Vancomycin binding to low-affinity ligands: delineating a minimum set of interactions necessary for high-affinity binding.

Bacterial resistance to vancomycin has been attributed to the loss of an intermolecular hydrogen bond between vancomycin and its peptidoglycan target when cell wall biosynthesis proceeds via depsipeptide intermediates rather than the usual polypeptide intermediates. To investigate the relative importance of this hydrogen bond to vancomycin binding, we have determined crystal structures at 1.0 A resolution for the vancomycin complexes with three ligands that mimic peptides and depsipeptides found in vancomycin-sensitive and vancomycin-resistant bacteria: N-acetylglycine, D-lactic acid, and 2-acetoxy-D-propanoic acid. These, in conjunction with structures that have been reported previously, indicate higher-affinity ligands elicit a structural change in the drug not seen with these low-affinity ligands. They also enable us to define a minimal set of drug-ligand interactions necessary to confer higher-affinity binding on a ligand. Most importantly, these structures point to factors in addition to the loss of an intermolecular hydrogen bond that must be invoked to explain the weaker affinity of vancomycin for depsipeptide ligands. These factors are important considerations for the design of vancomycin analogues to overcome vancomycin resistance.