Function and structure of GH13_31 α-glucosidase with high α-(1→4)-glucosidic linkage specificity and transglucosylation activity

Investigation of cholinesterase and α-glucosidase enzyme activities, and molecular docking and dft studies for 1,2-disubstituted cyclopentane derivatives with phenyl and benzyl units

Six known products (4-9) were prepared from reaction of adipoyl chloride with 1,2,3-trimethoxybenzene according to the literature. From (2,3,4-trimethoxyphenyl)(2-(2,3,4-trimethoxyphenyl)cyclopent-1-en-1-yl)methanone (4) of them, four new 1,2-disubstituted cyclopentane derivatives (10-13) with phenyl and benzyl units were synthesized by reactions such as hydrazonation, catalytic hydrogenation and bromination. The obtained compounds 4-13 were examined for their in vitro inhibitory activity against acetylcholinesterase (AChE), butyrylcholinesterase (BChE) and α-glucosidase enzymes. All compounds 4-13 showed inhibition at nanomolar level with Ki values in the range of 45.53 ± 7.35-631.96 ± 18.88 nM for AChE, 84.30 ± 9.92-622.10 ± 35.14 nM for BChE, and 25.47 ± 4.46-48.87 ± 7.33 for α-Glu. In silico molecular docking studies of the potent compounds were performed in the active sites of AChE (PDB: 1E66), BChE (PDB: 1P0I), and α-glucosidase (PDB: 5ZCC) to compare the effect of bromine atom on the inhibition mechanism. The optimized molecular structures, HOMO-LUMO energies and molecular electrostatic potential maps for the compounds were calculated by using density functional theory with B3LYP/6-31 + G(d,p).

Structural insights into α-(1→6)-linkage preference of GH97 glucodextranase from Flavobacterium johnsoniae

Glycoside hydrolase family 97 (GH97) comprises enzymes like anomer-inverting α-glucoside hydrolases (i.e., glucoamylase) and anomer-retaining α-galactosidases. In a soil bacterium, Flavobacterium johnsoniae, we previously identified a GH97 enzyme (FjGH97A) within the branched dextran utilization locus. It functions as an α-glucoside hydrolase, targeting α-(1→6)-glucosidic linkages in dextran and isomaltooligosaccharides (i.e., glucodextranase). FjGH97A exhibits a preference for α-(1→6)-glucoside linkages over α-(1→4)-linkages, while Bacteroides thetaiotaomicron glucoamylase SusB (with 69% sequence identity), which is involved in the starch utilization system, exhibits the highest specificity for α-(1→4)-glucosidic linkages. Here, we examined the crystal structures of FjGH97A in complexes with glucose, panose, or isomaltotriose, and analyzed the substrate preferences of its mutants to identify the amino acid residues that determine the substrate specificity for α-(1→4)- and α-(1→6)-glucosidic linkages. The overall structure of FjGH97A resembles other GH97 enzymes, with conserved catalytic residues similar to anomer-inverting GH97 enzymes. A comparison of active sites between FjGH97A and SusB revealed differences in amino acid residues at subsites +1 and +2 (specifically Ala195 and Ile378 in FjGH97A). Among the three mutants (A195S, I378F, and A195S-I378F), A195S and A195S-I378F exhibited increased activity toward α-(1→4)-glucoside bonds compared to α-(1→6)-glucoside bonds. This suggests that Ala195, located on the Gly184-Thr203 loop (named loop-N) conserved within the GH97 subgroup, including FjGH97A and SusB, holds significance in determining linkage specificity. The conservation of alanine in the active site of the GH97 enzymes, within the same gene cluster as the putative dextranase, indicates its crucial role in determining the specificity for α-(1→6)-glucoside linkage.

Screening of potential α-glucosidase inhibitors from astragalus membranaceus by affinity ultrafiltration screening coupled with UPLC-ESI-Orbitrap-MS method

Astragalus membranaceus is a traditional Chinese medicine with multiple pharmacological activities. Modern pharmacological research has found that Astragalus membranaceus extract has an inhibitory effect on α-glucosidase, however, which component can inhibit the activity of α-glucosidase and its degree of inhibition are unknown. To address this issue, this study used affinity ultrafiltration screening combined with UPLC-ESI-Orbitrap-MS technology to screen α-glucosidase inhibitors in Astragalus membranaceus. Using affinity ultrafiltration technology, we obtained the active components, and using UPLC-ESI-Orbitrap-MS technology, we quickly analyzed and identified them. As a result, a total of 8 ingredients were selected as α-glucosidase inhibitors.

