| Many widely used therapeutic natural products, including antibiotics (penicillin, vancomycin, bacitracin, etc), anticancer agents, immunosuppressants, antiparasitic agents, antifungals, and cardiovascular agents, are produced by polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs). PKSs and NRPSs are structurally and mechanistically similar to fatty acid synthases (FASs) which together comprise a class of large multimodular enzymes that catalyze the synthesis of biopolymers without the use of a nucleic acid template. Through the utilization of a “thiotemplate” mechanism and the modular organization of the large multienzyme systems, diverse structures are derived from the activation of acyl-coenzyme A or amino acid monomers. New natural products can be generated by introducing variability into the structure through the incorporation of alternative amino acid, primer or extender units. Establishing rules regarding donor and acceptor specificity allowing rational predictions as to how acyl chains are recognized and processed will aid in the making of polyketide and peptide combinatorial libraries.; Yersiniabactin (Ybt), the tetracyclic iron chelating siderophore produced as a virulence factor in infections by the plague bacterium Yersinia pestis, is encoded by the hybrid NRP/PK/NRP Ybt synthetase. The Ybt system encodes for many intriguing catalytic steps including: (1) the heterocyclization of N-acyl-cysteinyl-S-PCP intermediates to thiazolinyl rings; (2) the regioselective redox adjustment of the middle thiazoline of Ybt to thiazolidine; and (3) the introduction of three C-methyl groups by two methyl transferase (MT) domains. Additionally, Ybt is a member of the NRPS/PKS hybrid family and thus provides an opportunity to explore how the modules interface and how the chain elongation switches between carbon-carbon bond-forming keto synthase (KS) domains and peptide bond-forming condensation (C) domains. The work described here explores the rules, specificity, and mechanism of domains within both NRPS and PKS systems, which will be useful for potential portability and combinatorial biosynthesis in such hybrid natural products. |
