Effects of Saccharide Spacing and Chain Extension on Toxin Inhibition by Glycopolypeptides of Well-Defined Architecture

Abstract

Many recognition events important in biology are mediated via multivalent interactions between relevant oligosaccharides and multiple saccharide receptors present on lectins, viruses, toxins, and cell surfaces. Because of the important role played by protein−carbohydrate interactions in these pathogenic recognition events and in other human diseases, considerable effort has been devoted toward the development of multivalent polymeric ligands for carbohydrate-binding proteins. In this work, we report the synthesis of new polypeptide-based glycopolymers produced via a combination of protein engineering and chemical methods. These methodologies permit control over the number and the spacing of saccharides on the scaffold, as well as the conformation of the polymer backbone, and allow a more purposeful design of polymers for manipulation of multivalent binding events. Two families of galactose-bearing glycopolypeptides with random coil conformations, [(AG)₃PEG]_y (y = 10 and 16) and {[(AG)₂PSG]₂[(AG)₂PEG][(AG)₂PSG]₂}_y (y = 6), have been synthesized. The carboxylic acid functionality of the glutamic acid residues allowed subsequent modification with amino-saccharides to yield the desired glycopolypeptides; selective placement of the glutamic acid group permitted investigation of the effects of multivalency and saccharide spacing on toxin inhibition. In addition, a family of galactose-functionalized PGA-based glycopolymers of varying molecular weights was also synthesized to compare the effects of backbone flexibility and hydrodynamic volume, relative to the recombinant glycopolypeptides, on toxin inhibition. Glycopolypeptides were characterized via ¹H NMR, MALDI-TOF mass spectrometry, SDS-PAGE analysis, and spectrophotometric assays. They were tested as inhibitors of the binding of the cholera toxin B subunit via direct enzyme-linked assays. The data from these experiments confirm the relevance of appropriate saccharide spacing on controlling the binding event and also indicate the influence of chain extension in improving inhibition.