Functional and structural differences between isoflavonoid β-glycosidases from Dalbergia sp

A GH1 β-glucosidase from the Fervidobacterium pennivorans DSM9078 showed extraordinary thermostability and distinctive ability in the efficient transformation of ginsenosides

A novel GH1 β-glucosidase Fpglu1 from Fervidobacterium pennivorans DSM9078 was successfully cloned and expressed in Escherichia coli. This hyperthermophilic enzyme possesses unique features that make it valuable in biochemistry and pharmacology. It exhibited optimal activity at temperatures exceeding 100 °C, a trait rarely observed in other enzymes, and demonstrated extraordinary thermostability. It displayed multifunctional activity, with the highest activity observed for p-nitrophenyl-β-d-glucopyranoside (pNPGlu) at 92.47 U/mg. Furthermore, the distinctive capacity of Fpglu1 to transform ginsenosides (Rb1, Rb2, and Rc) into Compound-K (C-K) sets it apart from the other enzymes. It effectively cleaved the external β-(1-6) glycosidic linkage at the C-20 position of ginsenosides Rb1, Rb2, and Rc, followed by hydrolysis ofthe internal glycosidic bond connected to the C-3 position. The kcat/Km value of Fpglu1 for Rb1 was 453 ± 1.27 mM-1/s, significantly higher than those of Fpglu1 for other ginsenosides. The crystal structure of Fpglu1, determined at 1.85 Å resolution, provided a deeper understanding of its catalysis and substrate specificity. The evaluation of the binding conformation, hydrogen bond, and key amino acids of β-glucosidase Fpglu1 with different ginsenosides (Rb1, Rb2, and Rc) further elucidated the structural basis of its substrate-binding preference. In summary, Fpglu1, which had excellent thermostability and unique ginsenoside-transforming ability, was a highly promising catalyst for the industrial production of ginsenoside C-K. Additionally, structural studies have laid a theoretical foundation for further improving the catalytic properties of the enzyme through directed evolution in the future.

Crystal structures of Aspergillus oryzae exo-β-(1,3)-glucanase reveal insights into oligosaccharide binding, recognition, and hydrolysis

Exo-β-(1,3)-glucanases are promising enzymes for use in the biofuel industry as they hydrolyse sugars such as laminarin, a major constituent of the algal cell wall. This study reports structural and biochemical characterizations of Aspergillus oryzae exo-β-(1,3)-glucanase (AoBgl) belonging to the GH5 family. Purified AoBgl hydrolyses β-(1,3)-glycosidic linkages of the oligosaccharide laminaritriose and the polysaccharide laminarin effectively. We have determined three high-resolution structures of AoBgl: (a) the apo form at 1.75 Å, (b) the complexed form with bound cellobiose at 1.73 Å and (c) the glucose-bound form at 1.20 Å. The crystal structures, molecular dynamics simulation studies and site-directed mutagenesis reveal the mode of substrate binding and interactions at the active site. The results also indicate that AoBgl effectively hydrolyses trisaccharides and higher oligosaccharides. The findings from our structural and biochemical studies would aid in rational engineering efforts to generate superior AoBgl variants and similar GH5 enzymes for their industrial use.

Facilitating secretory expression of apple seed β-glucosidase in Komagataella phaffii for the efficient preparation of salidroside

Plant-derived β-glucosidases hold promise for glycoside biosynthesis via reverse hydrolysis because of their excellent glucose tolerance and robust stability. However, their poor heterologous expression hinders the development of large-scale production and applications. In this study, we overexpressed apple seed β-glucosidase (ASG II) in Komagataella phaffii and enhanced its production from 289 to 4322 U L-1 through expression cassette engineering and protein engineering. Upon scaling up to a 5-L high cell-density fermentation, the resultant mutant ASG IIV80A achieved a maximum protein concentration and activity in the secreted supernatant of 2.3 g L-1 and 41.4 kU L-1, respectively. The preparative biosynthesis of salidroside by ASG IIV80A exhibited a high space-time yield of 33.1 g L-1 d-1, which is so far the highest level by plant-derived β-glucosidase. Our work addresses the long-standing challenge of the heterologous expression of plant-derived β-glucosidase in microorganisms and presents new avenues for the efficient production of salidroside and other natural glycosides.

