Structural insight into potential cold adaptation mechanism through a psychrophilic glycoside hydrolase family 10 endo-β-1,4-xylanase

Delivery of Probiotic-Loaded Microcapsules in the Gastrointestinal Tract: A Review

Probiotics are live microorganisms that inhabit the gastrointestinal tract and confer health benefits to consumers. However, a sufficient number of viable probiotic cells must be delivered to the specific site of interest in the gastrointestinal tract to exert these benefits. Enhanced viability and tolerance to sublethal gastrointestinal stress can be achieved using appropriate coating materials and food matrices for orally consumed probiotics. The release mechanism and interaction of probiotic microcapsules with the gastrointestinal tract have been minimally explored in the literature to date. To the authors' knowledge, no review has been published to discuss the nature of release and the challenges in the targeted delivery of probiotics. This review addresses gastrointestinal-related complications in the formulation of targeted delivery and controlled release of probiotic strains. It investigates the impacts of environmental stresses during the transition stage and delivery to the target region in the gastrointestinal tract. The influence of factors such as pH levels, enzymatic degradation, and redox conditions on the release mechanisms of probiotics is presented. Finally, the available methods to evaluate the efficiency of a probiotic delivery system, including in vitro and in vivo, are reviewed and assessed. The paper concludes with a discussion highlighting the emerging technologies in the field and emphasising key areas in need of future study.

Characteristics of cold-adapted carbonic anhydrase and efficient carbon dioxide capture based on cell surface display technology

Carbonic anhydrase (CA) is currently under investigation because of its potential to capture CO2. A novel N-domain of ice nucleoproteins (INPN)-mediated surface display technique was developed to produce CA with low-temperature capture CO2 based on the mining and characterization of Colwellia sp. CA (CsCA) with cold-adapted enzyme structural features and catalytic properties. CsCA and INPN were effectively integrated into the outer membrane of the cell as fusion proteins. Throughout the display process, the integrity of the membrane of engineered bacteria BL21/INPN-CsCA was maintained. Notably, the study affirmed positive applicability, wherein 94 % activity persisted after 5 d at 15 °C, and 73 % of the activity was regained after 5 cycles of CO2 capture. BL21/INPN-CsCA displayed a high CO2 capture capacity of 52 mg of CaCO3/mg of whole-cell biocatalysts during CO2 mineralization at 25 °C. Therefore, the CsCA functional cell surface display technology could contribute significantly to environmentally friendly CO2 capture.

Extremozyme-Based Biosensors for Environmental Pollution Monitoring: Recent Developments

Extremozymes combine high specificity and sensitivity with the ability to withstand extreme operational conditions. This work presents an overview of extremozymes that show potential for environmental monitoring devices and outlines the latest advances in biosensors utilizing these unique molecules. The characteristics of various extremozymes described so far are presented, underlining their stability and operational conditions that make them attractive for biosensing. The biosensor design is discussed based on the detection of photosynthesis-inhibiting herbicides as a case study. Several biosensors for the detection of pesticides, heavy metals, and phenols are presented in more detail to highlight interesting substrate specificity, applications or immobilization methods. Compared to mesophilic enzymes, the integration of extremozymes in biosensors faces additional challenges related to lower availability and high production costs. The use of extremozymes in biosensing does not parallel their success in industrial applications. In recent years, the "collection" of recognition elements was enriched by extremozymes with interesting selectivity and by thermostable chimeras. The perspectives for biosensor development are exciting, considering also the progress in genetic editing for the oriented immobilization of enzymes, efficient folding, and better electron transport. Stability, production costs and immobilization at sensing interfaces must be improved to encourage wider applications of extremozymes in biosensors.

