Nonribosomal peptide synthetases (NRPSs) are responsible for the production of important biologically active peptides. The large, multidomain NRPSs operate through an assembly line strategy in which the growing peptide is tethered to carrier domains that deliver the intermediates to neighboring catalytic domains. While most NRPS domains catalyze standard chemistry of amino acid activation, peptide bond formation and product release, some canonical NRPS catalytic domains promote unexpected chemistry. The paradigm monobactam antibiotic sulfazecin is produced through the activity of a terminal thioesterase domain that catalyzes an unusual ��-lactam forming reaction in which the nitrogen of the C-terminal -sulfo-2,3-diaminopropionate residue attacks its thioester tether to release the ��-lactam product. We have determined the structure of the thioesterase domain as both a free-standing domain and a didomain complex with the upstream peptidyl-carrier domain. The structure illustrates a constrained active site that orients the substrate properly for ��-lactam formation. In this regard, the structure is similar to the ��-lactone forming thioesterase domain responsible for the production of obafluorin. Analysis of the structure identifies features that are responsible for this four-membered ring closure and enable bioinformatic analysis to identify additional, uncharacterized ��-lactam-forming biosynthetic gene clusters by genome mining.
