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

Thermotolerance Mechanism of Fungal GH6 Cellobiohydrolase. Part I. Characterization of Thermotolerant Mutant from the Basidiomycete Phanerochaete chrysosporium

Cellobiohydrolase (CBH), belonging to glycoside hydrolase family 6 (GH6), plays an essential role in cellulose saccharification, but its low thermotolerance presents a challenge in improving the reaction efficiency. Based on a report that chimeric CBH II (GH6) engineered to remove non-disulfide-bonded free Cys shows increased thermotolerance, we previously mutated the two free Cys residues to Ser in GH6 CBH from the basidiomycete Phanerochaete chrysosporium (PcCel6A) and obtained a thermotolerant double mutant, C240S/C393S (Yamaguchi et al., J. Appl. Glycosci. 2020; 67: 79-86). Here, characterization of the double mutant revealed that its activity towards both amorphous and crystalline cellulose was higher than that of the wild-type enzyme at elevated temperature, suggesting that the catalytic domain is the major contributor to the increased thermotolerance. To investigate the role of each free Cys residue, we prepared both single mutants, C240S and C393S, of the catalytic domain of PcCel6A and examined their residual activity at high temperature and the temperature-dependent changes of folding by means of circular dichroism measurements and thermal shift assay. The results indicate that the C393S mutation is the main contributor to both the increased thermotolerance of C240S/C393S and the increased activity of the catalytic domain at high temperature.

Thermotolerance Mechanism of Fungal GH6 Cellobiohydrolase. Part II. Structural Analysis of Thermotolerant Mutant from the Basidiomycete Phanerochaete chrysosporium

Glycoside hydrolase family 6 cellobiohydrolase (GH6 CBH) is a group of cellulases capable of hydrolyzing crystalline cellulose. However, the synergistic reaction of GH6 CBH with other cellulases is hindered by its relatively low thermotolerance. We previously obtained a thermotolerant double mutant, C240S/C393S, of GH6 CBH from the basidiomycete Phanerochaete chrysosporium (PcCel6A) by replacing the two free cysteine (Cys) residues, C240 and C393, with serine (Yamaguchi et al., J Appl Glycosci. 2020; 67;79-86). In the accompanying paper (Part I; Yamaguchi et al., J Appl Glycosci. 2024; 71: 55-62), we measured the temperature dependence of the activity and folding of C240S/C393S and its single mutants, C240S and C393S, and found that replacement of C393 was the major contributor to the increased thermotolerance of C240S/C393S. Here, in order to investigate the mechanism involved, we crystallized the wild-type and the mutant enzymes and compared their X-ray crystal structures. The overall structures of the wild-type and the three mutant enzymes were similar. However, C240S/C393S had the lowest relative B-factor at both the N-terminal loop (residues 172-177) and the C-terminal loop (residues 390-425). This result suggests that reduced structural fluctuation of the substrate-enclosing loops, possibly due to stronger hydrogen bonding involving C393, could account for the increased thermotolerance of C240S/C393S.

Improving the Heterologous Production of Fungal Peroxygenases through an Episomal Pichia pastoris Promoter and Signal Peptide Shuffling System

Fungal peroxygenases (UPOs) have emerged as oxyfunctionalization catalysts of tremendous interest in recent years. However, their widespread use in the field of biocatalysis is still hampered by their challenging heterologous production, substantially limiting the panel of accessible enzymes for investigation and enzyme engineering. Building upon previous work on UPO production in yeast, we have developed a combined promoter and signal peptide shuffling system for episomal high throughput UPO production in the industrially relevant, methylotrophic yeast Pichia pastoris. Eleven endogenous and orthologous promoters were shuffled with a diverse set of 17 signal peptides. Three previously described UPOs were selected as first test set, leading to the identification of beneficial promoter/signal peptide combinations for protein production. We applied the system then successfully to produce two novel UPOs: MfeUPO from Myceliophthora fergusii and MhiUPO from Myceliophthora hinnulea. To demonstrate the feasibility of the developed system to other enzyme classes, it was applied for the industrially relevant lipase CalB and the laccase Mrl2. In total, approximately 3200 transformants of eight diverse enzymes were screened and the best promoter/signal peptide combinations studied at various cofeeding, derepression, and induction conditions. High volumetric production titers were achieved by subsequent creation of stable integration lines and harnessing orthologous promoters from Hansenula polymorpha. In most cases promising yields were also achieved without the addition of methanol under derepressed conditions. To foster the use of the episomal high throughput promoter/signal peptide Pichia pastoris system, we made all plasmids available through Addgene.

