When substrate inhibits and inhibitor activates: implications of β-glucosidases

Molecular dynamics and solvation structures of the β-glucosidase from Humicola insolens (BGHI) in aqueous solutions containing glucose

The β-glucosidase enzyme is a glycosyl hydrolase that breaks down the β-1,4 linkage of cellobiose. It is inhibited by glucose at high concentrations due to competitive inhibition. However, at lower glucose concentrations, the glucose-tolerant β-glucosidase from Humicola insolens (BGHI) undergoes stimulation. Proteins, in aqueous sugar solutions, tend to be preferentially hydrated, which generally promotes their stabilization. Thus, solvation phenomena may contribute to both glucose tolerance and stimulation processes. We have performed atomistic classical Molecular Dynamics (MD) simulations of BGHI at different glucose concentrations to mimic the conditions found in the catalytic experiments. A detailed examination of the solvent environment through the calculation of minimum distance distribution functions (MDDFs) and Kirkwood-Buff (KB) integrals was performed. The enzyme is preferentially hydrated in the presence of glucose at all concentrations. Nevertheless, the hydration does not prevent the glucose from directly interacting with the BGHI surface or from entering the active site. Based on the obtained results, we hypothesize that preferential hydration is beneficial for enzyme activity. At the same time, product inhibition has little effect at lower concentrations of glucose, and at higher glucose concentrations, competition for the active site becomes predominant and the enzyme is primarily inhibited.

Analysis of phosphofructokinase-1 activity as affected by pH and ATP concentration

A key player in energy metabolism is phosphofructokinase-1 (PFK1) whose activity and behavior strongly influence glycolysis and thus have implications in many areas. In this research, PFK1 assays were performed to convert F6P and ATP into F-1,6-P and ADP for varied pH and ATP concentrations. PFK1 activity was assessed by evaluating F-1,6-P generation velocity in two ways: (1) directly calculating the time slope from the first two or more datapoints of measured product concentration (the initial-velocity method), and (2) by fitting all the datapoints with a differential equation explicitly representing the effects of ATP and pH (the modeling method). Similar general trends of inhibition were shown by both methods, but the former gives only a qualitative picture while the modeling method yields the degree of inhibition because the model can separate the two simultaneous roles of ATP as both a substrate of reaction and an inhibitor of PFK1. Analysis based on the model suggests that the ATP affinity is much greater to the PFK1 catalytic site than to the inhibitory site, but the inhibited ATP-PFK1-ATP complex is much slower than the uninhibited PFK1-ATP complex in product generation, leading to reduced overall reaction velocity when ATP concentration increases. The initial-velocity method is simple and useful for general observation of enzyme activity while the modeling method has advantages in quantifying the inhibition effects and providing insights into the process.

Strategy to mitigate substrate inhibition in wastewater treatment systems

Global urbanization requires more stable and sustainable wastewater treatment to reduce the burden on the water environment. To address the problem of substrate inhibition of microorganisms during wastewater treatment, which leads to unstable wastewater discharge, this study proposes an approach to enhance the tolerance of bacterial community by artificially setting up a non-lethal high substrate environment. And the feasibility of this approach was explored by taking the inhibition of anammox process by nitrite as an example. It was shown that the non-lethal high substrate environment could enhance the nitrite tolerance of anammox bacterial community, as the specific anammox activity increasing up to 24.71 times at high nitrite concentrations. Moreover, the system composed of anammox bacterial community with high nitrite tolerance also showed greater resistance (two-fold) in response to nitrite shock. The antifragility of the system was enhanced without affecting the operation of the main reactor, and the non-lethal high nitrite environment changed the dominant anammox genera to Candidatus Jettenia. This approach to enhance tolerance of bacterial community in a non-lethal high substrate environment not only allows the anammox system to operate stably, but also promises to be a potential strategy for achieving stable biological wastewater treatment processes to comply with standards.

