Coevolution of both Thermostability and Activity of Polyphosphate Glucokinase from Thermobifida fusca YX

ATP-free in vitro biotransformation of starch-derived maltodextrin into poly-3-hydroxybutyrate via acetyl-CoA

In vitro biotransformation (ivBT) facilitated by in vitro synthetic enzymatic biosystems (ivSEBs) has emerged as a highly promising biosynthetic platform. Several ivSEBs have been constructed to produce poly-3-hydroxybutyrate (PHB) via acetyl-coenzyme A (acetyl-CoA). However, some systems are hindered by their reliance on costly ATP, limiting their practicality. This study presents the design of an ATP-free ivSEB for one-pot PHB biosynthesis via acetyl-CoA utilizing starch-derived maltodextrin as the sole substrate. Stoichiometric analysis indicates this ivSEB can self-maintain NADP+/NADPH balance and achieve a theoretical molar yield of 133.3%. Leveraging simple one-pot reactions, our ivSEBs achieved a near-theoretical molar yield of 125.5%, the highest PHB titer (208.3 mM, approximately 17.9 g/L) and the fastest PHB production rate (9.4 mM/h, approximately 0.8 g/L/h) among all the reported ivSEBs to date, and demonstrated easy scalability. This study unveils the promising potential of ivBT for the industrial-scale production of PHB and other acetyl-CoA-derived chemicals from starch.

High-Throughput Screening Techniques for the Selection of Thermostable Enzymes

The acquisition of a thermostable enzyme is an indispensable prerequisite for its successful implementation in industrial applications and the development of novel functionalities. Various protein engineering approaches, including rational design, semirational design, and directed evolution, have been employed to enhance thermostability. However, all of these approaches require sensitive and reliable high-throughput screening (HTS) technologies to efficiently and rapidly identify variants with improved properties. While numerous reviews focus on modification strategies for enhancing enzyme thermostability, there is a dearth of literature reviewing HTS methods specifically aimed at this objective. Herein, we present a comprehensive overview of various HTS methods utilized for modifying enzyme thermostability across different screening platforms. Additionally, we highlight significant recent examples that demonstrate the successful application of these methods. Furthermore, we address the technical challenges associated with HTS technologies used for screening thermostable enzyme variants and discuss valuable perspectives to promote further advancements in this field. This review serves as an authoritative reference source offering theoretical support for selecting appropriate screening strategies tailored to specific enzymes with the aim of improving their thermostability.

Recent advances in simultaneous thermostability-activity improvement of industrial enzymes through structure modification

Engineered thermostable microbial enzymes are widely employed to catalyze chemical reactions in numerous industrial sectors. Although high thermostability is a prerequisite of industrial applications, enzyme activity is usually sacrificed during thermostability improvement. Therefore, it is vital to select the common and compatible strategies between thermostability and activity improvement to reduce mutants̕ libraries and screening time. Three functional protein engineering approaches, including directed evolution, rational design, and semi-rational design, are employed to manipulate protein structure on a genetic basis. From a structural standpoint, integrative strategies such as increasing substrate affinity; introducing electrostatic interaction; removing steric hindrance; increasing flexibility of the active site; N- and C-terminal engineering; and increasing intramolecular and intermolecular hydrophobic interactions are well-known to improve simultaneous activity and thermostability. The current review aims to analyze relevant strategies to improve thermostability and activity simultaneously to circumvent the thermostability and activity trade-off of industrial enzymes.

Bioelectrocatalysis for CO2 reduction: recent advances and challenges to develop a sustainable system for CO2 utilization

Activation and turning CO₂ into value added products is a promising orientation to address environmental issues caused by CO₂ emission. Currently, electrocatalysis has a potent well-established role for CO₂ reduction with fast electron transfer rate; but it is challenged by the poor selectivity and low faradic efficiency. On the other side, biocatalysis, including enzymes and microbes, has been also employed for CO₂ conversion to target C_n products with remarkably high selectivity; however, low solubility of CO₂ in the liquid reaction phase seriously affects the catalytic efficiency. Therefore, a new synergistic role in bioelectrocatalysis for CO₂ reduction is emerging thanks to its outstanding selectivity, high faradic efficiency, and desirable valuable C_n products under mild condition that are surveyed in this review. Herein, we comprehensively discuss the results already obtained for the integration craft of enzymatic-electrocatalysis and microbial-electrocatalysis technologies. In addition, the intrinsic nature of the combination is highly dependent on the electron transfer. Thus, both direct electron transfer and mediated electron transfer routes are modeled and concluded. We also explore the biocompatibility and synergistic effects of electrode materials, which emerge in combination with tuned enzymes and microbes to improve catalytic performance. The system by integrating solar energy driven photo-electrochemical technics with bio-catalysis is further discussed. We finally highlight the significant findings and perspectives that have provided strong foundations for the remarkable development of green and sustainable bioelectrocatalysis for CO₂ reduction, and that offer a blueprint for C_n valuable products originate from CO₂ under efficient and mild conditions.