LPMO AfAA9_B and Cellobiohydrolase AfCel6A from A. fumigatus Boost Enzymatic Saccharification Activity of Cellulase Cocktail

Cellulose is the most abundant polysaccharide in lignocellulosic biomass, where it is interlinked with lignin and hemicellulose. Bioethanol can be produced from biomass. Since breaking down biomass is difficult, cellulose-active enzymes secreted by filamentous fungi play an important role in degrading recalcitrant lignocellulosic biomass. We characterized a cellobiohydrolase (AfCel6A) and lytic polysaccharide monooxygenase LPMO (AfAA9_B) from Aspergillus fumigatus after they were expressed in Pichia pastoris and purified. The biochemical parameters suggested that the enzymes were stable; the optimal temperature was ~60 °C. Further characterization revealed high turnover numbers (kcat of 147.9 s-1 and 0.64 s-1, respectively). Surprisingly, when combined, AfCel6A and AfAA9_B did not act synergistically. AfCel6A and AfAA9_B association inhibited AfCel6A activity, an outcome that needs to be further investigated. However, AfCel6A or AfAA9_B addition boosted the enzymatic saccharification activity of a cellulase cocktail and the activity of cellulase Af-EGL7. Enzymatic cocktail supplementation with AfCel6A or AfAA9_B boosted the yield of fermentable sugars from complex substrates, especially sugarcane exploded bagasse, by up to 95%. The synergism between the cellulase cocktail and AfAA9_B was enzyme- and substrate-specific, which suggests a specific enzymatic cocktail for each biomass by up to 95%. The synergism between the cellulase cocktail and AfAA9_B was enzyme- and substrate-specific, which suggests a specific enzymatic cocktail for each biomass.

Random Mutagenesis by Insertion of Error-Prone PCR Products to the Chromosome of Bacillus subtilis

Bacillus subtilis is an attractive host for the directed evolution of the enzymes whose substrates cannot be transported across cell membrane. However, the generation of a mutant library in B. subtilis suffers problems of small library size, plasmid instability, and heterozygosity. Here, a large library of random mutant was created by inserting error-prone PCR (epPCR) products to the chromosome of B. subtilis. Specifically, the epPCR product was fused with flanking regions and antibiotic resistant marker using a PCR-based multimerization method, generating insertion construct. The epPCR product was integrated into the chromosome via homologous recombination after the insertion construct was transformed into the supercompetent cells of B. subtilis strain SCK6. The transformation efficiency of the insertion construct was improved through co-expressing homologous recombination-promoting protein NgAgo, raising the number of competent cells, and increasing the length of flanking regions. A library containing 5.31 × 105 random mutants was constructed using per μg insertion construct, which is sufficient for directed evolution. The library generation process was accomplished within 1 day. The effectiveness of this method was confirmed by improving the activity of Methyl Parathion Hydrolase (MPH) toward chlorpyrifos and by enhancing the secretion level of MPH in B. subtilis. Taken together, the present work provides a fast and efficient method to integrate epPCR products into the chromosome of B. subtilis, facilitating directed evolution and expression optimization of target proteins.

Thermostable Mutants of Glycoside Hydrolase Family 6 Cellobiohydrolase from the Basidiomycete Phanerochaete chrysosporium

Thermal inactivation of saccharifying enzymes is a crucial issue for the efficient utilization of cellulosic biomass as a renewable resource. Cellobiohydrolases (CBHs) are a kind of cellulase. In general, CBHs belonging to glycoside hydrolase (GH) family 6 (Cel6) act synergistically with CBHs of GH family 7 (Cel7) and other carbohydrate-active enzymes during the degradation of cellulosic biomass. However, while the catalytic rate of enzymes generally becomes faster at higher temperatures, Cel6 CBHs are inactivated at lower temperatures than Cel7 CBHs, and this represents a limiting factor for industrial utilization. In this study, we produced a series of mutants of the glycoside hydrolase family 6 cellobiohydrolase Pc Cel6A from the fungus Phanerochaete chrysosporium , and compared their thermal stability. Eight mutants from a random mutagenesis library and one rationally designed mutant were selected as candidate thermostable mutants and produced by heterologous expression in the yeast Pichia pastoris . Comparison of the hydrolytic activities at 50 and 60 °C indicated that the thermal stability of Pc Cel6A is influenced by the number and position of cysteine residues that are not involved in disulfide bonds.

