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.

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.

Diversion of a thioglycoligase for the synthesis of 1-O-acyl arabinofuranoses

An arabinofuranosylhydrolase from the GH51 family was transformed into an acyl transferase by mutation of the catalytic acid/base amino acid.

Biocatalyzed synthesis of difuranosides and their ability to trigger production of TNF-α

Transglycosylation reactions biocatalyzed by the native arabinofuranosidase Araf51 and using d-galactosyl, d-fucosyl and 6-deoxy-6-fluoro-D-galactosyl derivatives as donors and acceptors provided di-to pentahexofuranosides. The immunostimulatory potency of these compounds, and more especially their ability to induce production of TNF-α, was evaluated on the murine macrophage cell line, Raw 264.7. The results obtained showed concentration-dependent and most importantly, structure-dependent responses. Interestingly, oligoarabinofuranosides belonging to the oligopentafuranoside family displayed concentration-, chain length and aglycon-dependent bioactivities irrespective of their fine chemical variations. Thus, neo-oligofuranosides in D-Galf series, as well as their D-Fucf and 6-fluorinated counterparts are indeed potential sources of immunostimulating agents.