Thermostable cellulose saccharifying microbial enzymes: Characteristics, recent advances and biotechnological applications

Hydrolysis of ionic liquid-treated substrate with an Iocasia fonsfrigidae strain SP3-1 endoglucanase

Recently, we reported the discovery of a novel endoglucanase of the glycoside hydrolase family 12 (GH12), designated IfCelS12A, from the haloalkaliphilic anaerobic bacterium Iocasia fonsfrigidae strain SP3-1, which was isolated from a hypersaline pond in the Samut Sakhon province of Thailand (ca. 2017). IfCelS12A exhibits high substrate specificity on carboxymethyl cellulose and amorphous cellulose but low substrate specificity on b-1,3;1,4-glucan. Unlike some endoglucanases of the GH12 family, IfCelS12A does not exhibit hydrolytic activity on crystalline cellulose (i.e., Avicel™). High-Pressure Liquid Chromatography (HPLC) and Thin Layer Chromatography (TLC) analyses of products resulting from IfCelS12-mediated hydrolysis indicate mode of action for this enzyme. Notably, IfCelS12A preferentially hydrolyzes cellotetraoses, cellopentaoses, and cellohexaoses with negligible activity on cellobiose or cellotriose. Kinetic analysis with cellopentaose and barely b-D-glucan as cellulosic substrates were conducted. On cellopentaose, IfCelS12A demonstrates a 16-fold increase in activity (KM = 0.27 mM; kcat = 0.36 s-1; kcat/KM = 1.34 mM-1 s-1) compared to the enzymatic hydrolysis of barley b-D-glucan (KM: 0.04 mM, kcat: 0.51 s-1, kcat/KM = 0.08 mM-1 s-1). Moreover, IfCelS12A enzymatic efficacy is stable in hypersaline sodium chlorids (NaCl) solutions (up to 10% NaCl). Specifically, IfCel12A retains notable activity after 24 h at 2M NaCl (10% saline solution). IfCelS12A used as a cocktail component with other cellulolytic enzymes and in conjunction with mobile sequestration platform technology offers additional options for deconstruction of ionic liquid-pretreated cellulosic feedstock. KEY POINTS: • IfCelS12A from an anaerobic alkaliphile Iocasia fronsfrigidae shows salt tolerance • IfCelS12A in cocktails with other enzymes efficiently degrades cellulosic biomass • IfCelS12A used with mobile enzyme sequestration platforms enhances hydrolysis.

Enzyme immobilization with nanomaterials for hydrolysis of lignocellulosic biomass: Challenges and future Perspectives

Enzyme immobilization has emerged as a prodigious strategy in the enzymatic hydrolysis of lignocellulosic biomass (LCB) promising enhanced efficacy and stability of the enzymes. Further, enzyme immobilization on magnetic nanoparticles (MNPs) facilitates the easy recovery and reuse of biocatalysts. This results in the development of a nanobiocatalytic system, that serves as an eco-friendly and inexpensive LCB deconstruction approach. This review provides an overview of nanomaterials used for immobilization with special emphasis on the nanomaterial-enzyme interactions and strategies of immobilization. After the succinct outline of the immobilization procedures and supporting materials, a comprehensive assessment of the catalysis enabled by nanomaterial-immobilized biocatalysts for the conversion and degradation of lignocellulosic biomasses is provided by gathering state-of-the-art examples. The challenges and future directions associated with this technique providing a potential solution in the present article. Insight on the recent advancements in the process of nanomaterial-based immobilization for the hydrolysis of lignocellulosic biomass has also been highlighted in the article.

Screening of Bacillus spp. bacterial endophytes for protease production, and application in feather degradation and bio-detergent additive

Research on proteases and secondary metabolites from endophytes is an area that requires attention from researchers. In this study, proteases from Bacillus sp. strain MHSD16 and Bacillus sp. strain MHSD17 endophytes were characterised, and their potential biotechnological applications were investigated. Optimum protease production was achieved when isolates were grown in media containing (g/L): glucose 10g, casein 5g, yeast extract 5g, KH2PO4 2g, Na2CO3 10g at pH 9. The crude protease extracts were active in alkaline environments, thus referred to as alkaline proteases with optimal pH of 10. Additionally, Bacillus sp. strain MHSD 16 and Bacillus sp. strain MHSD17 proteases were active at high temperatures, with optimum enzyme activity at 50 °C. Thermostability profiles of these proteases showed that the enzymes were highly stable between (40-60 °C), maintaining over 85 % stability after 120 min incubation at 60 °C. Furthermore, the enzymes were stable and compatible with various household and laundry detergents. In the presence of commercial laundry detergent, OMO® 68 % and 72 % activity was retained for Bacillus sp. strain MHSD16 and Bacillus sp. strain MHSD17, respectively, while 67 % and 68 % activity were retained in the presence of Sunlight®. The potential application for use in detergents was investigated through the removal of blood stains with the crude alkaline extracts displaying efficient stain removal abilities. Feather degradation was also investigated and Bacillus sp. MHSD17 exhibited feather keratin degrading properties more effectively than Bacillus sp. MHSD16.

