Characterization ofMeiothermus taiwanensisGalactokinase and its Use in the One-Pot Enzymatic Synthesis of Uridine Diphosphate-Galactose and the Chemoenzymatic Synthesis of the Carbohydrate Antigen Stage Specific Embryonic Antigen-3

Comprehensive Modular Synthesis of Ganglioside Glycans and Evaluation of their Binding Affinities to Siglec-7 and Siglec-9

In the present work, bacterial glycosyltransferases are utilized to construct ganglioside glycans in a convergent approach via a sugar‒nucleotide regeneration system and one-pot multienzyme reactions. Starting from β-lactoside enables the diversification of both the glycan moieties and the linkages in the lower α-arm and upper β-arm. Overall, a comprehensive panel of 24 natural a-series (GM3, GM2, GM1a, GD1a, GT1a, and fucosyl-GM1), b-series (GD3, GD2, GD1b, GT1b, and GQ1b), c-series (GT3, GT2, GT1c, GQ1c, and GP1c), α-series (GM1α, GD1aα, and GT1aα), and o-series (GA2, GA1, GM1b, GalNAc-GM1b, and GD1c) ganglioside glycans are prepared, which are suitable for biological studies and further applications. Moreover, a microarray is constructed with these synthesized ganglioside glycans to investigate their binding specificity with recombinant Fc-fused Siglec-7 and Siglec-9, which are immune checkpoint-like glycan recognition proteins on natural killer cells. The microarray binding results reveal that GD3 and GT1aα are specific ligands for Siglec-7 and Siglec-9, respectively, and this discovery can lead to the identification of appropriate ligands for investigating the roles of these Siglecs in immunomodulation.

Efficient One-Pot Synthesis of Uridine Diphosphate Galactose Employing a Trienzyme System

The limited availability of high-cost nucleotide sugars is a significant constraint on the application of their downstream products (glycosides and prebiotics) in the food or pharmaceutical industry. To better solve the problem, this study presented a one-pot approach for the biosynthesis of UDP-Gal using a thermophilic multienzyme system consisting of GalK, UGPase, and PPase. Under optimal conditions, a 2 h reaction resulted in a UTP conversion rate of 87.4%. In a fed-batch reaction with Gal/ATP = 20 mM:10 mM, UDP-Gal accumulated to 33.76 mM with a space-time yield (STY) of 6.36 g/L·h-1 after the second feeding. In repetitive batch synthesis, the average yield of UDP-Gal over 8 cycles reached 10.80 g/L with a very low biocatalyst loading of 0.002 genzymes/gproduct. Interestingly, Galk (Tth0595) could synthesize Gal-1P using ADP as a donor of phosphate groups, which had never been reported before. This approach possessed the benefits of high synthesis efficiency, low cost, and superior reaction system stability, and it provided new insights into the rapid one-pot synthesis of UDP-Gal and high-value glycosidic compounds.

Chemoenzymatic Synthesis of Globo-series Glycosphingolipids and Evaluation of Their Immunosuppressive Activities

Glycosphingolipids (GSLs) play essential roles in many important biological processes, making them attractive synthetic targets. In this paper, a viable chemoenzymatic method is described for the synthesis of globo-series GSLs, namely, Gb4, Gb5, SSEA-4, and Globo H. The strategy uses a chemically synthesized lactoside acceptor equipped with a partial ceramide structure that is uniquely extended by glycosyltransferases in a highly efficient one-pot multiple engyme (OPME) procedure. A direct and quantitative conversion of Gb4 sphingosine to Globo H sphingosine is achieved by performing two-sequential OPME glycosylations. A reduction and N -acylation protocol allows facile incorporation of various fatty acids into the lipid portions of the GSLs. The chemically well-defined lipid-modified Globo H-GSLs displayed some differences in their immunosuprressive activities, which may benefit the structural modifications of Globo h ceramides in finding new types of immunosuppressive agents. The strategy outlined in this work should be applicable to rapid access to other complex GSLs.

