Fig. 3. Noncanonical polyketide synthase/nonribosomal peptide synthetase reactions involving tethered thioester intermediates.

a | Enoyl reductase (ER)-catalysed cyclopropanation during curacin biosynthesis. The curacin and jamaicamide biosynthetic pathways diverge at the β-branching step, which is initiated by decarboxylase (ECH₂)-catalysed decarboxylation, resulting in α–β double-bond and β–γ double-bond formation in respective curacin and jamaicamide enoyl-γ-chloro-acyl carrier protein (ACP) intermediates^45,46. Subsequent activity of an unusual cis-acting ER domain in CurF catalyses cyclopropane ring formation (highlighted in orange)⁴⁷. The homologous ER domain in JamJ is unreactive towards its respective ECH₂ product (unmodified ECH₂ thioester product, highlighted in blue). The new bond formed to close the cyclopropane ring is highlighted in red. b | On-line tailoring reactions catalysed by divergent condensation (C) and thioesterase (TE) domains during nocardicin biosynthesis. The terminal C domain in NocB catalyses initial elimination of water, followed by cyclization to yield the β-lactam pharmacophore⁴⁸. The resulting thioester intermediate is subsequently transferred to the NocB TE domain, which performs an unprecedented epimerization of the l-(p-hydroxyphenyl)glycine (l-Hpg) moiety, before canonical hydrolysis to release nocardicin G⁵⁰. Nonribosomal peptide synthetase domains catalysing the specific transformations illustrated are highlighted and new bonds formed to generate the β-lactam ring by the C domain are shown in red. Polyketide synthase domains and nonribosomal peptide synthetase domains are highlighted in red and blue, respectively. Trans-acting enzymes are highlighted in purple. A, adenylation; AT, acyltransferase; ECH₁, dehydratase; Hal, halogenase; HMGS, 3-hydroxy-3-methylglutaryl CoA synthase; KS, ketosynthase; PCP, peptidyl carrier protein.