Biomanufacturing by In Vitro Biotransformation (ivBT) Using Purified Cascade Multi-enzymes

In vitro biotransformation (ivBT) refers to the use of an artificial biological reaction system that employs purified enzymes for the one-pot conversion of low-cost materials into biocommodities such as ethanol, organic acids, and amino acids. Unshackled from cell growth and metabolism, ivBT exhibits distinct advantages compared with metabolic engineering, including but not limited to high engineering flexibility, ease of operation, fast reaction rate, high product yields, and good scalability. These characteristics position ivBT as a promising next-generation biomanufacturing platform. Nevertheless, challenges persist in the enhancement of bulk enzyme preparation methods, the acquisition of enzymes with superior catalytic properties, and the development of sophisticated approaches for pathway design and system optimization. In alignment with the workflow of ivBT development, this chapter presents a systematic introduction to pathway design, enzyme mining and engineering, system construction, and system optimization. The chapter also proffers perspectives on ivBT development.

Enhanced Thermal Stability of Polyphosphate-Dependent Glucomannokinase by Directed Evolution

Polyphosphate-dependent glucomannokinase (PPGMK) is able to utilize inorganic polyphosphate to synthesize mannose-6-phosphate (M6P) instead of highly costly ATP. This enzyme was modified and designed by combining error-prone PCR (EP-PCR) and site-directed saturation mutagenesis. Two mutants, H92L/A138V and E119V, were screened out from the random mutation library, and we used site-specific saturation mutations to find the optimal amino acid at each site. Finally, we found the optimal combination mutant, H92K/E119R. The thermal stability of H92K/E119R increased by 5.4 times at 50 °C, and the half-life at 50 °C increased to 243 min. Moreover, the enzyme activity of H92K/E119R increased to 16.6 U/mg, and its enzyme activity is twice that of WT. We analyzed the structure of the mutant using molecular dynamics simulation. We found that the shortening of the hydrogen bond distance and the formation of salt bridges can firmly connect the α-helix and β-sheet and improve the stability of the PPGMK structure.

Thermostability engineering of industrial enzymes through structure modification

Thermostability is an essential requirement of enzymes in the industrial processes to catalyze the reactions at high temperatures; thus, enzyme engineering through directed evolution, semi-rational design and rational design are commonly employed to construct desired thermostable mutants. Several strategies are implemented to fulfill enzymes' thermostability demand including decreasing the entropy of the unfolded state through substitutions Gly → Xxx or Xxx → Pro, hydrogen bond, salt bridge, introducing two different simultaneous interactions through single mutant, hydrophobic interaction, filling the hydrophobic cavity core, decreasing surface hydrophobicity, truncating loop, aromatic-aromatic interaction and introducing positively charged residues to enzyme surface. In the current review, horizons about compatibility between secondary structures and substitutions at preferable structural positions to generate the most desirable thermostability in industrial enzymes are broadened. KEY POINTS: • Protein engineering is a powerful tool for generating thermostable industrial enzymes. • Directed evolution and rational design are practical approaches in enzyme engineering. • Substitutions in preferable structural positions can increase thermostability.

Stoichiometric Conversion of Maltose for Biomanufacturing by In Vitro Synthetic Enzymatic Biosystems

Maltose is a natural α-(1,4)-linked disaccharide with wide applications in food industries and microbial fermentation. However, maltose has scarcely been used for in vitro biosynthesis, possibly because its phosphorylation by maltose phosphorylase (MP) yields β-glucose 1-phosphate (β-G1P) that cannot be utilized by α-phosphoglucomutase (α-PGM) commonly found in in vitro synthetic enzymatic biosystems previously constructed by our group. Herein, we designed an in vitro synthetic enzymatic reaction module comprised of MP, β-phosphoglucomutase (β-PGM), and polyphosphate glucokinase (PPGK) for the stoichiometric conversion of each maltose molecule to two glucose 6-phosphate (G6P) molecules. Based on this synthetic module, we further constructed two in vitro synthetic biosystems to produce bioelectricity and fructose 1,6-diphosphate (FDP), respectively. The 14-enzyme biobattery achieved a Faraday efficiency of 96.4% and a maximal power density of 0.6 mW/cm2, whereas the 5-enzyme in vitro FDP-producing biosystem yielded 187.0 mM FDP from 50 g/L (139 mM) maltose by adopting a fed-batch substrate feeding strategy. Our study not only suggests new application scenarios for maltose but also provides novel strategies for the high-efficient production of bioelectricity and value-added biochemicals.

