Nanocellulose isolation from lignocellulose is a tedious and expensive process with high energy and harsh chemical requirements, primarily due to the recalcitrance of the substrate, which otherwise would have been cost-effective due to its abundance. Replacing the chemical steps with biocatalytic processes offers opportunities to solve this bottleneck to a certain extent due to the enzymes substrate specificity and mild reaction chemistry. In this work, we demonstrate the isolation of sulphate-free nanocellulose from organosolv pretreated birch biomass using different glycosyl-hydrolases, along with accessory oxidative enzymes including a lytic polysaccharide monooxygenase (LPMO). The suggested process produced colloidal nanocellulose suspensions (ζ-potential −19.4 mV) with particles of 7–20 nm diameter, high carboxylate content and improved thermostability (T_o = 301 °C, T_max = 337 °C). Nanocelluloses were subjected to post-modification using LPMOs of different regioselectivity. The sample from chemical route was the least favorable for LPMO to enhance the carboxylate content, while that from the C1-specific LPMO treatment showed the highest increase in carboxylate content.

Increased environmental concerns over petroleum-based products triggered the quest to find a sustainable alternative for fuels, chemicals etc. Lignocellulose biomass, due to its abundance, is considered as one of the most promising sustainable sources for the production of fuels and chemicals, while replacing the traditional petroleum resources. In a biorefinery, by choosing a greener biochemical conversion process with cellulolytic enzymes, cellulose from biomass is depolymerized into monomeric sugars and residual fibers; which can be later converted into a spectra of value added products.

Lytic polysaccharide monooxygenases (LPMOs) are one of the essential groups of enzymes in the bioconversion of lignocellulose. They are copper active enzymes that are produced by different polysaccharide degrading organisms in nature, such as lignocellulolytic fungi. In lignocellulose degradation, they are different from the traditional hydrolytic cellulolytic enzymes with their unique way of oxidative breakage of cellulose, in the presence of a co-substrate such as oxygen, and a reducing agent like lignin in the biomass. Their ability to enhance the action of traditional cellulases in cellulose depolymerization make them an integral part of today’s commercial cellulosic cocktails.

Primary goals of biorefinery research include efficient liquefaction of lignocellulose in order to increase the release of monomeric sugars towards the production of various chemicals and fuels, together with the potential use of residual fibers for the production of value-added products; all by minimizing the release of undesired by-products and the environmental impact of the process. LPMOs, along with other cellulases, have been shown to be very much beneficial in this.

This thesis comprises the study of LPMOs from different fungal origin, in their depolymerization ability on various substrates, including both model substrates and natural biomass samples. The evaluation was done based on their ability to release neutral and oxidized sugars, as well as their capability to promote liquefaction. Effect of various pretreatment methods of lignocellulose on the action of LPMOs was studied, together with their capability to use lignin present in the wood as a reducing agent, which gives a better understanding about their function in nature. Lastly, their role in producing value added materials such as nanocellulose, the prebiotic disaccharide cellobiose, from lignocellulose was also evaluated. 

The main objective of this study focused on the sustainable production of cellobiose and other cellulose-derived oligosaccharides from non-edible sources, more specifically, from forest residues. For this purpose, a fine-tuning of the performance of the commercially available enzyme mixture Celluclast^® was conducted towards the optimization of cellobiose production. By enzyme reaction engineering (pH, multi-stage hydrolysis with buffer exchange, addition of β-glucosidase inhibitor), a cellobiose-rich product with a high cellobiose to glucose ratio (37.4) was achieved by utilizing organosolv-pretreated birch biomass. In this way, controlled enzymatic hydrolysis combined with efficient downstream processing, including product recovery and purification through ultrafiltration and nanofiltration, can potentially support the sustainable production of food-grade oligosaccharides from forest biomass. The potential of the hydrolysis product to support the growth of two Lactobacilli probiotic strains as a sole carbon source was also demonstrated

Lignocellulose biomass is considered as one of the most potential and sustainable sources for the production of value-added chemicals and fuels while replacing the traditional petroleum resources. In a biorefinery, by employing biochemical conversion processes,cellulose present in the biomass is broken down into monomeric sugars which can belater converted into fuels or chemicals. This process is done with the help of different cellulose digesting enzymes (cellulases), isolated from natural cellulolytic organisms suchas saprophytic fungi.