Helping proteins come in from the cold: 5 burning questions about cold-active enzymes

Enzymes from psychrophilic (cold-loving) organisms have attracted considerable interest over the past decades for their potential in various low-temperature industrial processes. However, we still lack large-scale commercialization of their activities. Here, we review their properties, limitations and potential. Our review is structured around answers to 5 central questions: 1. How do cold-active enzymes achieve high catalytic rates at low temperatures? 2. How is protein flexibility connected to cold-activity? 3. What are the sequence-based and structural determinants for cold-activity? 4. How does the thermodynamic stability of psychrophilic enzymes reflect their cold-active capabilities? 5. How do we effectively identify novel cold-active enzymes, and can we apply them in an industrial context? We conclude that emerging screening technologies combined with big-data handling and analysis make it reasonable to expect a bright future for our understanding and exploitation of cold-active enzymes.

Advances in cold-adapted enzymes derived from microorganisms

Cold-adapted enzymes, produced in cold-adapted organisms, are a class of enzyme with catalytic activity at low temperatures, high temperature sensitivity, and the ability to adapt to cold stimulation. These enzymes are largely derived from animals, plants, and microorganisms in polar areas, mountains, and the deep sea. With the rapid development of modern biotechnology, cold-adapted enzymes have been implemented in human and other animal food production, the protection and restoration of environments, and fundamental biological research, among other areas. Cold-adapted enzymes derived from microorganisms have attracted much attention because of their short production cycles, high yield, and simple separation and purification, compared with cold-adapted enzymes derived from plants and animals. In this review we discuss various types of cold-adapted enzyme from cold-adapted microorganisms, along with associated applications, catalytic mechanisms, and molecular modification methods, to establish foundation for the theoretical research and application of cold-adapted enzymes.

Effectiveness of ruminal xylanase with an extra proline-rich C-terminus on lignocellulosic biomass degradation

The efficient degradation of plant polysaccharides in agricultural waste requires xylanases with high catalytic activity. In this study, the C-terminal proline-rich GH10 xylanase XynA from sheep rumen was investigated using product analysis, structural characterization, truncated and site-directed mutagenesis, molecular dynamics simulation, and application evaluation, revealing that the proline-rich C-terminus contributes to the interaction at the substrate-binding pocket to reduce the binding free energy. Compared to the C-terminally truncated enzyme XynA-Tr, XynA has a more favorable conformation for proton transfer and affinity attack, facilitating the degradation of oligomeric and beechwood xylan without altering the hydrolysis pattern. Moreover, both the reduced sugar yield and weight loss of the pretreated wheat bran, corn cob, and corn stalk hydrolyzed by XynA for 12 h increased by more than 30 %. These findings are important to better understand the relationship between enzyme activities and their terminal regions and suggest candidate materials for lignocellulosic biomass utilization.

Improving the Specific Activity and Thermostability of Psychrophilic Xylosidase AX543 by Comparative Mutagenesis

Improving the specific activity and thermostability of psychrophilic xylosidase is important for improving its enzymatic performance and promoting its industrial application. Herein, a psychrophilic xylosidase AX543 exhibited activity in the temperature range between 0 and 35 °C, with optimum activity at 20 °C, which is lower than that of other reported psychrophilic xylosidases. The thermostability, specific activity, and catalytic efficiency of the site-directed variants G110S, Q201R, and L2 were significantly enhanced, without affecting the optimal reaction temperature. Comparative protein structural analysis and molecular dynamics simulation indicated that these improvements might be the result of the increased hydrogen bonds interaction and improved structural rigidity. Furthermore, homologous module substitution with four segments demonstrated that the psychrophilic characteristics of AX543 are the results of the whole protein structure, and the C-terminal segment A4 appears to be more essential in determining psychrophilic characteristics, exhibiting potentiality to produce more psychrophilic xylosidases. This study provides valuable structural information on psychrophilic xylosidases and also offers attractive modification strategies to modify catalytic activity, thermostability, and optimal reaction temperature.