Engineering substrate specificity of HAD phosphatases and multienzyme systems development for the thermodynamic-driven manufacturing sugars

Naturally, haloacid dehalogenase superfamily phosphatases have been evolved with broad substrate promiscuity; however, strong specificity to a particular substrate is required for developing thermodynamically driven routes for manufacturing sugars. How to alter the intrinsic substrate promiscuity of phosphatases and fit the “one enzyme-one substrate” model remains a challenge. Herein, we report the structure-guided engineering of a phosphatase, and successfully provide variants with tailor-made preference for three widespread phosphorylated sugars, namely, glucose 6-phosphate, fructose 6-phosphate, and mannose 6-phosphate, while simultaneously enhancement in catalytic efficiency. A 12000-fold switch from unfavorite substrate to dedicated one is generated. Molecular dynamics simulations reveal the origin of improved activity and substrate specificity. Furthermore, we develop four coordinated multienzyme systems and accomplish the conversion of inexpensive sucrose and starch to fructose and mannose in excellent yield of 94–96%. This innovative sugar-biosynthesis strategy overcomes the reaction equilibrium of isomerization and provides the promise of high-yield manufacturing of other monosaccharides and polyols.

Haloacid dehalogenase-like phosphatases are widespread across all domains of life and play a crucial role in the regulation of levels of sugar phosphate metabolites in cells. The authors report on the structure-guided engineering of phosphatases for dedicated substrate specificity for the conversion of sucrose and starch into fructose and mannose.

Engineering of a thermophilic dihydroxy-acid dehydratase toward glycerate dehydration for in vitro biosystems

Dihydroxy-acid dehydratase (DHAD) plays an important role in the utilization of glycerol or glucose for the production of value-added chemicals in the in vitro synthetic enzymatic biosystem. The low activity of DHAD in the dehydration of glycerate to pyruvate hampers its applications in biosystems. Protein engineering of a thermophilic DHAD from Sulfolobus solfataricus (SsDHAD) was performed to increase its dehydration activity. A triple mutant (I161M/Y145S/G205K) with a 10-fold higher activity on glycerate dehydration was obtained after three rounds of iterative saturation mutagenesis (ISM) based on computational analysis. The shrunken substrate-binding pocket and newly formed hydrogen bonds were the reason for the activity improvement of the mutant. For the in vitro synthetic enzymatic biosystems of converting glucose or glycerol to L-lactate, the biosystems with the mutant SsDHAD showed 3.32- and 2.34-fold higher reaction rates than the wild type, respectively. This study demonstrates the potential of protein engineering to improve the efficiency of in vitro synthetic enzymatic biosystems by enhancing the enzyme activity of rate-limited enzymes. KEY POINTS: • A screening method was established for the protein engineering of SsDHAD. • A R3 mutant of SsDHAD with 10-fold higher activity was obtained. • The R3 mutant exhibits higher productivity in the in vitro biosystems.

Biomass sugar-powered enzymatic fuel cells based on a synthetic enzymatic pathway

Biomass stores a tremendous amount of chemical energy and is considered as an abundant and sustainable alternative to fossil fuels. However, the use of biomass to produce mW-level electricity for portable devices suffers from its structural complexity and therefore low energy conversion efficiency. In this study, we design an enzymatic pathway that could co-utilize and completely oxidize glucose and xylose from biomass hydrolysate to achieve high energy density in EFC. Faraday efficiency of 92% and maximum power density of 0.14 mW cm^-2 are achieved in this EFC. After the systematically optimization of enzyme loading and temperature as well as the removal of enzyme inhibitor from biomass hydrolysate by activated charcoal, the biomass sugar-powered EFC could reach a maximum power density of 0.5 mW cm^-2 and remain 60% of the initial value after 10 days. These results offer a feasible way to extract the energy stored in biomass as much as possible without the side effects of biomass hydrolysate on EFC.