Acuminosylation of Tyrosol by a Commercial Diglycosidase

A commercial glycosidase mixture obtained from Penicillium multicolor (Aromase H2) was found to comprise a specific diglycosidase activity, β-acuminosidase, alongside undetectable levels of β-apiosidase. The enzyme was tested in the transglycosylation of tyrosol using 4-nitrophenyl β-acuminoside as the diglycosyl donor. The reaction was not chemoselective, providing a mixture of Osmanthuside H and its counterpart regioisomer 4-(2-hydroxyethyl)phenyl β-acuminoside in 58% yield. Aromase H2 is therefore the first commercial β-acuminosidase which is also able to glycosylate phenolic acceptors.

Comparative Cytological and Transcriptome Analyses of Anther Development in Nsa Cytoplasmic Male Sterile (1258A) and Maintainer Lines in Brassica napus Produced by Distant Hybridization

1258A is a new line of B.napus with Nsa cytoplasmic male sterility (CMS) with potential applications in hybrid rapeseed breeding. Sterile cytoplasm was obtained from XinJiang Sinapis arvensis through distant hybridization and then backcrossed with 1258B for many generations. However, the characteristics and molecular mechanisms underlying pollen abortion in this sterile line are poorly understood. In this study, a cytological analysis revealed normal microsporogenesis and uninucleate pollen grain formation. Pollen abortion was due to non-programmed cell death in the tapetum and the inability of microspores to develop into mature pollen grains. Sucrose, soluble sugar, and adenosine triphosphate (ATP) contents during microspore development were lower than those of the maintainer line, along with an insufficient energy supply, reduced antioxidant enzyme activity, and substantial malondialdehyde (MDA) accumulation in the anthers. Transcriptome analysis revealed that genes involved in secondary metabolite biosynthesis, glutathione metabolism, phenylpropane biosynthesis, cyanoamino acid metabolism, starch and sucrose metabolism, and glycerolipid metabolism may contribute to pollen abortion. The down regulation of nine cytochrome P450 monooxygenases genes were closely associated with pollen abortion. These results suggest that pollen abortion in 1258A CMS stems from abnormalities in the chorioallantoic membranes, energy deficiencies, and dysfunctional antioxidant systems in the anthers. Our results provide insight into the molecular mechanism underlying pollen abortion in Nsa CMS and provide a theoretical basis for better heterosis utilization in B.napus.

Structural and Catalytic Characterization of TsBGL, a β-Glucosidase From Thermofilum sp. ex4484_79

Beta-glucosidase is an enzyme that catalyzes the hydrolysis of the glycosidic bonds of cellobiose, resulting in the production of glucose, which is an important step for the effective utilization of cellulose. In the present study, a thermostable β-glucosidase was isolated and purified from the Thermoprotei Thermofilum sp. ex4484_79 and subjected to enzymatic and structural characterization. The purified β-glucosidase (TsBGL) exhibited maximum activity at 90°C and pH 5.0 and displayed maximum specific activity of 139.2μmol/min/mg_zne against p-nitrophenyl β-D-glucopyranoside (pNPGlc) and 24.3μmol/min/mg_zen against cellobiose. Furthermore, TsBGL exhibited a relatively high thermostability, retaining 84 and 47% of its activity after incubation at 85°C for 1.5h and 90°C for 1.5h, respectively. The crystal structure of TsBGL was resolved at a resolution of 2.14Å, which revealed a classical (α/β)₈-barrel catalytic domain. A structural comparison of TsBGL with other homologous proteins revealed that its catalytic sites included Glu210 and Glu414. We provide the molecular structure of TsBGL and the possibility of improving its characteristics for potential applications in industries.

Apiose-Relevant Glycosidases

Apiose is a branched pentose naturally occurring either as a component of the plant cell wall polysaccharides or as a sugar moiety present in numerous plant secondary metabolites such as flavonoid and phenylethanoid glycosides, substrates in plant defense systems or as glycosylated aroma precursors. The enzymes catalyzing hydrolysis of such apiosylated substances (mainly glycosidases specific towards apiose or acuminose) have promising applications not only in hydrolysis (flavor development), but potentially also in the synthesis of apiosides and apioglucosides with pharmaceutical relevance. This review summarizes the actual knowledge of glycosidases recognizing apiose and their potential application in biocatalysis.