Effects of heterologous expression and N-glycosylation on the hyperthermostable endoglucanase of Pyrococcus furiosus

Hyperthermostable endoglucanases of glycoside hydrolase family 12 from the archaeon Pyrococcus furiosus (EGPf) catalyze the hydrolysis of β-1,4-glucosidic linkages in cellulose and β-glucan structures that contain β-1,3- and β-1,4-mixed linkages. In this study, EGPf was heterologously expressed with Aspergillus niger and the recombinant enzyme was characterized. The successful expression of EGPf resulted as N-glycosylated protein in its secretion into the culture medium. The glycosylation of the recombinant EGPf positively impacted the kinetic characterization of EGPf, thereby enhancing its catalytic efficiency. Moreover, glycosylation significantly boosted the thermostability of EGPf, allowing it to retain over 80% of its activity even after exposure to 100 °C for 5 h, with the optimal temperature being above 120 °C. Glycosylation did not affect the pH stability or salt tolerance of EGPf, although the glycosylated compound exhibited a high tolerance to ionic liquids. EGPf displayed the highest specific activity in the presence of 20% (v/v) 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), reaching approximately 2.4 times greater activity than that in the absence of [Bmim]Cl. The specific activity was comparable to that without the ionic liquid even in the presence of 40% (v/v) [Bmim]Cl. Glycosylated EGPf has potential as an enzyme for saccharifying cellulose under high-temperature conditions or with ionic liquid treatment due to its exceptional thermostability and ionic liquid tolerance. These results underscore the potential of N-glycosylation as an effective strategy to further enhance both the thermostability of highly thermostable archaeal enzymes and the hydrolysis of barley cellulose in the presence of [Bmim]Cl.

Enhanced crystalline cellulose degradation by a novel metagenome-derived cellulase enzyme

Metagenomics has revolutionized access to genomic information of microorganisms inhabiting the gut of herbivorous animals, circumventing the need for their isolation and cultivation. Exploring these microorganisms for novel hydrolytic enzymes becomes unattainable without utilizing metagenome sequencing. In this study, we harnessed a suite of bioinformatic analyses to discover a novel cellulase-degrading enzyme from the camel rumen metagenome. Among the protein-coding sequences containing cellulase-encoding domains, we identified and subsequently cloned and purified a promising candidate cellulase enzyme, Celcm05-2, to a state of homogeneity. The enzyme belonged to GH5 subfamily 4 and exhibited robust enzymatic activity under acidic pH conditions. It maintained hydrolytic activity under various environmental conditions, including the presence of metal ions, non-ionic surfactant Triton X-100, organic solvents, and varying temperatures. With an optimal temperature of 40 °C, Celcm05-2 showcased remarkable efficiency when deployed on crystalline cellulose (> 3.6 IU/mL), specifically Avicel, thereby positioning it as an attractive candidate for a myriad of biotechnological applications spanning biofuel production, paper and pulp processing, and textile manufacturing. Efficient biodegradation of waste paper pulp residues and the evidence of biopolishing suggested that Celcm05-2 can be used in the bioprocessing of cellulosic craft fabrics in the textile industry. Our findings suggest that the camel rumen microbiome can be mined for novel cellulase enzymes that can find potential applications across diverse biotechnological processes.

