Abstract
Aklanonic acid, an anthraquinone natural product, is a common advanced intermediate in the biosynthesis of several antitumor polyketide antibiotics, including doxorubicin and aclacinomycin A. Intensive semisynthetic and biosynthetic efforts have been directed toward developing improved analogues of these clinically important compounds. The primer unit of such polyfunctional aromatic polyketides is an attractive site for introducing novel chemical functionality, and attempts have been made to modify the primer unit by precursor-directed biosynthesis or protein engineering of the polyketide synthase (PKS). We have previously demonstrated the feasibility of engineering bimodular aromatic PKSs capable of synthesizing unnatural hexaketides and octaketides. In this report, we extend this ability by preparing analogues of aklanonic acid, a decaketide, and its methyl ester. For example, by recombining the R1128 initiation module with the dodecaketide-specific pradimicin PKS, the isobutyryl-primed analogue of aklanonic acid (YT296b, 10) and its methyl ester (YT299b, 12) were prepared. In contrast, elongation modules from dodecaketide-specific spore pigment PKSs were unable to interact with the R1128 initiation module. Thus, in addition to revealing a practical route to new anthracycline antibiotics, we also observed a fundamental incompatibility between antibiotic and spore pigment biosynthesis in the actinomycetes bacteria.