Expression of a thermostable glucose-stimulated β-glucosidase from a hot-spring metagenome and its promising application to produce gardenia blue

This study reports a thermostable glucose-stimulated β-glucosidase, BglY442, from hot-spring metagenomic data that was cloned and expressed in Escherichia coli BL21 (DE3). The molecular mass of recombinant BglY442 was 69.9 kDa and was used in the production of gardenia blue. The recombinant BglY442 showed its maximum activity at pH 6.0 and 75 °C, maintained 50 % activity at 70 °C for 36 h, presented over 90 % activity in a broad pH range and a wide range of pH stability. Moreover, BglY442 exhibited excellent tolerance toward methanol and ethanol. The specific activity of BglY442 was 235 U/mg at pH 6.0 and 75 °C with 10 mM pNPG as substrate. BglY442 activity increased by over fourfold with 2 M glucose or xylose. Specifically, the enzyme kinetics of BglY442 seem to be non-Michaelis-Menten kinetics or atypical kinetics because the Michaelis-Menten saturation kinetics were not observed with pNPG, oNPG or geniposide as substrates. Under optimum conditions, geniposide was dehydrated by BglY442 and reacted with nine amino acids respectively by the one-pot method. Only the Arg or Met derived pigments showed bright blue, and these two pigments had similar ultraviolet absorption spectra. The OD590 nm of GB was detected to be 1.06 after 24 h with the addition of Arg and 1.61 after 36 h with the addition of Met. The intermediate was elucidated and identified as ginipin. Molecular docking analysis indicated that the enzyme had a similar catalytic mechanism to the reported GH1 Bgls. BglY442 exhibited potential for gardenia blue production by the one-pot method. With outstanding thermostability and glucose tolerance, BglY442 should be considered a potential β-glucosidase in biotechnology applications.

Exploring the diversity of β-glucosidase: Classification, catalytic mechanism, molecular characteristics, kinetic models, and applications

High-value chemicals and energy-related products can be produced from biomass. Biorefinery technology offers a sustainable and cost-effective method for this high-value conversion. β-glucosidase is one of the key enzymes in biorefinery processes, catalyzing the production of glucose from aryl-glycosides and cello-oligosaccharides via the hydrolysis of β-glycosidic bonds. Although β-glucosidase plays a critical catalytic role in the utilization of cellulosic biomass, its efficacy is often limited by substrate or product inhibitions, low thermostability, and/or insufficient catalytic activity. To provide a detailed overview of β-glucosidases and their benefits in certain desired applications, we collected and summarized extensive information from literature and public databases, covering β-glucosidases in different glycosidase hydrolase families and biological kingdoms. These β-glucosidases show differences in amino acid sequence, which are translated into varying degrees of the molecular properties critical in enzymatic applications. This review describes studies on the diversity of β-glucosidases related to the classification, catalytic mechanisms, key molecular characteristics, kinetics models, and applications, and highlights several β-glucosidases displaying high stability, activity, and resistance to glucose inhibition suitable for desired biotechnological applications.

Binding interactions of hydrophobically-modified flavonols with β-glucosidase: fluorescence spectroscopy and molecular modelling study

Natural flavonoids are capable of inhibiting glucosidase activity, so they can be used for treating diabetes mellitus and hypertension. However, molecular-level details of their interactions with glucosidase enzymes remain poorly understood. This paper describes the synthesis and spectral characterization of a series of fluorescent flavonols and their interaction with the β-glucosidase enzyme. To tune flavonol-enzyme interaction modes and affinity, we introduced different polar halogen-containing groups or bulky aromatic/alkyl substituents in the peripheral 2-aryl ring of a flavonol moiety. Using fluorescence spectroscopy methods in combination with molecular docking and molecular dynamics simulations, we examined the binding affinity and identified probe binding patterns, which are critical for steric blockage of the key catalytic residues of the enzyme. Using a fluorescent assay, we demonstrated that the binding of flavonol 2e to β-glucosidase decreased its enzymatic activity up to 3.5 times. In addition, our molecular docking and all-atom molecular dynamics simulations suggest that the probe binding is driven by hydrophobic interactions with aromatic Trp and Tyr residues within the catalytic glycone binding pockets of β-glucosidase. Our study provides a new insight into structure-property relations for flavonol-protein interactions, which govern their enzyme binding, and outlines a framework for a rational design of new flavonol-based potent inhibitors for β-glucosidases.

Bifunctional Mutant of Oligo-1,6-glucosidase for the Production of Glucose from Glucose Mother Liquor

Glucose mother liquor (GML) is a byproduct of the glucose (G1) crystallization process. However, the presence of maltooligosaccharides and isomaltooligosaccharides within GML imposes limitations on its reutilization. Furthermore, the high concentration of G1 in GML leads to product inhibition of G1-producing enzymes. To overcome these challenges, a variant enzyme called V219A was developed through genetic mutation. The V219A exhibits the ability to hydrolyze both maltooligosaccharides and isomaltooligosaccharides. Product inhibition kinetics showed that the IC50 value of V219A was 7 times higher than that of the wild type. Upon subjecting primary, secondary, and tertiary GML to treatment with V219A, the G1 content exhibited notable increases, reaching 96.88, 95.70, and 90.46%, respectively. These significant findings not only establish an innovative and environmentally conscious approach for G1 production from GML but also provide a promising strategy for enzyme construction that caters to the demands of industrial-scale production.

