BIOPHYSICS SEMINAR<br>Cell-Wall Architecture of Staphylococcus Aureus and the Mode of Action of Glycopeptide Antibiotics by Solid-State NMR
The chemical structure of the Staphylococcus aureus peptidoglycan (PG) repeat unit consists of a disaccharide, a pentapeptide stem, and a bridge connecting adjacent stems. The pentapeptide stem has the amino acid sequence, l-alanine-d-iso-glutamine-l-lysine-d-alanine-d-alanine, and the bridge is a pentaglycyl segment. PG units form a cross-linked three-dimensional cell-wall lattice by the cooperative action of a transglycosylase-transpeptidase. While the chemical structure of the PG repeat unit is known, the three-dimensional structure of the PG lattice is unknown.
We have examined the PG tertiary structure in intact whole-cells of S. aureus using solid-state NMR. The PG was selectively labeled by growing bacteria in the presence of an alanine racemase inhibitor (alaphosphin) in defined media containing d-[1-13C]alanine and l-[15N]alanine. Rotational-echo double-resonance (REDOR) NMR was used to determine heteronuclear dipolar coupling, hence internuclear distances, between the 13C and 15N labels in the PG stems, and from these labels to the 19F of a glycopeptide drug bound specifically to the PG lattice. These REDOR distance constraints are consistent with a model for local order in a repeating structural motif from which the PG lattice of S. aureus is assembled such that membrane-bound protein complexes and other large functional units are accomodated. Within this motif, the glycan chains (repeating disaccharides) (i) have a 4-fold screw-axis helical conformation with a 40-Å periodicity, (ii) have cross-linked stems that are parallel to one another, and (iii) are themselves proximate to the cross-linking bridges. Glycopeptide drugs that bind to nascent PG (incompletely cross-linked PG that is just entering the lattice) interfere with the ordering of the structural motif and are therefore potent antibiotics.