Acceptor-mediated regioselective enzyme catalyzed sialylation: chemoenzymatic synthesis of GAA-7 ganglioside glycan

Herein, we applied PmST1 (a sialyltransferase) to achieve acceptor-mediated regioselective sialylation (AMRS) on the nonreducing end GalNH₂ or GalAz (2-azido-2-deoxy galactose). Thus, C5 and C8-modified sialic acid was efficiently assembled on GalNH₂ (or GalAz) to achieve the synthesis of the GAA-7 (one of the echinodermatous gangliosides with higher neuritogenic activity) glycan moiety.

A Modular Chemoenzymatic Synthesis of Disialosyl Globopentaosylceramide (DSGb5Cer) Glycan

The total synthesis of the oligosaccharide moiety of disialosyl globopentaosylceramide (DSGb5 Cer), a dominant ganglioside isolated from malignant renal cell carcinoma tissues, is reported. The synthetic strategy relies on a chemical α(2,6)-sialylation at the internal GalNAc unit of a Gb5 pentasaccharide backbone that furnishes a Neu5Acα(2,6)GalNAc-linked hexasaccharide, suitable for an enzymatic α(2,3)-sialylation of the terminal Gal residue to construct a heptasaccharide glycan. Convergent access to this key α(2,6)-sialylated hexasaccharide was also achieved through a [3+3] glycosylation building upon a Galβ(1,3)[Neu5Acα(2,6)]GalNAc-based trisaccharide donor and a Gb3 acceptor. The synthetic DSGb5 glycan bearing a 6-azidohexyl aglycon at the reducing end could undergo further regioselective functionalization. This approach represents a viable chemoenzymatic method for accessing complex ganglioside glycans and should be useful for the synthesis and biological investigation of DSGb5 derivatives.

Chemoenzymatic synthesis of the oligosaccharide moiety of the tumor-associated antigen disialosyl globopentaosylceramide

Disialosyl globopentaosylceramide (DSGb5) is often expressed by renal cell carcinomas. To investigate properties of DSGb5, we have prepared its oligosaccharide moiety by chemically synthesizing Gb5 which was enzymatically sialylated using the mammalian sialyltransferases ST3Gal1 and ST6GalNAc5. Glycan microarray binding studies indicate that Siglec-7 does not recognize DSGb5, and preferentially binds Neu5Acα(2,8)Neu5Ac containing glycans.

Chemoenzymatic Synthesis of DSGb5 and Sialylated Globo-series Glycans

Sialic-acid-binding, immunoglobulin-type lectin-7 (Siglec-7) is present on the surface of natural killer cells. Siglec-7 shows preference for disialylated glycans, including α(2,8)-α(2,3)-disialic acids or internally branched α(2,6)-NeuAc, such as disialosylglobopentaose (DSGb5). Herein, DSGb5 was synthesized by a one-pot multiple enzyme method from Gb5 by α2,3-sialylation (with PmST1) followed by α2,6-sialylation (with Psp2,6ST) in 23 % overall yield. DSGb5 was also chemoenzymatically synthesized. The protection of the nonreducing-end galactose of Gb5 as 3,4-O-acetonide, 3,4-O-benzylidene, and 4,6-O-benzylidene derivatives provided DSGb5 in overall yields of 26 %, 12 %, and 19 %, respectively. Gb3, Gb4, and Gb5 were enzymatically sialylated to afford a range of globo-glycans. Surprisingly, DSGb5 shows a low affinity for Siglec-7 in a glycan microarray binding affinity assay. Among the synthesized globo-series glycans, α6α3DSGb4 shows the highest binding affinity for Siglec-7.

Carbohydrate Kinases: A Conserved Mechanism Across Differing Folds

Carbohydrate kinases activate a wide variety of monosaccharides by adding a phosphate group, usually from ATP. This modification is fundamental to saccharide utilization, and it is likely a very ancient reaction. Modern organisms contain carbohydrate kinases from at least five main protein families. These range from the highly specialized inositol kinases, to the ribokinases and galactokinases, which belong to families that phosphorylate a wide range of substrates. The carbohydrate kinases utilize a common strategy to drive the reaction between the sugar hydroxyl and the donor phosphate. Each sugar is held in position by a network of hydrogen bonds to the non-reactive hydroxyls (and other functional groups). The reactive hydroxyl is deprotonated, usually by an aspartic acid side chain acting as a catalytic base. The deprotonated hydroxyl then attacks the donor phosphate. The resulting pentacoordinate transition state is stabilized by an adjacent divalent cation, and sometimes by a positively charged protein side chain or the presence of an anion hole. Many carbohydrate kinases are allosterically regulated using a wide variety of strategies, due to their roles at critical control points in carbohydrate metabolism. The evolution of a similar mechanism in several folds highlights the elegance and simplicity of the catalytic scheme.

