Structural basis for reversible phosphorolysis and hydrolysis reactions of 2-O-α-glucosylglycerol phosphorylase

The many functions of carbohydrate-active enzymes in family GH65: diversity and application

Glycoside Hydrolase family 65 (GH65) is a unique family of carbohydrate-active enzymes. It is the first protein family to bring together glycoside hydrolases, glycoside phosphorylases and glycosyltransferases, thereby spanning a broad range of reaction types. These enzymes catalyze the hydrolysis, reversible phosphorolysis or synthesis of various α-glucosides, typically α-glucobioses or their derivatives. In this review, we present a comprehensive overview of the diverse reaction types and substrate specificities found in family GH65. We describe the determinants that control this remarkable diversity, as well as the applications of GH65 enzymes for carbohydrate synthesis.

Complete In Vitro Reconstitution of the Apramycin Biosynthetic Pathway Demonstrates the Unusual Incorporation of a β-d-Sugar Nucleotide in the Final Glycosylation Step

Apramycin is a widely used aminoglycoside antibiotic with applications in veterinary medicine. It is composed of a 4-amino-4-deoxy-d-glucose moiety and the pseudodisaccharide aprosamine, which is an adduct of 2-deoxystreptamine and an unusual eight-carbon bicyclic dialdose. Despite its extensive study and relevance to medical practice, the biosynthetic pathway of this complex aminoglycoside nevertheless remains incomplete. Herein, the remaining unknown steps of apramycin biosynthesis are reconstituted in vitro, thereby leading to a comprehensive picture of its biological assembly. In particular, phosphomutase AprJ and nucleotide transferase AprK are found to catalyze the conversion of glucose 6-phosphate to NDP-β-d-glucose as a critical biosynthetic intermediate. Moreover, the dehydrogenase AprD5 and transaminase AprL are identified as modifying this intermediate via introduction of an amino group at the 4″ position without requiring prior 6″-deoxygenation as is typically encountered in aminosugar biosynthesis. Finally, the glycoside hydrolase family 65 protein AprO is shown to utilize NDP-β-d-glucose or NDP-4"-amino-4"-deoxy-β-d-glucose to form the 8',1″-O-glycosidic linkage of saccharocin or apramycin, respectively. As the activated sugar nucleotides in all known natural glycosylation reactions involve either NDP-α-d-hexoses or NDP-β-l-hexoses, the reported chemistry expands the scope of known biological glycosylation reactions to NDP-β-d-hexoses, with important implications for the understanding and repurposing of aminoglycoside biosynthesis.

Biological sources, metabolism, and production of glucosylglycerols, a group of natural glucosides of biotechnological interest

Glucosylglycerols (GGs) are a group of functional heterosides comprising glycerol and glucose. In nature, they are mainly produced by many moderately salt-tolerant cyanobacteria as compatible solutes in a salt-dependent manner and synthesized in a few higher plants and fermentation processes. Because of their many interesting physicochemical properties and biological activities, such as low sweetness, low hygroscopicity, high water-holding capacity, excellent biocompatibility, favorable performance in protecting macromolecules, and antitumor activity, GGs exhibit large application potential in the fields of cosmetics, health care, food service, enzyme production, and pharmaceuticals. Many in vitro systems using different members of the GH (glycoside hydrolase) family have been established for the enzymatic synthesis of GGs, and a few of them are in use for commercial production. Based on a good understanding of the genetic bases, biochemical processes, and regulatory mechanisms of GG metabolism in microorganisms (mainly cyanobacteria), in recent years GG production technologies with in vivo systems have also been developed by applying metabolic and bioprocess engineering to a few native or heterologous microbial cell factories. This successfully provides the market GG products with an alternative source. With the further elucidation of details about the biological functions of GGs and related mechanisms, the application scope of GGs will be greatly expanded. In the present review, the biological sources and physiological roles of GGs, the molecular bases and regulation of GG metabolism, and the recent progress in GG production and application are systematically summarized. A few new questions that have arisen in the basic research of GGs and perspectives on GG application are also discussed.