α-Glucosidase inhibitory potential of Oroxylum indicum using molecular docking, molecular dynamics, and in vitro evaluation

Background

According to the International Diabetes Federation, there will be 578 million individuals worldwide with diabetes by 2030 and 700 million by 2045. One of the promising drug targets to fight diabetes is α-glucosidase (AG), and its inhibitors may be used to manage diabetes by reducing the breakdown of complex carbohydrates into simple sugars. The study aims to identify and validate potential AG inhibitors in natural sources to combat diabetes.

Methods

Computational techniques such as structure-based virtual screening and molecular dyncamic simulation were employed to predict potential AG inhibitors from compounds of Oroxylum indicum. Finally, in silico results were validated by in vitro analysis using n-butanol fraction of crude methanol extracts.

Results

The XP glide scores of top seven hits OI_13, OI_66, OI_16, OI_44, OI_43, OI_20, OI_78 and acarbose were -14.261, -13.475, -13.074, -13.045, -12.978, -12.659, -12.354 and -12.296 kcal/mol, respectively. These hits demonstrated excellent binding affinity towards AG, surpassing the known AG inhibitor acarbose. The MM-GBSA dG binding energies of OI_13, OI_66, and acarbose were -69.093, -62.950, and -53.055 kcal/mol, respectively. Most of the top hits were glycosides, indicating that active compounds lie in the n-butanol fraction of the extract. The IC50 value for AG inhibition by n-butanol fraction was 248.1 μg/ml, and for that of pure acarbose it was 89.16 μg/ml. The predicted oral absorption rate in humans for the top seven hits was low like acarbose, which favors the use of these compounds as anti-diabetes in the small intestine.

Conclusion

In summary, the study provides promising insights into the use of natural compounds derived from O. indicum as potential AG inhibitors to manage diabetes. However, further research, including clinical trials and pharmacological studies, would be necessary to validate their efficacy and safety before clinical use.

Conserved residues at the family and subfamily levels determine enzyme activity and substrate binding in glycoside hydrolase family 13

Site-directed mutagenesis is a valuable strategy for modifying enzymes, but the lack of understanding of conserved residues regulating glycosidase function hinders enzyme design. We analyzed 1662 enzyme sequences to identify conserved amino acids in maltohexaose-forming amylase at both family and subfamily levels. Several conserved residues at the family level (G37, P45, R52, Y57, D101, V103, H106, G230, R232, D234, E264, H330, D331, and G360) were found, mutations of which resulted in reduced enzyme activity or inactivation. At the subfamily level, several conserved residues (L65, E67, F68, D111, E114, R126, R147, F154, W156, F161, G163, D165, W218H, V342, W345, and F346) were identified, which primarily facilitate substrate binding in the enzyme's active site, as shown by molecular dynamics and kinetic assays. Our findings provide critical insights into conserved residues essential for catalysis and can inform targeted enzyme design in protein engineering.

Analyzing Current Trends and Possible Strategies to Improve Sucrose Isomerases' Thermostability

Due to their ability to produce isomaltulose, sucrose isomerases are enzymes that have caught the attention of researchers and entrepreneurs since the 1950s. However, their low activity and stability at temperatures above 40 °C have been a bottleneck for their industrial application. Specifically, the instability of these enzymes has been a challenge when it comes to their use for the synthesis and manufacturing of chemicals on a practical scale. This is because industrial processes often require biocatalysts that can withstand harsh reaction conditions, like high temperatures. Since the 1980s, there have been significant advancements in the thermal stabilization engineering of enzymes. Based on the literature from the past few decades and the latest achievements in protein engineering, this article systematically describes the strategies used to enhance the thermal stability of sucrose isomerases. Additionally, from a theoretical perspective, we discuss other potential mechanisms that could be used for this purpose.

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.