Cold-adaptive mechanism of psychrophilic bacteria in food and its application

Psychrophilic bacteria are a type of microorganisms that normally grow in low-temperature environments. They are usually found in extremely cold environments. However, as people's demand for low-temperature storage of food becomes higher, psychrophilic bacteria have also begun to appear in cold storage and refrigerators, which has become a food safety hazard. In this paper, the optimal cooling strategies of psychrophilic bacteria are reviewed from the aspects of the cell membrane, psychrophilic enzymes, antifreeze proteins, cold shock proteins, gene regulation, metabolic levels and antifreeze agents, and the principle of psychrophilic mechanism is briefly described. The application of thermophilic bacteria and its products adapted to cold environments in food fields are analyzed. The purpose of this paper is to provide ideas for future research on psychrophilic bacteria based on the mechanism and application of psychrophilic bacteria.

Molecular and Biochemical Analyses of a Novel Trifunctional Endoxylanase/Endoglucanase/Feruloyl Esterase from the Human Colonic Bacterium Bacteroides intestinalis DSM 17393

A novel enzyme Bi76 comprising GH10, E_set_Esterase_N, and CE1 modules was identified, with the highest homology (62.9%) with a bifunctional endoxylanase/feruloyl esterase among characterized enzymes. Interestingly, Bi76 hydrolyzed glucan substrates besides xylans and feruloylated substrates, suggesting that it is the first characterized trifunctional endoxylanase/endoglucanase/feruloyl esterase. Analyses of truncation variants revealed that GH10 and E_set_Esterase_N + CE1 modules encoded endoxylanase/endoglucanase and feruloyl esterase activities, respectively. Synergism analyses indicated that endoxylanase, α-l-arabinofuranosidase, and feruloyl esterase acted cooperatively in releasing ferulic acid (FA) and xylooligosaccharides from feruloylated arabinoxylan. The interdomain synergism of Bi76 overmatched the intermolecular synergism of TM1 and TM2. Importantly, Bi76 exhibited good capacity in producing FA, releasing 5.20, 4.38, 2.12, 1.35, 0.46, and 0.19 mg/g from corn bran, corn cob, wheat bran, corn stover, rice husk, and rice bran, respectively. This study expands the trifunctional endoxylanase/endoglucanase/feruloyl esterase repertoire and demonstrates the great potential of Bi76 in agricultural residue utilization.

Extremophilic Prokaryotic Endoxylanases: Diversity, Applicability, and Molecular Insights

Extremophilic endoxylanases grabbed attention in recent years due to their applicability under harsh conditions of several industrial processes. Thermophilic, alkaliphilic, and acidophilic endoxylanases found their employability in bio-bleaching of paper pulp, bioconversion of lignocellulosic biomass into xylooligosaccharides, bioethanol production, and improving the nutritious value of bread and other bakery products. Xylanases obtained from extremophilic bacteria and archaea are considered better than fungal sources for several reasons. For example, enzymatic activity under broad pH and temperature range, low molecular weight, cellulase-free activity, and longer stability under extreme conditions of prokaryotic derived xylanases make them a good choice. In addition, a short life span, easy cultivation/harvesting methods, higher yield, and rapid DNA manipulations of bacterial and archaeal cells further reduces the overall cost of the product. This review focuses on the diversity of prokaryotic endoxylanases, their characteristics, and their functional attributes. Besides, the molecular mechanisms of their extreme behavior have also been presented here.

Microbial enrichment and meta-omics analysis identify CAZymes from mangrove sediments with unique properties