Discovery and Biotechnological Exploitation of Glycoside-Phosphorylases

Among carbohydrate active enzymes, glycoside phosphorylases (GPs) are valuable catalysts for white biotechnologies, due to their exquisite capacity to efficiently re-modulate oligo- and poly-saccharides, without the need for costly activated sugars as substrates. The reversibility of the phosphorolysis reaction, indeed, makes them attractive tools for glycodiversification. However, discovery of new GP functions is hindered by the difficulty in identifying them in sequence databases, and, rather, relies on extensive and tedious biochemical characterization studies. Nevertheless, recent advances in automated tools have led to major improvements in GP mining, activity predictions, and functional screening. Implementation of GPs into innovative in vitro and in cellulo bioproduction strategies has also made substantial advances. Herein, we propose to discuss the latest developments in the strategies employed to efficiently discover GPs and make the best use of their exceptional catalytic properties for glycoside bioproduction.

Discovery of a Kojibiose Hydrolase by Analysis of Specificity-Determining Correlated Positions in Glycoside Hydrolase Family 65

The Glycoside Hydrolase Family 65 (GH65) is an enzyme family of inverting α-glucoside phosphorylases and hydrolases that currently contains 10 characterized enzyme specificities. However, its sequence diversity has never been studied in detail. Here, an in-silico analysis of correlated mutations was performed, revealing specificity-determining positions that facilitate annotation of the family’s phylogenetic tree. By searching these positions for amino acid motifs that do not match those found in previously characterized enzymes from GH65, several clades that may harbor new functions could be identified. Three enzymes from across these regions were expressed in E. coli and their substrate profile was mapped. One of those enzymes, originating from the bacterium Mucilaginibacter mallensis, was found to hydrolyze kojibiose and α-1,2-oligoglucans with high specificity. We propose kojibiose glucohydrolase as the systematic name and kojibiose hydrolase or kojibiase as the short name for this new enzyme. This work illustrates a convenient strategy for mapping the natural diversity of enzyme families and smartly mining the ever-growing number of available sequences in the quest for novel specificities.

Whole cell-based catalyst for enzymatic production of the osmolyte 2-O-α-glucosylglycerol

Background

Glucosylglycerol (2-O-α-d-glucosyl-sn-glycerol; GG) is a natural osmolyte from bacteria and plants. It has promising applications as cosmetic and food-and-feed ingredient. Due to its natural scarcity, GG must be prepared through dedicated synthesis, and an industrial bioprocess for GG production has been implemented. This process uses sucrose phosphorylase (SucP)-catalyzed glycosylation of glycerol from sucrose, applying the isolated enzyme in immobilized form. A whole cell-based enzyme formulation might constitute an advanced catalyst for GG production. Here, recombinant production in Escherichia coli BL21(DE3) was compared systematically for the SucPs from Leuconostoc mesenteroides (LmSucP) and Bifidobacterium adolescentis (BaSucP) with the purpose of whole cell catalyst development.

Results

Expression from pQE30 and pET21 plasmids in E. coli BL21(DE3) gave recombinant protein at 40–50% share of total intracellular protein, with the monomeric LmSucP mostly soluble (≥ 80%) and the homodimeric BaSucP more prominently insoluble (~ 40%). The cell lysate specific activity of LmSucP was 2.8-fold (pET21; 70 ± 24 U/mg; N = 5) and 1.4-fold (pQE30; 54 ± 9 U/mg, N = 5) higher than that of BaSucP. Synthesis reactions revealed LmSucP was more regio-selective for glycerol glycosylation (~ 88%; position O2 compared to O1) than BaSucP (~ 66%), thus identifying LmSucP as the enzyme of choice for GG production. Fed-batch bioreactor cultivations at controlled low specific growth rate (µ = 0.05 h⁻¹; 28 °C) for LmSucP production (pET21) yielded ~ 40 g cell dry mass (CDM)/L with an activity of 2.0 × 10⁴ U/g CDM, corresponding to 39 U/mg protein. The same production from the pQE30 plasmid gave a lower yield of 6.5 × 10³ U/g CDM, equivalent to 13 U/mg. A single freeze–thaw cycle exposed ~ 70% of the intracellular enzyme activity for GG production (~ 65 g/L, ~ 90% yield from sucrose), without releasing it from the cells during the reaction.