Crystal structure of a GH1 β-glucosidase from Hamamotoa singularis

A GH1 β-glucosidase from the fungus Hamamotoa singularis (HsBglA) has high transgalactosylation activity and efficiently converts lactose to galactooligosaccharides. Consequently, HsBglA is among the most widely used enzymes for industrial galactooligosaccharide production. Here, we present the first crystal structures of HsBglA with and without 4'-galactosyllactose, a tri-galactooligosaccharide, at 3.0 and 2.1 Å resolutions, respectively. These structures reveal details of the structural elements that define the catalytic activity and substrate binding of HsBglA, and provide a possible interpretation for its high catalytic potency for transgalactosylation reaction.

Flavonoids from the stems of Millettia pachyloba Drake mediate cytotoxic activity through apoptosis and autophagy in cancer cells

In this study, systematic separation and subsequent pharmacological activity studies were carried out to identify cytotoxic natural products from the dried stems of Millettia pachyloba Drake. Five previously undescribed isoflavones, pachyvones A-E; one previously undescribed xanthone, pachythone A; and twenty-two known compounds were obtained. The structures of these compounds were assigned on the basis of 1D/2D NMR data and high-resolution electrospray ionization mass spectroscopy analysis. Preliminary activity screening with HeLa and MCF-7 cells showed that ten compounds (3-5, 9, 12, 17-19, 24, and 25) had potential cytotoxicity. Further in-depth activity studies with five cancer cell lines (HeLa, HepG2, MCF-7, Hct116, and MDA-MB-231) and one normal cell line (HUVEC) revealed that these ten compounds showed specific cytotoxicity in cancer cells, with IC50 values ranging from 5 to 40 μM, while they had no effect on normal cell lines. To investigate whether the cytotoxicity of these ten compounds was associated with autophagy, their autophagic effects were evaluated in GFP-LC3-HeLa cells. The results demonstrated that compound 9 (durmillone) significantly induced autophagy in a concentration-dependent manner and had the best activity as an autophagy inducer among all of the compounds. Therefore, compound 9 was selected for further study. The PI/Annexin V double staining assay and Western blotting results revealed that compound 9 also induced obvious apoptosis in HeLa and MCF-7 cells, which suggests that it mediates cytotoxic activity through activation of both apoptosis and autophagy. Taken together, this study identified ten natural cytotoxic products from the dried stems of Millettia pachyloba Drake, of which compound 9 induced apoptosis and autophagy and could be an anticancer drug candidate.

Molecular Characterization and Potential Synthetic Applications of GH1 β-Glucosidase from Higher Termite Microcerotermes annandalei

A novel β-glucosidase from higher termite Microcerotermes annandalei (MaBG) was obtained via a screening method targeting β-glucosidases with increased activities in the presence of glucose. The purified natural MaBG showed a subunit molecular weight of 55 kDa and existed in a native form as a dimer without any glycosylation. Gene-specific primers designed from its partial amino acid sequences were used to amplify the corresponding 1,419-bp coding sequence of MaBG which encodes a 472-amino acid glycoside hydrolase family 1 (GH1) β-glucosidase. When expressed in Komagataella pastoris, the recombinant MaBG appeared as a ~ 55-kDa protein without glycosylation modifications. Kinetic parameters as well as the lack of secretion signal suggested that MaBG is an intracellular enzyme and not involved in cellulolysis. The hydrolytic activities of MaBG were enhanced in the presence of up to 3.5-4.5 M glucose, partly due to its strong transglucosylation activity, which suggests its applicability in biosynthetic processes. The potential synthetic activities of the recombinant MaBG were demonstrated in the synthesis of para-nitrophenyl-β-D-gentiobioside via transglucosylation and octyl glucoside via reverse hydrolysis. The information obtained from this study has broadened our insight into the functional characteristics of this variant of termite GH1 β-glucosidase and its applications in bioconversion and biotechnology.

β-Glucosidases: Multitasking, moonlighting or simply misunderstood?

β-Glucosidases have a wide range of functions in plants, including roles in recycling of cell-wall oligosaccharides, defense, phytohormone signaling, secondary metabolism, and scent release, among others. It is not always clear which one is responsible for a specific function, as plants contain a large set of β-glucosidases. However, progress has been made in recent years in elucidating these functions. To help understand what is known and what remains ambiguous, we review the general approaches to investigating plant β-glucosidase functions. We consider information that has been gained regarding glycoside hydrolase family 1 enzyme functions utilizing these approaches in the past decade. In several cases, one enzyme has been assigned different biological functions by different research groups. We suggest that, at least in some cases, the ambiguity of an enzyme's function may come from having multiple functions that may help coordinate the response to injury or other stresses.