Although lignocellulose is the most abundant and renewable natural resource for biofuel production, its use remains under exploration because of its highly recalcitrant structure. Its deconstruction into sugar monomers is mainly driven by carbohydrate-active enzymes (CAZymes). To develop highly efficient and fast strategies to discover biomass-degrading enzymes for biorefinery applications, an enrichment process combined with integrative omics approaches was used to identify new CAZymes. The lignocellulolytic-enriched mangrove microbial community (LignoManG) established on sugarcane bagasse (SB) was enriched with lignocellulolytic bacteria and fungi such as Proteobacteria, Bacteroidetes, Basidiomycota, and Ascomycota. These microbial communities were able to degrade up to 55 % of the total SB, indicating the production of lignocellulolytic enzymes. Metagenomic analysis revealed that the LignoManG harbors 18.042 CAZyme sequences such as of cellulases, hemicellulases, carbohydrate esterases, and lytic polysaccharide monooxygenase. Similarly, our metaproteomic analysis depicted several enzymes from distinct families of different CAZy families. Based on the LignoManG data, a xylanase (coldXynZ) was selected, amplified, cloned, expressed, and biochemically characterized. The enzyme displayed psicrofilic properties, with the highest activity at 15 °C, retaining 77 % of its activity when incubated at 0 °C. Moreover, molecular modeling in silico indicated that coldXynZ is composed of a TIM barrel, which is a typical folding found in the GH10 family, and displayed similar structural features related to cold-adapted enzymes. Collectively, the data generated in this study represent a valuable resource for lignocellulolytic enzymes with potential biotechnological applications.

Cloning, Expression, Characterization, and Antioxidant Protection of Glutaredoxin3 From Psychrophilic Bacterium Psychrobacter sp. ANT206

Glutaredoxins (Grxs) are proteins that catalyze the glutathione (GSH)-dependent reduction of protein disulfides. In this study, a Grx-related gene (264 bp), encoding a Ps-Grx3, was cloned from Psychrobacter sp. ANT206. Sequence analysis indicated the presence of the active site motif CPYC in this protein. Homology modeling showed that Ps-Grx3 had fewer hydrogen bonds and salt bridges, as well as a lower Arg/(Arg + Lys) ratio than its mesophilic homologs, indicative of an improved catalytic ability at low temperatures. Site-directed mutagenesis demonstrated that the Cys13, Pro14, and Cys16 sites were essential for the catalytic activity of Ps-Grx3, while circular dichroism (CD) spectroscopy confirmed that point mutations in these amino acid residues led to the loss or reduction of enzyme activity. Furthermore, analysis of the biochemical properties of Ps-Grx3 showed that the optimum temperature of this enzyme was 25 °C. Importantly, Ps-Grx3 was more sensitive to tBHP and CHP than to H₂O₂, and retained approximately 40% activity even when the H₂O₂ concentration was increased to 1 mm Regarding substrate specificity, Ps-Grx3 had a higher affinity for HED, L-cystine, and DHA than for S-sulfocysteine and BSA. We also investigated the DNA-protective ability of Ps-Grx3 using the pUC19 plasmid, and found that Ps-Grx3 could protect supercoiled DNA from oxidation-induced damage at 15°C for 1.5 h. This study provides new insights into the structure and catalytic activity of a cold-adapted Grx3.

Mapping the Transglycosylation Relevant Sites of Cold-Adapted β-d-Galactosidase from Arthrobacter sp. 32cB

β-Galactosidase from Arthrobacter sp. 32cB (ArthβDG) is a cold-adapted enzyme able to catalyze hydrolysis of β-d-galactosides and transglycosylation reaction, where galactosyl moiety is being transferred onto an acceptor larger than a water molecule. Mutants of ArthβDG: D207A and E517Q were designed to determine the significance of specific residues and to enable formation of complexes with lactulose and sucrose and to shed light onto the structural basis of the transglycosylation reaction. The catalytic assays proved loss of function mutation E517 into glutamine and a significant drop of activity for mutation of D207 into alanine. Solving crystal structures of two new mutants, and new complex structures of previously presented mutant E441Q enables description of introduced changes within active site of enzyme and determining the importance of mutated residues for active site size and character. Furthermore, usage of mutants with diminished and abolished enzymatic activity enabled solving six complex structures with galactose, lactulose or sucrose bounds. As a result, not only the galactose binding sites were mapped on the enzyme's surface but also the mode of lactulose, product of transglycosylation reaction, and binding within the enzyme's active site were determined and the glucopyranose binding site in the distal of active site was discovered. The latter two especially show structural details of transglycosylation, providing valuable information that may be used for engineering of ArthβDG or other analogous galactosidases belonging to GH2 family.