Enhancing thermostability of tryptophan hydroxylase via protein engineering and its application in 5-hydroxytryptophan production

5-Hydroxytryptophan (5-HTP), as the precursor of serotonin and melatonin in animals, can regulate mood, sleep, and behavior, which is widely used in pharmaceutical and health products industry. The enzymatic production of 5-hydroxytryptophan (5-HTP) from L-tryptophan (L-Trp) using tryptophan hydroxylase (TPH) show huge potential in application due to its advantages, such as mild reaction conditions, avoidance of protection/deprotection processes, excellent regioselectivity and considerable catalytic efficiency, compared with chemical synthesis and natural extraction. However, the low thermostability of TPH restricted its hydroxylation efficiency toward L-Trp. In this study, we aimed to improve the thermostability of TPH via semi-rational design guided by (folding free energy) ΔΔG fold calculation. After two rounds of evolution, two beneficial mutants M1 (S422V) and M30 (V275L/I412K) were obtained. Thermostability evaluation showed that M1 and M30 possessed 5.66-fold and 6.32-fold half-lives (t1/2) at 37 °C, and 4.2 °C and 6.0 °C higher melting temperature (Tm) than the WT, respectively. The mechanism behind thermostability improvement was elucidated with molecular dynamics simulation. Furthermore, biotransformation of 5-HTP from L-Trp was performed, M1 and M30 displayed 1.80-fold and 2.30-fold than that of WT, respectively. This work provides important insights into the thermostability enhancement of TPH and generate key mutants that could be robust candidates for practical production of 5-HTP.

Komagataella phaffii as a Platform for Heterologous Expression of Enzymes Used for Industry

In the 1980s, Escherichia coli was the preferred host for heterologous protein expression owing to its capacity for rapid growth in complex media; well-studied genetics; rapid and direct transformation with foreign DNA; and easily scalable fermentation. Despite the relative ease of use of E. coli for achieving the high expression of many recombinant proteins, for some proteins, e.g., membrane proteins or proteins of eukaryotic origin, this approach can be rather ineffective. Another microorganism long-used and popular as an expression system is baker's yeast, Saccharomyces cerevisiae. In spite of a number of obvious advantages of these yeasts as host cells, there are some limitations on their use as expression systems, for example, inefficient secretion, misfolding, hyperglycosylation, and aberrant proteolytic processing of proteins. Over the past decade, nontraditional yeast species have been adapted to the role of alternative hosts for the production of recombinant proteins, e.g., Komagataella phaffii, Yarrowia lipolytica, and Schizosaccharomyces pombe. These yeast species' several physiological characteristics (that are different from those of S. cerevisiae), such as faster growth on cheap carbon sources and higher secretion capacity, make them practical alternative hosts for biotechnological purposes. Currently, the K. phaffii-based expression system is one of the most popular for the production of heterologous proteins. Along with the low secretion of endogenous proteins, K. phaffii efficiently produces and secretes heterologous proteins in high yields, thereby reducing the cost of purifying the latter. This review will discuss practical approaches and technological solutions for the efficient expression of recombinant proteins in K. phaffii, mainly based on the example of enzymes used for the feed industry.

Enzymatic and biophysical characterization of a novel modular cellulosomal GH5 endoglucanase multifunctional from the anaerobic gut fungus Piromyces finnis

Cellulases from anaerobic fungi are enzymes less-studied biochemically and structurally than cellulases from bacteria and aerobic fungi. Currently, only thirteen GH5 cellulases from anaerobic fungi were biochemically characterized and two crystal structures were reported. In this context, here, we report the functional and biophysical characterization of a novel multi-modular cellulosomal GH5 endoglucanase from the anaerobic gut fungus Piromyces finnis (named here PfGH5). Multiple sequences alignments indicate that PfGH5 is composed of a GH5 catalytic domain and a CBM1 carbohydrate-binding module connected through a CBM10 dockerin module. Our results showed that PfGH5 is an endoglucanase from anaerobic fungus with a large spectrum of activity. PfGH5 exhibited preference for hydrolysis of oat β-glucan, followed by galactomannan, carboxymethyl cellulose, mannan, lichenan and barley β-glucan, therefore displaying multi-functionality. For oat β-glucan, PfGH5 reaches its optimum enzymatic activity at 40 °C and pH 5.5, with Km of 7.1 μM. Ion exchange chromatography analyzes revealed the production of oligosaccharides with a wide degree of polymerization indicated that PfGH5 has endoglucanase activity. The ability to bind and cleave different types of carbohydrates evidence the potential of PfGH5 for use in biotechnology and provide a useful basis for future investigation and application of new anaerobic fungi enzymes.