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.

Human soluble CD39 displays substrate inhibition in a substrate-specific manner

CD39 (ectonucleoside triphosphate diphosphohydrolase-1; ENTPD1) metabolizes extracellular ATP and ADP to AMP. AMP is subsequently metabolized by CD79 to adenosine. CD39 activity is therefore a key regulator of purinergic signalling in cancer, thrombosis, and autoimmune diseases. In this study we demonstrate that soluble, recombinant CD39 shows substrate inhibition with ADP or ATP as the substrate. Although CD39 activity initially increased with increasing substrate concentration, at high concentrations of ATP or ADP, CD39 activity was markedly reduced. Although the reaction product, AMP, inhibits CD39 activity, insufficient AMP was generated under our conditions to account for the substrate inhibition seen. In contrast, inhibition was not seen with UDP or UTP as substrates. 2-methylthio-ADP also showed no substrate inhibition, indicating the nucleotide base is an important determinant of substrate inhibition. Molecular dynamics simulations revealed that ADP can undergo conformational rearrangements within the CD39 active site that were not seen with UDP or 2-methylthio-ADP. Appreciating the existence of substrate inhibition of CD39 will help the interpretation of studies of CD39 activity, including investigations into drugs that modulate CD39 activity.

Enzyme kinetics by GH7 cellobiohydrolases on chromogenic substrates is dictated by non-productive binding: insights from crystal structures and MD simulation

Cellobiohydrolases (CBHs) in the glycoside hydrolase family 7 (GH7) (EC3.2.1.176) are the major cellulose degrading enzymes both in industrial settings and in the context of carbon cycling in nature. Small carbohydrate conjugates such as p-nitrophenyl-β-d-cellobioside (pNPC), p-nitrophenyl-β-d-lactoside (pNPL) and methylumbelliferyl-β-d-cellobioside have commonly been used in colorimetric and fluorometric assays for analysing activity of these enzymes. Despite the similar nature of these compounds the kinetics of their enzymatic hydrolysis vary greatly between the different compounds as well as among different enzymes within the GH7 family. Through enzyme kinetics, crystallographic structure determination, molecular dynamics simulations, and fluorometric binding studies using the closely related compound o-nitrophenyl-β-d-cellobioside (oNPC), in this work we examine the different hydrolysis characteristics of these compounds on two model enzymes of this class, TrCel7A from Trichoderma reesei and PcCel7D from Phanerochaete chrysosporium. Protein crystal structures of the E212Q mutant of TrCel7A with pNPC and pNPL, and the wildtype TrCel7A with oNPC, reveal that non-productive binding at the product site is the dominating binding mode for these compounds. Enzyme kinetics results suggest the strength of non-productive binding is a key determinant for the activity characteristics on these substrates, with PcCel7D consistently showing higher turnover rates (k_cat ) than TrCel7A, but higher Michaelis-Menten (K_M ) constants as well. Furthermore, oNPC turned out to be useful as an active-site probe for fluorometric determination of the dissociation constant for cellobiose on TrCel7A but could not be utilized for the same purpose on PcCel7D, likely due to strong binding to an unknown site outside the active site.

Analysis of Retronasal Flavor Alterations in Smoke-Affected Wines and the Efficacy of Various Inter-Stimulus Rinse Protocols in Clearing Smoke-Related Attributes

Wildfires produce smoke, which can then encounter wine grapes, causing the fruit to absorb smoke-related volatile organic compounds. These compounds impact the sensorial profiles of the resulting wines, introducing an uncharacteristic smokey flavor and ashy finish. Since these off-flavor attributes are observed to have longer-lasting perception, a proper inter-stimulus protocol is necessary to ensure an accurate sensory analysis. Previous work has indicated that a 1 g/L pectin rinse with 120 s of separation is effective for clearing the smoke flavor to mitigate potential carryover effects. The purpose of this work was to determine if there was a more efficient rinsing protocol to lessen the time taken between samples. By using wines with various levels of smoke exposure (high, moderate, and none), the efficacy of four different rinse systems were evaluated with a fixed-time-point evaluation system. These results indicate that a 4 g/L glucose solution is more efficient than pectin, requiring only 90 s of separation to clear smoke flavor perception. Additionally, this work identified appropriate references for the retronasal attributes associated with smoke taint in wine. These results can be used to guide a sensory analysis of wildfire-affected wines to ensure effective and accurate results.

