Total Synthesis of N-Acetylglucosamine-1,6-anhydro-N-acetylmuramylpentapeptide and Evaluation of Its Turnover by AmpD from Escherichia coli

The bacterial cell wall is recycled extensively during the course of cell growth. The first recycling event involves the catalytic action of the lytic transglycosylase enzymes, which produce an uncommon 1,6-anhydropyranose moiety during separation of the muramyl residues from the peptidoglycan, the major constituent of the cell wall. This product, an N-acetyl-β-d-glucosamine-(1→4)-1,6-anhydro-N-acetyl-β-d-muramylpeptide, is either internalized to initiate the recycling process or diffuses into the milieu to cause stimulation of the pro-inflammatory responses by the host. We report the total syntheses of N-acetyl-β-d-glucosamine-(1→4)-1,6-anhydro-N-acetyl-β-d-muramyl-l-Ala-γ-d-Glu-meso-DAP-d-Ala-d-Ala (compound 1, the product of lytic transglycosylase action on the cell wall of Gram-negative bacteria) and N-acetyl-β-d-glucosamine-(1→4)-1,6-anhydro-N-acetyl-β-d-muramyl-l-Ala-γ-d-Glu-l-Lys-d-Ala-d-Ala (compound 2, from lytic transglycosylase action on the cell wall of Gram-positive bacteria). The syntheses were accomplished in 15 linear steps. Compound 1 is shown to be a substrate of the AmpD enzyme of the Gram-negative bacterium Escherichia coli, an enzyme that removes the peptide from the disaccharide scaffold in the early cytoplasmic phase of cell wall turnover.