Isolation and Characterization of a Novel Cold-Active, Halotolerant Endoxylanase from Echinicola rosea sp. Nov. JL3085T

We cloned a xylanase gene (xynT) from marine bacterium Echinicola rosea sp. nov. JL3085^T and recombinantly expressed it in Escherichia coli BL21. This gene encoded a polypeptide with 379 amino acid residues and a molecular weight of ~43 kDa. Its amino acid sequence shared 45.3% similarity with an endoxylanase from Cellvibrio mixtus that belongs to glycoside hydrolases family 10 (GH10). The XynT showed maximum activity at 40 °C and pH 7.0, and a maximum velocity of 62 μmoL min⁻¹ mg⁻¹. The XynT retained its maximum activity by more than 69%, 51%, and 26% at 10 °C, 5 °C, and 0 °C, respectively. It also exhibited the highest activity of 135% in the presence of 4 M NaCl and retained 76% of its activity after 24 h incubation with 4 M NaCl. This novel xylanase, XynT, is a cold-active and halotolerant enzyme that may have promising applications in drug, food, feed, and bioremediation industries.

A New Cold-Adapted and Salt-Tolerant Glutathione Reductase from Antarctic Psychrophilic Bacterium Psychrobacter sp. and Its Resistance to Oxidation

A new glutathione reductase gene (psgr) coding for glutathione reductase (GR) from an Antarctic bacterium was cloned and overexpressed into Escherichia coli (E. coli). A sequence analysis revealed that PsGR is a protein consisting of 451 amino acids, and homology modeling demonstrated that PsGR has fewer hydrogen bonds and salt bridges, which might lead to improved conformational flexibility at low temperatures. PsGR possesses the flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide phosphate (NADPH) binding motifs. Recombinant PsGR (rPsGR) was purified using Ni-NTA affinity chromatography and was found to have a molecular mass of approximately 53.5 kDa. rPsGR was found to be optimally active at 25 °C and a pH of 7.5. It was found to be a cold-adapted enzyme, with approximately 42% of its optimal activity remaining at 0 °C. Moreover, rPsGR was most active in 1.0 M NaCl and 62.5% of its full activity remained in 3.0 M NaCl, demonstrating its high salt tolerance. Furthermore, rPsGR was found to have a higher substrate affinity for NADPH than for GSSG (oxidized glutathione). rPsGR provided protection against peroxide (H₂O₂)-induced oxidative stress in recombinant cells, and displayed potential application as an antioxidant protein. The results of the present study provide a sound basis for the study of the structural characteristics and catalytic characterization of cold-adapted GR.

A Novel Cold-Adapted and Salt-Tolerant RNase R from Antarctic Sea-Ice Bacterium Psychrobacter sp. ANT206

A novel RNase R, psrnr, was cloned from the Antarctic bacterium Psychrobacter sp. ANT206 and expressed in Escherichia coli (E. coli). A bioinformatics analysis of the psrnr gene revealed that it contained an open reading frame of 2313 bp and encoded a protein (PsRNR) of 770 amino acids. Homology modeling indicated that PsRNR had reduced hydrogen bonds and salt bridges, which might be the main reason for the catalytic efficiency at low temperatures. A site directed mutation exhibited that His 667 in the active site was absolutely crucial for the enzyme catalysis. The recombinant PsRNR (rPsRNR) showed maximum activity at 30 °C and had thermal instability, suggesting that rPsRNR was a cold-adapted enzyme. Interestingly, rPsRNR displayed remarkable salt tolerance, remaining stable at 0.5-3.0 M NaCl. Furthermore, rPsRNR had a higher k_cat value, contributing to its efficient catalytic activity at a low temperature. Overall, cold-adapted RNase R in this study was an excellent candidate for antimicrobial treatment.