Production of Glucose 6-Phosphate From a Cellulosic Feedstock in a One Pot Multi-Enzyme Synthesis

Glucose 6-phosphate is the phosphorylated form of glucose and is used as a reagent in enzymatic assays. Current production occurs via a multi-step chemical synthesis. In this study we established a fully enzymatic route for the synthesis of glucose 6-phosphate from cellulose. As the enzymatic phosphorylation requires ATP as phosphoryl donor, the use of a cofactor regeneration system is required. We evaluated Escherichia coli glucokinase and Saccharomyces cerevisiae hexokinase (HK) for the phosphorylation reaction and Pseudomonas aeruginosa polyphosphate kinase 2 (PPK2) for ATP regeneration. All three enzymes were characterized in terms of temperature and pH optimum and the effects of substrates and products concentrations on enzymatic activities. After optimization of the conditions, we achieved a 85% conversion of glucose into glucose 6-phosphate using the HK/PPK2 activities within a 24 h reaction resulting in 12.56 g/l of glucose 6-phosphate. Finally, we demonstrated the glucose 6-phosphate formation from microcrystalline cellulose in a one-pot reaction comprising Aspergillus niger cellulase for glucose release and HK/PPK2 activities. We achieved a 77% conversion of released glucose into glucose 6-phosphate, however at the expense of a lower glucose 6-phosphate yield of 1.17 g/l. Overall, our study shows an alternative approach for synthesis of glucose 6-phosphate that can be used to valorize biomass derived cellulose.

An ATP-free in vitro synthetic enzymatic biosystem facilitating one-pot stoichiometric conversion of starch to mannitol

D-Mannitol (hereinafter as mannitol) is a six-carbon sugar alcohol with diverse applications in food and pharmaceutical industries. To overcome the drawbacks of the chemical hydrogenation method commonly used for mannitol production at present, there is a need to search for novel prospective mannitol production strategies that are of high yield and low cost. In this study, we present a novel approach for the stoichiometric synthesis of mannitol via an in vitro synthetic enzymatic biosystem using the low-cost starch as substrate. By dividing the overall reaction pathway into three modules which could be executed sequentially in one pot, our design aimed at the stoichiometric conversion of starch-based materials into mannitol in an ATP-independent and cofactor-balanced manner. At optimized conditions, high product yields of around 95-98% were achieved using both 10 g/L and 50 g/L maltodextrin as substrate, indicating the potential of our designed system for industrial applications. This study not only provides a high-efficient strategy for the synthesis of mannitol but also expands the product scope of sugar alcohols by the in vitro synthetic enzymatic biosystems using low-cost starch-based materials as the input. KEY POINTS : • We described a design-build-test-learn pipeline to construct in vitro biosystems. • The designed system comprised six key enzymes and another three enzymes. • The system converted maltodextrin stoichiometrically to mannitol in one pot.

Enzymatic regeneration and conservation of ATP: challenges and opportunities

Adenosine triphosphate (ATP), the universal energy currency of life, has a central role in numerous biochemical reactions with potential for the synthesis of numerous high-value products. ATP can be regenerated by three types of mechanisms: substrate level phosphorylation, oxidative phosphorylation, and photophosphorylation. Current ATP regeneration methods are mainly based on substrate level phosphorylation catalyzed by one enzyme, several cascade enzymes, or in vitro synthetic enzymatic pathways. Among them, polyphosphate kinases and acetate kinase, along with their respective phosphate donors, are the most popular approaches for in vitro ATP regeneration. For in vitro artificial pathways, either ATP-free or ATP-balancing strategies can be implemented via smart pathway design by choosing ATP-independent enzymes. Also, we discuss some remaining challenges and suggest perspectives, especially for industrial biomanufacturing. Development of ATP regeneration systems featuring low cost, high volumetric productivity, long lifetime, flexible compatibility, and great robustness could be one of the bottom-up strategies for cascade biocatalysis and in vitro synthetic biology.

Artificially designed routes for the conversion of starch to value-added mannosyl compounds through coupling in vitro and in vivo metabolic engineering strategies

Starch/cellulose has become the major feedstock for manufacturing biofuels and biochemicals because of their abundance and sustainability. In this study, we presented an artificially designed "starch-mannose-fermentation" biotransformation process through coupling the advantages of in vivo and in vitro metabolic engineering strategies together. Starch was initially converted into mannose via an in vitro metabolic engineering biosystem, and then mannose was fermented by engineered microorganisms for biomanufacturing valuable mannosyl compounds. The in vitro metabolic engineering biosystem based on phosphorylation/dephosphorylation reactions was thermodynamically favorable and the conversion rate reached 81%. The mannose production using whole-cell biocatalysts reached 75.4 g/L in a 30-L reactor, indicating the potential industrial application. Furthermore, the produced mannose in the reactor was directly served as feedstock for the fermentation process to bottom-up produced 19.2 g/L mannosyl-oligosaccharides (MOS) and 7.2 g/L mannosylglycerate (MG) using recombinant Corynebacterium glutamicum strains. Notably, such a mannose fermentation process facilitated the synthesis of MOS, which has not been achieved under glucose fermentation and improved MG production by 2.6-fold than that using the same C-mole of glucose. This approach also allowed access to produce other kinds of mannosyl derivatives from starch.