Immobilization-Stabilization of β-Glucosidase for Implementation of Intensified Hydrolysis of Cellobiose in Continuous Flow Reactors

Cellulose saccharification to glucose is an operation of paramount importance in the bioenergy sector and the chemical and food industries, while glucose is a critical platform chemical in the integrated biorefinery. Among the cellulose degrading enzymes, β-glucosidases are responsible for cellobiose hydrolysis, the final step in cellulose saccharification, which is usually the critical bottleneck for the whole cellulose saccharification process. The design of very active and stable β-glucosidase-based biocatalysts is a key strategy to implement an efficient saccharification process. Enzyme immobilization and reaction engineering are two fundamental tools for its understanding and implementation. Here, we have designed an immobilized-stabilized solid-supported β-glucosidase based on the glyoxyl immobilization chemistry applied in porous solid particles. The biocatalyst was stable at operational temperature and highly active, which allowed us to implement 25 °C as working temperature with a catalyst productivity of 109 mmol/min/gsupport. Cellobiose degradation was implemented in discontinuous stirred tank reactors, following which a simplified kinetic model was applied to assess the process limitations due to substrate and product inhibition. Finally, the reactive process was driven in a continuous flow fixed-bed reactor, achieving reaction intensification under mild operation conditions, reaching full cellobiose conversion of 34 g/L in a reaction time span of 20 min.

Functional properties and potential application of ethanol tolerant β-glucosidases from Pichia ofunaensis and Trichosporon multisporum yeasts

β-Glucosidases have been extensively investigated to integrate the enzyme complex for cellulose fiber saccharification and for improving the aroma of wine. To produce these enzymes, greater attention has been given to filamentous fungi and bacteria, and few investigations have targeted the potential applications of enzymes secreted by yeasts. Addressing this issue, in this study, β-glucosidases were produced by the Pichia ofunaensis and Trichosporon multisporum yeasts, via solid state fermentation with wheat bran as a substrate. When using p-Nitrophenyl β-d-glucopyranoside (pNPG) as an enzyme substrate, maximum β-glucosidase activities were detected at pH 5.5-6.0 and 50-60 °C for P. ofunaensis, and pH 5-6 and 55 °C for T. multisporum. Both enzymes were able to hydrolyze cellobiose and exhibited stability over a wide range of pH (3.5-9.0) for 24 h at 4 °C, thermostability up to 50 °C for 1 h and tolerance to 10 mM phenolic compounds. Negative modulation on enzyme activity was observed in the presence of Cu2+, Fe3+, Zn2+, Al3+ and Hg2+, while both β-glucosidases were tolerant to 30% methanol, isopropanol and acetone. In the presence of ethanol and glucose, enzymes from P. ofunaensis were the more active and stable of the two. These enzymes, especially the P. ofunaensis β-glucosidases, could be tested in enology for improving the aroma of wine and for integrating a cellulolytic complex to produce 2G ethanol.

Role of Conformational Change and Glucose Binding Sites in the Enhanced Glucose Tolerance of Agrobacterium tumefaciens 5A GH1 β-Glucosidase Mutants

β-Glucosidases are often inhibited by their reaction product glucose and a barrier to the efficient lignocellulosic biomass hydrolysis to glucose. We had previously reported the mutants, C174V, and H229S, with a nearly 2-fold increased glucose tolerance over the wild type (WT), H0HC94, encoded in Agrobacterium tumefaciens 5A (apparent K_i,_Glc = 686 mM). We report our steady-state and time-resolved intrinsic fluorescence spectroscopy, circular dichroism, and isothermal titration calorimetry (ITC) studies to further understand increased glucose tolerance. Changes in the mutants' emission intensity and the differential change in quenching rate in the absence and presence of glucose reflect changes in protein conformation by glucose. Time-resolved lifetime and anisotropy measurements further indicated the microenvironment differences across solvent-exposed tryptophan residues and a higher hydrodynamic radius due to glucose binding, respectively. ITC measurements confirmed the increase of glucose binding sites in the mutants. The experiment results were supported by molecular dynamics simulations, which revealed significant variations in the glucose-protein hydrogen-bonding profiles. Protein structure network analysis of the simulated structures further indicates the mutants' conformation change than the WT. Computational studies also indicated additional glucose binding sites in mutants. Our results indicate the role of glucose binding in modulating the enzyme response to glucose.

