Galactofuranose-Related Enzymes: Challenges and Hopes

Crystal structure of β-d-galactofuranosidase from Streptomyces sp. JHA19 in complex with an inhibitor provides insights into substrate specificity

d-Galactofuranose (Galf) is widely distributed in glycoconjugates of pathogenic microbes. β-d-Galactofuranosidase (Galf-ase) from Streptomyces sp. JHA19 (ORF1110) belongs to glycoside hydrolase (GH) family 2 and is the first identified Galf-specific degradation enzyme. Here, the crystal structure of ORF1110 in complex with a mechanism-based potent inhibitor, d-iminogalactitol (Ki = 65 μm) was solved. ORF1110 binds to the C5-C6 hydroxy groups of d-iminogalactitol with an extensive and integral hydrogen bond network, a key interaction that discriminates the substrates. The active site structure of ORF1110 is largely different from those of β-glucuronidases and β-galactosidases in the same GH2 family. A C-terminal domain of ORF1110 is predicted to be a carbohydrate-binding module family 42 that may bind Galf. The structural insights into Galf-ase will contribute to the investigation of therapeutic tools against pathogens.

Substrate binding and catalytic mechanism of UDP-α-D-galactofuranose: β-galactofuranoside β-(1→5)-galactofuranosyltransferase GfsA

UDP-α-D-galactofuranose (UDP-Galf): β-galactofuranoside β-(1→5)-galactofuranosyltransferase, known as GfsA, is essential in synthesizing β-(1→5)-galactofuranosyl oligosaccharides that are incorporated into the cell wall of pathogenic fungi. This study analyzed the structure and function of GfsA from Aspergillus fumigatus. To provide crucial insights into the catalytic mechanism and substrate recognition, the complex structure was elucidated with manganese (Mn2+), a donor substrate product (UDP), and an acceptor sugar molecule (β-galactofuranose). In addition to the typical GT-A fold domain, GfsA has a unique domain formed by the N and C termini. The former interacts with the GT-A of another GfsA, forming a dimer. The active center that contains Mn2+, UDP, and galactofuranose forms a groove structure that is highly conserved in the GfsA of Pezizomycotina fungi. Enzymatic assays using site-directed mutants were conducted to determine the roles of specific active-site residues in the enzymatic activity of GfsA. The predicted enzyme-substrate complex model containing UDP-Galf characterized a specific β-galactofuranosyltransfer mechanism to the 5'-OH of β-galactofuranose. Overall, the structure of GfsA in pathogenic fungi provides insights into the complex glycan biosynthetic processes of fungal pathogenesis and may inform the development of novel antifungal therapies.

Advances in Engineering Nucleotide Sugar Metabolism for Natural Product Glycosylation in Saccharomyces cerevisiae

Glycosylation is a ubiquitous modification present across all of biology, affecting many things such as physicochemical properties, cellular recognition, subcellular localization, and immunogenicity. Nucleotide sugars are important precursors needed to study glycosylation and produce glycosylated products. Saccharomyces cerevisiae is a potentially powerful platform for producing glycosylated biomolecules, but it lacks nucleotide sugar diversity. Nucleotide sugar metabolism is complex, and understanding how to engineer it will be necessary to both access and study heterologous glycosylations found across biology. This review overviews the potential challenges with engineering nucleotide sugar metabolism in yeast from the salvage pathways that convert free sugars to their associated UDP-sugars to de novo synthesis where nucleotide sugars are interconverted through a complex metabolic network with governing feedback mechanisms. Finally, recent examples of engineering complex glycosylation of small molecules in S. cerevisiae are explored and assessed.

