Antimicrobial resistance is a serious and growing problem for the public health, the number of infections from antibiotic-resistant microorganisms has increased significantly in the last few years. The pathogenic gram-negative bacteria, such as Pseudomonas aeruginosa and Acinetobacter baumannii are found resistant to many antibiotics, for this reason the design of new antibiotics, or new strategies to against this class of bacteria are urgently required . Bacterial surface polysaccharide antigens represent one of the most important targets of immunotherapy to prevent bacterial infections, unfortunately the variability of the external part of the polysaccharide antigens, restrict the use of monoclonal antibodies (mAb), while an additional drawback is the lower bactericidal capacity of mAb.
Recently a new engineered mAb (VSX) targeting the inner-core glycan of the Lipopolysaccharide (LPS) of P. aeruginosa, was biologically proven to recognise the less-variable part of the glycan antigen, and for this reason potentially targeting a wider class of bacteria. An Antibody-Drug-Coniugate (ADC) system was than designed connecting by covalent link an α-helical peptide able to affect the membrane integrity of P. aeruginosa. Biological tests in-vitro and in-vivo indicate that ADC is active against P. aeruginosa strains and protects mice from P. aeruginosa lung infection. Bacterial cell ELISA test indicates that VSX binds to 23 diverse P. aeruginosa strains, while it does not bind to other common human pathogenic gram-negative bacteria including multiple strains of Escherichia coli, Klebsiella pneumoniae and Salmonella typhimurium.
These, and additional molecular biology evidences indicate that VSX recognize the conserved inner-core oligosaccharide, particularly the 2,4-diphospho α-D-Heptose moiety in the LPS present on the outer membrane of P. aeruginosa strains. To understand the molecular reasons by which VSX recognize the inner-core glycan of LPS, and possibly predicting how to tune it, the application of complementary structural biology techniques experimental and theoretical are strongly required. In a first stage the 2,4-diphospho α-D-Heptose monosaccharide is used as a model for the glycan part of LPS. 1H, 1H-STD, and 31P NMR spectra run on high sensitivity spectrometer cryo-probe 600MHz, allow to obtain a nearly atomic description of the interaction by which VSX recognize this glycan, depicting how the two phosphate groups, in particular 4-P, are required for the binding. Molecular Docking, and MD simulation with the “state of the art” GLYCAM06  Force-Field are applied to build a 3D static and dynamic interpretation of the recognition event, which comprehension will be considered preparatory before to include bigger size glycan antigen.
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