Structural features of cold-adapted dimeric GH2 β-D-galactosidase from Arthrobacter sp. 32cB

Crystal structures of cold-adapted β-d-galactosidase (EC 3.2.1.23) from the Antarctic bacterium Arthrobacter sp. 32cB (ArthβDG) have been determined in an unliganded form resulting from diffraction experiments conducted at 100 K (at resolution 1.8 Å) and at room temperature (at resolution 3.0 Å). A detailed comparison of those two structures of the same enzyme was performed in order to estimate differences in their molecular flexibility and rigidity and to study structural rationalization for the cold-adaptation of the investigated enzyme. Furthermore, a comparative analysis with structures of homologous enzymes from psychrophilic, mesophilic, and thermophilic sources has been discussed to elucidate the relationship between structure and cold-adaptation in a wider context. The performed studies confirm that the structure of cold-adapted ArthβDG maintains balance between molecular stability and structural flexibility, which can be observed independently on the temperature of conducted X-ray diffraction experiments. Obtained information about proper protein function under given conditions provide a guideline for rational engineering of proteins in terms of their temperature optimum and thermal stability.

Increase in the thermostability of Bacillus sp. strain TAR-1 xylanase using a site saturation mutagenesis library

Site saturation mutagenesis library is a recently developed technique, in which any one out of all amino acid residues in a target region is substituted into other 19 amino acid residues. In this study, we used this technique to increase the thermostability of a GH10 xylanase, XynR, from Bacillus sp. strain TAR-1. We hypothesized that the substrate binding region of XynR is flexible, and that the thermostability of XynR will increase if the flexibility of the substrate binding region is decreased without impairing the substrate binding ability. Site saturation mutagenesis libraries of amino acid residues Tyr43–Lys115 and Ala300–Asn325 of XynR were constructed. By screening 480 clones, S92E was selected as the most thermostable one, exhibiting the residual activity of 80% after heat treatment at 80°C for 15 min in the hydrolysis of Remazol Brilliant Blue-xylan. Our results suggest that this strategy is effective for stabilization of GH10 xylanase.

Abbreviations: DNS: 3,5-dinitrosalicylic acid; RBB-xylan: Remazol Brilliant Blue-xylan

Tyr51: Key Determinant of the Low Thermostability of the Colwellia psychrerythraea Cold-Shock Protein

Cold-shock proteins (Csps) are expressed at lower-than-optimum temperatures, and they function as RNA chaperones; however, no structural studies on psychrophilic Csps have been reported. Here, we aimed to investigate the structure and dynamics of the Csp of psychrophile Colwellia psychrerythraea 34H, ( Cp-Csp). Although Cp-Csp shares sequence homology, common folding patterns, and motifs, including a five β-stranded barrel, with its thermophilic counterparts, its thermostability (37 °C) was markedly lower than those of other Csps. Cp-Csp binds heptathymidine with an affinity of 10^-7 M, thereby increasing its thermostability to 50 °C. Nuclear magnetic resonance spectroscopic analysis of the Cp-Csp structure and backbone dynamics revealed a flexible structure with only one salt bridge and 10 residues in the hydrophobic cavity. Notably, Cp-Csp contains Tyr51 instead of the conserved Phe in the hydrophobic core, and its phenolic hydroxyl group projects toward the surface. The Y51F mutation increased the stability of hydrophobic packing and may have allowed for the formation of a K3-E21 salt bridge, thereby increasing its thermostability to 43 °C. Cp-Csp exhibited conformational exchanges in its ribonucleoprotein motifs 1 and 2 (754 and 642 s^-1), and heptathymidine binding markedly decreased these motions. Cp-Csp lacks salt bridges and has longer flexible loops and a less compact hydrophobic cavity resulting from Tyr51 compared to mesophilic and thermophilic Csps. These might explain the low thermostability of Cp-Csp. The conformational flexibility of Cp-Csp facilitates its accommodation of nucleic acids at low temperatures in polar oceans and its function as an RNA chaperone for cold adaptation.