Galf-Specific Neolectins: Towards Promising Diagnostic Tools

In the absence of naturally available galactofuranose-specific lectin, we report herein the bioengineering of GalfNeoLect, from the first cloned wild-type galactofuranosidase (Streptomyces sp. strain JHA19), which recognises and binds a single monosaccharide that is only related to nonmammalian species, usually pathogenic microorganisms. We kinetically characterised the GalfNeoLect to confirm attenuation of hydrolytic activity and used competitive inhibition assay, with close structural analogues of Galf, to show that it conserved interaction with its original substrate. We synthetised the bovine serum albumin-based neoglycoprotein (GalfNGP), carrying the multivalent Galf units, as a suitable ligand and high-avidity system for the recognition of GalfNeoLect which we successfully tested directly with the galactomannan spores of Aspergillus brasiliensis (ATCC 16404). Altogether, our results indicate that GalfNeoLect has the necessary versatility and plasticity to be used in both research and diagnostic lectin-based applications.

Characterization of the need for galactofuranose during the Neurospora crassa life cycle

Galactofuranose is a constituent of the cell walls of filamentous fungi. The galactofuranose can be found as a component of N-linked oligosaccharides, in O-linked oligosaccharides, in GPI-anchored galactomannan, and in free galactomannan. The Neurospora genome contains a single UDP-galactose mutase gene (ugm-1/NCU01824) and two UDP-galactofuranose translocases used to import UDP-galactofuranose into the lumen of the Golgi apparatus (ugt-1/NCU01826 and ugt-2/NCU01456). Our results demonstrate that loss of galactofuranose synthesis or its translocation into the lumen of the secretory pathway affects the morphology and growth rate of the vegetative hyphae, the production of conidia (asexual spores), and dramatically affects the sexual stages of the life cycle. In mutants that are unable to make galactofuranose or transport it into the lumen of the Golgi apparatus, ascospore development is aborted soon after fertilization and perithecium maturation is aborted prior to the formation of the neck and ostiole. The Neurospora genome contains three genes encoding possible galactofuranosyltransferases from the GT31 family of glycosyltransferases (gfs-1/NCU05878, gfs-2/NCU07762, and gfs-3/NCU02213) which might be involved in generating galactofuranose-containing oligosaccharide structures. Analysis of triple KO mutants in GT31 glycosyltransferases shows that these mutants have normal morphology, suggesting that these genes do not encode vital galactofuranosyltransferases.

Identification of the furanose ring conformations and the factors driving their adoption

Three groups of furanoses with restricted freedom of rotation on the C3-C4, C2-C3, and C1-C2 bonds, respectively, are presented. Conformational analysis of these furanoses is conducted based on the proton nuclear magnetic resonance (¹H NMR) spectroscopy, density functional theory (DFT) calculations, and X-ray analysis. It is shown that the particular group of the presented furanoses is locked in the specific conformation. These are the ¹T₂-like, the ⁰E-like, and the ³T₄-like conformation, respectively. Characteristic ¹H NMR spectra of these three conformations are presented and the factors influencing the conformational preferences of the analyzed furanoses are discussed.

Biochemical characterization of a glycoside hydrolase family 43 β-D-galactofuranosidase from the fungus Aspergillus niger

β-D-Galactofuranose (Galf) and its polysaccharides are found in bacteria, fungi and protozoa but do not occur in mammalian tissues, and thus represent a specific target for anti-pathogenic drugs. Understanding the enzymatic degradation of these polysaccharides is therefore of great interest, but the identity of fungal enzymes with exclusively galactofuranosidase activity has so far remained elusive. Here we describe the identification and characterization of a galactofuranosidase from the industrially important fungus Aspergillus niger. Analysis of glycoside hydrolase family 43 subfamily 34 (GH43_34) members via conserved unique peptide patterns and phylogeny, revealed the occurrence of distinct clusters and, by comparison with specificities of characterized bacterial members, suggested a basis for prediction of enzyme specificity. Using this rationale, in tandem with molecular docking, we identified a putative β-D-galactofuranosidase from A. niger which was recombinantly produced in Escherichia coli. The Galf-specific hydrolase, encoded by xynD demonstrates maximum activity at pH 5, 25 °C towards 4-nitrophenyl-β-galactofuranoside (pNP-β-Galf), with a K_m of 17.9 ± 1.9 mM and V_max of 70.6 ± 5.3 µM min^-1. The characterization of this first fungal GH43 galactofuranosidase offers further molecular insight into the degradation of Galf-containing structures.