Thermophilic bacterial communities inhabiting the microbial mats of "indifferent" and chalybeate (iron-rich) thermal springs: Diversity and biotechnological analysis

Microbial mats are occasionally reported in thermal springs and information on such mats is very scarce. In this study, microbial mats were collected from two hot springs (Brandvlei (BV) and Calitzdorp (CA)), South Africa and subjected to scanning electron microscopy (SEM) and targeted 16S rRNA gene amplicon analysis using Next Generation Sequencing (NGS). Spring water temperature was 55°C for Brandvlei and 58°C for Calitzdorp while the pH of both springs was slightly acidic, with an almost identical pH range (6.2-6.3). NGS analysis resulted in a total of 4943 reads, 517 and 736 OTUs for BV and CA at, respectively, a combined total of 14 different phyla in both samples, 88 genera in CA compared to 45 in BV and 37.64% unclassified sequences in CA compared to 27.32% recorded in BV. Dominant bacterial genera in CA microbial mat were Proteobacteria (29.19%), Bacteroidetes (9.41%), Firmicutes (9.01%), Cyanobacteria (6.89%), Actinobacteria (2.65%), Deinococcus-Thermus (2.57%), and Planctomycetes (1.94%) while the BV microbial mat was dominated by Bacteroidetes (47.3%), Deinococcus-Thermus (12.35%), Proteobacteria (7.98%), and Planctomycetes (2.97%). Scanning electron microscopy results showed the presence of microbial filaments possibly resembling cyanobacteria, coccids, rod-shaped bacteria and diatoms in both microbial mats. Dominant genera that were detected in this study have been linked to different biotechnological applications including hydrocarbon degradation, glycerol fermentation, anoxic-fermentation, dehalogenation, and biomining processes. Overall, the results of this study exhibited thermophilic bacterial community structures with high diversity in microbial mats, which have a potential for biotechnological exploitation.

Characterisation of a Bacterial Galactokinase with High Activity and Broad Substrate Tolerance for Chemoenzymatic Synthesis of 6-Aminogalactose-1-Phosphate and Analogues

Glycosyl phosphates are important intermediates in many metabolic pathways and are substrates for diverse carbohydrate-active enzymes. Thus, there is a need to develop libraries of structurally similar analogues that can be used as selective chemical probes in glycomics. Here, we explore chemoenzymatic cascades for the fast generation of glycosyl phosphate libraries without protecting-group strategies. The key enzyme is a new bacterial galactokinase (LgGalK) cloned from Leminorella grimontii, which was produced in Escherichia coli and shown to catalyse 1-phosphorylation of galactose. LgGalK displayed a broad substrate tolerance, being able to catalyse the 1-phosphorylation of a number of galactose analogues, including 3-deoxy-3-fluorogalactose and 4-deoxy-4-fluorogalactose, which were first reported to be substrates for wild-type galactokinase. LgGalK and galactose oxidase variant M₁ were combined in a one-pot, two-step system to synthesise 6-oxogalactose-1-phosphate and 6-oxo-2-fluorogalactose-1-phosphate, which were subsequently used to produce a panel of 30 substituted 6-aminogalactose-1-phosphate derivatives by chemical reductive amination in a one-pot, three-step chemoenzymatic process.

Efficient chemoenzymatic synthesis of novel galacto-N-biose derivatives and their sialylated forms

Galacto-N-biose (GNB) derivatives were efficiently synthesized from galactose derivatives via a one-pot two-enzyme system containing two promiscuous enzymes from Bifidobacterium infantis: a galactokinase (BiGalK) and a d-galactosyl-β1-3-N-acetyl-d-hexosamine phosphorylase (BiGalHexNAcP). Mono-sialyl and di-sialyl galacto-N-biose derivatives were then prepared using a one-pot two-enzyme system containing a CMP-sialic acid synthetase and an α2-3-sialyltransferase or an α2-6-sialyltransferase.