As a growing number of clinical isolates of are resistant to most antibiotics, new treatment options that are effective against these drug-resistant strains are desperately needed. The majority of the linkages in the cell wall peptidoglycan of are synthesized by nonclassical transpeptidases, namely, the l,d-transpeptidases. Emerging evidence suggests that these enzymes represent a new molecular vulnerability in this pathogen. Recent studies have demonstrated that inhibition of these enzymes by the carbapenem class of β-lactams determines their activity against Here, we studied the interactions of β-lactams with two l,d-transpeptidases in, namely, Ldt and Ldt, and found that both the carbapenem and cephalosporin, but not penicillin, subclasses of β-lactams inhibit these enzymes. Contrary to the commonly held belief that combination therapy with β-lactams is redundant, doripenem and cefdinir exhibit synergy against both pansusceptible and clinical isolates that are resistant to most antibiotics, which suggests that dual-β-lactam therapy has potential for the treatment of Finally, we solved the first crystal structure of an l,d-transpeptidase, Ldt, and using substitutions of critical amino acids in the catalytic site and computational simulations, we describe the key molecular interactions between this enzyme and β-lactams, which provide an insight into the molecular basis for the relative efficacy of different β-lactams against .

Nonribosomal peptide synthetases (NRPSs) underlie the biosynthesis of many natural products that have important medicinal utility. Protection of the NRPS peptide products from proteolysis is critical to these pathways and is often achieved by structural modification, principally the introduction of d -amino acid residues into the elongating peptide. These amino acids are generally formed in situ from their l -stereoisomers by epimerization domains or dual-function condensation/epimerization domains. In singular contrast, the thioesterase domain of nocardicin biosynthesis mediates both the effectively complete l - to d -epimerization of its C -terminal amino acid residue (≥100:1) and hydrolytic product release. We report herein high-resolution crystal structures of the nocardicin thioesterase domain in ligand-free form and reacted with a structurally precise fluorophosphonate substrate mimic that identify the complete peptide binding pocket to accommodate both stereoisomers. These structures combined with additional functional studies provide detailed mechanistic insight into this unique dual-function NRPS domain.

SUMMARY Diphenylphosphonates (DPPs) have been used for 50 years to inactivate serine hydrolases, creating adducts representative of tetrahedral intermediates of this important class of enzymes. Failure to react at active site serine residues, however, can thwart their usefulness. Here we describe a facile route and allied mechanistic studies to highly reactive, structurally complex organofluorophosphonates (FPs) by direct fluorinative hydrolysis of DPPs. Advantages over current preparations of FPs are exemplified by the synthesis of a β-lactam containing peptide substrate analogue capable of modifying the C-terminal, dual-function thioesterase involved in nocardicin A biosynthesis. Although this serine hydrolase was found to resist modification by classical DPP inhibitors, active site selective phosphonylation by the corresponding FP occurs rapidly to form a stable adduct. This simple, late-stage method enables the ready preparation of FP probes that retain important structural motifs of native substrates, thus promoting efforts in mechanistic enzymology by accessing biologically relevant enzyme-inhibitor co-structures.