The Synthesis and Glycoside Formation of Polyfluorinated Carbohydrates

Fluorinated carbohydrates have found many applications in the glycosciences. Typically, these contain fluorination at a single position. There are not many applications involving polyfluorinated carbohydrates, here defined as monosaccharides in which more than one carbon has at least one fluorine substituent directly attached to it, with the notable exception of their use as mechanism-based inhibitors. The increasing attention to carbohydrate physical properties, especially around lipophilicity, has resulted in a surge of interest for this class of compounds. This review covers the considerable body of work toward the synthesis of polyfluorinated hexoses, pentoses, ketosugars, and aminosugars including sialic acids and nucleosides. An overview of the current state of the art of their glycosidation is also provided.

Synthesis of α-l-Araf and β-d-Galf series furanobiosides using mutants of a GH51 α-l-arabinofuranosidase

The GH-51 α-l-arabinofuranosidase from Thermobacillus xylanilyticus (TxAbf) possesses versatile catalytic properties, displaying not only the ability to hydrolyze glycosidic linkages but also to synthesize furanobiosides in α-l-Araf and β-d-Galf series. Herein, mutants are investigated to evaluate their ability to perform self-condensation, assessing both yield improvements and changes in regioselectivity. Overall yields of oligo-α-l-arabino- and oligo-β-d-galactofuranosides were increased up to 4.8-fold compared to the wild-type enzyme. In depth characterization revealed that the mutants exhibit increased transfer rates and thus a hydrolysis/self-condensation ratio in favor of synthesis. The consequence of the substitution N216W is the creation of an additional binding subsite that provides the basis for an alternative acceptor substrate binding mode. As a result, mutants bearing N216W synthesize not only (1,2)-linked furanobiosides, but also (1,3)- and even (1,5)-linked furanobiosides. Since the self-condensation is under kinetic control, the yield of homo-disaccharides was maximized using higher substrate concentrations. In this way, the mutant R69H-N216W produced oligo-β-d-galactofuranosides in > 70% yield. Overall, this study further demonstrates the potential usefulness of TxAbf mutants for glycosynthesis and shows how these might be used to synthesize biologically-relevant glycoconjugates.

The Glycan Structure of T. cruzi mucins Depends on the Host. Insights on the Chameleonic Galactose

Trypanosoma cruzi, the protozoa that causes Chagas disease in humans, is transmitted by insects from the Reduviidae family. The parasite has developed the ability to change the structure of the surface molecules, depending on the host. Among them, the mucins are the most abundant glycoproteins. Structural studies have focused on the epimastigotes and metacyclic trypomastigotes that colonize the insect, and on the mammal trypomastigotes. The carbohydrate in the mucins fulfills crucial functions, the most important of which being the accepting of sialic acid from the host, a process catalyzed by the unique parasite trans-sialidase. The sialylation of the parasite influences the immune response on infection. The O-linked sugars have characteristics that differentiate them from human mucins. One of them is the linkage to the polypeptide chain by the hexosamine, GlcNAc, instead of GalNAc. The main monosaccharide in the mucins oligosaccharides is galactose, and this may be present in three configurations. Whereas β-d-galactopyranose (β-Galp) was found in the insect and the human stages of Trypanosoma cruzi, β-d-galactofuranose (β-Galf) is present only in the mucins of some strains of epimastigotes and α-d-galactopyranose (α-Galp) characterizes the mucins of the bloodstream trypomastigotes. The two last configurations confer high antigenic properties. In this review we discuss the different structures found and we pose the questions that still need investigation on the exchange of the configurations of galactose.