Characterization of a PA14 domain-containing galactofuranose-specific β-d-galactofuranosidase from Streptomyces sp

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.

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.

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.

Identification and characterization of β-d-galactofuranosidases from Aspergillus nidulans and Aspergillus fumigatus

Although β-d-galactofuranosidases (Galf-ases) that hydrolyze β-d-galactofuranose (Galf)-containing oligosaccharides have been characterized in various organisms, to date no Galf-specific Galf-ase-encoding genes have been reported in Aspergillus fungi. Based on the amino acid sequences of previously identified bacterial Galf-ases, here we found two candidate Galf-specific Galf-ase genes AN2395 (gfgA) and AN3200 (gfgB) in the genome of Aspergillus nidulans. Indeed, recombinant GfgA and GfgB proteins exhibited Galf-specific Galf-ase activity, but no detectable α-l-arabinofuranosidase (Araf-ase) activity. Phylogenetic analysis of GfgA and GfgB orthologs indicated that there are two types of Aspergillus species: those containing one ortholog each for GfgA and GfgB; and those containing only one ortholog in total, among which Aspergillus fumigatus there is a representative with a single ortholog Galf-ase Afu2g14520. Unlike GfgA and GfgB, the recombinant Afu2g14520 protein showed higher Araf-ase activity than Galf-ase activity. An assay of substrate specificity revealed that although GfgA and GfgB are both exo-type Galf-ases and hydrolyze β-(1,5) and β-(1,6) linkages, GfgA hydrolyzes β-(1,6)-linked Galf-oligosaccharide more effectively as compared with GfgB. Collectively, our findings indicate that Galf-ases in Aspergillus species may have a role in cooperatively degrading Galf-containing oligosaccharides depending on environmental conditions.

Galactofuranose-Related Enzymes: Challenges and Hopes

Galactofuranose is a rare form of the well-known galactose sugar, and its occurrence in numerous pathogenic micro-organisms makes the enzymes responsible for its biosynthesis interesting targets. Herein, we review the role of these carbohydrate-related proteins with a special emphasis on the galactofuranosidases we recently characterized as an efficient recombinant biocatalyst.

Galactofuranosidase from JHA 19 Streptomyces sp.: subcloning and biochemical characterization

Despite the crucial role of the rare galactofuranose (Galf) in many pathogenic micro-organisms and our increased knowledge of its metabolism, there is still a lack of recombinant and efficient galactofuranoside hydrolase available for chemo-enzymatic synthetic purposes of specific galactofuranosyl-conjugates. Subcloning of the Galf-ase from JHA 19 Streptomyces sp. and its further overexpression lead us to the production of this enzyme with a yield of 0.5 mg/L of culture. It exhibits substrate specificity exclusively towards pNP β-d-Galf, giving a K_M value of 250 μM, and the highest enzymatic efficiency ever observed of 14 mM^-1  s^-1. It proved to be stable to temperature up to 60 °C and to at least 4 freeze-thaw's cycles. Thus, Galf-ase demonstrated to be an efficient and stable biocatalyst with greatly improved specificity toward the galactofuranosyl entity, thus paving the way to the further development of transglycosylation and thioligation reactions.

Improvement of the versatility of an arabinofuranosidase against galactofuranose for the synthesis of galactofuranoconjugates

A new performant biocatalyst was developed for the synthesis ofO-,S- and acyl-galactofuranoconjugates.

Amino Acid Metabolism and Transport Mechanisms as Potential Antifungal Targets

Discovering new drugs for treatment of invasive fungal infections is an enduring challenge. There are only three major classes of antifungal agents, and no new class has been introduced into clinical practice in more than a decade. However, recent advances in our understanding of the fungal life cycle, functional genomics, proteomics, and gene mapping have enabled the identification of new drug targets to treat these potentially deadly infections. In this paper, we examine amino acid transport mechanisms and metabolism as potential drug targets to treat invasive fungal infections, including pathogenic yeasts, such as species of Candida and Cryptococcus, as well as molds, such as Aspergillus fumigatus. We also explore the mechanisms by which amino acids may be exploited to identify novel drug targets and review potential hurdles to bringing this approach into clinical practice.

Biosynthesis of galactomannans found in filamentous fungi belonging to Pezizomycotina

The galactomannans (GMs) that are produced by filamentous fungi belonging to Pezizomycotina, many of which are pathogenic for animals and plants, are polysaccharides consisting of α-(1→2)-/α-(1→6)-mannosyl and β-(1→5)-/β-(1→6)-galactofuranosyl residues. GMs are located at the outermost layer of the cell wall. When a pathogenic fungus infects a host, its cell surface must be in contact with the host. The GMs on the cell surface may be involved in the infection mechanism of a pathogenic fungus or the defense mechanism of a host. There are two types of GMs in filamentous fungi, fungal-type galactomannans and O-mannose type galactomannans. Recent biochemical and genetic advances have facilitated a better understanding of the biosynthesis of both types. This review summarizes our current information on their biosynthesis.

Draft Genome Sequence of Streptomyces sp. JHA26, a Strain That Harbors a PA14 Domain Containing β-d-Galactofuranosidase

The genome sequence of Streptomyces sp. strain JHA26, the culture supernatant of which exhibited β-d-galactofuranosidase (Galf-ase) activity, was analyzed to search for a Galf-ase-encoding gene. We report here the results of whole-genome shotgun sequencing and reveal the identity of a new Galf-ase gene.