Sialic acids in infection and their potential use in detection and protection against pathogens.

In structural terms, the sialic acids are a large family of nine carbon sugars based around an alpha-keto acid core. They are widely spread in nature, where they are often found to be involved in molecular recognition processes, including in development, immunology, health and disease. The prominence of sialic acids in infection is a result of their exposure at the non-reducing terminus of glycans in diverse glycolipids and glycoproteins. Herein, we survey representative aspects of sialic acid structure, recognition and exploitation in relation to infectious diseases, their diagnosis and prevention or treatment. Examples covered span influenza virus and Covid-19, Leishmania and Trypanosoma, algal viruses, Campylobacter, Streptococci and Helicobacter, and commensal Ruminococci.

Reverse thiophosphorylase activity of a glycoside phosphorylase in the synthesis of an unnatural Manß1,4GlcNAc library.

ß-Mannosides are ubiquitous in nature, with diverse roles in many biological processes. Notably, Manß1,4GlcNAc a constituent of the core N-glycan in eukaryotes was recently identified as an immune activator, highlighting its potential for use in immunotherapy. Despite their biological significance, the synthesis of ß-mannosidic linkages remains one of the major challenges in glycoscience. Here we present a chemoenzymatic strategy that affords a series of novel unnatural Manß1,4GlcNAc analogues using the ß-1,4-d-mannosyl-N-acetyl-d-glucosamine phosphorylase, BT1033. We show that the presence of fluorine in the GlcNAc acceptor facilitates the formation of longer ß-mannan-like glycans. We also pioneer a "reverse thiophosphorylase" enzymatic activity, favouring the synthesis of longer glycans by catalysing the formation of a phosphorolysis-stable thioglycoside linkage, an approach that may be generally applicable to other phosphorylases.

Virtual screening, identification and in vitro validation of small molecule GDP-mannose dehydrogenase inhibitors.

Upon undergoing mucoid conversion within the lungs of cystic fibrosis patients, the pathogenic bacterium Pseudomonas aeruginosa synthesises copious quantities of the virulence factor and exopolysaccharide alginate. The enzyme guanosine diphosphate mannose dehydrogenase (GMD) catalyses the rate-limiting step and irreversible formation of the alginate sugar nucleotide building block, guanosine diphosphate mannuronic acid. Since there is no corresponding enzyme in humans, strategies that could prevent its mechanism of action could open a pathway for new and selective inhibitors to disrupt bacterial alginate production. Using virtual screening, a library of 1447 compounds within the Known Drug Space parameters were evaluated against the GMD active site using the Glide, FRED and GOLD algorithms. Compound hit evaluation with recombinant GMD refined the panel of 40 potential hits to 6 compounds which reduced NADH production in a time-dependent manner; of which, an usnic acid derivative demonstrated inhibition six-fold stronger than a previously established sugar nucleotide inhibitor, with an IC50 value of 17 µM. Further analysis by covalent docking and mass spectrometry confirm a single site of GMD alkylation.

ß-1,2-Oligomannan phosphorylase-mediated synthesis of potential oligosaccharide vaccine candidates.

ß-(1,2)-Mannan antigens incorporated into vaccines candidates for immunization studies, showed that antibodies raised against ß-(1,2)-mannotriose antigens can protect against disseminated candidiasis. Until recently, ß-(1,2)- mannans could only be obtained by isolation from microbial cultures, or by lengthy synthetic strategies involving protecting group manipulation. The discovery of two ß-(1,2)-mannoside phosphorylases, Teth514_1788 and Teth514_1789, allowed efficient access to these compounds. In this work, Teth514_1788 was utilised to generate ß-(1,2)-mannan antigens, tri- and tetra-saccharides, decorated with a conjugation tether at the reducing end, suitable to be incorporated on a carrier en-route to novel vaccine candidates, illustrated here by conjugation of the trisaccharide to BSA.

Synthesis of C6-modified mannose 1-phosphates and evaluation of derived sugar nucleotides against GDP-mannose dehydrogenase.

Sufferers of cystic fibrosis are at significant risk of contracting chronic bacterial lung infections. The dominant pathogen in these cases is mucoid Pseudomonas aeruginosa. Such infections are characterised by overproduction of the exopolysaccharide alginate. We present herein the design and chemoenzymatic synthesis of sugar nucleotide tools to probe a critical enzyme within alginate biosynthesis, GDP-mannose dehydrogenase (GMD). We first synthesise C6-modified glycosyl 1-phosphates, incorporating 6-amino, 6-chloro and 6-sulfhydryl groups, followed by their evaluation as substrates for enzymatic pyrophosphorylative coupling. The development of this methodology enables access to GDP 6-chloro-6-deoxy-ᴅ-mannose and its evaluation against GMD.

Studies towards the Synthesis of (+)-Dictyoxetane.

