The Linker Region Promotes Activity and Binding Efficiency of Modular LPMO towards Polymeric Substrate

The Effect of CBM1 and Linker on the Oxidase, Peroxidase and Monooxygenase Activities of AA9 LPMOs: Insight into Their Correlation with the Nature of Reductants and Crystallinity of Celluloses

This study explores the effect of carbohydrate-binding module 1 (CBM1) and the linker on the function of auxiliary activity 9 (AA9) lytic polysaccharide monooxygenases (LPMOs), with a particular focus on monooxygenase activity, using different crystallinity celluloses and electron donors. The tested C1/C4-oxidizing AA9 LPMOs exhibited higher oxidase and peroxidase activities compared to those of the C4-oxidizing AA9 LPMOs. While the presence of CBM1 promoted cellulose-binding affinity, it reduced the oxidase activity of modular AA9 LPMOs. The effect of CBM1 on peroxidase activity was variable and enzyme-specific. Its influence on monooxygenase activity was linked to the type of reductants and the crystallinity of celluloses. Overall, CBM1 improved the monooxygenase activity on high-, medium-, and low-crystallinity celluloses when ascorbic acid (AscA) was used as the electron donor. CBM1 also facilitated monooxygenase activity on high-crystallinity cellulose, but significantly inhibited monooxygenase activity on low-crystallinity cellulose when cellobiose dehydrogenase (CDH) was the electron donor. Linker truncation of NcLOMO9C enhanced the cellulose-binding affinity but decreased both the oxidase and peroxidase activities. Linker truncation also impacted the monooxygenase activity in both the AscA-AA9 LPMO and AfCDH-AA9 LPMO systems, though its effect was less pronounced compared to that of CBM1. This work provides new insights into the role of the reductant type and cellulose crystallinity in the functionality of CBM1 and the linker in AA9 LPMOs.

The impact of the carbohydrate-binding module on how a lytic polysaccharide monooxygenase modifies cellulose fibers

BACKGROUND

In recent years, lytic polysaccharide monooxygenases (LPMOs) that oxidatively cleave cellulose have gained increasing attention in cellulose fiber modification. LPMOs are relatively small copper-dependent redox enzymes that occur as single domain proteins but may also contain an appended carbohydrate-binding module (CBM). Previous studies have indicated that the CBM "immobilizes" the LPMO on the substrate and thus leads to more localized oxidation of the fiber surface. Still, our understanding of how LPMOs and their CBMs modify cellulose fibers remains limited.

RESULTS

Here, we studied the impact of the CBM on the fiber-modifying properties of NcAA9C, a two-domain family AA9 LPMO from Neurospora crassa, using both biochemical methods as well as newly developed multistep fiber dissolution methods that allow mapping LPMO action across the fiber, from the fiber surface to the fiber core. The presence of the CBM in NcAA9C improved binding towards amorphous (PASC), natural (Cell I), and alkali-treated (Cell II) cellulose, and the CBM was essential for significant binding of the non-reduced LPMO to Cell I and Cell II. Substrate binding of the catalytic domain was promoted by reduction, allowing the truncated CBM-free NcAA9C to degrade Cell I and Cell II, albeit less efficiently and with more autocatalytic enzyme degradation compared to the full-length enzyme. The sequential dissolution analyses showed that cuts by the CBM-free enzyme are more evenly spread through the fiber compared to the CBM-containing full-length enzyme and showed that the truncated enzyme can penetrate deeper into the fiber, thus giving relatively more oxidation and cleavage in the fiber core.

CONCLUSIONS

These results demonstrate the capability of LPMOs to modify cellulose fibers from surface to core and reveal how variation in enzyme modularity can be used to generate varying cellulose-based materials. While the implications of these findings for LPMO-based cellulose fiber engineering remain to be explored, it is clear that the presence of a CBM is an important determinant of the three-dimensional distribution of oxidation sites in the fiber.

Carbohydrate-binding modules enhance H2O2 tolerance by promoting lytic polysaccharide monooxygenase active site H2O2 consumption

Lytic polysaccharide monooxygenases (LPMOs) oxidatively depolymerize recalcitrant polysaccharides, which is important for biomass conversion. The catalytic domains of many LPMOs are linked to carbohydrate-binding modules (CBMs) through flexible linkers, but the function of these CBMs in LPMO catalysis is not well understood. In this study, we utilized MtLPMO9L and MtLPMO9G derived from Myceliophthora thermophila to investigate the impact of CBMs on LPMO activity, with particular emphasis on their influence on H2O2 tolerance. Using truncated forms of MtLPMO9G generated by removing the CBM, we found reduced substrate binding affinity and enzymatic activity. Conversely, when the CBM was fused to the C terminus of the single-domain MtLPMO9L to create MtLPMO9L-CBM, we observed a substantial improvement in substrate binding affinity, enzymatic activity, and notably, H2O2 tolerance. Furthermore, molecular dynamics simulations confirmed that the CBM fusion enhances the proximity of the active site to the substrate, thereby promoting multilocal cleavage and impacting the exposure of the copper active site to H2O2. Importantly, the fusion of CBM resulted in more efficient consumption of H2O2 by LPMO, leading to improved enzymatic activity and reduced auto-oxidative damage of the copper active center.

