Funding for this research was generously provided by the National Institutes of Health (AI51629).
Polyketides, Microbial metabolites
The majority of modular polyketide synthase (PKS) systems which generate unsaturated products do so with trans double bonds. Phoslactomycin B (PLM B) presents a class of antitumor and antiviral natural polyketide products that have unique structural features, including a linear unsaturated backbone with one trans and three cis double bonds. There is substantial evidence that trans double bonds are established by ketoreductase-dehydratase (KR-DH) didomains within a PKS module. In cases where modules containing these didomains appear to generate product containing a cis double bond there is no experimental evidence to determine if they do so directly, or if they also form a trans double bond with a subsequent isomerization step. A critical step in addressing this issue is establishing the stereochemistry of the polyketide intermediate which passes to the subsequent module. Herein, we demonstrate through a series of experiments that an activated cis-3-cyclohexylpropenoic acid is the diketide intermediate which passes from module 1 to module 2 of the PLM PKS. The trans isomer of the diketide intermediate could not be processed directly into PLM B by module 2, but could be converted to PLM B by degradation to cyclohexanecarboxylic acid and elongation by the entire PLM PKS. These observations indicate not only that module 1 with a DH-KR didomain is responsible for establishing C14–C15 cis double bond of PLM B, but that the subsequent modules of the PKS clearly discriminate between the cis and trans-diketide intermediate and do not contain domains capable of catalyzing double bond isomerization.
Alhamadsheh, Mamoun M.; Palaniappan, Nadaraj; DasChouduri, Suparna; and Reynolds, Kevin A., "Modular Polyketide Synthases and cis Double Bond Formation: Establishment of Activated cis-3-Cyclohexylpropenoic Acid as the Diketide Intermediate in Phoslactomycin Biosynthesis" (2007). Chemistry Faculty Publications and Presentations. 172.