Insight into the cold adaptation and hemicellulose utilization of Cladosporium neopsychrotolerans from genome analysis and biochemical characterization

The occurrence of Cladosporium in cold ecosystems has been evidenced long before, and most of the knowledge about nutrient utilization of this genus is sporadic. An alpine soil isolate C. neopsychrotolerans SL-16, showing great cold tolerance and significant lignocellulose-degrading capability, was sequenced to form a 35.9 Mb genome that contains 13,456 predicted genes. Functional annotation on predicted genes revealed a wide array of proteins involved in the transport and metabolism of carbohydrate, protein and lipid. Large numbers of transmembrane proteins (967) and CAZymes (571) were identified, and those related to hemicellulose degradation was the most abundant. To undermine the hemicellulose (xyaln as the main component) utilization mechanism of SL-16, the mRNA levels of 23 xylanolytic enzymes were quantified, and representatives of three glycoside hydrolase families were functionally characterized. The enzymes showed similar neutral, cold active and thermolabile properties and synergistic action on xylan degradation (the synergy degree up to 15.32). Kinetic analysis and sequence and structure comparison with mesophilic and thermophilic homologues indicated that these cold-active enzymes employed different cold adaptation strategies to function well in cold environment. These similar and complementary advantages in cold adaptation and catalysis might explain the high efficiency of lignocellulose conversion observed in SL-16 under low temperatures.

Structure features of GH10 xylanase from Caldicellulosiruptor bescii: implication for its thermophilic adaption and substrate binding preference

Caldicellulosiruptor bescii is the most thermophilic cellulolytic species of organisms known to date. In our previous study, GH10 xylanase CbXyn10B from C. bescii displayed outstanding hydrolytic activity toward various xylans at high temperatures. To understand the structural basis for this protein's catalysis and thermostability, we solved the crystal structures of CbXyn10B and its complexes with xylooligosaccharides. These structural models were used to guide comparison with its mesophilic counterpart PbXyn10B. A distinctive structural feature is that thermophilic CbXyn10B presents a relatively stable interaction between the extended loops L7 and L8 in the catalytic cleft by an extensive hydrogen bonding network, which is mediated by Lys³⁰⁶, Arg³¹⁴ and three well-ordered water molecules. Moreover, a unique aromatic cluster consisting of Try¹⁷, Phe²⁰, Phe²¹, and Phe³³⁷ may enhance the interaction between the N- and C- terminus. Targeted mutagenesis demonstrated that these interactions substantially contribute to enzyme stabilization, as indicated by a considerable decrease in the melting temperature (T_m) of CbXyn10B by substituting critical residues with Ala. Therefore, it was shown that not only the aromatic interaction connecting protein termini but also the extensive hydrogen bonding network formed between surface loops could restrict the local structural flexibility and contribute significantly to the overall stability of enzymes. Furthermore, the xylooligosaccharides were found to tightly bind to the glycone subsites of xylanase, indicating higher affinities at these subsites and reflecting its substrate binding preference. Our results suggest that CbXyn10B is stabilized with distinct rigidity at the catalytic cleft as well as the terminal regions, which provides insights into the evolutionary strategy for accommodating the functional needs of GH10 enzymes to high temperature.

Discovery, Molecular Mechanisms, and Industrial Applications of Cold-Active Enzymes

Cold-active enzymes constitute an attractive resource for biotechnological applications. Their high catalytic activity at temperatures below 25°C makes them excellent biocatalysts that eliminate the need of heating processes hampering the quality, sustainability, and cost-effectiveness of industrial production. Here we provide a review of the isolation and characterization of novel cold-active enzymes from microorganisms inhabiting different environments, including a revision of the latest techniques that have been used for accomplishing these paramount tasks. We address the progress made in the overexpression and purification of cold-adapted enzymes, the evolutionary and molecular basis of their high activity at low temperatures and the experimental and computational techniques used for their identification, along with protein engineering endeavors based on these observations to improve some of the properties of cold-adapted enzymes to better suit specific applications. We finally focus on examples of the evaluation of their potential use as biocatalysts under conditions that reproduce the challenges imposed by the use of solvents and additives in industrial processes and of the successful use of cold-adapted enzymes in biotechnological and industrial applications.