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

Corn-derived Expansin synergistically promotes enzymatic hydrolysis of corn cob

The enzymatic hydrolysis of lignocellulose is hindered by challenges such as high enzyme usage and associated costs. It is essential to explore effective approaches to improve the efficiency of enzymatic hydrolysis while reducing costs. Expansins are non-enzymatic proteins that can interact with lignocellulose and facilitate the loosening of plant cell walls. Given their natural affinity to plant cell walls, we hypothesized that a corn Expansin could enhance the enzymatic hydrolysis of corn lignocellulose. In this study, we expressed a corn (Zea mays) Expansin, EXPA17, in yeast cells and explored the synergistic effect between EXPA17 and commercial cellulase, and found that EXPA17 exhibited a pronounced synergistic effect on the enzymatic hydrolysis of corn cobs. The addition of 0.015 mg/mL EXPA17 resulted in a 14.00 % increase in glucose yield. Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analysis revealed that EXPA17 proteins disrupted hydrogen bonds in the amorphous regions of corn cobs, leading to a more porous and looser structure, thereby enhancing cellulose accessibility. Our work leveraged the synergistic effect between Expansin and lignocellulose from the same source of corn, providing a novel strategy to improve the efficiency of enzymatic hydrolysis of corn lignocellulose while potentially reducing the associated costs.

Plant Cell Wall Loosening by Expansins

Expansins comprise an ancient group of cell wall proteins ubiquitous in land plants and their algal ancestors. During cell growth, they facilitate passive yielding of the wall's cellulose networks to turgor-generated tensile stresses, without evidence of enzymatic activity. Expansins are also implicated in fruit softening and other developmental processes and in adaptive responses to environmental stresses and pathogens. The major expansin families in plants include α-expansins (EXPAs), which act on cellulose-cellulose junctions, and β-expansins, which can act on xylans. EXPAs mediate acid growth, which contributes to wall enlargement by auxin and other growth agents. The genomes of diverse microbes, including many plant pathogens, also encode expansins designated expansin-like X. Expansins are proposed to disrupt noncovalent bonding between laterally aligned polysaccharides (notably cellulose), facilitating wall loosening for a variety of biological roles.

Pretreatment of chitin depolymerization by expansin-like protein to improve hydrolysis efficiency

Chitin is a polysaccharide similar to cellulose that contains abundant hydrogen bonds. Expansin-like proteins disrupt hydrogen bond networks, causing cellulose to swell and accelerating its degradation. We examined the effects of pretreatment with two expansin-like proteins, CxEXL22 (Arthrobotrys sp. CX1) and HcEXL (Hahella chejuensis), on chitin depolymerisation and enzymatic degradation. The efficiency of chitin degradation increased more than two-fold after pretreatment with expansin-like proteins. Following pretreatment with expansin-like proteins, chitin had a lower crystallinity index, greater d-spacing and crystallite size, and weaker hydrogen bonds, and the loosened porous microfibrils were more exposed than in untreated chitin. The rupture characterisation of crystalline chitin indicated that expansin-like proteins loosened the hydrogen bonds of the chitin polysaccharide chains, causing significant depolymerisation to expose more porous structures and enhance chitin accessibility.

Insights into the action of phylogenetically diverse microbial expansins on the structure of cellulose microfibrils

BACKGROUND

Microbial expansins (EXLXs) are non-lytic proteins homologous to plant expansins involved in plant cell wall formation. Due to their non-lytic cell wall loosening properties and potential to disaggregate cellulosic structures, there is considerable interest in exploring the ability of microbial expansins (EXLX) to assist the processing of cellulosic biomass for broader biotechnological applications. Herein, EXLXs with different modular structure and from diverse phylogenetic origin were compared in terms of ability to bind cellulosic, xylosic, and chitinous substrates, to structurally modify cellulosic fibrils, and to boost enzymatic deconstruction of hardwood pulp.

RESULTS

Five heterogeneously produced EXLXs (Clavibacter michiganensis; CmiEXLX2, Dickeya aquatica; DaqEXLX1, Xanthomonas sacchari; XsaEXLX1, Nothophytophthora sp.; NspEXLX1 and Phytophthora cactorum; PcaEXLX1) were shown to bind xylan and hardwood pulp at pH 5.5 and CmiEXLX2 (harboring a family-2 carbohydrate-binding module) also bound well to crystalline cellulose. Small-angle X-ray scattering revealed a 20-25% increase in interfibrillar distance between neighboring cellulose microfibrils following treatment with CmiEXLX2, DaqEXLX1, or NspEXLX1. Correspondingly, combining xylanase with CmiEXLX2 and DaqEXLX1 increased product yield from hardwood pulp by ~ 25%, while supplementing the TrAA9A LPMO from Trichoderma reesei with CmiEXLX2, DaqEXLX1, and NspEXLX1 increased total product yield by over 35%.

CONCLUSION

This direct comparison of diverse EXLXs revealed consistent impacts on interfibrillar spacing of cellulose microfibers and performance of carbohydrate-active enzymes predicted to act on fiber surfaces. These findings uncover new possibilities to employ EXLXs in the creation of value-added materials from cellulosic biomass.

Exploring the cellulolytic and hemicellulolytic activities of manganese peroxidase for lignocellulose deconstruction

BACKGROUND

A cost-effective pretreatment and saccharification process is a necessary prerequisite for utilizing lignocellulosic biomass (LCB) in biofuel and biomaterials production. Utilizing a multifunctional enzyme with both pretreatment and saccharification functions in a single step for simultaneous biological pretreatment and saccharification process (SPS) will be a green method of low cost and high efficiency. Manganese peroxidase (MnP, EC 1.11.1.13), a well-known lignin-degrading peroxidase, is generally preferred for the biological pretreatment of biomass. However, exploring the role and performance of MnP in LCB conversion will promote the application of MnP for lignocellulose-based biorefineries.