High-efficiency transformation of archaea by direct PCR products with its application to directed evolution of a thermostable enzyme

Hyperthermophilic archaea with unique biochemical and physiological characteristics are important organisms for fundamental research of life science and have great potential for biotechnological applications. However, low transformation efficiency of foreign DNA molecules impedes developments in genetic modification tools and industrial applications. In this study, we applied prolonged overlap extension PCR (POE-PCR) to generate multimeric DNA molecules and then transformed them into two hyperthermophilic archaea, Thermococcus kodakarensis KOD1 and Pyrococcus yayanosii A1. This study was the first example to demonstrate the enhanced transformation efficiencies of POE-PCR products by a factor of approximately 100 for T. kodakarensis KOD1 and 8 for P. yayanosii A1, respectively, relative to circular shuttle plasmids. Furthermore, directed evolution of a modestly thermophilic enzyme, Methanothermococcus okinawensis 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), was conducted to obtain more stable ones due to high transformation efficiency of T. kodakarensis (i.e. ~3 × 10⁴  CFU per μg DNA). T. kodakarensis harbouring the most thermostable MoHMGR mutant can grow in the presence of a thermostable antibiotic simvastatin at 85°C and even higher temperatures. This high transformation efficiency technique could not only help develop more hyperthermophilic enzyme mutants via directed evolution but also simplify genetical modification of archaea, which could be novel hosts for industrial biotechnology.

Engineering of β-Glucosidase Bgl15 with Simultaneously Enhanced Glucose Tolerance and Thermostability To Improve Its Performance in High-Solid Cellulose Hydrolysis

In this study, a Petri-dish-based double-layer high-throughput screening method was established to improve glucose tolerance of β-glucosidase Bgl15. Two beneficial mutations were identified, and the joint mutant 2R1 improved the half-maximal inhibitory concentration of glucose from 0.04 to 2.1 M. The crystal structure of 2R1 was subsequently determined at 2.7 Å. Structure analysis revealed that enhancement of glucose tolerance may be due to improved transglycosylation activity made possible by a hydrophobic binding site for glucose as an acceptor and more stringent control of a putative water channel. To further ameliorate the application potential of the enzyme, it was engineered to increase the half-life at 50 °C from 0.8 h (Bgl15) to 180 h (mutant 5R1). Furthermore, supplementation of 5R1 to the cellulase cocktail significantly improved glucose production from pretreated sugar cane bagasse by 38%. Consequently, this study provided an efficient approach to enhance glucose tolerance and generated a promising catalyst for cellulose saccharification.

Efficient Multi-Enzymes Immobilized on Porous Microspheres for Producing Inositol From Starch

In vitro synthetic enzymatic biosystem is considered to be the next generation of biomanufacturing platform. This biosystem contains multiple enzymes for the implementation of complicated biotransformatiom. However, the hard-to-reuse and instability of multiple enzymes limit the utilization of this biosystem in industrial process. Multi-enzyme immobilization might be a feasible alternative to address these problems. Herein, porous microspheres are used as carriers to co-immobilize multiple enzymes for producing inositol from starch. At first, all the enzymes (i.e., α-glucan phosphorylase aGP, phosphoglucose mutase PGM, inositol 1-phosphate synthase IPS, and inositol monophosphatase IMP) for converting starch to inositol were immobilized on porous microspheres individually to check the effect of immobilization, then all the enzymes are co-immobilized on porous microspheres. Compared to reaction system containing all the individual immobilized enzymes, the reaction system containing the co-immobilized enzymes exhibit ∼3.5 fold of reaction rate on producing inositol from starch. This reaction rate is comparable to that by free enzyme mixture. And the co-immobilized multi-enzyme system show higher thermal stability and recovery stability than free enzyme mixture. After 7 batches, the immobilized enzymes retain 45.6% relative yield, while the free enzyme mixture only retain 13.3% relative yield after 3 batches. Co-immobilization of multiple enzymes on porous microspheres for biomanufacturing would shed light on the application of in vitro synthetic enzymatic biosystem in industrial scale.