A novel soybean (Glycine max) gene encoding a family 3 β-glucosidase has high isoflavone 7-O-glucoside-hydrolyzing activity in transgenic rice

A previous study demonstrated that purified Glycine max β-glucosidase (GmBGL) could hydrolyze glucosyl isoflavone to the aglyconic form. This study reports the cloning and functional characterization of a soybean cDNA encoding the β-glucosidase. GmBGL was isolated by use of a purified soybean N-terminal amino acid sequence and conserved sequences of β-glucosidase genes from other plants. Sequence analysis of GmBGL revealed an open reading frame of 1884 bp encoding a polypeptide of 627 amino acids with a calculated molecular mass of 69 kDa. Phylogenetic analysis classified the GmBGL into the glycosyl hydrolase 3 family. In soybean, the GmBGL transcript was predominantly accumulated in roots and leaves. To examine the enzymatic activity and substrate specificity, GmBGL was ectopically expressed in transgenic rice. Purified GmBGL protein from transgenic rice could catalyze the hydrolysis of genistin and daidzin to produce genistein and daidzein, respectively, which confirmed GmBGL as a functional β-glucosidase with isoflavone glucoside-hydrolyzing activity. This paper reveals that GmBGL is a key enzyme in transforming glucosyl isoflavones to aglycones in soybean, which may help in genetic manipulation of aglycone-rich soybean seeds.

Crystal structures of β-primeverosidase in complex with disaccharide amidine inhibitors

β-Primeverosidase (PD) is a disaccharide-specific β-glycosidase in tea leaves. This enzyme is involved in aroma formation during the manufacturing process of oolong tea and black tea. PD hydrolyzes β-primeveroside (6-O-β-d-xylopyranosyl-β-d-glucopyranoside) at the β-glycosidic bond of primeverose to aglycone, and releases aromatic alcoholic volatiles of aglycones. PD only accepts primeverose as the glycone substrate, but broadly accepts various aglycones, including 2-phenylethanol, benzyl alcohol, linalool, and geraniol. We determined the crystal structure of PD complexes using highly specific disaccharide amidine inhibitors, N-β-primeverosylamidines, and revealed the architecture of the active site responsible for substrate specificity. We identified three subsites in the active site: subsite -2 specific for 6-O-β-d-xylopyranosyl, subsite -1 well conserved among β-glucosidases and specific for β-d-glucopyranosyl, and wide subsite +1 for hydrophobic aglycone. Glu-470, Ser-473, and Gln-477 act as the specific hydrogen bond donors for 6-O-β-d-xylopyranosyl in subsite -2. On the other hand, subsite +1 was a large hydrophobic cavity that accommodates various aromatic aglycones. Compared with aglycone-specific β-glucosidases of the glycoside hydrolase family 1, PD lacks the Trp crucial for aglycone recognition, and the resultant large cavity accepts aglycone and 6-O-β-d-xylopyranosyl together. PD recognizes the β-primeverosides in subsites -1 and -2 by hydrogen bonds, whereas the large subsite +1 loosely accommodates various aglycones. The glycone-specific activity of PD for broad aglycone substrates results in selective and multiple release of temporally stored alcoholic volatile aglycones of β-primeveroside.

β-Glucosidases

β-Glucosidases (3.2.1.21) are found in all domains of living organisms, where they play essential roles in the removal of nonreducing terminal glucosyl residues from saccharides and glycosides. β-Glucosidases function in glycolipid and exogenous glycoside metabolism in animals, defense, cell wall lignification, cell wall β-glucan turnover, phytohormone activation, and release of aromatic compounds in plants, and biomass conversion in microorganisms. These functions lead to many agricultural and industrial applications. β-Glucosidases have been classified into glycoside hydrolase (GH) families GH1, GH3, GH5, GH9, and GH30, based on their amino acid sequences, while other β-glucosidases remain to be classified. The GH1, GH5, and GH30 β-glucosidases fall in GH Clan A, which consists of proteins with (β/α)(8)-barrel structures. In contrast, the active site of GH3 enzymes comprises two domains, while GH9 enzymes have (α/α)(6) barrel structures. The mechanism by which GH1 enzymes recognize and hydrolyze substrates with different specificities remains an area of intense study.

