its possible auxiliary role in the enzymatic saccharification of lignocellulosic substrates.

RESULTS

The swo1 gene was overexpressed in T. reesei QM9414 Δxyr1 mutant, featuring downregulated cellulase production, and the protein was purified from culture supernatant. SWO1 was N-glycosylated and its circular dichroism spectrum suggested a folded protein. Adsorption isotherms (25 °C, pH 5.0, 1.0 mg substrate/mL) revealed SWO1 to be 120- and 20-fold more specific for binding to birchwood xylan and kraft lignin, respectively, than for binding to Avicel PH-101. The SWO1 binding capacity on lignin (25 µmol/g) exceeded 12-fold that on Avicel PH-101 (2.1 µmol/g). On xylan, not only the binding capacity (22 µmol/g) but also the affinity of SWO1 (K d = 0.08 µM) was enhanced compared to Avicel PH-101 (K d = 0.89 µM). SWO1 caused rapid release of a tiny amount of reducing sugars (<1 % of total) from different substrates (Avicel PH-101, nanocrystalline cellulose, steam-pretreated wheat straw, barley β-glucan, cellotetraose) but did not promote continued saccharification. Atomic force microscopy revealed that amorphous cellulose films were not affected by SWO1. Also with AFM, binding of SWO1 to cellulose nanocrystallites was demonstrated at the single-molecule level, but adsorption did not affect this cellulose. SWO1 exhibited no synergy with T. reesei cellulases in the hydrolysis of the different celluloses. However, SWO1 boosted slightly (1.5-fold) the reducing sugar release from a native grass substrate.

CONCLUSIONS

SWO1 is a strongly glycosylated protein, which has implications for producing it in heterologous hosts. Although SWO1 binds to crystalline cellulose, its adsorption to xylan is much stronger. SWO1 is not an auxiliary factor of the enzymatic degradation of a variety of cellulosic substrates. Effect of SWO1 on sugar release from intact plant cell walls might be exploitable with certain (e.g., mildly pretreated) lignocellulosic feedstocks.

Molecular dynamics of the Bacillus subtilis expansin EXLX1: interaction with substrates and structural basis of the lack of activity of mutants

Expansins are disruptive proteins that loosen growing plant cell walls and can enhance the enzymatic hydrolysis of cellulose.

Bacterial expansins and related proteins from the world of microbes

The discovery of microbial expansins emerged from studies of the mechanism of plant cell growth and the molecular basis of plant cell wall extensibility. Expansins are wall-loosening proteins that are universal in the plant kingdom and are also found in a small set of phylogenetically diverse bacteria, fungi, and other organisms, most of which colonize plant surfaces. They loosen plant cell walls without detectable lytic activity. Bacterial expansins have attracted considerable attention recently for their potential use in cellulosic biomass conversion for biofuel production, as a means to disaggregate cellulosic structures by nonlytic means ("amorphogenesis"). Evolutionary analysis indicates that microbial expansins originated by multiple horizontal gene transfers from plants. Crystallographic analysis of BsEXLX1, the expansin from Bacillus subtilis, shows that microbial expansins consist of two tightly packed domains: the N-terminal domain D1 has a double-ψ β-barrel fold similar to glycosyl hydrolase family-45 enzymes but lacks catalytic residues usually required for hydrolysis; the C-terminal domain D2 has a unique β-sandwich fold with three co-linear aromatic residues that bind β-1,4-glucans by hydrophobic interactions. Genetic deletion of expansin in Bacillus and Clavibacter cripples their ability to colonize plant tissues. We assess reports that expansin addition enhances cellulose breakdown by cellulase and compare expansins with distantly related proteins named swollenin, cerato-platanin, and loosenin. We end in a speculative vein about the biological roles of microbial expansins and their potential applications. Advances in this field will be aided by a deeper understanding of how these proteins modify cellulosic structures.

A novel expansin protein from the white-rot fungus Schizophyllum commune

A novel expansin protein (ScExlx1) was found, cloned and expressed from the Basidiomycete fungus Schizophylum commune. This protein showed the canonical features of plant expansins. ScExlx1 showed the ability to form “bubbles” in cotton fibers, reduce the size of avicel particles and enhance reducing sugar liberation from cotton fibers pretreated with the protein and then treated with cellulases. ScExlx1 was able to bind cellulose, birchwood xylan and chitin and this property was not affected by different sodium chloride concentrations. A novel property of ScExlx1 is its capacity to enhance reducing sugars (N-acetyl glucosamine) liberation from pretreated chitin and further added with chitinase, which has not been reported for any expansin or expansin-like protein. To the best of our knowledge, this is the first report of a bona fide fungal expansin found in a basidiomycete and we could express the bioactive protein in Pichia pastoris.

Synergistic proteins for the enhanced enzymatic hydrolysis of cellulose by cellulase

Reducing the enzyme loadings for enzymatic saccharification of lignocellulose is required for economically feasible production of biofuels and biochemicals. One strategy is addition of small amounts of synergistic proteins to cellulase mixtures. Synergistic proteins increase the activity of cellulase without causing significant hydrolysis of cellulose. Synergistic proteins exert their activity by inducing structural modifications in cellulose. Recently, synergistic proteins from various biological sources, including bacteria, fungi, and plants, were identified based on genomic data, and their synergistic activities were investigated. Currently, an up-to-date overview of several aspects of synergistic proteins, such as their functions, action mechanisms and synergistic activity, are important for future industrial application. In this review, we summarize the current state of research on four synergistic proteins: carbohydrate-binding modules, plant expansins, expansin-like proteins, and Auxiliary Activity family 9 (formerly GH61) proteins. This review provides critical information to aid in promoting research on the development of efficient and industrially feasible synergistic proteins.