RESULTS

In this study, we explored the ability of an MnP from Moniliophthora roreri, MrMnP, in LCB degradation. With Mn2+ and H2O2, MrMnP decomposed 5.0 g/L carboxymethyl cellulose to 0.14 mM of reducing sugar with a conversion yield of 5.0 mg/g, including 40 μM cellobiose, 70 μM cellotriose, 20 μM cellotetraose, and 10 μM cellohexaose, and degraded 1.0 g/L mannohexaose to 0.33 μM mannose, 4.08 μM mannotriose, and 4.35 μM mannopentaose. Meanwhile, MrMnP decomposed 5.0 g/L lichenan to 0.85 mM of reducing sugar with a conversion yield of 30.6 mg/g, including 10 μM cellotriose, 20 μM cellotetraose, and 80 μM cellohexose independently of Mn2+ and H2O2. Moreover, the versatility of MrMnP in LCB deconstruction was further verified by decomposing locust bean gum and wheat bran into reducing sugars with a conversion yield of 54.4 mg/g and 29.5 mg/g, respectively, including oligosaccharides such as di- and tri-saccharides. The catalytic mechanism underlying MrMnP degraded lignocellulose was proposed as that with H2O2, MrMnP oxidizes Mn2+ to Mn3+. Subsequently, it forms a complex with malonate, facilitating the degradation of CMC and mannohexaose into reducing sugars. Without H2O2, MrMnP directly oxidizes malonate to hydroperoxyl acetic acid radical to form compound I, which then attacks the glucosidic bond of lichenan.

CONCLUSION

This study identified a new function of MrMnP in the hydrolysis of cellulose and hemicellulose, suggesting that MrMnP exhibits its versatility in the pretreatment and saccharification of LCB. The results will lead to an in-depth understanding of biocatalytic saccharification and contribute to forming new enzymatic systems for using lignocellulose resources to produce sustainable and economically viable products and the long-term development of biorefinery, thereby increasing the productivity of LCB as a green resource.

BsEXLX of engineered Trichoderma reesei strain as dual-active expansin to boost cellulases secretion for synergistic enhancement of biomass enzymatic saccharification in corn and Miscanthus straws

In this study, bacterial BsEXLE1 gene was overexpressed into T. reesei (Rut-C30) to generate a desirable engineered TrEXLX10 strain. While incubated with alkali-pretreated Miscanthus straw as carbon source, the TrEXLX10 secreted the β-glucosidases, cellobiohydrolases and xylanses with activities raised by 34%, 82% and 159% compared to the Rut-C30. Supplying EXLX10-secreted crude enzymes and commercial mixed-cellulases for two-step lignocellulose hydrolyses of corn and Miscanthus straws after mild alkali pretreatments, this work measured consistently higher hexoses yields released by the EXLX10-secreted enzymes for synergistic enhancements of biomass saccharification in all parallel experiments examined. Meanwhile, this study detected that the expansin, purified from EXLX10-secreted solution, was of exceptionally high binding activities with wall polymers, and further determined its independent enhancement for cellulose hydrolysis. Therefore, this study raised a mechanism model to highlight EXLX/expansin dual-activation roles for both secretion of stable biomass-degradation enzymes at high activity and biomass enzymatic saccharification in bioenergy crops.

Thermostable enzyme research advances: a bibliometric analysis

Thermostable enzymes are enzymes that can withstand elevated temperatures as high as 50 °C without altering their structure or distinctive features. The potential of thermostable enzymes to increase the conversion rate at high temperature has been identified as a key factor in enhancing the efficiency of industrial operations. Performing procedures at higher temperatures with thermostable enzymes minimises the risk of microbial contamination, which is one of the most significant benefits. In addition, it helps reduce substrate viscosity, improve transfer speeds, and increase solubility during reaction operations. Thermostable enzymes offer enormous industrial potential as biocatalysts, especially cellulase and xylanase, which have garnered considerable amount of interest for biodegradation and biofuel applications. As the usage of enzymes becomes more common, a range of performance-enhancing applications are being explored. This article offers a bibliometric evaluation of thermostable enzymes. Scopus databases were searched for scientific articles. The findings indicated that thermostable enzymes are widely employed in biodegradation as well as in biofuel and biomass production. Japan, the United States, China, and India, as along with the institutions affiliated with these nations, stand out as the academically most productive in the field of thermostable enzymes. This study's analysis exposed a vast number of published papers that demonstrate the industrial potential of thermostable enzymes. These results highlight the significance of thermostable enzyme research for a variety of applications.

Role of ascomycete and basidiomycete fungi in meeting established and emerging sustainability opportunities: a review

Fungal biomass is the future's feedstock. Non-septate Ascomycetes and septate Basidiomycetes, famously known as mushrooms, are sources of fungal biomass. Fungal biomass, which on averagely comprises about 34% protein and 45% carbohydrate, can be cultivated in bioreactors to produce affordable, safe, nontoxic, and consistent biomass quality. Fungal-based technologies are seen as attractive, safer alternatives, either substituting or complementing the existing standard technology. Water and wastewater treatment, food and feed, green technology, innovative designs in buildings, enzyme technology, potential health benefits, and wealth production are the key sectors that successfully reported high-efficiency performances of fungal applications. This paper reviews the latest technical know-how, methods, and performance of fungal adaptation in those sectors. Excellent performance was reported indicating high potential for fungi utilization, particularly in the sectors, yet to be utilized and improved on the existing fungal-based applications. The expansion of fungal biomass in the industrial-scale application for the sustainability of earth and human well-being is in line with the United Nations' Sustainable Development Goals.