Structural and enzymatic characterization of Os3BGlu6, a rice beta-glucosidase hydrolyzing hydrophobic glycosides and (1->3)- and (1->2)-linked disaccharides

Glycoside hydrolase family 1 (GH1) beta-glucosidases play roles in many processes in plants, such as chemical defense, alkaloid metabolism, hydrolysis of cell wall-derived oligosaccharides, phytohormone regulation, and lignification. However, the functions of most of the 34 GH1 gene products in rice (Oryza sativa) are unknown. Os3BGlu6, a rice beta-glucosidase representing a previously uncharacterized phylogenetic cluster of GH1, was produced in recombinant Escherichia coli. Os3BGlu6 hydrolyzed p-nitrophenyl (pNP)-beta-d-fucoside (k(cat)/K(m) = 67 mm(-1) s(-1)), pNP-beta-d-glucoside (k(cat)/K(m) = 6.2 mm(-1) s(-1)), and pNP-beta-d-galactoside (k(cat)/K(m) = 1.6 mm(-1)s(-1)) efficiently but had little activity toward other pNP glycosides. It also had high activity toward n-octyl-beta-d-glucoside and beta-(1-->3)- and beta-(1-->2)-linked disaccharides and was able to hydrolyze apigenin beta-glucoside and several other natural glycosides. Crystal structures of Os3BGlu6 and its complexes with a covalent intermediate, 2-deoxy-2-fluoroglucoside, and a nonhydrolyzable substrate analog, n-octyl-beta-d-thioglucopyranoside, were solved at 1.83, 1.81, and 1.80 A resolution, respectively. The position of the covalently trapped 2-F-glucosyl residue in the enzyme was similar to that in a 2-F-glucosyl intermediate complex of Os3BGlu7 (rice BGlu1). The side chain of methionine-251 in the mouth of the active site appeared to block the binding of extended beta-(1-->4)-linked oligosaccharides and interact with the hydrophobic aglycone of n-octyl-beta-d-thioglucopyranoside. This correlates with the preference of Os3BGlu6 for short oligosaccharides and hydrophobic glycosides.

The beta-glucosidases responsible for bioactivation of hydroxynitrile glucosides in Lotus japonicus

Lotus japonicus accumulates the hydroxynitrile glucosides lotaustralin, linamarin, and rhodiocyanosides A and D. Upon tissue disruption, the hydroxynitrile glucosides are bioactivated by hydrolysis by specific beta-glucosidases. A mixture of two hydroxynitrile glucoside-cleaving beta-glucosidases was isolated from L. japonicus leaves and identified by protein sequencing as LjBGD2 and LjBGD4. The isolated hydroxynitrile glucoside-cleaving beta-glucosidases preferentially hydrolyzed rhodiocyanoside A and lotaustralin, whereas linamarin was only slowly hydrolyzed, in agreement with measurements of their rate of degradation upon tissue disruption in L. japonicus leaves. Comparative homology modeling predicted that LjBGD2 and LjBGD4 had nearly identical overall topologies and substrate-binding pockets. Heterologous expression of LjBGD2 and LjBGD4 in Arabidopsis (Arabidopsis thaliana) enabled analysis of their individual substrate specificity profiles and confirmed that both LjBGD2 and LjBGD4 preferentially hydrolyze the hydroxynitrile glucosides present in L. japonicus. Phylogenetic analyses revealed a third L. japonicus putative hydroxynitrile glucoside-cleaving beta-glucosidase, LjBGD7. Reverse transcription-polymerase chain reaction analysis showed that LjBGD2 and LjBGD4 are expressed in aerial parts of young L. japonicus plants, while LjBGD7 is expressed exclusively in roots. The differential expression pattern of LjBGD2, LjBGD4, and LjBGD7 corresponds to the previously observed expression profile for CYP79D3 and CYP79D4, encoding the two cytochromes P450 that catalyze the first committed step in the biosyntheis of hydroxynitrile glucosides in L. japonicus, with CYP79D3 expression in aerial tissues and CYP79D4 expression in roots.

beta-Glucosidases as detonators of plant chemical defense

Some plant secondary metabolites are classified as phytoanticipins. When plant tissue in which they are present is disrupted, the phytoanticipins are bio-activated by the action of beta-glucosidases. These binary systems--two sets of components that when separated are relatively inert--provide plants with an immediate chemical defense against protruding herbivores and pathogens. This review provides an update on our knowledge of the beta-glucosidases involved in activation of the four major classes of phytoanticipins: cyanogenic glucosides, benzoxazinoid glucosides, avenacosides and glucosinolates. New aspects of the role of specific proteins that either control oligomerization of the beta-glucosidases or modulate their product specificity are discussed in an evolutionary perspective.