Structure of Extended Lipopolysaccharide Glycoforms Containing Two Globotriose Units in Haemophilus influenzae Serotype b Strain RM7004

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Abstract
Lipopolysaccharide (LPS) is a major virulence determinant of the human bacterial pathogen Haemophilus influenzae. Structural elucidation of the LPS from H. influenzae type b strain RM7004 was achieved by using electrospray ionization mass spectrometry (ESI-MS) and high-field NMR techniques on delipidated LPS and core oligosaccharide samples of LPS. It was found that the organism elaborates a series of related LPS glycoforms having a common inner-core structure, but differing in the number and position of attached hexose residues. LPS glycoforms containing between four and nine hexose residues were structurally characterized. The inner-core element was determined to be l-α-d-Hepp-(1→2)-[PEA→6]-l-α-d-Hepp-(1→3)-[β-d-Glcp-(1→4)]-l-α-d-Hepp-(1→5)-[P→4]-α-KDOp-(2→, a structural feature which has been identified in every H. influenzae strain investigated to date. Two major groups of isomeric glycoforms were characterized in which the terminal Hepp residue of the inner-core element was either substituted at the O-2 position with a β-d-Galp residue or not. The structures of the major LPS glycoforms were found to have oligosaccharide chain extensions from O-3 of the middle Hepp residue. Glycoforms containing five and six hexose residues were most abundant and were shown to carry the tetrasaccharide unit α-d-Galp-(1→4)-β-d-Galp-(1→4)-β-d-Glcp-(1→4)-α-d-Glcp at the O-3 position of the middle heptose. This tetrasaccharide displays the globoside trisaccharide (globotriose) as a terminal epitope, a structure that is found on many human cells (Pk blood group antigen) and which is thought to be an important virulence determinant for H. influenzae. LPS glycoforms were characterized that had further chain extension from the β-d-Glcp-(1→ residue of the proximal Hepp. In the fully extended LPS (Hex9/Hex8‘ glycoforms), both the proximal and middle heptose residues carried tetrasaccharide chains displaying terminal globotriose epitopes. In addition, the LPS was found to carry phosphorylcholine and O-acetyl groups.

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