Biochemical characteristics of Myceliophthora thermophila recombinant β-glucosidase (MtBgl3c) applicable in cellulose bioconversion

The GH3 β-glucosidase gene of Myceliophthora thermophila (MtBgl3c) has been cloned and heterologously expressed in E. coli for the first time. This study highlights the important characteristics of recombinant MtBgl3c (rMtBgl3c) which make it a promising candidate in industrial applications. Optimization of the production of rMtBgl3c led to 28,000 U L-1. On purification, it has a molecular mass of ∼100 kDa. It is a broad substrate specific thermostable enzyme that exhibits pH and temperature optima at 5.0 and 55 °C, respectively. The amino acid residues Asp287 and Glu514 act as nucleophile and catalytic acid/base, respectively in the enzyme catalysis. Its low Km value (1.28 mM) indicates a high substrate affinity as compared to those previously reported. The rMtBgl3c displays a synergistic action with the commercial enzyme cocktail in the saccharification of sugarcane bagasse suggesting its utility in the cellulose bioconversion. Tolerance to solvents, detergents as well as glucose make this enzyme applicable in wine, detergent, paper and textile industries too.

Comparative genome analysis of the freshwater fungus Filosporella fistucella indicates potential for plant-litter degradation at cold temperatures

Freshwater fungi play an important role in the decomposition of organic matter of leaf litter in rivers and streams. They also possess the necessary mechanisms to endure lower temperatures caused by habitat and weather variations. This includes the production of cold-active enzymes and antifreeze proteins. To better understand the physiological activities of freshwater fungi in their natural environment, different methods are being applied, and genome sequencing is one in the spotlight. In our study, we sequenced the first genome of the freshwater fungus Filosporella fistucella (45.7 Mb) and compared the genome with the evolutionary close-related species Tricladium varicosporioides (48.2 Mb). The genomes were annotated using the carbohydrate-active enzyme database where we then filtered for leaf-litter degradation-related enzymes (cellulase, hemicellulase, laccase, pectinase, cutinase, amylase, xylanase, and xyloglucanase). Those enzymes were analyzed for antifreeze properties using a machine-learning approach. We discovered that F. fistucella has more enzymes to participate in the breakdown of sugar, leaf, and wood than T. varicosporioides (855 and 719, respectively). Filosporella fistucella shows a larger set of enzymes capable of resisting cold temperatures than T. varicosporioides (75 and 66, respectively). Our findings indicate that in comparison with T. varicosporioides, F. fistucella has a greater capacity for aquatic growth, adaptability to freshwater environments, and resistance to low temperatures.

Structural and thermostability insights into cellobiohydrolase of a thermophilic mould Myceliophthora thermophila: in-silico studies

Cellobiohydrolase (CBH) is one of the cellulases with a wide range of industrial applications; it plays a pivotal role in cellulose hydrolysis and thus in biofuel production. The structural and thermostability analysis of a CBHII of the thermophilic mold Myceliophthora thermophila (MtCel6A) had been carried out using various in-silico approaches. The validation of 3 D model by the Ramachandran plot indicated 88.5% amino acid residues in the favoured regions. Docking analysis suggested MtCel6A to display a high affinity towards cellotetraose as compared to other substrates. The enzyme exhibited a high tolerance to the end product, cellobiose. The thermostability evaluation by molecular dynamic simulations and principal component analysis confirmed its tolerance to elevated temperatures. The identified thermolabile regions could be targeted for site-directed mutagenesis in order to ameliorate thermostability further. Our experimental data published earlier confirmed the present findings of in-silico studies. The structural and functional characteristics of MtCel6A highlighted its critical features that make it a useful biocatalyst in several industrial processes.Communicated by Ramaswamy H. Sarma.

Profiling of the β-glucosidases identified in the genome of Penicillium funiculosum: insights from genomics, transcriptomics, proteomics, and homology-modeling studies

