Conformational and quantitative characterization of oritavancin-peptidoglycan complexes in whole cells of Staphylococcus aureus by in vivo 13C and 15N labeling

Solid-state NMR has been used to examine the cell walls of intact whole cells of Staphyloccus aureus grown on media containing D-1-(13)C]alanine, (15)N]glycine, and the alanine racemase inhibitor, alaphosphin. The results of in situ site-selective, four-frequency NMR experiments show directly for the first time that (i) 54% of the cell-wall peptidoglycan stems have D-alanine termini and 46%, D-alanine-D-alanine termini; (ii) the molar ratio of stems ending in D-alanine to esterified alditol repeats of cell-wall teichoic and lipoteichoic acids is 3:2; and (iii) 50% of the mature cell-wall binding sites for a fluorinated oritavancin analogue consist of two nearest-neighbor peptide stems of different glycan strands. The drug is bound to the D-Ala-D-Ala terminus of one stem and is proximate to the bridging pentaglycyl segment that cross-links the two stems. Structural details of the binding site are revealed in a model of the glycopeptide-peptidoglycan interaction produced by molecular dynamics simulations with internuclear distance restraints determined by NMR.