Conclusions

Compared to BaSucP, LmSucP is preferred for regio-selective GG production. Expression from pET21 and pQE30 plasmids enables high-yield bioreactor production of the enzyme as a whole cell catalyst. The freeze–thaw treated cells represent a highly active, solid formulation of the LmSucP for practical synthesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01569-4.

Disaccharide phosphorylases: Structure, catalytic mechanisms and directed evolution

Disaccharide phosphorylases (DSPs) are carbohydrate-active enzymes with outstanding potential for the biocatalytic conversion of common table sugar into products with attractive properties. They are modular enzymes that form active homo-oligomers. From a mechanistic as well as a structural point of view, they are similar to glycoside hydrolases or glycosyltransferases. As the majority of DSPs show strict stereo- and regiospecificities, these enzymes were used to synthesize specific disaccharides. Currently, protein engineering of DSPs is pursued in different laboratories to broaden the donor and acceptor substrate specificities or improve the industrial particularity of naturally existing enzymes, to eventually generate a toolbox of new catalysts for glycoside synthesis. Herein we review the characteristics and classifications of reported DSPs and the glycoside products that they have been used to synthesize.

High-Yield Biosynthesis of Glucosylglycerol through Coupling Phosphorolysis and Transglycosylation Reactions

Glucosylglycerol is a powerful osmolyte that has attracted attention as a useful moisturizing ingredient in the cosmetic industry. This study demonstrates two artificially designed synthetic routes for manufacturing glucosylglycerol by combining phosphorolysis and transglycosylation reactions. The overall Gibbs energy change of the synthetic routes was negative, indicating that they are thermodynamically favorable. In vitro biosystems were constructed through combining the phosphorolysis ability of sucrose/maltose phosphorylase and the transglycosylation capacity of glucosylglycerol phosphorylases from different organisms. A near-stoichiometric conversion of sucrose and glycerol with a high product yield of 98% was achieved under optimal reaction conditions. The large-scale glucosylglycerol production of this biosystem was investigated under a high concentration of substrates (2 mol/L sucrose and 2.4 mol/L glycerol), and the titer reached 1.78 mol/L (452 g/L) with a productivity of 24.3 g/L/h. To the best of our knowledge, this value presented the highest glucosylglycerol production level until now, which indicated a great industrial application potential for glucosylglycerol manufacturing.

Diverse and common features of trehalases and their contributions to microbial trehalose metabolism

Trehalose is a stable disaccharide that consists of two glucose units linked primarily by an α,α-(1 → 1)-linkage, and it has been found in a wide variety of organisms. In these organisms, trehalose functions not only as a source of carbon energy but also as a protector against various stress conditions. In addition, this disaccharide is attractive for use in a wide range of applications due to its bioactivities. In trehalose metabolism, direct trehalose-hydrolyzing enzymes are known as trehalases, which have been reported for bacteria, archaea, and eukaryotes, and are classified into glycoside hydrolase 37 (GH37), GH65, and GH15 families according to the Carbohydrate-Active enZyme (CAZy) database. The catalytic domains (CDs) of these enzymes commonly share (α/α)₆-barrel structures and have two amino acid residues, Asp and/or Glu, that function as catalytic residues in an inverting mechanism. In this review, I focus on diverse and common features of trehalases within different GH families and their contributions to microbial trehalose metabolism.

Biochemical characteristics of maltose phosphorylase MalE from Bacillus sp. AHU2001 and chemoenzymatic synthesis of oligosaccharides by the enzyme