Single-Point Mutant Inverts the Stereoselectivity of a Carbonyl Reductase toward β-Ketoesters with Enhanced Activity

Enzyme stereoselectivity control is still a major challenge. To gain insight into the molecular basis of enzyme stereo-recognition and expand the source of antiPrelog carbonyl reductase toward β-ketoesters, rational enzyme design aiming at stereoselectivity inversion was performed. The designed variant Q139G switched the enzyme stereoselectivity toward β-ketoesters from Prelog to antiPrelog, providing corresponding alcohols in high enantiomeric purity (89.1-99.1 % ee). More importantly, the well-known trade-off between stereoselectivity and activity was not found. Q139G exhibited higher catalytic activity than the wildtype enzyme, the enhancement of the catalytic efficiency (k_cat /K_m ) varied from 1.1- to 27.1-fold. Interestingly, the mutant Q139G did not lead to reversed stereoselectivity toward aromatic ketones. Analysis of enzyme-substrate complexes showed that the structural flexibility of β-ketoesters and a newly formed cave together facilitated the formation of the antiPrelog-preferred conformation. In contrast, the relatively large and rigid structure of the aromatic ketones prevents them from forming the antiPrelog-preferred conformation.

A novel mechanism of β-glucosidase stimulation through a monosaccharide binding-induced conformational change

It is urgent the transition from a fossil fuel-based economy to a sustainable bioeconomy based on bioconversion technologies using renewable plant biomass feedstocks to produce high chemicals, bioplastics, and biofuels. β-Glucosidases are key enzymes responsible for degrading the plant cell wall polymers, as they cleave glucan-based oligo- and polysaccharides to generate glucose. Monosaccharide-tolerant or -stimulated β-glucosidases have been reported in the past decade. Here, we describe a novel mechanism of β-glucosidase stimulation by glucose and xylose. The glycoside hydrolase 1 family β-glucosidase from Thermotoga petrophila (TpBgl1) displays a typical glucose stimulation mechanism based on an increased V_max and decreased K_m in response to glucose. Through molecular docking and dynamics analyses, we mapped putative monosaccharide binding regions (BRs) on the surface of TpBgl1. Our results indicate that after interaction with glucose or xylose at BR1 site, an adjacent loop region assumes an extended conformation, which increases the entrance to the TpBgl1 active site, improving product formation. Biochemical assays with TpBgl1 BR1 mutants, TpBgl1^D49A/Y410A and TpBgl1^D49K/Y410H, resulted in decreasing and abolishing monosaccharide stimulation, respectively. These mutations also impaired the BR1 looping extension responsible for monosaccharide stimulation. This study provides a molecular basis for the rational design of β-glucosidases for biotechnological applications.

Enzymatic processing of lignocellulosic biomass: principles, recent advances and perspectives

Efficient saccharification of lignocellulosic biomass requires concerted development of a pretreatment method, an enzyme cocktail and an enzymatic process, all of which are adapted to the feedstock. Recent years have shown great progress in most aspects of the overall process. In particular, increased insights into the contributions of a wide variety of cellulolytic and hemicellulolytic enzymes have improved the enzymatic processing step and brought down costs. Here, we review major pretreatment technologies and different enzyme process setups and present an in-depth discussion of the various enzyme types that are currently in use. We pay ample attention to the role of the recently discovered lytic polysaccharide monooxygenases (LPMOs), which have led to renewed interest in the role of redox enzyme systems in lignocellulose processing. Better understanding of the interplay between the various enzyme types, as they may occur in a commercial enzyme cocktail, is likely key to further process improvements.

Towards Controlling the Reactivity of Enzymes in Mechanochemistry: Inert Surfaces Protect β-Glucosidase Activity During Ball Milling

The activity of β-glucosidases-the enzymes responsible for the final step in the enzymatic conversion of cellulose to glucose-can be maintained and manipulated under mechanochemical conditions in the absence of bulk solvent, either through an unexpected stabilization effect of inert surfaces, or by altering the enzymatic equilibrium. The reported results illustrate unique aspects of mechanoenzymatic reactions that are not observable in conventional aqueous solutions. They also represent the first reported strategies to enhance activity and favor either direction of the reaction under mechanochemical conditions.

