Investigating the Molecular Basis for the Virulence of Escherichia coli K5 by Nuclear Magnetic Resonance Analysis of the Capsule Polysaccharide

The capsular polysaccharide of Escherichia coli K5 has been hypothesised to promote virulence through its molecular mimicry of host heparan sulphate. To test this hypothesis, we have produced pure oligosaccharides from K5 capsular polysaccharide and investigated their conformational properties with ultra-high-field nuclear magnetic resonance (NMR) (900 MHz). Ultra-high-field affords a significant resolution enhancement over previous studies and allowed a full-atomic assignment of the K5 hexasaccharide for the first time. All carbohydrate rings adopt a C-4(1) conformation, the amide sidechains have a trans orientation and the hydroxymethyl group is freely exposed to bulk solvent. Initial models of the glycosidic linkage conformation based upon simple interpretation of NOE cross-peaks suggests that the beta 1 -> 4 linkage adopts a 3D geometry of phi approximate to 60 degrees, psi approximate to 0 degrees and the alpha 1 -> 4 linkage prefers phi approximate to -30 degrees, psi approximate to -30 degrees (phi and psi being defined by dihedral angles involving linkage protons). In this conformation the overall molecular geometries of K5 polysaccharide, heparan sulphate and even fully-sulphated heparin are remarkably similar. These results substantiate the hypothesis that the K5 capsular polysaccharide confers virulence to E. coli K5 by being a 3D molecular mimetic of host heparan sulphate, helping it to evade detection by the mammalian immune system. Copyright (C) 2009 S. Karger AG, Basel