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 structurally-defined polymeric glycosylated phosphoprenols as potential lipopolysaccharide biosynthetic probes

The biosynthesis of lipopolysaccharide (LPS), a key immunomodulatory molecule produced by gram-negative bacteria, has been a topic of long-term interest. To date, the chemical probes used as tools to study LPS biosynthetic pathways have consisted primarily of small fragments of the larger structure (e.g., the O-chain repeating unit). While such compounds have helped to provide significant insight into many aspects of LPS assembly, understanding other aspects will require larger, more complex probes. For example, the molecular interactions between polymeric LPS biosynthetic intermediates and the proteins that transfer them across the inner and outer membrane remain largely unknown. We describe the synthesis of two lipid-linked polysaccharides, containing 11 and 27 monosaccharide residues, that are related to LPS O-chain biosynthesis in Escherichia coli O9a. This work has led not only to multi-milligram quantities of two biosynthetic probes, but also provided insights into challenges that must be overcome in the chemical synthesis of structurally-defined polysaccharides.

The synthesis of lipid-linked polysaccharides containing 11 and 27 monosaccharides via a ‘frame-shift’ strategy is described. The work provides biosynthetic probes and highlights challenges in synthesizing structurally-defined polymeric glycans.

Recent progress in chemical synthesis of bacterial surface glycans

With the continuing advancement of carbohydrate chemical synthesis, bacterial glycomes have become increasingly attractive and accessible synthetic targets. Although bacteria also produce carbohydrate-containing secondary metabolites, our review here will cover recent chemical synthetic efforts on bacterial surface glycans. The obtained compounds are excellent candidates for the development of improved structurally defined glycoconjugate vaccines to combat bacterial infections. They are also important probes for investigating glycan-protein interactions. Glycosylation strategies applied for the formation of some challenging glycosidic bonds of various uncommon sugars in a number of recently synthesized bacterial surface glycans are highlighted.

Chemical ADP-ribosylation: mono-ADPr-peptides and oligo-ADP-ribose

ADP-ribosylation is an important post-translational modification that plays a pivotal role in many cellular processes, including cell signaling, DNA repair, gene regulation and apoptosis. Although chemical synthesis of mono- or poly-ADP-ribosylated biomolecules is extremely difficult due to the challenges in regio- and stereoselective glycosylation, suitable protective group manipulations and pyrophosphate construction, synthetic procedures towards these bio-related targets have been reported in recent years. Chemically synthesized well-defined ADP-ribose derivatives serve as useful tools in biological experiments aimed to further elucidate native ADP-ribosylation. In this review, we will discuss the synthetic studies on mono-ADP-ribosylated proteins and oligo-ADP-ribose chains. Future possible synthetic targets and upcoming new methods for the synthesis of these molecules are also included.

Biosynthesis of Galactan in Mycobacterium tuberculosis as a Viable TB Drug Target?

While target-based drug design has proved successful in several therapeutic areas, this approach has not yet provided compelling outcomes in the field of antibacterial agents. This statement remains especially true for the development of novel therapeutic interventions against tuberculosis, an infectious disease that is among the top ten leading causes of death globally. Mycobacterial galactan is an important component of the protective cell wall core of the tuberculosis pathogen and it could provide a promising target for the design of new drugs. In this review, we summarize the current knowledge on galactan biosynthesis in Mycobacterium tuberculosis, including landmark findings that led to the discovery and understanding of three key enzymes in this pathway: UDP-galactose mutase, and galactofuranosyl transferases GlfT1 and GlfT2. Moreover, we recapitulate the efforts aimed at their inhibition. The predicted common transition states of the three enzymes provide the lucrative possibility of multitargeting in pharmaceutical development, a favourable property in the mitigation of drug resistance. We believe that a tight interplay between target-based computational approaches and experimental methods will result in the development of original inhibitors that could serve as the basis of a new generation of drugs against tuberculosis.

A Route to Polyprenol Pyrophosphate-Based Probes of O-Polysaccharide Biosynthesis in Klebsiella pneumoniae O2a

An approach for the assembly of polyprenol pyrophosphate-based probes of O-polysaccharide biosynthesis in Klebsiella pneumoniae serotype O2a is described. This convergent route features high-yielding, diastereoselective glycosylations and the late-stage installation of the polyprenol pyrophosphate moiety. Although applied to the synthesis of a nonasaccharide bearing a farnesyl group (1), the modular nature of the route makes it amenable to the synthesis of additional derivatives containing either larger glycans or different lipid domains.