Multivalent carbohydrate inhibitors against bacterial toxins

Diksha Bansi Haksar

Research output: ThesisDoctoral thesis 1 (Research UU / Graduation UU)

Abstract

Cholera and shiga toxin belong to the AB5 class of bacterial toxins thereby sharing an A subunit which is responsible for the toxicity, surrounded by the pentameric B subunit. The GM1 oligosaccharide which is a part of the natural cell-surface receptor of cholera toxin, has been used extensively as a potent ligand to inhibit the toxin in suitable bioassays. However, the structural complexity of GM1 and therefore the costs involved in preparing it do not make it an ideal candidate for development of an affordable therapeutic. Affordability is key, especially considering that the therapeutic needs to be repeatedly administered due to the natural flow of the intestinal tract, in order to maintain protection during epidemics. Keeping these considerations at the forefront of our experimental design, we focused on a simpler yet effective ligand that could be further strengthened when conjugated to a multivalent scaffold. Thus, we chose meta-nitrophenyl α-galactoside (MNPG) as the main candidate for our monovalent compound, which was discovered by Minke et al. The usage of easily available (i.e. polyacrylamide, dextran) and easy to synthesize polymers (i.e. hyperbranched polyglycerol or hPG) further helped us in realizing economical final compounds. We explored the cholera toxin further by diving into the mounting evidence regarding the presence of a secondary binding site. To account for this secondary binding site, we envisioned a “hybrid” molecule that could block both GM1-based and fucose-based intoxication. To this end we also synthesized the hybrid inhibitor for the cholera toxin that could potentially inhibit both the GM1 based adhesion of the primary binding site and the fucose-based adhesion of the secondary binding site. Shigellosis or bacterial dysentry is caused by gram negative bacterium of four species of Shigella: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei through the fecal-oral route. The pathology can include bloody diarrhea (hemorraghic colitis) followed by the often fatal hemolytic uremic syndrome (HUS). borne with the largest ever reported in Germany (2011), linked to sprout consumption. The pentameric B subunit of shiga toxin binds to globotriaosylceramide (Gb3; Galα1-4Galβ1-4Glcβ1-ceramide, also known as CD77 or the Pk blood group antigen). In order to find a simple yet effective toxin inhibitor we compared three classes of carbohydrate- -based inhibitors: glycodendrimers (di- and tetravalent), glycopolymers (hPG-based) and oligosaccharides (chitosan, alginate, fructose and galactose). Keeping the theme of affordability in mind, we decided to use an attenuated ligand thereby opting for the disaccharide (Galα1-4Galβ; galabiose). The glycopolymers conjugated to galabiose emerged as the most potent of the series.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Utrecht University
Supervisors/Advisors
  • Pieters, Roland, Primary supervisor
Award date22 Nov 2021
Publisher
DOIs
Publication statusPublished - 22 Nov 2021

Keywords

  • carbohydrates
  • bacterial toxins
  • multvalency
  • cholera toxin
  • shiga toxin
  • polymers
  • hyperbranched polymers

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