The dolabellane-type diterpene dictyoxetane represents a significant challenge to synthetic organic chemistry. Methodology directed towards the total synthesis of naturally occurring (+)-dictyoxetane is reported. Catalytic asymmetric synthesis of the trans-hydrindane ring system is achieved through chemoselective deoxygenation of the Hajos-Parrish ketone. An alternative to the Garst-Spencer furan annulation is developed for the synthesis of a 2,5-dimethyl, tetrasubstituted furan, employing a tandem 5-exo-dig alcohol to alkyne cyclisation/aromatisation reaction as a key step. The (4+3) cycloaddition reaction of an oxyallyl cation with a tetrasubstituted furan is established on a cyclohexanone-derived model system, and a range of related (4+3) cycloadditions investigated on a homochiral, trans-hydrindane-fused furan, where regio- and diastereoselectivity is required for the natural product synthesis. In an alternative (4+2) Diels-Alder approach, a C2 -symmetric vinyl sulfoxide-based chiral ketene equivalent is used to prepare oxanorbornenes with the same oxygen bridge stereochemistry found in the 2,7-dioxatricyclo[4.2.1.03,8 ]nonane ring system of the natural product.

Anomeric 1,2,3-triazole-linked sialic acid derivatives show selective inhibition towards a bacterial neuraminidase over a trypanosome trans-sialidase.

Sialic acid is the natural substrate for sialidases and its chemical modification has been a useful approach to generate potent and selective inhibitors. Aiming at advancing the discovery of selective Trypanosoma cruzi trans-sialidase (TcTS) inhibitors, we have synthesised a small series of anomeric 1,2,3-triazole-linked sialic acid derivatives in good yields and high purity via copper-catalysed azide-alkyne cycloaddition (CuAAC, click chemistry) and evaluated their activity towards TcTS and neuraminidase. Surprisingly, the compounds showed practically no TcTS inhibition, whereas ca. 70% inhibition was observed for neuraminidase in relation to the analogues bearing hydrophobic substituents and ca. 5% for more polar substituents. These results suggest that polarity changes are less tolerated by neuraminidase due to the big difference in impact of hydrophobicity upon inhibition, thus indicating a simple approach to differentiate both enzymes. Moreover, such selectivity might be reasoned based on a possible steric hindrance caused by a bulky hydrophobic loop that sits over the TcTS active site and may prevent the hydrophobic inhibitors from binding. The present study is a step forward in exploiting subtle structural differences in sialidases that need to be addressed in order to achieve selective inhibition.

Prospects for anti-Candida therapy through targeting the cell wall: A mini-review.

The impact of fungal infections on humans is a serious public health issue that has received much less attention than bacterial infection and treatment, despite ever-increasing incidence exacerbated by an increased incidence of immunocompromised individuals in the population. Candida species, in particular, cause some of the most prevalent hospital-related fungal infections. Fungal infections are also detrimental to the well-being of grazing livestock, with milk production in dairy cows, and body and coat condition adversely affected by fungal infections. Fungal cell walls are essential for viability, morphogenesis and pathogenesis: numerous anti-fungal drugs rely on targeting either the cell wall or cell membrane, but the pipeline of available bioactives is limited. There is a clear and unmet need to identify novel targets and develop new classes of anti-fungal agents. This mini review focuses on fungal cell wall structure, composition and biosynthesis in Candida spp., including C. auris. In addition, an overview of current advances in the development of cell wall targeted therapies is considered.

Inhibition of the GDP-d-Mannose Dehydrogenase from Pseudomonas aeruginosa Using Targeted Sugar Nucleotide Probes.

Sufferers of cystic fibrosis are at extremely high risk for contracting chronic lung infections. Over their lifetime, one bacterial strain in particular, Pseudomonas aeruginosa, becomes the dominant pathogen. Bacterial strains incur loss-of-function mutations in the mucA gene that lead to a mucoid conversion, resulting in copious secretion of the exopolysaccharide alginate. Strategies that stop the production of alginate in mucoid Pseudomonas aeruginosa infections are therefore of paramount importance. To aid in this, a series of sugar nucleotide tools to probe an enzyme critical to alginate biosynthesis, guanosine diphosphate mannose dehydrogenase (GMD), have been developed. GMD catalyzes the irreversible formation of the alginate building block, guanosine diphosphate mannuronic acid. Using a chemoenzymatic strategy, we accessed a series of modified sugar nucleotides, identifying a C6-amide derivative of guanosine diphosphate mannose as a micromolar inhibitor of GMD. This discovery provides a framework for wider inhibition strategies against GMD to be developed.

Chemoenzymatic Synthesis of C6-Modified Sugar Nucleotides To Probe the GDP-d-Mannose Dehydrogenase from Pseudomonas aeruginosa.

The chemoenzymatic synthesis of a series of C6-modified GDP-d-Man sugar nucleotides is described. This provides the first structure-function tools for the GDP-d-ManA producing GDP-d-mannose dehydrogenase (GMD) from Pseudomonas aeruginosa. Using a common C6 aldehyde functionalization strategy, chemical synthesis introduces deuterium enrichment, alongside one-carbon homologation at C6 for a series of mannose 1-phosphates. These materials are shown to be substrates for the GDP-mannose pyrophosphorylase from Salmonella enterica, delivering the required toolbox of modified GDP-d-Mans. C6-CH3 modified sugar-nucleotides are capable of reversibly preventing GDP-ManA production by GMD. The ketone product from oxidation of a C6-CH3 modified analogue is identified by high-resolution mass spectrometry.