A detailed biochemical characterization, toxicological assessment and molecular docking studies of Launaea fragilis: An important medicinal xero-halophyte

Launaea fragilis (Asso) Pau (Family: Asteraceae) is a wild medicinal plant that has been used in the folklore as a potential treatment for numerous ailments such as skin diseases, diarrhea, infected wounds, inflammation, child fever and hepatic pain. This study explored the chemical constitution, in-vivo toxicity, antimicrobial, antioxidant, and enzyme inhibition potential of ethanolic extract of L. fragilis (EELF). Additionally, in-silico docking studies of predominant compounds were performed against in-vitro tested enzymes. Similarly, in-silico ADMET properties of the compounds were performed to determine their pharmacokinetics, physicochemical properties, and toxicity profiles. The EELF was found rich in TFC (73.45 ± 0.25 mg QE/g) and TPC (109.02 ± 0.23 mg GAE/g). GC-MS profiling of EELF indicated the presence of a total of 47 compounds mainly fatty acids and essential oil. EELF showed no toxicity or growth retardation in chicks up to 300 mg/kg with no effect on the biochemistry and hematology of the chicks. EELF gave promising antioxidant activity through the CUPRAC method with an IC₅₀ value of 13.14 ± 0.18 µg/ml. The highest inhibition activity against tyrosinase followed by acetylcholinesterase and α-Glucosidase was detected. Similarly, the antimicrobial study revealed the extract with good antibacterial and antiviral activity. A good docking score was observed in the in silico computational study of the predominant compounds. The findings revealed L. fragilis as a biocompatible, potent therapeutic alternative and suggest isolation and further in vivo pharmacological studies.

Alteration of Substrate Specificity and Transglucosylation Activity of GH13_31 α-Glucosidase from Bacillus sp. AHU2216 through Site-Directed Mutagenesis of Asn258 on β→α Loop 5

α-Glucosidase catalyzes the hydrolysis of α-d-glucosides and transglucosylation. Bacillus sp. AHU2216 α-glucosidase (BspAG13_31A), belonging to the glycoside hydrolase family 13 subfamily 31, specifically cleaves α-(1→4)-glucosidic linkages and shows high disaccharide specificity. We showed previously that the maltose moiety of maltotriose (G3) and maltotetraose (G4), covering subsites +1 and +2 of BspAG13_31A, adopts a less stable conformation than the global minimum energy conformation. This unstable d-glucosyl conformation likely arises from steric hindrance by Asn258 on β→α loop 5 of the catalytic (β/α)8-barrel. In this study, Asn258 mutants of BspAG13_31A were enzymatically and structurally analyzed. N258G/P mutations significantly enhanced trisaccharide specificity. The N258P mutation also enhanced the activity toward sucrose and produced erlose from sucrose through transglucosylation. N258G showed a higher specificity to transglucosylation with p-nitrophenyl α-d-glucopyranoside and maltose than the wild type. E256Q/N258G and E258Q/N258P structures in complex with G3 revealed that the maltose moiety of G3 bound at subsites +1 and +2 adopted a relaxed conformation, whereas a less stable conformation was taken in E256Q. This structural difference suggests that stabilizing the G3 conformation enhances trisaccharide specificity. The E256Q/N258G-G3 complex formed an additional hydrogen bond between Met229 and the d-glucose residue of G3 in subsite +2, and this interaction may enhance transglucosylation.

Screening of Potential α-Glucosidase Inhibitors from the Roots and Rhizomes of Panax Ginseng by Affinity Ultrafiltration Screening Coupled with UPLC-ESI-Orbitrap-MS Method

Panax ginseng was a traditional Chinese medicine with various pharmacological activities and one of its important activities was hypoglycemic activity; therefore, panax ginseng has been used in China as an adjuvant in the treatment of diabetes mellitus. In vivo and in vitro tests have revealed that ginsenosides, which are derived from the roots and rhizomes of panax ginseng have anti-diabetic effects and produce different hypoglycemic mechanisms by acting on some specific molecular targets, such as SGLT1, GLP-1, GLUTs, AMPK, and FOXO1. α-Glucosidase is another important hypoglycemic molecular target, and its inhibitors can inhibit the activity of α-Glucosidase so as to delay the absorption of dietary carbohydrates and finally reduce postprandial blood sugar. However, whether ginsenosides have the hypoglycemic mechanism of inhibiting α-Glucosidase activity, and which ginsenosides exactly attribute to the inhibitory effect as well as the inhibition degree are not clear, which needs to be addressed and systematically studied. To solve this problem, affinity ultrafiltration screening coupled with UPLC-ESI-Orbitrap-MS technology was used to systematically select α-Glucosidase inhibitors from panax ginseng. The ligands were selected through our established effective data process workflow based on systematically analyzing all compounds in the sample and control specimens. As a result, a total of 24 α-Glucosidase inhibitors were selected from panax ginseng, and it was the first time that ginsenosides were systematically studied for the inhibition of α-Glucosidase. Meanwhile, our study revealed that inhibiting α-Glucosidase activity probably was another important mechanism for ginsenosides treating diabetes mellitus. In addition, our established data process workflow can be used to select the active ligands from other natural products using affinity ultrafiltration screening.