Lytic polysaccharide monooxygenases (LPMOs) are considered as one of the vital classesof enzymes in the bio-conversion of lignocellulose. They are copper active enzymes present naturally in cellulose degrading fungi. Unlike the traditional cellulases, they havea unique way of breaking cellulose using molecular oxygen or hydrogen peroxide as cosubstratein the presence of a reducing agent. Their ability to enhance the action of other cellulases in depolymerizing the cellulose, make them an integral part of today’s commercial cellulase cocktails.

This thesis comprises the study about the action of lytic polysaccharide monooxygenaseson various cellulose substrates, both model and natural. The first part of the thesis focuses on the ability of an LPMO (MtLPMO9) and a traditional cellulase (MtEG5A), to act insynergism. The evaluation was done based on the release of oxidized and non-oxidized sugars and also on the ability to liquefy the substrates. It was observed that together, these two enzymes resulted in enhanced release of oxidized and non-oxidized sugars. Both were able to reduce viscosity of the substrates but no further synergistic effect was observed when added together.

The second part focuses on the ability of LPMOs to accept electrons from lignins for their action of breaking cellulose chains. Three LPMOs, MtLPMO9, PcLPMO9D and NcLPMO9C, lignins from agricultural and forest biomass pretreated by various pretreatment methods were selected. It was demonstrated that lignins, both in isolatedand substrate bound form were able to act indirectly as reducing agents, by releasingsoluble low-molecular-weight molecules that act as mediators between enzyme and bulklignins. The structural and compositional properties of lignins also affected their ability toact as electron donors. In addition, the effect of biomass pretreatment methods on the lignin properties was also studied. The lignins from acid catalyzed organosolv pretreatment were found as the best candidates in supplying electrons to the enzymes.Interestingly, NcLPMO9C was not able to utilize lignins as electron donors requiring further investigation on their mechanism both in vivo and in vitro.

Background

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that oxidatively cleave recalcitrant lignocellulose in the presence of oxygen or hydrogen peroxide as co-substrate and a reducing agent as electron donor. One of the possible systems that provide electrons to the LPMOs active site and promote the polysaccharide degradation involves the mediation of phenolic agents, such as lignin, low-molecular-weight lignin-derived compounds and other plant phenols. In the present work, the interaction of the bulk insoluble lignin fraction extracted from pretreated biomass with LPMOs and the ability to provide electrons to the active site of the enzymes is studied.

Results

The catalytic efficiency of three LPMOs, namely MtLPMO9 with C1/C4 regioselectivity, PcLPMO9D which is a C1 active LPMO and NcLPMO9C which is a C4 LPMO, was evaluated in the presence of different lignins. It was correlated with the physicochemical and structural properties of lignins, such as the molecular weight and the composition of aromatic and aliphatic hydroxyl groups. Moreover, the redox potential of lignins was determined with the use of large amplitude Fourier Transform alternating current cyclic voltammetry method and compared to the formal potential of the Cu (II) center in the active site of the LPMOs, providing more information about the lignin-LPMO interaction. The results demonstrated the existence of low-molecular weight lignin-derived compounds that are diffused in the reaction medium, which are able to reduce the enzyme active site and subsequently utilize additional electrons from the insoluble lignin fraction to promote the LPMO oxidative activity. Regarding the bulk lignin fractions, those isolated from the organosolv pretreated materials served as the best candidates in supplying electrons to the soluble compounds and, finally, to the enzymes. This difference, based on biomass pretreatment, was also demonstrated by the activity of LPMOs on natural substrates in the presence and absence of ascorbic acid as additional reducing agent.

Conclusions

Lignins can support the action of LPMOs and serve indirectly as electron donors through low-molecular-weight soluble compounds. This ability depends on their physicochemical and structural properties and is related to the biomass source and pretreatment method.

