Azide-Functionalized Derivatives of the Virulence-Associated Sugar Pseudaminic Acid: Chiral Pool Synthesis and Labeling of Bacteria

Chemical biology tools to probe bacterial glycans

Bacterial cells are covered by a complex carbohydrate coat of armor that allows bacteria to thrive in a range of environments. As a testament to the importance of bacterial glycans, effective and heavily utilized antibiotics including penicillin and vancomycin target and disrupt the bacterial glycocalyx. Despite their importance, the study of bacterial glycans lags far behind their eukaryotic counterparts. Bacterial cells use a large palette of monosaccharides to craft glycans, leading to molecules that are significantly more complex than eukaryotic glycans and that are refractory to study. Fortunately, chemical tools designed to probe bacterial glycans have yielded insights into these molecules, their structures, their biosynthesis, and their functions.

Co-factor prosthesis facilitates biosynthesis of azido-pseudaminic acid probes for use as glycosyltransferase reporters

Truncated thioester N,S-diacetylcysteamine (SNAc) was utilised as a co-factor mimic for PseH, an acetyl-coA dependent aminoglycoside N-acetyltransferase, in the biosynthesis of the bacterial sugar, pseudaminic acid. Additionally, an azido-SNAc analogue was used to smuggle N7-azide functionality into the pseudaminic acid backbone, facilitating its use as a reporter of pseudaminyltransferase activity.

Surface-modified bacteria: synthesis, functionalization and biomedical applications

The past decade has witnessed a great leap forward in bacteria-based living agents, including imageable probes, diagnostic reagents, and therapeutics, by virtue of their unique characteristics, such as genetic manipulation, rapid proliferation, colonization capability, and disease site targeting specificity. However, successful translation of bacterial bioagents to clinical applications remains challenging, due largely to their inherent susceptibility to environmental insults, unavoidable toxic side effects, and limited accumulation at the sites of interest. Cell surface components, which play critical roles in shaping bacterial behaviors, provide an opportunity to chemically modify bacteria and introduce different exogenous functions that are naturally unachievable. With the help of surface modification, a wide range of functionalized bacteria have been prepared over the past years and exhibit great potential in various biomedical applications. In this article, we mainly review the synthesis, functionalization, and biomedical applications of surface-modified bacteria. We first introduce the approaches of chemical modification based on the bacterial surface structure and then highlight several advanced functions achieved by modifying specific components on the surface. We also summarize the advantages as well as limitations of surface chemically modified bacteria in the applications of bioimaging, diagnosis, and therapy and further discuss the current challenges and possible solutions in the future. This work will inspire innovative design thinking for the development of chemical strategies for preparing next-generation biomedical bacterial agents.

Chemical tools to study bacterial glycans: a tale from discovery of glycoproteins to disruption of their function

Bacteria coat themselves with a dense array of cell envelope glycans that enhance bacterial fitness and promote survival. Despite the importance of bacterial glycans, their systematic study and perturbation remains challenging. Chemical tools have made important inroads toward understanding and altering bacterial glycans. This review describes how pioneering discoveries from Prof. Carolyn Bertozzi's laboratory inspired our laboratory to develop sugar probes to facilitate the study of bacterial glycans. As described below, we used metabolic glycan labelling to install bioorthogonal reporters into bacterial glycans, ultimately permitting the discovery of a protein glycosylation system, the identification of glycosylation genes, and the development of metabolic glycan inhibitors. Our results have provided an approach to screen bacterial glycans and gain insight into their function, even in the absence of detailed structural information.

Syntheses of Legionaminic Acid, Pseudaminic Acid, Acetaminic Acid, 8-epi-Acetaminic Acid, and 8-epi-Legionaminic Acid Glycosyl Donors from N-Acetylneuraminic Acid by Side Chain Exchange

Metaperiodate cleavage of the glycerol side chain from an N-acetyl neuraminic acid-derived thioglycoside and condensation with the two enantiomers of the Ellman sulfinamide afford two diastereomeric N-sulfinylimines from which bacterial sialic acid donors with the legionaminic and acetaminic acid configurations and their 8-epi-isomers are obtained by samarium iodide-mediated coupling with acetaldehyde and subsequent manipulations. A variation on the theme, with inversion of the configuration at C5, similarly provides two differentially protected pseudaminic acid donors.

Synthesis and Application of Rare Deoxy Amino l-Sugar Analogues to Probe Glycans in Pathogenic Bacteria

Bacterial cell envelope glycans are compelling antibiotic targets as they are critical for strain fitness and pathogenesis yet are virtually absent from human cells. However, systematic study and perturbation of bacterial glycans remains challenging due to their utilization of rare deoxy amino l-sugars, which impede traditional glycan analysis and are not readily available from natural sources. The development of chemical tools to study bacterial glycans is a crucial step toward understanding and altering these biomolecules. Here we report an expedient methodology to access azide-containing analogues of a variety of unusual deoxy amino l-sugars starting from readily available l-rhamnose and l-fucose. Azide-containing l-sugar analogues facilitated metabolic profiling of bacterial glycans in a range of Gram-negative bacteria and revealed differential utilization of l-sugars in symbiotic versus pathogenic bacteria. Further application of these probes will refine our knowledge of the glycan repertoire in diverse bacteria and aid in the design of novel antibiotics.

Chemical Biology Tools for Modulating and Visualizing Gram-Negative Bacterial Surface Polysaccharides

Bacterial cells present a wide diversity of saccharides that decorate the cell surface and help mediate interactions with the environment. Many Gram-negative cells express O-antigens, which are long sugar polymers that makeup the distal portion of lipopolysaccharide (LPS) that constitutes the surface of the outer membrane. This review highlights chemical biology tools that have been developed in recent years to facilitate the modulation of O-antigen synthesis and composition, as well as related bacterial polysaccharide pathways, and the detection of unique glycan sequences. Advances in the biochemistry and structural biology of O-antigen biosynthetic machinery are also described, which provide guidance for the design of novel chemical and biomolecular probes. Many of the tools noted here have not yet been utilized in biological systems and offer researchers the opportunity to investigate the complex sugar architecture of Gram-negative cells.

Azide-Functionalized Derivatives of the Virulence-Associated Sugar Pseudaminic Acid: Chiral Pool Synthesis and Labeling of Bacteria

Pseudaminic acid (Pse) is a significant prokaryotic monosaccharide found in important Gram-negative and Gram-positive bacteria. This unique sugar serves as a component of cell-surface-associated glycans or glycoproteins and is associated with their virulence. We report the synthesis of azidoacetamido-functionalized Pse derivatives as part of a search for Pse-derived metabolic labeling reagents. The synthesis was initiated with d-glucose (Glc), which served as a cost-effective chiral pool starting material. Key synthetic steps involve the conversion of C1 of Glc into the terminal methyl group of Pse, and inverting deoxyaminations at C3 and C5 of Glc followed by backbone elongation with a three-carbon unit using the Barbier reaction. Metabolic labeling experiments revealed that, of the four Pse derivatives, ester-protected C5 azidoacetamido-Pse successfully labeled cells of Pse-expressing Gram-positive and Gram-negative strains. No labeling was observed in cells of non-Pse-expressing strains. The ester-protected and C5 azidoacetamido-functionalized Pse is thus a useful reagent for the identification of bacteria expressing this unique virulence-associated nonulosonic acid.