The enzymatic conversion of lignocellulosic biomass to bioethanol depends on efficient enzyme systems with β-glucosidase as one of the key components. In this study, we performed in-depth profiling of the various β-glucosidases present in the genome of the hypercellulolytic fungus Penicillium funiculosum using genomics, transcriptomics, proteomics, and molecular dynamics simulation approaches. Of the eight β-glucosidase genes identified in the P. funiculosum genome, three were predicted to be extracellular based on signal peptide prediction and abundance in the secretome. Among the three secreted β-glucosidases, two belonged to the GH3 family and one belonged to the GH1 family. Homology models of these proteins predicted a deep and narrow active site for the GH3 β-glucosidases (PfBgl3A and PfBgl3B) and a shallow open active site for the GH1 β-glucosidase (PfBgl1A). The enzymatic assays indicated that P. funiculosum-secreted proteins showed high β-glucosidase activities with prominent bands on the 4-methylumbelliferyl β-D-glucopyranoside zymogram. To understand the contributory effects of each of the three secreted β-glucosidases (PfBgls), the corresponding gene was deleted separately, and the effect of the deletion on the β-glucosidase activity of the secretome was examined. Although not the most abundant, PfBgl3A was found to be one of the most important β-glucosidases, as evidenced by a 42% reduction in β-glucosidase activity in the ΔPfBgl3A strain. Our results advance the understanding of the genetic and biochemical nature of all β-glucosidases produced by P. funiculosum and pave the way to design a superior biocatalyst for the hydrolysis of lignocellulosic biomass. IMPORTANCE Commercially available cellulases are primarily produced from Trichoderma reesei. However, external supplementation of the cellulase cocktail from this host with exogenous β-glucosidase is often required to achieve the desired optimal saccharification of cellulosic feedstocks. This challenge has led to the exploration of other cellulase-producing strains. The nonmodel hypercellulolytic fungus Penicillium funiculosum has been studied in recent times and identified as a promising source of industrial cellulases mainly due to its ability to produce a balanced concoction of cellulolytic enzymes, including β-glucosidases. Various genetic interventions targeted at strain improvement for cellulase production have been performed; however, the β-glucosidases of this strain have remained largely understudied. This study, therefore, reports profiling of all eight β-glucosidases of P. funiculosum via molecular and computational approaches. The results of this study provide useful insights that will establish the background for future engineering strategies to transform this fungus into an industrial workhorse.

Recent Advances in β-Glucosidase Sequence and Structure Engineering: A Brief Review

β-glucosidases (BGLs) play a crucial role in the degradation of lignocellulosic biomass as well as in industrial applications such as pharmaceuticals, foods, and flavors. However, the application of BGLs has been largely hindered by issues such as low enzyme activity, product inhibition, low stability, etc. Many approaches have been developed to engineer BGLs to improve these enzymatic characteristics to facilitate industrial production. In this article, we review the recent advances in BGL engineering in the field, including the efforts from our laboratory. We summarize and discuss the BGL engineering studies according to the targeted functions as well as the specific strategies used for BGL engineering.

Inactivation and process intensification of β-glucosidase in biomass utilization

Lignocellulosic biomass has emerged as a promising environmental resource. Enzyme catalysis, as one of the most environmentally friendly and efficient tools among various treatments, is used for the conversion of biomass into chemicals and fuels. Cellulase is a complex enzyme composed of β-glucosidase (BGL), endo-β-1,4-glucanase (EG), and exo-β-1,4-glucanase (CBH), which synergistically hydrolyzes cellulose into monosaccharides. BGL, which further deconstructs cellobiose and short-chain cellooligosaccharides obtained by EG and CBH catalysis into glucose, is the most sensitive component of the synergistic enzyme system constituted by the three enzymes and is highly susceptible to inactivation by external conditions, becoming the rate-limiting component in biomass conversion. This paper firstly introduces the source and catalytic mechanism of BGL used in the process of biomass resource utilization. The focus is on the review of various factors affecting BGL activity during hydrolysis, including competitive adsorption of lignin, gas-liquid interface inactivation, thermal inactivation, and solvent effect. And the methods to improve BGL inactivation are proposed from two aspects-substrate initiation and enzyme initiation. In particular, the screening, modification, and alteration of the enzyme molecules themselves are discussed with emphasis. This review can provide novel ideas for studies of BGL inactivation mechanism, containment of inactivation, and activity enhancement. KEY POINTS: • Factors affecting β-glucosidase inactivation are described. • Process intensification is presented in terms of substrate and enzyme. • Solvent selection, protein engineering, and immobilization remain topics of interest.