A High-Throughput Method for Directed Evolution of NAD(P)+-Dependent Dehydrogenases for the Reduction of Biomimetic Nicotinamide Analogues

Engineering flavin-free NAD(P)⁺-dependent dehydrogenases to reduce biomimetic nicotinamide analogues (mNAD⁺s) is of importance for eliminating the need for costly NAD(P)⁺ in coenzyme regeneration systems. Current redox dye-based screening methods for engineering the mNAD⁺ specificity of dehydrogenases are frequently encumbered by a background signal from endogenous NAD(P) and intracellular reducing compounds, making the detection of low mNAD⁺-based activities a limiting factor for directed evolution. Here, we develop a high-throughput screening method, NAD(P)-eliminated solid-phase assay (NESPA), which can reliably identify mNAD⁺-active mutants of dehydrogenases with a minimal background signal. This method involves (1) heat lysis of colonies to permeabilize the cell membrane, (2) colony transfer onto filter paper, (3) washing to remove endogenous NAD(P) and reducing compounds, (4) enzyme-coupled assay for mNADH-dependent color production, and (5) digital imaging of colonies to identify mNAD⁺-active mutants. This method was used to improve the activity of 6-phosphogluconate dehydrogenase on nicotinamide mononucleotide (NMN⁺). The best mutant obtained after six rounds of directed evolution exhibits a 50-fold enhancement in catalytic efficiency (k _cat/K _M) and a specific activity of 17.7 U/mg on NMN⁺, which is comparable to the wild-type enzyme on its natural coenzyme, NADP⁺. The engineered dehydrogenase was then used to construct an NMNH regeneration system to drive an ene-reductase catalysis. A comparable level of turnover frequency and product yield was observed using the engineered system relative to NADPH regeneration by using the wild-type dehydrogenase. NESPA provides a simple and accurate readout of mNAD⁺-based activities and the screening at high-throughput levels (approximately tens of thousands per round), thus opening up an avenue for the evolution of dehydrogenases with specific activities on mNAD⁺s similar to the levels of natural enzyme/coenzyme pairs.

Effects of Random Mutagenesis and In Vivo Selection on the Specificity and Stability of a Thermozyme

Factors that give enzymes stability, activity, and substrate recognition result from the combination of few weak molecular interactions, which can be difficult to study through rational protein engineering approaches. We used irrational random mutagenesis and in vivo selection to test if a β-glycosidase from the thermoacidophile Saccharolobus solfataricus (Ssβ-gly) could complement an Escherichia coli strain unable to grow on lactose. The triple mutant of Ssβ-gly (S26L, P171L, and A235V) was more active than the wild type at 85 °C, inactivated at this temperature almost 300-fold quicker, and showed a 2-fold higher kcat on galactosides. The three mutations, which were far from the active site, were analyzed to test their effect at the structural level. Improved activity on galactosides was induced by the mutations. The S26L and P171L mutations destabilized the enzyme through the removal of a hydrogen bond and increased flexibility of the peptide backbone, respectively. However, the flexibility added by S26L mutation improved the activity at T > 60 °C. This study shows that random mutagenesis and biological selection allowed the identification of residues that are critical in determining thermal activity, stability, and substrate recognition.

Enhancing the Thermostability of Highly Active and Glucose-Tolerant β-Glucosidase Ks5A7 by Directed Evolution for Good Performance of Three Properties

A high-performance β-glucosidase for efficient cellulose hydrolysis needs to excel in thermostability, catalytic efficiency, and resistance to glucose inhibition. However, it is challenging to achieve superb properties in all three aspects in a single enzyme. In this study, a hyperactive and glucose-tolerant β-glucosidase Ks5A7 was employed as the starting point. Four rounds of random mutagenesis were then performed, giving rise to a thermostable mutant 4R1 with five amino acid substitutions. The half-life of 4R1 at 50 °C is 8640-fold that of Ks5A7 (144 h vs 1 min). Meanwhile, 4R1 had a higher specific activity (374.26 vs 243.18 units·mg^-1) than the wild type with a similar glucose tolerance. When supplemented to Celluclast 1.5L, the mutant significantly enhanced the hydrolysis of pretreated sugar cane bagasse, improving the released glucose concentration by 44%. With excellent performance in thermostability, activity, and glucose tolerance, 4R1 will serve as an exceptional catalyst for industrial applications.