Maltose phosphorylase (MP), a glycoside hydrolase family 65 enzyme, reversibly phosphorolyzes maltose. In this study, we characterized Bacillus sp. AHU2001 MP (MalE) that was produced in Escherichia coli. The enzyme exhibited phosphorolytic activity to maltose, but not to other α-linked glucobioses and maltotriose. The optimum pH and temperature of MalE for maltose-phosphorolysis were 8.1 and 45°C, respectively. MalE was stable at a pH range of 4.5-10.4 and at ≤40°C. The phosphorolysis of maltose by MalE obeyed the sequential Bi-Bi mechanism. In reverse phosphorolysis, MalE utilized d-glucose, 1,5-anhydro-d-glucitol, methyl α-d-glucoside, 2-deoxy-d-glucose, d-mannose, d-glucosamine, N-acetyl-d-glucosamine, kojibiose, 3-deoxy-d-glucose, d-allose, 6-deoxy-d-glucose, d-xylose, d-lyxose, l-fucose, and l-sorbose as acceptors. The k_cat(app)/K_m(app) value for d-glucosamine and 6-deoxy-d-glucose was comparable to that for d-glucose, and that for other acceptors was 0.23-12% of that for d-glucose. MalE synthesized α-(1→3)-glucosides through reverse phosphorolysis with 2-deoxy-d-glucose and l-sorbose, and synthesized α-(1→4)-glucosides in the reaction with other tested acceptors.

Extensive Turnover of Compatible Solutes in Cyanobacteria Revealed by Deuterium Oxide (D2O) Stable Isotope Probing

Cyanobacteria are important primary producers of organic matter in diverse environments on a global scale. While mechanisms of CO₂ fixation are well understood, the distribution of the flow of fixed organic carbon within individual cells and complex microbial communities is less well characterized. To obtain a general overview of metabolism, we describe the use of deuterium oxide (D₂O) to measure deuterium incorporation into the intracellular metabolites of two physiologically diverse cyanobacteria: a terrestrial filamentous strain (Microcoleus vaginatus PCC 9802) and a euryhaline unicellular strain (Synechococcus sp. PCC 7002). D₂O was added to the growth medium during different phases of the diel cycle. Incorporation of deuterium into metabolites at nonlabile positions, an indicator of metabolite turnover, was assessed using liquid chromatography mass spectrometry. Expectedly, large differences in turnover among metabolites were observed. Some metabolites, such as fatty acids, did not show significant turnover over 12-24 h time periods but did turn over during longer time periods. Unexpectedly, metabolites commonly regarded to act as compatible solutes, including glutamate, glucosylglycerol, and a dihexose, showed extensive turnover compared to most other metabolites already after 12 h, but only during the light phase in the cycle. The observed extensive turnover is surprising considering the conventional view on compatible solutes as biosynthetic end points given the relatively slow growth and constant osmotic conditions. This suggests the possibility of a metabolic sink for some compatible solutes (e.g., into glycogen) that allows for rapid modulation of intracellular osmolarity. To investigate this, uniformly ¹³C-labeled Synechococcus sp. PCC 7002 were exposed to ¹²C glucosylglycerol. Following metabolite extraction, amylase treatment of methanol-insoluble polymers revealed ¹²C labeling of glycogen. Overall, our work shows that D₂O probing is a powerful method for analysis of cyanobacterial metabolism including discovery of novel metabolic processes.

Heterosides--compatible solutes occurring in prokaryotic and eukaryotic phototrophs

The acclimation to osmotic and/or salt stress conditions induces an integrated response at different cellular levels. One acclimation strategy relies on the massive accumulation of low molecular mass compounds, so-called compatible solutes, to balance osmotic gradients and to directly protect critical macromolecules. Heterosides are compounds composed of a sugar and a polyol moiety that represent one chemical class of compatible solutes with interesting features. Well-investigated examples are glucosylglycerol, which is found in many cyanobacteria, and galactosylglycerols (floridoside and isofloridoside), which are accumulated by eukaryotic algae under salt stress conditions. Here, we review knowledge on physiology, biochemistry and genetics of heteroside accumulation in pro- and eukaryotic photoautotrophic organisms.

Diversity of phosphorylases in glycoside hydrolase families

Phosphorylases are useful catalysts for the practical preparation of various sugars. The number of known specificities was 13 in 2002 and is now 30. The drastic increase in available genome sequences has facilitated the discovery of novel activities. Most of these novel phosphorylase activities have been identified through the investigations of glycoside hydrolase families containing known phosphorylases. Here, the diversity of phosphorylases in each family is described in detail.