Sustainable and Green Production of Nanostructured Cellulose by a 2-Step Mechano-Enzymatic Process

Nanostructured cellulose (NC) represents an emerging sustainable biomaterial for diverse biotechnological applications; however, its production requires hazardous chemicals that render the process ecologically unfriendly. Using commercial plant-derived cellulose, an innovative strategy for NC production based on the combination of mechanical and enzymatic approaches was proposed as a sustainable alternative to conventional chemical procedures. After ball milling, the average length of the fibers was reduced by one order of magnitude (down to 10-20 μm) and the crystallinity index decreased from 0.54 to 0.07-0.18. Moreover, a 60 min ball milling pre-treatment followed by 3 h Cellic Ctec2 enzymatic hydrolysis led to NC production (15% yield). Analysis of the structural features of NC obtained by the mechano-enzymatic process revealed that the diameters of the obtained cellulose fibrils and particles were in the range of 200-500 nm and approximately 50 nm, respectively. Interestingly, the film-forming property on polyethylene (coating ≅ 2 μm thickness) was successfully demonstrated and a significant reduction (18%) of the oxygen transmission rate was obtained. Altogether, these findings demonstrated that nanostructured cellulose could be successfully produced using a novel, cheap, and rapid 2-step physico-enzymatic process that provides a potential green and sustainable route that could be exploitable in future biorefineries.

Archaea as a Model System for Molecular Biology and Biotechnology

Archaea represents the third domain of life, displaying a closer relationship with eukaryotes than bacteria. These microorganisms are valuable model systems for molecular biology and biotechnology. In fact, nowadays, methanogens, halophiles, thermophilic euryarchaeota, and crenarchaeota are the four groups of archaea for which genetic systems have been well established, making them suitable as model systems and allowing for the increasing study of archaeal genes' functions. Furthermore, thermophiles are used to explore several aspects of archaeal biology, such as stress responses, DNA replication and repair, transcription, translation and its regulation mechanisms, CRISPR systems, and carbon and energy metabolism. Extremophilic archaea also represent a valuable source of new biomolecules for biological and biotechnological applications, and there is growing interest in the development of engineered strains. In this review, we report on some of the most important aspects of the use of archaea as a model system for genetic evolution, the development of genetic tools, and their application for the elucidation of the basal molecular mechanisms in this domain of life. Furthermore, an overview on the discovery of new enzymes of biotechnological interest from archaea thriving in extreme environments is reported.

Insight into CAZymes of Alicyclobacillus mali FL18: Characterization of a New Multifunctional GH9 Enzyme

In the bio-based era, cellulolytic and hemicellulolytic enzymes are biocatalysts used in many industrial processes, playing a key role in the conversion of recalcitrant lignocellulosic waste biomasses. In this context, many thermophilic microorganisms are considered as convenient sources of carbohydrate-active enzymes (CAZymes). In this work, a functional genomic annotation of Alicyclobacillus mali FL18, a recently discovered thermo-acidophilic microorganism, showed a wide reservoir of putative CAZymes. Among them, a novel enzyme belonging to the family 9 of glycosyl hydrolases (GHs), named AmCel9, was identified; in-depth in silico analyses highlighted that AmCel9 shares general features with other GH9 members. The synthetic gene was expressed in Escherichia coli and the recombinant protein was purified and characterized. The monomeric enzyme has an optimal catalytic activity at pH 6.0 and has comparable activity at temperatures ranging from 40 °C to 70 °C. It also has a broad substrate specificity, a typical behavior of multifunctional cellulases; the best activity is displayed on β-1,4 linked glucans. Very interestingly, AmCel9 also hydrolyses filter paper and microcrystalline cellulose. This work gives new insights into the properties of a new thermophilic multifunctional GH9 enzyme, that looks a promising biocatalyst for the deconstruction of lignocellulose.

Recent progress in key lignocellulosic enzymes: Enzyme discovery, molecular modifications, production, and enzymatic biomass saccharification

Lignocellulose, the most prevalent biomass on earth, can be enzymatically converted into carbohydrates for bioethanol production and other uses. Among lignocellulosic enzymes, endoglucanase, xylanase, and laccase are the key enzymes, owing to their ability to disrupt the main structure of lignocellulose. Recently, new discovery methods have been established to obtain key lignocellulosic enzymes with excellent enzymatic properties. Molecular modification of enzymes to modulate their thermostability, catalytic activity, and substrate specificity has been performed with protein engineering technology. In addition, the enzyme expression has been effectively improved through expression element screening and host modification, as well as fermentation optimization. Immobilization of enzymes, use of surfactants, synergistic degradation, and optimization of reaction conditions have addressed the inefficiency of enzymatic saccharification. In this review, recent advances in key lignocellulosic enzymes are summarized, along with future prospects for the development of super-engineered strains and integrative technologies for enzymatic biomass saccharification.