Heterologous expression and functional characterization of a novel cellulose-disruptive protein LeEXP2 from Lycopersicum esculentum

There has been little research reported on the synergism of plant expansins in cellulose hydrolysis because of the difficulty of heterologous expression. In this study, the expansin gene LeExp2 of Lycopersicum esculentum was expressed for the first time in Pichia pastoris to identify its function in vitro. The optimal expression level was obtained by adding PMSF and a commercial protease inhibitor, and the maximum expression level was 70.9 mg/L. The affinity-purified LeEXP2 displayed cellulose-weakening activity and synergism with cellulase, and the reducing sugar yield in the reaction mixture with LeEXP2 was 1.4-9.8-fold that of control. The optimum pH and temperature for reducing-sugar liberation by LeEXP2 and endoglucanase were pH 4.8 and 50 °C. Cu(2+) and Fe(2+) notably increased the synergistic activity of LeEXP2, and Co(2+) enhanced the activity of endoglucanase but failed to improve synergism. LeEXP2 exhibited a high level of robustness to reagents and heat, indicating its potential application for cellulose decomposition. LeEXP2 preferentially bound to cellulose over lignin, and the binding capability to given substrates was not related to the crystallinity. This is the first study to characterize the functional role of LeEXP2 in cellulose degradation.

PcExl1 a novel acid expansin-like protein from the plant pathogen Pectobacterium carotovorum, binds cell walls differently to BsEXLX1

Microbial expansins act on plant cell walls similarly to plant expansins, albeit their loosening activity levels are tenfold lesser compared to plant expansins. We report the characterization of an expansin-like gene from the plant pathogen Pectobacterium carotovorum, named exl1. PcExl1 is an acidic protein that binds cellulose (Avicel), and weakens filter paper. The acidic nature of PcExl1 confers different binding properties when compared to Bacillus subtilis BsEXLX1, which is a basic protein. PcExl1 binding to wheat cell wall increased when acidic components were depleted, reaching a similar level to the binding to Avicel, indicating that cellulose is the target of PcExl1.

One-pot pretreatment, saccharification and ethanol fermentation of lignocellulose based on acid–base mixture pretreatment

One-pot pretreatment, saccharification and ethanol fermentation of lignocellulose, which is based on acid–base mixture pretreament, will greatly reduces the overall processing costs not only for the production of cellulosic ethanol but also for the lignocellulose-based biorefinery.

An expansin from the marine bacterium Hahella chejuensis acts synergistically with xylanase and enhances xylan hydrolysis

HcEXLX2 is a bacterial expansin found in a marine bacterium, Hahella chejuensis. Previously, HcEXLX2 was reported to act synergistically with a commercial cellulase preparation on the cellulose hydrolysis. The aim of the present study was to investigate the possible synergistic activity of HcEXLX2 with an endo-type xylanase from Saccharophagus degradans 2-40(T) (Xyn10C) in the hydrolysis of xylan. When 160 μg of HcEXLX2 was incubated with 12 μg of Xyn10C, the yield of reducing sugar increased 3.1 times when compared to that without HcEXLX2. The optimal temperature and pH for the synergism of HcEXLX2 with Xyn10C were 30°C and pH 7, respectively. In addition, binding experiments revealed that HcEXLX2 binds to xylan more preferentially than to Avicel. These results imply that HcEXLX2 could be used as an accessory protein to boost the activity of xylanase if its synergistic effect is strengthened at lower dosages.

Evaluation of bacterial expansin EXLX1 as a cellulase synergist for the saccharification of lignocellulosic Agro-industrial wastes

Various types of lignocellulosic wastes extensively used in biofuel production were provided to assess the potential of EXLX1 as a cellulase synergist. Enzymatic hydrolysis of natural wheat straw showed that all the treatments using mixtures of cellulase and an optimized amount of EXLX1, released greater quantities of sugars than those using cellulase alone, regardless of cellulase dosage and incubation time. EXLX1 exhibited different synergism and binding characteristics for different wastes, but this can be related to their lignocellulosic components. The cellulose proportion could be one of the important factors. However, when the cellulose proportion of different biomass samples exhibited no remarkable differences, a higher synergism of EXLX1 is prone to occur on these materials, with a high proportion of hemicellulose and a low proportion of lignin. The information could be favorable to assess whether EXLX1 is effective as a cellulase synergist for the hydrolysis of the used materials. Binding assay experiments further suggested that EXLX1 bound preferentially to alkali pretreated materials, as opposed to acid pretreated materials under the assay condition and the binding preference would be affected by incubation temperature.

Binding characteristics of a bacterial expansin (BsEXLX1) for various types of pretreated lignocellulose

BsEXLX1 from Bacillus subtilis is the first discovered bacterial expansin as a structural homolog of a plant expansin, and it exhibited synergism with cellulase on the cellulose hydrolysis in a previous study. In this study, binding characteristics of BsEXLX1 were investigated using pretreated and untreated Miscanthus x giganteus in comparison with those of CtCBD3, a cellulose-binding domain from Clostridium thermocellum. The amounts of BsEXLX1 bound to cellulose-rich substrates were significantly lower than those of CtCBD3. However, the amounts of BsEXLX1 bound to lignin-rich substrates were much higher than those of CtCBD3. A binding competition assay between BsEXLX1 and CtCBD3 revealed that binding of BsEXLX1 to alkali lignin was not affected by the presence of CtCBD3. This preferential binding of BsEXLX1 to lignin could be related to root colonization in plants by bacteria, and the bacterial expansin could be used as a lignin blocker in the enzymatic hydrolysis of lignocellulose.