The First Chemical Synthesis of UDP[6-3H]-α-D-galactofuranose

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.

Efficient Synthesis of UDP-Furanoses via 4,5-Dicyanoimidazole(DCI)-Promoted Coupling of Furanosyl-1-Phosphates with Uridine Phosphoropiperidate

A P(V)-N activation method based on nucleoside phosphoropiperidate/DCI system has been developed for improved synthesis of diverse UDP-furanoses. The reaction conditions including temperature, amount of activator, and reaction time were optimized to alleviate the degradation of UDP-furanoses to cyclic phosphates. In addition, an efficient and facile phosphoramidite route was employed for the preparation of furanosyl-1-phosphates.

Synthesis and Evaluation of Bicyclo[3.1.0]hexane-Based UDP-Galf Analogues as Inhibitors of the Mycobacterial Galactofuranosyltransferase GlfT2

UDP-galactofuranose (UDP-Galf) is the donor substrate for both bifunctional galactofuranosyltransferases, GlfT1 and GlfT2, which are involved in the biosynthesis of mycobacterial galactan. In this paper, a group of UDP-Galf mimics were synthesized via reductive amination of a bicyclo[3.1.0]hexane-based amine by reacting with aromatic, linear, or uridine-containing aldehydes. These compounds were evaluated against GlfT2 using a coupled spectrophotometric assay, and were shown to be weak inhibitors of the enzyme.

Synthetic UDP-furanoses inhibit the growth of the parasite Leishmania

The chemical synthesis of UDP-6-NHAc-6-deoxy-Galf was performed and it led to the isolation of both pure anomers. They were then evaluated together with the previously prepared UDP-furanoses for their anti-parasitic properties against Leishmania donovani promastigotes, one of the agents responsible for visceral leishmaniasis. Amongst them, the unnatural 1,2-trans UDP-6-NHAc-Galf demonstrated a high potency in inhibiting the growth of the parasite.

Chemistry and biology of galactofuranose-containing polysaccharides

The thermodynamically less stable form of galactose-galactofuranose (Galf)-is essential for the viability of several pathogenic species of bacteria and protozoa but absent in this form in mammals, so the biochemical pathways by which Galf-containing glycans are assembled and catabolysed are attractive sites for drug action. This potential has led to increasing interest in the synthesis of molecules containing Galf residues, their subsequent use in studies directed towards understanding the enzymes that process these residues and the identification of potential inhibitors of these pathways. Major achievements of the past several years have included an in-depth understanding of the mechanism of UDP-galactopyranose mutase (UGM), the enzyme that produces UDP-Galf, which is the donor species for galactofuranosyltransferases. A number of methods for the synthesis of galactofuranosides have also been developed, and practitioners in the field now have many options for the initiation of a synthesis of glycoconjugates containing either alpha- or beta-Galf residues. UDP-Galf has also been prepared by a number of approaches, and it appears that a chemoenzymatic approach is currently the most viable method for producing multi-milligram amounts of this important intermediate. Recent advances both in the understanding of the mechanism of UGM and in the synthesis of galactofuranose and its derivatives are highlighted in this review.

A single UDP-galactofuranose transporter is required for galactofuranosylation in Aspergillus fumigatus

Galactofuranose (Galf) containing molecules have been described at the cell surface of several eukaryotes and shown to contribute to the virulence of the parasite Leishmania major and the fungus Aspergillus fumigatus. It is anticipated that a number of the surface glycoconjugates such as N-glycans or glycolipids are galactofuranosylated in the Golgi apparatus. This raises the question of how the substrate for galactofuranosylation reactions, UDP-Galf, which is synthesized in the cytosol, translocates into the organelles of the secretory pathway. Here we report the first identification of a Golgi-localized nucleotide sugar transporter, named GlfB, with specificity for a UDP-Galf. In vitro transport assays established binding of UDP-Galf to GlfB and excluded transport of several other nucleotide sugars. Furthermore, the implication of glfB in the galactofuranosylation of A. fumigatus glycoconjugates and galactomannan was demonstrated by a targeted gene deletion approach. Our data reveal a direct connection between galactomannan and the organelles of the secretory pathway that strongly suggests that the cell wall-bound polysaccharide originates from its glycosylphosphatidylinositol-anchored form.

Development of a coupled spectrophotometric assay for GlfT2, a bifunctional mycobacterial galactofuranosyltransferase

As a key constituent of their protective cell wall all mycobacteria produce a large structural component, the mycolyl-arabinogalactan (mAG) complex, which has at its core a galactan moiety of alternating beta-(1-->5) and beta-(1-->6) galactofuranosyl residues. Galactan biosynthesis is essential for mycobacterial viability and thus inhibitors of the enzymes involved in its assembly are potential drugs for the treatment of mycobacterial diseases, including tuberculosis. Only two galactofuranosyltransferases, GlfT1 and GlfT2, are responsible for the biosynthesis of the entire galactan domain of the mAG and we report here the first high-throughput assay for GlfT2. Successful implementation of the assay required the synthesis of multi-milligram amounts of the donor for the enzyme, UDP-Galf, 1, which was achieved using a chemoenzymatic approach. We also describe an improved expression system for GlfT2, which provides a larger amount of active protein for the assay. Kinetic analysis of 1 and a known trisaccharide acceptor for the enzyme, 2, have been carried out and the apparent K(m) and k(cat) values obtained for the latter are in agreement with those obtained using a previously reported radiochemical assay. The assay has been implemented in 384-well microtiter plates, which will facilitate the screening of large numbers of potential GlfT2 inhibitors, with possible utility as novel anti-TB drugs.

Versatile synthesis of rare nucleotide furanoses

Direct activation of unprotected thioimidoyl furanosides yielded in only one step and few minutes a panel of rare uridine 5'-diphospho-furanoses. Diastereoselectivity of the reaction was tightly connected with reaction time, temperature, and nature of the furanosyl donor. This approach was totally selective since no ring expansion from the initial five-membered ring to the more stable pyranose form was observed.

Facile synthesis of benzyl β-d-galactofuranoside. A convenient intermediate for the synthesis of d-galactofuranose-containing molecules

Benzyl beta-D-galactofuranoside was efficiently obtained from 1,2,3,5,6-penta-O-benzoyl-alpha,beta-D-galactofuranose, via benzyl 2,3,5,6-tetra-O-benzoyl-beta-D-galactofuranoside. Conditions for the O-debenzylation were investigated in order to evaluate the synthetic application of the benzyl group as an anomeric protector of a galactofuranose moiety in synthetic strategies involving galactofuranose.