Capturing Natural Product Factories in Action

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Polyketides are biosynthesised by large, multi-modular polypeptides known as polyketide synthases (PKS). These contain diverse catalytic domains, that are programmed to function in a highly controlled fashion. Nature employs a range of strategies to introduce structural diversity into polyketides with exquisite control of regio- and stereochemistry. One such example is the introduction of alkyl chains at the electrophilic beta-position of the growing polyketide - so called beta-branching. This process is under the control of a set of enzymes known collectively as a 3-hydroxy-3-methylglutaryl synthase (HMGS) cassette, usually yielding a beta-methyl branched alpha,beta-unsaturated product. Bongkrekic acid is one example of a polyketide which contains two beta-branches. One of these, the carboxymethyl beta-branch, appears to omit the final ECH2 (BonI)-catalysed decarboxylation. It is not currently known how this is controlled. Utilising an in vitro NMR assay, we aim to monitor [13C]-labelled acyl carrier protein (ACP)-bound intermediates, in real time, as they proceed through an enzyme cascade. We aim to synthesise [13C]-labelled ACP-bound substrates which incorporate a sub-section of bongkrekic acid and apply NMR to monitor the fate of this intermediate. Bongkrekic acid is just one of many examples of beta-branching in polyketides and the development and implementation of this NMR assay could be applied to a variety of systems.