Figure 3.

Individual reactions in the catalytic cycle of DEBS module 3. Each module of DEBS catalyzes a set of reactions that can be categorized as follows: (1) intermodular chain translocation involving transthioesterification from the ACP domain of the upstream module to the KS domain of the target module, (2) transfer of an acyl group from an α-carboxyacyl-CoA extender unit to the ACP domain of the target module, (3) chain elongation involving decarboxylative condensation of the growing polyketide chain onto the extender unit, (4) modification of the newly elongated chain at the α- and β-carbon atoms, and (5) intermodular chain translocation involving transthioesterification from the ACP domain of the target module to the KS domain of the downstream module. In the case of the representative DEBS module 3 shown here, chain modification simply involves epimerization of the α-carbon, a reaction that is catalyzed by KR⁰. Note that the three proposed states of the ketosynthase are highlighted as KS, KS*, and KS**. For details, see the text.