Ca2+-mediated higher-order assembly of heterodimers in amino acid transport system b0,+ biogenesis and cystinuria

Cystinuria is a genetic disorder characterized by overexcretion of dibasic amino acids and cystine, causing recurrent kidney stones and kidney failure. Mutations of the regulatory glycoprotein rBAT and the amino acid transporter b^0,+AT, which constitute system b^0,+, are linked to type I and non-type I cystinuria respectively and they exhibit distinct phenotypes due to protein trafficking defects or catalytic inactivation. Here, using electron cryo-microscopy and biochemistry, we discover that Ca²⁺ mediates higher-order assembly of system b^0,+. Ca²⁺ stabilizes the interface between two rBAT molecules, leading to super-dimerization of b^0,+AT-rBAT, which in turn facilitates N-glycan maturation and protein trafficking. A cystinuria mutant T216M and mutations of the Ca²⁺ site of rBAT cause the loss of higher-order assemblies, resulting in protein trapping at the ER and the loss of function. These results provide the molecular basis of system b^0,+ biogenesis and type I cystinuria and serve as a guide to develop new therapeutic strategies against it. More broadly, our findings reveal an unprecedented link between transporter oligomeric assembly and protein-trafficking diseases.

Elucidating the inhibitory mechanism of yeast α-glucosidase by phytocompounds from Scoparia dulcis through in vitro and in silico approach

Antidiabetic activity of herb Scoparia dulcis Linn (SD) used in traditional medicine is well established, yet, the molecular mechanism is not understood. In this study, in vitro α-glucosidase inhibitory effects of SD aqueous extract and its kinetics were investigated and in silico analysis was carried out. SD showed potent inhibition of α-glucosidase with low IC₅₀value (30 μg/mL). Enzyme kinetics analysis revealed the inhibition to be a mixed type of inhibition. From literature screening, we found that six compounds of SD to exhibit potent anti-diabetic activity, namely apigenin, betulinic acid, hispidulin, luteolin, scopadulcic-acid-B and scutellarein. These compounds were subjected to molecular docking. Docking studies revealed scopadulcic acid B and betulunic acid to show optimum binding constant and low free energy. Molecular dynamics simulation was carried out to further understand the interaction and stability between glucosidase and ligands of SD. Taken together, the study reveals that the potency of SD is due to synergistic effect of active phytochemicals in it and suggest that their properties can be utilized for anti-diabetic treatment strategies.Communicated by Ramaswamy H. Sarma.

Structural Insight into a Yeast Maltase-The BaAG2 from Blastobotrys adeninivorans with Transglycosylating Activity

An early-diverged yeast, Blastobotrys (Arxula) adeninivorans (Ba), has biotechnological potential due to nutritional versatility, temperature tolerance, and production of technologically applicable enzymes. We have biochemically characterized from the Ba type strain (CBS 8244) the GH13-family maltase BaAG2 with efficient transglycosylation activity on maltose. In the current study, transglycosylation of sucrose was studied in detail. The chemical entities of sucrose-derived oligosaccharides were determined using nuclear magnetic resonance. Several potentially prebiotic oligosaccharides with α-1,1, α-1,3, α-1,4, and α-1,6 linkages were disclosed among the products. Trisaccharides isomelezitose, erlose, and theanderose, and disaccharides maltulose and trehalulose were dominant transglycosylation products. To date no structure for yeast maltase has been determined. Structures of the BaAG2 with acarbose and glucose in the active center were solved at 2.12 and 2.13 Å resolution, respectively. BaAG2 exhibited a catalytic domain with a (β/α)₈-barrel fold and Asp216, Glu274, and Asp348 as the catalytic triad. The fairly wide active site cleft contained water channels mediating substrate hydrolysis. Next to the substrate-binding pocket an enlarged space for potential binding of transglycosylation acceptors was identified. The involvement of a Glu (Glu309) at subsite +2 and an Arg (Arg233) at subsite +3 in substrate binding was shown for the first time for α-glucosidases.