Our comparative studies reveal that the two lytic polysaccharide monooxygenases HiLP-MO9B and HiLPMO9I from the white-rot conifer pathogen Heterobasidion irregulare display clear difference with respect to their activity against crystalline cellulose and glucomannan. HiLPMO9I produced very little soluble sugar on bacterial microcrystalline cellulose (BMCC). In contrast, HiLPMO9B was much more active against BMCC and even released more soluble sugar than the H. irregulare cellobiohydrolase I, HiCel7A. Furthermore, HiLPMO9B was shown to cooperate with and stimulate the activity of HiCel7A, both when the BMCC was first pretreated with HiLPMO9B, as well as when HiLPMO9B and HiCel7A were added together. No such stimulation was shown by HiLPMO9I. On the other hand, HiLPMO9I was shown to degrade glucomannan, using a C4-oxidizing mechanism, whereas no oxidative cleavage activity of glucomannan was detected for HiLPMO9B. Structural modeling and comparison with other glucomannan-active LPMOs suggest that conserved sugar-interacting residues on the L2, L3 and LC loops may be essential for glucomannan binding, where 4 out of 7 residues are shared by HiLPMO9I, but only one is found in HiLPMO9B. The difference shown between these two H. irregulare LPMOs may reflect distinct biological roles of these enzymes within deconstruction of different plant cell wall polysaccharides during fungal colonization of softwood.

Lytic Polysaccharide Monooxygensaes have now been evolved as one of the most promising enzymes, attracting huge research attention due to their potential use in saccharification of lignocellulosic biomass for the production of fuels and value added chemicals. In the presence of molecular oxygen, these copper depended enzymes break the recalcitrant cellulose chain by a combined oxidative and hydrolytic action, and increase the substrate accessibility for other cellulases to work. This ‘boosting effect’ and ability to act in synergy makes them important subject to research, towards the future goal of sustainable bioeconomy. Diversity of this enzyme group ranges from early discovered chitin and cellulose active ones, to the recently identified hemicellulose and starch active ones. In this chapter we present a brief summary about LPMOs and the findings related to them from their discovery to the recent developments.

Background

Filamentous fungi are among the most powerful cellulolytic organisms in terrestrial ecosystems. To perform the degradation of lignocellulosic substrates, these microorganisms employ both hydrolytic and oxidative mechanisms that involve the secretion and synergism of a wide variety of enzymes. Interactions between these enzymes occur on the level of saccharification, i.e., the release of neutral and oxidized products, but sometimes also reflected in the substrate liquefaction. Although the synergism regarding the yield of neutral sugars has been extensively studied, further studies should focus on the oxidized sugars, as well as the effect of enzyme combinations on the viscosity properties of the substrates.

Results

In the present study, the heterologous expression of an endoglucanase (EG) and its combined activity together with a lytic polysaccharide monooxygenase (LPMO), both from the thermophilic fungus Myceliophthora thermophila, are described. The EG gene, belonging to the glycoside hydrolase family 5, was functionally expressed in the methylotrophic yeast Pichia pastoris. The produced MtEG5A (75 kDa) featured remarkable thermal stability and showed high specific activity on microcrystalline cellulose compared to CMC, which is indicative of its processivity properties. The enzyme was capable of releasing high amounts of cellobiose from wheat straw, birch, and spruce biomass. Addition of MtLPMO9 together with MtEG5A showed enhanced enzymatic hydrolysis yields against regenerated amorphous cellulose (PASC) by improving the release not only of the neutral but also of the oxidized sugars. Assessment of activity of MtEG5A on the reduction of viscosity of PASC and pretreated wheat straw using dynamic viscosity measurements revealed that the enzyme is able to perform liquefaction of the model substrate and the natural lignocellulosic material, while when added together with MtLPMO9, no further synergistic effect was observed.

Conclusions

The endoglucanase MtEG5A from the thermophilic fungus M. thermophila exhibited excellent properties that render it a suitable candidate for use in biotechnological applications. Its strong synergism with LPMO was reflected in sugars release, but not in substrate viscosity reduction. Based on the level of oxidative sugar formation, this is the first indication of synergy between LPMO and EG reported.