Rozhkova A, N. Zorov I, Korotkova O, P. Sinitsyn A, Schwaneberg U, D. Davari M. Engineering Robust Cellulases for Tailored Lignocellulosic Degradation Cocktails

Lignocellulosic biomass is a most promising feedstock in the production of second-generation biofuels. Efficient degradation of lignocellulosic biomass requires a synergistic action of several cellulases and hemicellulases. Cellulases depolymerize cellulose, the main polymer of the lignocellulosic biomass, to its building blocks. The production of cellulase cocktails has been widely explored, however, there are still some main challenges that enzymes need to overcome in order to develop a sustainable production of bioethanol. The main challenges include low activity, product inhibition, and the need to perform fine-tuning of a cellulase cocktail for each type of biomass. Protein engineering and directed evolution are powerful technologies to improve enzyme properties such as increased activity, decreased product inhibition, increased thermal stability, improved performance in non-conventional media, and pH stability, which will lead to a production of more efficient cocktails. In this review, we focus on recent advances in cellulase cocktail production, its current challenges, protein engineering as an efficient strategy to engineer cellulases, and our view on future prospects in the generation of tailored cellulases for biofuel production.

A biochemical comparison of fungal GH6 cellobiohydrolases

Cellobiohydrolases (CBHs) from glycoside hydrolase family 6 (GH6) make up an important part of the secretome in many cellulolytic fungi. They are also of technical interest, particularly because they are part of the enzyme cocktails that are used for the industrial breakdown of lignocellulosic biomass. Nevertheless, functional studies of GH6 CBHs are scarce and focused on a few model enzymes. To elucidate functional breadth among GH6 CBHs, we conducted a comparative biochemical study of seven GH6 CBHs originating from fungi living in different habitats, in addition to one enzyme variant. The enzyme sequences were investigated by phylogenetic analyses to ensure that they were not closely related phylogenetically. The selected enzymes were all heterologously expressed in Aspergillus oryzae, purified and thoroughly characterized biochemically. This approach allowed direct comparisons of functional data, and the results revealed substantial variability. For example, the adsorption capacity on cellulose spanned two orders of magnitude and kinetic parameters, derived from two independent steady-state methods also varied significantly. While the different functional parameters covered wide ranges, they were not independent since they changed in parallel between two poles. One pole was characterized by strong substrate interactions, high adsorption capacity and low turnover number while the other showed weak substrate interactions, poor adsorption and high turnover. The investigated enzymes essentially defined a continuum between these two opposites, and this scaling of functional parameters raises interesting questions regarding functional plasticity and evolution of GH6 CBHs.

Structure-Guided Directed Evolution of Glycosidases: A Case Study in Engineering a Blood Group Antigen-Cleaving Enzyme

Directed evolution is an incredibly powerful strategy for engineering enzyme function. Applying this approach to glycosidases offers enormous potential for the development of highly specialized tools in chemical glycobiology. Performing enzyme directed evolution requires the generation, by random mutagenesis, of mutant libraries from which large numbers of variant enzymes must be screened in high-throughput assays. A structure-guided "semirational" method for library creation allows researchers to target specific amino acid positions for mutagenesis, concentrating mutations where they might be most effective in order to produce mutant libraries of a manageable size, minimizing screening effort while maximizing the chances of finding improved mutants. Well-designed assays, which may use specially prepared substrates, enable efficient screening of these mutant libraries. This chapter will detail general methods in the structure-guided directed evolution of glycosidases, which have previously been employed in engineering a blood group antigen-cleaving enzyme.

Directed evolution of the bacterial endo-β-1,4-glucanase from Streptomyces sp. G12 towards improved catalysts for lignocellulose conversion

With the aim to develop biocatalysts for enhanced hydrolysis of (hemi)cellulose into monosaccharides, random diversity by directed evolution was introduced in the gene coding for the endo-β-1,4-glucanase from Streptomyces sp. G12 which had been recombinantly expressed in Escherichia coli and named rCelStrep. The main objectives were therefore to set up a complete strategy for creation and automated screening of rCelStrep evolved direct mutants and to apply it to generate and screen a library of 10,000 random mutants to select the most active variants. The diversity was introduced in the gene by error-prone polymerase chain reaction. A primary qualitative screening on solid plates containing carboxymethylcellulose as the substrate allowed selecting 2200 active clones that were then subjected to a secondary quantitative screening towards AZO-CMC for the selection of 76 improved variants that were cultured in flasks and characterized. Five rCelStrep mutants exhibiting the highest hydrolytic activities than the wild-type enzyme were further characterized and applied to the bioconversion of the pretreated Arundo donax lignocellulosic biomass. It is worth of noting that one of the five tested mutants exhibited a 30% improvement in bioconversion yields compared to the wild-type enzyme, despite the absence of the carbohydrate binding module domain in this variant. Homology models of the three-dimensional structures of the catalytic and binding modules of rCelStrep were obtained and localization of mutations on these models allowed us to speculate on the structure-function relationships of the mutants.