Oxidative Machinery of basidiomycetes as potential enhancers in lignocellulosic biorefineries: A lytic polysaccharide monooxygenases approach

Basidiomycetes are renowned as highly effective decomposers of plant materials, due to their extensive array of oxidative enzymes, which enable them to efficiently break down complex lignocellulosic biomass structures. Among the oxidative machinery of industrially relevant basidiomycetes, the role of lytic polysaccharide monooxygenases (LPMO) in lignocellulosic biomass deconstruction is highlighted. So far, only a limited number of basidiomycetes LPMOs have been identified and heterologously expressed. These LPMOs have presented activity on cellulose and hemicellulose, as well as participation in the deconstruction of lignin. Expanding on this, the current review proposes both enzymatic and non-enzymatic mechanisms of LPMOs for biomass conversion, considering the significance of the Carbohydrate-Binding Modules and other C-terminal regions domains associated with their structure, which is involved in the deconstruction of lignocellulosic biomass.

Current insights of factors interfering the stability of lytic polysaccharide monooxygenases

Cellulose and chitin are two of the most abundant biopolymers in nature, but they cannot be effectively utilized in industry due to their recalcitrance. This limitation was overcome by the advent of lytic polysaccharide monooxygenases (LPMOs), which promote the disruption of biopolymers through oxidative mechanism and provide a breakthrough in the action of hydrolytic enzymes. In the application of LPMOs to biomass degradation, the key to consistent and effective functioning lies in their stability. The efficient transformation of biomass resources using LPMOs depends on factors that interfere with their stability. This review discussed three aspects that affect LPMO stability: general external factors, structural factors, and factors in the enzyme-substrate reaction. It explains how these factors impact LPMO stability, discusses the resulting effects, and finally presents relevant measures and considerations, including potential resolutions. The review also provides suggestions for the application of LPMOs in polysaccharide degradation.

Novel Design of an α-Amylase with an N-Terminal CBM20 in Aspergillus niger Improves Binding and Processing of a Broad Range of Starches

In the starch processing industry including the food and pharmaceutical industries, α-amylase is an important enzyme that hydrolyses the α-1,4 glycosidic bonds in starch, producing shorter maltooligosaccharides. In plants, starch molecules are organised in granules that are very compact and rigid. The level of starch granule rigidity affects resistance towards enzymatic hydrolysis, resulting in inefficient starch degradation by industrially available α-amylases. In an approach to enhance starch hydrolysis, the domain architecture of a Glycoside Hydrolase (GH) family 13 α-amylase from Aspergillus niger was engineered. In all fungal GH13 α-amylases that carry a carbohydrate binding domain (CBM), these modules are of the CBM20 family and are located at the C-terminus of the α-amylase domain. To explore the role of the domain order, a new GH13 gene encoding an N-terminal CBM20 domain was designed and found to be fully functional. The starch binding capacity and enzymatic activity of N-terminal CBM20 α-amylase was found to be superior to that of native GH13 without CBM20. Based on the kinetic parameters, the engineered N-terminal CBM20 variant displayed surpassing activity rates compared to the C-terminal CBM20 version for the degradation on a wide range of starches, including the more resistant raw potato starch for which it exhibits a two-fold higher Vmax underscoring the potential of domain engineering for these carbohydrate active enzymes.

Recent advances in the efficient degradation of lignocellulosic metabolic networks by lytic polysaccharide monooxygenase

Along with long-term evolution, the plant cell wall generates lignocellulose and other anti-degradation barriers to confront hydrolysis by fungi. Lytic polysaccharide monooxygenase (LPMO) is a newly defined oxidase in lignocellulosic degradation systems that significantly fuels hydrolysis. LPMO accepts electrons from wide sources, such as cellobiose dehydrogenase (CDH), glucose-methanol-choline (GMC) oxidoreductases, and small phenols. In addition, the extracellular cometabolic network formed by cosubstrates improves the degradation efficiency, forming a stable and efficient lignocellulose degradation system. In recent years, using structural proteomics to explore the internal structure and the complex redox system of LPMOs has become a research hotspot. In this review, the diversity of LPMOs, catalytic domains, carbohydrate binding modules, direct electron transfer with CDH, cosubstrates, and degradation networks of LPMOs are explored, which can provide a systematic reference for the application of lignocellulosic degradation systems in industrial approaches.