Synergistic hydrolysis of filter paper by recombinant cellulase cocktails leveraging a key cellobiase, Cba2, of Cellulomonas biazotea

Cellulomonas biazotea, a Gram-positive cellulolytic bacterium isolated from soil, is capable of producing a complete cellulase complex exhibiting endoglucanase, exoglucanase, and cellobiase activities. Despite the presence of a full complement of all three types of cellulases, samples prepared from both cell lysates and culture media of C. biazotea showed only weak synergistic activities formed among the cellulase components, as reflected by their inefficient performance in filter paper hydrolysis. However, when the five previously characterized recombinant cellobiases of C. biazotea were mixed individually or in different combinations with recombinant enzyme preparations (CenA/Cex) containing an endoglucanase, CenA, and an exoglucanase, Cex, of another Cellulomonas species, C. fimi, the cellulase cocktails exhibited not only much higher but also synergistic activities in filter paper hydrolysis. Among the 5 C. biazotea cellobiases studied, Cba2 was shown to perform 2.8 to 3.8 times better than other homologous isozymes when acting individually with CenA/Cex. More noteworthy is that when Cba2 and Cba4 were added together to the reaction mixture, an even better synergistic effect was achieved. The filter paper activities resulting from Cba2 and Cba4 interacting with CenA/Cex are comparable to those obtained from some commercial fungal cellulase mixtures. To our knowledge, our results represent the first demonstration of synergistic effects on filter paper hydrolysis achieved using recombinant bacterial cellulases.

Enhancement of thermostability and catalytic properties of ammonia lyase through disulfide bond construction and backbone cyclization

Ammonia lyases have great application potential in food and pharmaceuticals owing to their unique ammonia addition reaction and atom economy. A novel methylaspartate ammonia-lyase, EcMAL, from E. coli O157:H7 showed high catalytic activity. To further strengthen its thermostability and activity, disulfide bond and backbone cyclization (cyclase) variants were constructed by rational design, respectively. Among them, variant M3, with a disulfide bond introduced, exhibited a 2.3-fold increase in half-life at 50 °C, while cyclase variant M8 showed better performance, with 25.9-fold increases. The synergistic promotion effect of this combinational strategy on activity and stability was also investigated, and the combined mutant M9 exhibited a 1.1-fold improvement in catalytic efficiency while maintaining good thermostability. Circular dichroism analysis and molecular dynamics simulation confirmed that the main sources of improved thermostability were reduced atomic fluctuation and a more stable secondary structure. To our knowledge, this is the first example of combining the introduction of disulfide bonds with cyclase construction to improve enzyme stability, which was characterized by modification away from the enzyme active center, and provided a new method for adjusting enzyme thermostability.

Coupled Biohydrogen Production and Bio-Nanocatalysis for Dual Energy from Cellulose: Towards Cellulosic Waste Up-Conversion into Biofuels

Hydrogen, an emergent alternative energy vector to fossil fuels, can be produced sustainably by fermentation of cellulose following hydrolysis. Fermentation feedstock was produced hydrolytically using hot compressed water. The addition of CO2 enhanced hydrolysis by ~26% between 240 and 260 °C with comparable hydrolysis products as obtained under N2 but at a 10 °C lower temperature. Co-production of inhibitory 5-hydromethyl furfural was mitigated via activated carbon sorption, facilitating fermentative biohydrogen production from the hydrolysate by Escherichia coli. Post-fermentation E. coli cells were recycled to biomanufacture supported Pd/Ru nanocatalyst to up-convert liquid-extracted 5-HMF to 2,5-dimethyl furan, a precursor of ‘drop in’ liquid fuel, in a one-pot reaction. This side stream up-valorisation mitigates against the high ‘parasitic’ energy demand of cellulose bioenergy, potentially increasing process viability via the coupled generation of two biofuels. This is discussed with respect to example data obtained via a hydrogen biotechnology with catalytic side stream up-conversion from cellulose feedstock.

Expression and characterization of a novel lytic polysaccharide monooxygenase, PdLPMO9A, from the edible fungus Pleurotus djamor and its synergistic interactions with cellulase in corn straw biomass saccharification

Lytic polysaccharide monooxygenases play a unique role in biomass saccharification. A novel gene, PdLPMO9A, from Pleurotus djamor, was reported to be upregulated during the degradation of corn straw in our previous research. However, very little information is available on PdLPMO9A. Therefore, PdLPMO9A was heterologously expressed in Pichia pastoris, and biophysical characterisitics of the recombinant protein PdLPMO9A were investigated; it was shown to have superior thermostability and pH stability. PdLPMO9A markedly improved the cellulase-mediated saccharification of corn straw, when the dosage of PdLPMO9A was 0.66 mg/g corn straw and hydrolysis time was 48 h. When CuSO₄ was added at a concentration of 0.1 mM, glucose yield rose by a further 28.16%. In light of these findings, it was concluded that PdLPMO9A has the potential to function as an essential component of a cellulase cocktail capable of ensuring the saccharification of corn straw biomass.