Lignocellulose degrading extremozymes produced by Pichia pastoris: current status and future prospects

In this review article, extremophilic lignocellulosic enzymes with special interest on xylanases, β-mannanases, laccases and finally cellulases, namely, endoglucanases, exoglucanases and β-glucosidases produced by Pichia pastoris are reviewed for the first time. Recombinant lignocellulosic extremozymes are discussed from the perspectives of their potential application areas; characteristics of recombinant and native enzymes; the effects of P. pastoris expression system on recombinant extremozymes; and their expression levels and applied strategies to increase the enzyme expression yield. Further, effects of enzyme domains on activity and stability, protein engineering via molecular dynamics simulation and computational prediction, and site-directed mutagenesis and amino acid modifications done are also focused. Superior enzyme characteristics and improved stability due to the proper post-translational modifications and better protein folding performed by P. pastoris make this host favourable for extremozyme production. Especially, glycosylation contributes to the structure, function and stability of enzymes, as generally glycosylated enzymes produced by P. pastoris exhibit better thermostability than non-glycosylated enzymes. However, there has been limited study on enzyme engineering to improve catalytic efficiency and stability of lignocellulosic enzymes. Thus, in the future, studies should focus on protein engineering to improve stability and catalytic efficiency via computational modelling, mutations, domain replacements and fusion enzyme technology. Also metagenomic data need to be used more extensively to produce novel enzymes with extreme characteristics and stability.

Evaluation of microbially enhanced composting of sophora flavescens residues

The effects of inoculants on the composting of Sophora flavescens residues were evaluated based on several physical, chemical and biological parameters, as well as the infrared spectra. Compared to the control compost without inoculants, the treatment compost with inoculants (Bacillus subtilis strain G-13 and Chaetomium thermophilum strain GF-1) had a significantly longer thermophilic duration, higher cellulase activity and a higher degradation rate of cellulose, hemicellulose and lignin (P < 0.05). Thus, a higher maturity degree of compost with apparently lower C:N ratio (15.88 vs. 17.77) and NH₄-N:NO₃-N ratio (0.16 vs. 0.20) was obtained with the inoculation comparing with the control (P < 0.05). Besides, the inoculants could markedly accelerate the composting process and increase the maturity degree of compost as indicated by the germination index (GI) in which the treatment reached the highest GI of 133.2% at day 15 while the control achieved the highest GI of 125.7% at day 30 of the composting. Inoculation with B. subtilis and C. thermophilum is a useful method to enhance the S. flavescens residues composting according to this study.

Directed evolution of an exoglucanase facilitated by a co-expressed β-glucosidase and construction of a whole engineered cellulase system in Escherichia coli

A novel high-throughput screening method is proposed for the directed evolution of exoglucanase facilitated by the co-expression of β-glucosidase, using the glucose released from filter paper as the screening indicator. Three transformants (B1, D6 and G10) with improved activity were selected from 4,000 colonies. The specific activities of B1, D6 and G10 for releasing glucose were, respectively, 1.4-, 1.3- and 1.6-fold higher than that of the wild type. The engineered exoglucanase gene was inserted into an expression vector carrying the previously engineered endoglucanase and β-glucosidase genes, and transformed into Escherichia coli to form a completely engineered cellulase system that showed 8.2-fold increase in glucose production (relative activity) compared to the cells equipped with wild-type enzymes. To our knowledge, this is the first report for directed evolution of an exoglucanase using insoluble cellulose as the screening substrate.

Metabolic engineering of yeasts by heterologous enzyme production for degradation of cellulose and hemicellulose from biomass: a perspective

This review focuses on current approaches to metabolic engineering of ethanologenic yeast species for the production of bioethanol from complex lignocellulose biomass sources. The experimental strategies for the degradation of the cellulose and xylose-components of lignocellulose are reviewed. Limitations to the current approaches are discussed and novel solutions proposed.