Genome-scale metabolic modeling reveals key features of a minimal gene set

Mesoplasma florum, a fast-growing near-minimal organism, is a compelling model to explore rational genome designs. Using sequence and structural homology, the set of metabolic functions its genome encodes was identified, allowing the reconstruction of a metabolic network representing ˜ 30% of its protein-coding genes. Growth medium simplification enabled substrate uptake and product secretion rate quantification which, along with experimental biomass composition, were integrated as species-specific constraints to produce the functional iJL208 genome-scale model (GEM) of metabolism. Genome-wide expression and essentiality datasets as well as growth data on various carbohydrates were used to validate and refine iJL208. Discrepancies between model predictions and observations were mechanistically explained using protein structures and network analysis. iJL208 was also used to propose an in silico reduced genome. Comparing this prediction to the minimal cell JCVI-syn3.0 and its parent JCVI-syn1.0 revealed key features of a minimal gene set. iJL208 is a stepping-stone toward model-driven whole-genome engineering.

A novel pH and thermo-tolerant halophilic alpha-amylase from moderate halophile Nesterenkonia sp. strain F: gene analysis, molecular cloning, heterologous expression and biochemical characterization

A novel pH and thermo-tolerate halophilic alpha-amylase from moderately halophilic bacterium, Nesterenkonia sp.strain F was cloned and expressed in Escherichia coli. 16S rRNA sequence of the strain shared 99.46% similarities with closely related type species. Also, the genome sequence shared ANI values below 92% and dDDH values below 52% with the closely related type species. Consequently, it is proposed that strain F represents a novel species. The AmyF gene was 1390 bp long and encodes an alpha-amylase of 463 amino acid residues with pI of 4.62. The deduced AmyF shared very low sequence similarity (< 24%) with functionally characterized recombinant halophilic alpha-amylases. The recombinant alpha-amylase was successfully purified from Ni-NTA columns with a molecular mass of about 52 KDa on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The enzyme was active over a wide range of temperature (25-75 °C) and pH (4-9) with optimum activity at 45 °C and 7.5, respectively. Also, although it was active over a various concentrations of NaCl and KCl (0-4 M), increasing activity of the enzyme was observed with increasing concentration of these salts. Low concentrations of Ca²⁺ ion had no activating effect, but high concentrations of the ion (40-200 mM) enhanced activity of AmyF. The enzyme activity was increased by increasing concentrations of Mg²⁺, Zn²⁺, Hg²⁺ and Fe³⁺. However, it was inhibited only at very high concentrations of these metal ions. Cu²⁺ did not decrease the amylase activity and the highest activity was observed at 100 mM of the ion. These properties indicate wide potential applications of this recombinant enzyme in starch processing industries. This is the first isolation, cloning and characterization of a gene encoding alpha-amylase from Nesternkonia genus.

An 1,4-α-Glucosyltransferase Defines a New Maltodextrin Catabolism Scheme in Lactobacillus acidophilus

The maltooligosaccharide (MOS) utilization locus in Lactobacillus acidophilus NCFM, a model for human small-intestine lactobacilli, encodes three glycoside hydrolases (GHs): a putative maltogenic α-amylase of family 13, subfamily 20 (LaGH13_20), a maltose phosphorylase of GH65 (LaGH65), and a family 13, subfamily 31, member (LaGH13_31B), annotated as a 1,6-α-glucosidase. Here, we reveal that LaGH13_31B is a 1,4-α-glucosyltransferase that disproportionates MOS with a degree of polymerization of ≥2, with a preference for maltotriose. Kinetic analyses of the three GHs encoded by the MOS locus revealed that the substrate preference of LaGH13_31B toward maltotriose complements the ~40-fold lower k _cat of LaGH13_20 toward this substrate, thereby enhancing the conversion of odd-numbered MOS to maltose. The concerted action of LaGH13_20 and LaGH13_31B confers the efficient conversion of MOS to maltose that is phosphorolyzed by LaGH65. Structural analyses revealed the presence of a flexible elongated loop that is unique for a previously unexplored clade of GH13_31, represented by LaGH13_31B. The identified loop insertion harbors a conserved aromatic residue that modulates the activity and substrate affinity of the enzyme, thereby offering a functional signature of this clade, which segregates from 1,6-α-glucosidases and sucrose isomerases previously described within GH13_31. Genomic analyses revealed that the LaGH13_31B gene is conserved in the MOS utilization loci of lactobacilli, including acidophilus cluster members that dominate the human small intestine.IMPORTANCE The degradation of starch in the small intestine generates short linear and branched α-glucans. The latter are poorly digestible by humans, rendering them available to the gut microbiota, e.g., lactobacilli adapted to the small intestine and considered beneficial to health. This study unveils a previously unknown scheme of maltooligosaccharide (MOS) catabolism via the concerted activity of an 1,4-α-glucosyltransferase together with a classical hydrolase and a phosphorylase. The intriguing involvement of a glucosyltransferase likely allows the fine-tuning of the regulation of MOS catabolism for optimal harnessing of this key metabolic resource in the human small intestine. The study extends the suite of specificities that have been identified in GH13_31 and highlights amino acid signatures underpinning the evolution of 1,4-α-glucosyl transferases that have been recruited in the MOS catabolism pathway in lactobacilli.

