Complex polyketides are characterized by multiple chiral centers harboring hydroxyl and alkyl substituents. To investigate the mechanisms by which these stereocenters are set, several high-resolution structures of the ketoreductase (KR) domain from the second module of the amphotericin modular polyketide synthase (PKS) were solved. This first structural analysis of an A-type KR helps reveal how these KRs direct polyketide intermediates into their active sites from the side opposite that used by B-type KRs, resulting in a beta-hydroxyl group of opposite stereochemistry. A comparison of structures obtained in the absence and presence of ligands reveals an induced fit mechanism that is important for catalysis. Activity assays of mutants of KRs from the first and second modules of the amphotericin PKS reveal the relative contributions of several active site residues toward catalysis and stereocontrol. Together, these results highlight the possibility of region-specific modification of polyketides through active site engineering of KRs.

Chiral building blocks are valuable intermediates in the syntheses of natural products and pharmaceuticals. A scalable chemoenzymatic route to chiral diketides has been developed that includes the general synthesis of α-substituted, β-ketoacyl N-acetylcysteamine thioesters followed by a biocatalytic cycle in which a glucose-fueled NADPH-regeneration system drives reductions catalyzed by isolated modular polyketide synthase (PKS) ketoreductases (KRs). To identify KRs that operate as active, stereospecific biocatalysts, 11 isolated KRs were incubated with 5 diketides and their products were analyzed by chiral chromatography. KRs that naturally reduce small polyketide intermediates were the most active and stereospecific toward the panel of diketides. Several biocatalytic reactions were scaled up to yield more than 100 mg of product. These syntheses demonstrate the ability of PKS enzymes to economically and greenly generate diverse chiral building blocks on a preparative scale.

The enoylreductase (ER) is the final common enzyme from modular polyketide synthases (PKSs) to be structurally characterized. The 3.0 Å resolution structure of the didomain comprised of the ketoreductase (KR) and ER from the second module of the spinosyn PKS reveals that ER shares an ~600 Å2 interface with KR distinct from that of the related mammalian fatty acid synthase (FAS). In contrast to the ER domains of the mammalian FAS, the ER domains of the second module of the spinosyn PKS do not make contact across the twofold axis of the synthase. This monomeric organization may have been necessary in the evolution of multimodular PKSs to enable acyl carrier proteins (ACPs) to access each of their cognate enzymes. The isolated ER domain showed activity towards a substrate analog, enabling the contributions of its active site residues to be determined.

Jadomycin B, an antifungal antibiotic with a unique 8H-benz[b]oxazolo[3,2-f]phenanthridine pentacyclic skeleton produced by the bacterium Streptomyces venezuelae ISP 5230, exists in a dynamic equilibrium of two diastereomers differing in the configuration of C-3a. Several novel jadomycins with various amino acid-derived 1-side chains could be generated, by replacing isoleucine in the production medium of S. venezuelae with other amino acids. These two findings led to the conclusion that a nonenzymatic reaction with the amino acid followed by a likewise nonenzymatic cyclization cascade are crucial for its late biosynthesis.

Modular polyketide synthase ketoreductases often set two stereocenters when reducing intermediates in the biosynthesis of a complex polyketide. Here we report the 2.60 Å-resolution structure of an A2-type ketoreductase from the eleventh module of the amphotericin polyketide synthase that sets a combination of l-α-methyl and l-β-hydroxyl stereochemistries and represents the final catalytically-competent ketoreductase type to be structurally elucidated. Through structure-guided mutagenesis a double mutant of an A1-type ketoreductase was generated that functions as an A2-type ketoreductase on a diketide substrate analog, setting an α-alkyl substituent in an l- orientation rather than in the d-orientation set by the unmutated ketoreductase. When the activity of the double-mutant was examined in the context of an engineered triketide lactone synthase, the anticipated triketide lactone was not produced even though the ketoreductase-containing module still reduced the diketide substrate analog as expected. These findings suggest that re-engineered ketoreductases may be catalytically outcompeted within engineered polyketide synthase assembly lines.

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Aurora kinases have emerged as promising targets for cancer therapy because of their critical role in mitosis. These kinases are well-conserved in all eukaryotes, and IPL1 gene encodes the single Aurora kinase in budding yeast. In a virtual screening attempt, 22 compounds were identified from nearly 15,000 microbial natural products as potential small-molecular inhibitors of human Aurora-B kinase. One compound, Jadomycin B, inhibits the growth of ipl1-321 temperature-sensitive mutant more dramatically than wild-type yeast cells, raising the possibility that this compound is an Aurora kinase inhibitor. Further in vitro biochemical assay using purified recombinant human Aurora-B kinase shows that Jadomycin B inhibits Aurora-B activity in a dose-dependent manner. Our results also indicate that Jadomycin B competes with ATP for the kinase domain, which is consistent with our docking prediction. Like other Aurora kinase inhibitors, Jadomycin B blocks the phosphorylation of histone H3 on Ser10 in vivo. We also present evidence suggesting that Jadomycin B induces apoptosis in tumor cells without obvious effects on cell cycle. All the results indicate that Jadomycin B is a new Aurora-B kinase inhibitor worthy of further investigation. [Mol Cancer Ther 2008;7(8):2386-93]

Jadomycin B is a member of atypical angucycline antibiotics whose biosynthesis involves a unique ring opening C-C bond cleavage reaction. Here, we firmly identified JadG as the enzyme responsible for the B ring opening reaction in jadomycin biosynthesis. In vitro analysis of the JadG catalyzed reaction revealed that it requires FMNH(2) or FADH(2) as cofactors in the conversion of dehydrorabelomycin to jadomycin A. The cofactors could be supplied by either a cluster-situated flavin reductase JadY or the Escherichia coli Fre. JadY was characterized as a NAD(P)H-dependent FMN/FAD reductase, with FMN as the preferred substrate. Disruption mutant of jadY still produced jadomycin, indicating that the function of JadY could be substituted by other enzymes in the host. JadG represents the biochemically verified member of an enzyme class catalyzing an unprecedented C-C bond cleavage reaction.