Synthetic chain terminators off-load intermediates from a type I polyketide synthase.

Modular biocatalysis is responsible for the generation of countless bioactive products and its mining remains a major focus for drug discovery purposes. One of the enduring hurdles is the isolation of biosynthetic intermediates in a readily-analysed form. We prepared a series of nonhydrolysable pantetheine and N-acetyl cysteamine mimics of the natural (methyl)malonyl extender units recruited for polyketide formation. Using these analogues as competitive substrates, we were able to trap and off-load diketide and triketide species directly from an in vitro reconstituted type I polyketide synthase, the 6-deoxyerythronolide B synthase 3 (DEBS3). The putative intermediates, which were extracted in organic solvent and characterised by LC-HR-ESI-MS, are the first of their kind and prove that small-molecule chain terminators can be used as convenient probes of the biosynthetic process.