Molecular engineering of fungal GH5 and GH26 beta-(1,4)-mannanases toward improvement of enzyme activity

Microbial mannanases are biotechnologically important enzymes since they target the hydrolysis of hemicellulosic polysaccharides of softwood biomass into simple molecules like manno-oligosaccharides and mannose. In this study, we have implemented a strategy of molecular engineering in the yeast Yarrowia lipolytica to improve the specific activity of two fungal endo-mannanases, PaMan5A and PaMan26A, which belong to the glycoside hydrolase (GH) families GH5 and GH26, respectively. Following random mutagenesis and two steps of high-throughput enzymatic screening, we identified several PaMan5A and PaMan26A mutants that displayed improved kinetic constants for the hydrolysis of galactomannan. Examination of the three-dimensional structures of PaMan5A and PaMan26A revealed which of the mutated residues are potentially important for enzyme function. Among them, the PaMan5A-G311S single mutant, which displayed an impressive 8.2-fold increase in kcat /KM due to a significant decrease of KM, is located within the core of the enzyme. The PaMan5A-K139R/Y223H double mutant revealed modification of hydrolysis products probably in relation to an amino-acid substitution located nearby one of the positive subsites. The PaMan26A-P140L/D416G double mutant yielded a 30% increase in kcat /KM compared to the parental enzyme. It displayed a mutation in the linker region (P140L) that may confer more flexibility to the linker and another mutation (D416G) located at the entrance of the catalytic cleft that may promote the entrance of the substrate into the active site. Taken together, these results show that the directed evolution strategy implemented in this study was very pertinent since a straightforward round of random mutagenesis yielded significantly improved variants, in terms of catalytic efiiciency (kcat/KM).

Engineering of family-5 glycoside hydrolase (Cel5A) from an uncultured bacterium for efficient hydrolysis of cellulosic substrates

Cel5A, an endoglucanase, was derived from the metagenomic library of vermicompost. The deduced amino acid sequence of Cel5A shows high sequence homology with family-5 glycoside hydrolases, which contain a single catalytic domain but no distinct cellulose-binding domain. Random mutagenesis and cellulose-binding module (CBM) fusion approaches were successfully applied to obtain properties required for cellulose hydrolysis. After two rounds of error-prone PCR and screening of 3,000 mutants, amino acid substitutions were identified at various positions in thermotolerant mutants. The most heat-tolerant mutant, Cel5A_2R2, showed a 7-fold increase in thermostability. To enhance the affinity and hydrolytic activity of Cel5A on cellulose substrates, the family-6 CBM from Saccharophagus degradans was fused to the C-terminus of the Cel5A_2R2 mutant using overlap PCR. The Cel5A_2R2-CBM6 fusion protein showed 7-fold higher activity than the native Cel5A on Avicel and filter paper. Cellobiose was a major product obtained from the hydrolysis of cellulosic substrates by the fusion enzyme, which was identified by using thin layer chromatography analysis.

Consistent mutational paths predict eukaryotic thermostability

BACKGROUND

Proteomes of thermophilic prokaryotes have been instrumental in structural biology and successfully exploited in biotechnology, however many proteins required for eukaryotic cell function are absent from bacteria or archaea. With Chaetomium thermophilum, Thielavia terrestris and Thielavia heterothallica three genome sequences of thermophilic eukaryotes have been published.

RESULTS

Studying the genomes and proteomes of these thermophilic fungi, we found common strategies of thermal adaptation across the different kingdoms of Life, including amino acid biases and a reduced genome size. A phylogenetics-guided comparison of thermophilic proteomes with those of other, mesophilic Sordariomycetes revealed consistent amino acid substitutions associated to thermophily that were also present in an independent lineage of thermophilic fungi. The most consistent pattern is the substitution of lysine by arginine, which we could find in almost all lineages but has not been extensively used in protein stability engineering. By exploiting mutational paths towards the thermophiles, we could predict particular amino acid residues in individual proteins that contribute to thermostability and validated some of them experimentally. By determining the three-dimensional structure of an exemplar protein from C. thermophilum (Arx1), we could also characterise the molecular consequences of some of these mutations.

CONCLUSIONS

The comparative analysis of these three genomes not only enhances our understanding of the evolution of thermophily, but also provides new ways to engineer protein stability.

Directed evolution: selection of the host organism

Directed evolution has become a well-established tool for improving proteins and biological systems. A critical aspect of directed evolution is the selection of a suitable host organism for achieving functional expression of the target gene. To date, most directed evolution studies have used either Escherichia coli or Saccharomyces cerevisiae as a host; however, other bacterial and yeast species, as well as mammalian and insect cell lines, have also been successfully used. Recent advances in synthetic biology and genomics have opened the possibility of expanding the use of directed evolution to new host organisms such as microalgae. This review focuses on the different host organisms used in directed evolution and highlights some of the recent directed evolution strategies used in these organisms.