Improving the Substrate Affinity and Catalytic Efficiency of β-Glucosidase Bgl3A from Talaromyces leycettanus JCM12802 by Rational Design

Improving the substrate affinity and catalytic efficiency of β-glucosidase is necessary for better performance in the enzymatic saccharification of cellulosic biomass because of its ability to prevent cellobiose inhibition on cellulases. Bgl3A from Talaromyces leycettanus JCM12802, identified in our previous work, was considered a suitable candidate enzyme for efficient cellulose saccharification with higher catalytic efficiency on the natural substrate cellobiose compared with other β-glucosidase but showed insufficient substrate affinity. In this work, hydrophobic stacking interaction and hydrogen-bonding networks in the active center of Bgl3A were analyzed and rationally designed to strengthen substrate binding. Three vital residues, Met36, Phe66, and Glu168, which were supposed to influence substrate binding by stabilizing adjacent binding site, were chosen for mutagenesis. The results indicated that strengthening the hydrophobic interaction between stacking aromatic residue and the substrate, and stabilizing the hydrogen-bonding networks in the binding pocket could contribute to the stabilized substrate combination. Four dominant mutants, M36E, M36N, F66Y, and E168Q with significantly lower K_m values and 1.4–2.3-fold catalytic efficiencies, were obtained. These findings may provide a valuable reference for the design of other β-glucosidases and even glycoside hydrolases.

Recombinant cellobiohydrolase of Myceliophthora thermophila: characterization and applicability in cellulose saccharification

A codon optimized cellobiohydrolase (CBH) encoding synthetic gene of 1188 bp from a thermophilic mold Myceliophthora thermophila (MtCel6A) was cloned and heterologously expressed in Escherichia coli for the first time. In silico analysis suggested that MtCel6A is a GH6 CBH and belongs to CBHII family, which is structurally similar to Cel6A of Humicola insolens. The recombinant MtCel6A is expressed as active inclusion bodies, and the molecular mass of the purified enzyme is ~ 45 kDa. The rMtCel6A is active in a wide range of pH (4-12) and temperatures (40-100 °C) with optima at pH 10.0 and 60 °C. It exhibits T1/2 of 6.0 and 1.0 h at 60 and 90 °C, respectively. The rMtCel6A is an extremozyme with organic solvent, salt and alkali tolerance. The Km, Vmax, kcat and kcat/Km values of the enzyme are 3.2 mg mL-1, 222.2 μmol mg-1 min-1, 2492 s-1 and 778.7 s-1 mg-1 mL-1, respectively. The product analysis of rMtCel6A confirmed that it is an exoenzyme that acts from the non-reducing end of cellulose. The addition of rMtCel6A to the commercial cellulase mix (Cellic CTec2) led to 1.9-fold increase in saccharification of the pre-treated sugarcane bagasse. The rMtCel6A is a potential CBH that finds utility in industrial processes such as in bioethanol, paper pulp and textile industries.

Glycoside Hydrolases and Glycosyltransferases from Hyperthermophilic Archaea: Insights on Their Characteristics and Applications in Biotechnology

Hyperthermophilic Archaea colonizing unnatural habitats of extremes conditions such as volcanoes and deep-sea hydrothermal vents represent an unmeasurable bioresource for enzymes used in various industrial applications. Their enzymes show distinct structural and functional properties and are resistant to extreme conditions of temperature and pressure where their mesophilic homologs fail. In this review, we will outline carbohydrate-active enzymes (CAZymes) from hyperthermophilic Archaea with specific focus on the two largest families, glycoside hydrolases (GHs) and glycosyltransferases (GTs). We will present the latest advances on these enzymes particularly in the light of novel accumulating data from genomics and metagenomics sequencing technologies. We will discuss the contribution of these enzymes from hyperthermophilic Archaea to industrial applications and put the emphasis on newly identifed enzymes. We will highlight their common biochemical and distinct features. Finally, we will overview the areas that remain to be explored to identify novel promising hyperthermozymes.