Structural and Functional Insights Into CmGH1, a Novel GH39 Family β-Glucosidase From Deep-Sea Bacterium

Glucosidases play key roles in many diseases and are limiting enzymes during cellulose degradation, which is an important part of global carbon cycle. Here, we identified a novel β-glucosidase, CmGH1, isolated from marine bacterium Croceicoccus marinus E4A9^T. In spite of its high sequence and structural similarity with β-xylosidase family members, CmGH1 had enzymatic activity toward p-nitrophenyl-β-D-glucopyranoside (p-NPG) and cellobiose. The K_m and K_cat values of CmGH1 toward p-NPG were 0.332 ± 0.038 mM and 2.15 ± 0.081 min^–1, respectively. CmGH1 was tolerant to high concentration salts, detergents, as well as many kinds of organic solvents. The crystal structure of CmGH1 was resolved with a 1.8 Å resolution, which showed that CmGH1 was composed of a canonical (α/β)₈-barrel catalytic domain and an auxiliary β-sandwich domain. Although no canonical catalytic triad residues were found in CmGH1, structural comparison and mutagenesis analysis suggested that residues Gln157 and Tyr264 of CmGH1 were the active sites. Mutant Q157E significantly increased its hydrolase activity up to 15-fold, whereas Y264E totally abolished its enzymatic activity. These results might provide new insights into understanding the different catalytic mechanism during evolution for β-glucosidases and β-xylosidases.

Multifunctional elastin-like polypeptide renders β-glucosidase enzyme phase transition and high stability

BACKGROUND

In the enzymatic conversion of biomass, it becomes an important issue to efficiently and cost-effectively degrade cellulose into fermentable glucose. β-Glucosidase (Bgluc), an essential member of cellulases, plays a critical role in cellulosic biomass degradation. The difficulty in improving the stability of Bgluc has been a bottleneck in the enzyme-dependent cellulose degradation. The traditional method of protein purification, however, leads to higher production cost and a decrease in activity. To simplify and efficiently purify Bgluc with modified special properties, Bgluc-tagged ELP and His with defined phase transitions was designed to facilitate the process.

RESULTS

Here, a novel binary ELP and His tag was fused with Bgluc from termite Coptotermes formosanus to construct a Bgluc-linker-ELP-His recombinant fusion protein (BglucLEH). The recombinant plasmid Bgluc expressing a His tag (BglucH) was also constructed. The BglucLEH and BglucH were expressed in E. coli BL21 and purified using inverse transition cycling (ITC) or Ni-NTA resin. The optimum salt concentration for the ITC purification of BglucLEH was 0.5 M (NH₄)₂SO₄ and the specific activity of BglucLEH purified by ITC was 75.5 U/mg for substrate p-NPG, which was slightly higher than that of BglucLEH purified by Ni-NTA (68.2 U/mg). The recovery rate and purification fold of BglucLEH purified by ITC and Ni-NTA were 77.8%, 79.1% and 12.60, 11.60, respectively. The results indicated that purification with ITC was superior to the traditional Ni-NTA. The K _m of BglucLEH and BglucH for p-NPG was 5.27 and 5.73 mM, respectively. The K _ca t/K _m (14.79 S^-1 mM^-1) of BglucLEH was higher than that of BglucH (12.10 S^-1 mM^-1). The effects of ELP tag on the enzyme activity, secondary structure and protein stability were also studied. The results showed that ELP tag did not affect the secondary structure or enzyme activity of Bgluc. More importantly, ELP improved the protein stability in harsh conditions such as heating and exposure to denaturant.

CONCLUSION

The Bgluc-linker-ELP-His system shows wide application prospect in maintaining the activity, efficient purification and improving the stability of Bgluc. These properties of BglucLEH make it an interesting tool to reduce cost, to improve the efficiency of biocatalyst and potentially to enhance the degradation of lignocellulosic biomass.

Glucose tolerant and glucose stimulated β-glucosidases - A review

The β-glucosidases (β-D-glucoside glucohydrolase, EC 3.2.1.21) hydrolyze glycosidic bonds of alkyl-, amino-, or aryl-β-D-glucosides, cyanogenic glucosides, disaccharides and short oligosaccharides and can also catalyze the synthesis of glycosyl-bonds between different molecules via transglycosylation. Due to their ubiquitous phylogenetic distribution, substrate diversity and ability to both hydrolyze and synthesize glycosidic bonds, the catalysis and regulation of β-glucosidases have been extensively studied. Many β-glucosidases are inhibited by the reaction product glucose, and reduced catalytic activity may limit the biotechnological and industrial applications of these enzymes and this has stimulated the search for β-glucosidases that maintain their activity at high glucose concentrations. Studies of many glucose tolerant enzymes have been reported and due to the ongoing interest in these enzymes, here it has been reviewed this accumulated body of knowledge which provides valuable insights as to the kinetics, structure, regulation and evolution of glucose tolerant and glucose stimulated β-glucosidases.