Enzyme synergy for plant cell wall polysaccharide degradation

Valorizing plant cell wall, marine and algal polysaccharides is of utmost importance for the development of the circular bioeconomy. This is because polysaccharides are by far the most abundant organic molecules found in nature with complex chemical structures that require a large set of enzymes for their degradation. Microorganisms produce polysaccharide-specific enzymes that act in synergy when performing hydrolysis. Although discovered since decades enzyme synergy is still poorly understood at the molecular level and thus it is difficult to harness and optimize. In the last few years, more attention has been given to improve and characterize enzyme synergy for polysaccharide valorization. In this review, we summarize literature to provide an overview of the different type of synergy involving carbohydrate modifying enzymes and the recent advances in the field exemplified by plant cell-wall degradation.

Effect of multimodularity and spatial organization of glycoside hydrolases on catalysis

The wide diversity among the carbohydrate-active enzymes (CAZymes) reflects the equally broad versatility in terms of composition and chemicals bonds found in the plant cell wall polymers on which they are active. This diversity is also expressed through the various strategies developed to circumvent the recalcitrance of these substrates to biological degradation. Glycoside hydrolases (GHs) are the most abundant of the CAZymes and are expressed as isolated catalytic modules or in association with carbohydrate-binding module (CBM), acting in synergism within complex arrays of enzymes. This multimodularity can be even more complex. The cellulosome presents a scaffold protein immobilized to the outer membrane of some microorganisms on which enzymes are grafted to prevent their dispersion and increase catalytic synergism. In polysaccharide utilization loci (PUL), GHs are also distributed across the membranes of some bacteria to co-ordinate the deconstruction of polysaccharides and the internalization of metabolizable carbohydrates. Although the study and characterization of these enzymatic activities need to take into account the entirety of this complex organization-in particular because of the dynamics involved in it-technical problems limit the present study to isolated enzymes. However, these enzymatic complexes also have a spatiotemporal organization, whose still neglected aspect must be considered. In the present review, the different levels of multimodularity that can occur in GHs will be reviewed, from its simplest forms to the most complex. In addition, attempts to characterize or study the effect on catalytic activity of the spatial organization within GHs will be addressed.

On the impact of carbohydrate-binding modules (CBMs) in lytic polysaccharide monooxygenases (LPMOs)

Lytic polysaccharide monooxygenases (LPMOs) have revolutionized our understanding of how enzymes degrade insoluble polysaccharides. Compared with the substantial knowledge developed on the structure and mode of action of the catalytic LPMO domains, the (multi)modularity of LPMOs has received less attention. The presence of other domains, in particular carbohydrate-binding modules (CBMs), tethered to LPMOs has profound implications for the catalytic performance of the full-length enzymes. In the last few years, studies on LPMO modularity have led to advancements in elucidating how CBMs, other domains, and linker regions influence LPMO structure and function. This mini review summarizes recent literature, with particular focus on comparative truncation studies, to provide an overview of the diversity in LPMO modularity and the functional implications of this diversity.

A thermostable and CBM2-linked GH10 xylanase from Thermobifida fusca for paper bleaching

Xylanases have the potential to be used as bio-deinking and bio-bleaching materials and their application will decrease the consumption of the chlorine-based chemicals currently used for this purpose. However, xylanases with specific properties could act effectively, such as having significant thermostability and alkali resistance, etc. In this study, we found that TfXyl10A, a xylanase from Thermobifida fusca, was greatly induced to transcript by microcrystalline cellulose (MCC) substrate. Biochemical characterization showed that TfXyl10A is optimally effective at temperature of 80 °C and pH of 9.0. After removing the carbohydrate-binding module (CBM) and linker regions, the optimum temperature of TfXyl10A-CD was reduced by 10°C (to 70°C), at which the enzyme’s temperature tolerance was also weakened. While truncating only the CBM domain (TfXyl10AdC) had no significant effect on its thermostability. Importantly, polysaccharide-binding experiment showed that the auxiliary domain CBM2 could specifically bind to cellulose substrates, which endowed xylanase TfXyl10A with the ability to degrade xylan surrounding cellulose. These results indicated that TfXyl10A might be an excellent candidate in bio-bleaching processes of paper industry. In addition, the features of active-site architecture of TfXyl10A in GH10 family were further analyzed. By mutating each residue at the -2 and -1 subsites to alanine, the binding force and enzyme activity of mutants were observably decreased. Interestingly, the mutant E51A, locating at the distal -3 subsite, exhibited 90% increase in relative activity compared with wild-type (WT) enzyme TfXyl10A-CD (the catalytic domain of TfXyl110A). This study explored the function of a GH10 xylanase containing a CBM2 domain and the contribution of amino acids in active-site architecture to catalytic activity. The results obtained provide guidance for the rational design of xylanases for industrial applications under high heat and alkali-based operating conditions, such as paper bleaching.