Molecular characterization and heterologous expression of two α-glucosidases from Metschnikowia spp, both producers of honey sugars

BACKGROUND

α-Glucosidases are widely distributed enzymes with a varied substrate specificity that are traditionally used in biotechnological industries based on oligo- and polysaccharides as starting materials. According to amino acid sequence homology, α-glucosidases are included into two major families, GH13 and GH31. The members of family GH13 contain several α-glucosidases with confirmed hydrolytic activity on sucrose. Previously, a sucrose splitting activity from the nectar colonizing yeast Metschnikowia reukaufii which produced rare sugars with α-(1→1), α-(1→3) and α-(1→6) glycosidic linkages from sucrose was described.

RESULTS

In this study, genes codifying for α-glucosidases from the nectaries yeast M. gruessii and M. reukaufii were characterised and heterologously expressed in Escherichia coli for the first time. Recombinant proteins (Mg-αGlu and Mr-αGlu) were purified and biochemically analysed. Both enzymes mainly displayed hydrolytic activity towards sucrose, maltose and p-nitrophenyl-α-D-glucopyranoside. Structural analysis of these proteins allowed the identification of common features from the α-amylase family, in particular from glycoside hydrolases that belong to family GH13. The three acidic residues comprising the catalytic triad were identified and their relevance for the protein hydrolytic mechanism confirmed by site-directed mutagenesis. Recombinant enzymes produced oligosaccharides naturally present in honey employing sucrose as initial substrate and gave rise to mixtures with the same products profile (isomelezitose, trehalulose, erlose, melezitose, theanderose and esculose) previously obtained with M. reukaufii cell extracts. Furthermore, the same enzymatic activity was detected with its orthologous Mg-αGlu from M. gruessii. Interestingly, the isomelezitose amounts obtained in reactions mediated by the recombinant proteins, ~ 170 g/L, were the highest reported so far.

CONCLUSIONS

Mg/Mr-αGlu were heterologously overproduced and their biochemical and structural characteristics analysed. The recombinant α-glucosidases displayed excellent properties in terms of mild reaction conditions, in addition to pH and thermal stability. Besides, the enzymes produced a rare mixture of hetero-gluco-oligosaccharides by transglucosylation, mainly isomelezitose and trehalulose. These compounds are natural constituents of honey which purification from this natural source is quite unviable, what make these enzymes very interesting for the biotechnological industry. Finally, it should be remarked that these sugars have potential applications as food additives due to their suitable sweetness, viscosity and humectant capacity.

Structure-function analysis of silkworm sucrose hydrolase uncovers the mechanism of substrate specificity in GH13 subfamily 17 exo-α-glucosidases

The domestic silkworm Bombyx mori expresses two sucrose-hydrolyzing enzymes, BmSUH and BmSUC1, belonging to glycoside hydrolase family 13 subfamily 17 (GH13_17) and GH32, respectively. BmSUH has little activity on maltooligosaccharides, whereas other insect GH13_17 α-glucosidases are active on sucrose and maltooligosaccharides. Little is currently known about the structural mechanisms and substrate specificity of GH13_17 enzymes. In this study, we examined the crystal structures of BmSUH without ligands; in complexes with substrates, products, and inhibitors; and complexed with its covalent intermediate at 1.60-1.85 Å resolutions. These structures revealed that the conformations of amino acid residues around subsite -1 are notably different at each step of the hydrolytic reaction. Such changes have not been previously reported among GH13 enzymes, including exo- and endo-acting hydrolases, such as α-glucosidases and α-amylases. Amino acid residues at subsite +1 are not conserved in BmSUH and other GH13_17 α-glucosidases, but subsite -1 residues are absolutely conserved. Substitutions in three subsite +1 residues, Gln¹⁹¹, Tyr²⁵¹, and Glu⁴⁴⁰, decreased sucrose hydrolysis and increased maltase activity of BmSUH, indicating that these residues are key for determining its substrate specificity. These results provide detailed insights into structure-function relationships in GH13 enzymes and into the molecular evolution of insect GH13_17 α-glucosidases.