New Insights into the Inhibition Mechanism of Betulinic Acid on α-Glucosidase

Betulinic acid (BA), an important pentacyclic triterpene widely distributed in many foods, possesses high antidiabetic activity. In this study, BA was found to exhibit stronger inhibition of α-glucosidase than acarbose with an IC₅₀ value of (1.06 ± 0.02) × 10^-5 mol L^-1 in a mixed-type manner. BA bound with α-glucosidase to form a BA-α-glucosidase complex, resulting in a more compact structure of the enzyme. The obtained concentrations and spectra profiles of the components resolved by the multivariate-curve resolution-alternating least-squares confirmed the formation of the BA-α-glucosidase complex. Molecular docking showed that BA tightly bound to the active cavity of α-glucosidase, which might hinder the entrance of the substrate leading to a decline in enzyme activity. The chemical modification of α-glucosidase verified the results of the computer simulation that the order of importance of the four amino acid residues in the binding process was His > Tyr > Lys > Arg.

The inhibition of hemicellulosic sugars on cellulose hydrolysis are highly dependant on the cellulase productive binding, processivity, and substrate surface charges

In this study, the influence of major hemicellulosic sugars (mannose and xylose) on cellulose hydrolysis and major enzyme activities were evaluated by using both commercial enzyme cocktail and purified cellulase monocomponents over a "library" of cellulosic substrates. Surprisingly, the results showed that unlike glucose, mannose/xylose did not inhibit individual cellulase activities but significantly decreased their hydrolytic performance on cellulose substrates. When various enzyme-substrate interactions (e.g. adsorption/desorption, productive binding, and processive moving) were evaluated, it appeared that these hemicellulosic sugars significantly reduced the productive binding and processivity of Cel7A, which in turn limited cellulase hydrolytic efficacy. Among a range of major cellulose characteristics (e.g. crystallinity, degree of polymerization, accessibility, and surface charges), the acid group content of the cellulosic substrates seemed to be the main driver that determined the extent of hemicellulosic sugar inhibition. Our results provided new insights for better understanding the sugar inhibition mechanisms of cellulose hydrolysis.

Inhibitory mechanism of two allosteric inhibitors, oleanolic acid and ursolic acid on α-glucosidase

Glycemic control which can be efficaciously regulated by inhibiting α-glucosidase activity is an effective therapy for diabetes mellitus. This work is to investigate the kinetics and inhibition mechanism of oleanolic acid and ursolic acid on α-glucosidase. Oleanolic acid and ursolic acid exhibited potent inhibitory activities with IC50 values of (6.35±0.02)×10-6 and (1.69±0.03)×10-5molL-1 respectively in a reversible and non-competitive manner. Both of them binding to α-glucosidase induced the conformational change and intrinsic fluorescence quenching of α-glucosidase. The binding constants of oleanolic acid and ursolic acid with α-glucosidase at 298K were (2.04±0.02)×103 and (1.87±0.02)×103Lmol-1, respectively. Docking results showed that oleanolic acid and ursolic acid bound in different allosteric sites of cavity 2 and cavity 4 on α-glucosidase, respectively, which triggered allosteric regulation to perturb conformational dynamics of α-glucosidase, eventually leading to a decrease of catalytic activity of the enzyme. The substrate was not catalyzed by α-glucosidase to generate further products due to formation of a nonreactive ternary complex of oleanolic acid- or ursolic acid-α-glucosidase-substrate. The combination of oleanolic acid and ursolic acid displayed a significant synergistic inhibition on α-glucosidase.

Genetic modification: A tool for enhancing beta-glucosidase production for biofuel application

Beta-glucosidase (BGL) is a rate-limiting enzyme for cellulose hydrolysis as it acts in the final step of lignocellulosic biomass conversion to convert cellobiose into glucose, the final end product. Most of the fungal strains used for cellulase production are deficient in BGL hence BGL is supplemented into cellulases to have an efficient biomass conversion. Genetic engineering has enabled strain modification to produce BGL optimally with desired properties to be employed for biofuel applications. It has been cloned either directly into the host strains lacking BGL or into another expression system, to be overexpressed so as to be blended into BGL deficient cellulases. In this article, role of genetic engineering to overcome BGL limitations in the cellulase cocktail and its significance for biofuel applications has been critically reviewed.

Development and characterization of an α-l-rhamnosidase mutant with improved thermostability and a higher efficiency for debittering orange juice

The glycoside hydrolase, α-l-rhamnosidase, could remove the bitter taste of naringin from citrus juices. However, most α-l-rhamnosidases are easily deactivated at high temperatures, limiting the practice in debittering citrus juices. The V529A mutant of the α-l-rhamnosidase r-Rha1 from Aspergillus niger JMU-TS528 was developed with improved thermostability using directed evolution technology and site-directed mutagenesis. The enzyme mutant had a half-live of thermal inactivation T_(1/2) of 1.92 h, 25.00 min, and 2 min at 60, 65, and 70 °C, respectively. In addition, it had improved substrate affinity and better resistance to the inhibition of glucose. The improved substrate affinity was related to its lowered binding energy. Most significantly, the naringin content was reduced to below the bitter taste threshold by treatment with 75 U/mL of the mutant during the preheating process of orange juice production. The comprehensive results indicate that thermostability improvement could promote the practical value of α-l-rhamnosidase in citrus juice processing.

Engineering the GH1 β-glucosidase from Humicola insolens: Insights on the stimulation of activity by glucose and xylose

The activity of the GH1 β-glucosidase from Humicola insolens (Bglhi) against p-nitrophenyl-β-D-glucopyranoside (pNP-Glc) and cellobiose is enhanced 2-fold by glucose and/or xylose. Kinetic and transglycosylation data showed that hydrolysis is preferred in the absence of monosaccharides. Stimulation involves allosteric interactions, increased transglycosylation and competition of the substrate and monosaccharides for the -1 glycone and the +1/+2 aglycone binding sites. Protein directed evolution has been used to generate 6 mutants of Bglhi with altered stimulation patterns. All mutants contain one of three substitutions (N235S, D237V or H307Y) clustered around the +1/+2 aglycone binding sites. Two mutants with the H307Y substitution preferentially followed the transglycosylation route in the absence of xylose or glucose. The strong stimulation of their pNP-glucosidase and cellobiase activities was accompanied by increased transglycosylation and higher monosaccharide tolerance. The D237V mutation favoured hydrolysis over transglycosylation and the pNP-glucosidase activity, but not the cellobiase activity, was stimulated by xylose. The substitution N235S abolished the preference for hydrolysis or transglycosylation; the cellobiase, but not the pNP-glucosidase activity of the mutants was strongly inhibited by xylose. Both the D237V and N235S mutations lowered tolerance to the monosaccharides. These results provide evidence that the fine modulation of the activity of Bglhi and mutants by glucose and/or xylose is regulated by the relative affinities of the glycone and aglycone binding sites for the substrate and the free monosaccharides.

Human Chitotriosidase Is an Endo-Processive Enzyme

Human chitotriosidase (HCHT) is involved in immune response to chitin-containing pathogens in humans. The enzyme is able to degrade chitooligosaccharides as well as crystalline chitin. The catalytic domain of HCHT is connected to the carbohydrate binding module (CBM) through a flexible hinge region. In humans, two active isoforms of HCHT are found-the full length enzyme and its truncated version lacking CBM and the hinge region. The active site architecture of HCHT is reminiscent to that of the reducing-end exo-acting processive chitinase ChiA from bacterium Serratia marcescens (SmChiA). However, the presence of flexible hinge region and occurrence of two active isoforms are reminiscent to that of non-processive endo-chitinase from S. marcescens, SmChiC. Although the studies on soluble chitin derivatives suggest the endo-character of HCHT, the mode of action of the enzyme on crystalline chitin is not known. Here, we made a thorough characterization of HCHT in terms of the mode of action, processivity, binding, and rate constants for the catalysis and dissociation using α-chitin as substrate. HCHT efficiently released the end-label from reducing-end labelled chitin and had also high probability (95%) of endo-mode initiation of processive run. These results qualify HCHT as an endo-processive enzyme. Processivity and the rate constant of dissociation of HCHT were found to be in-between those, characteristic to processive exo-enzymes, like SmChiA and randomly acting non-processive endo-enzymes, like SmChiC. Apart from increasing the affinity for chitin, CBM had no major effect on kinetic properties of HCHT.