Wound-healing assay-guided fractionation of an EtOAc extract of the fungal strain Fusarium oxysporum EPH2RAA endophytic in Ephedra fasciculata afforded beauvericin (1), (-)-oxysporidinone (2), and two new N-methyl-2-pyridones, (-)-4,6'-anhydrooxysporidinone (3) and (-)-6-deoxyoxysporidinone (4). Beauvericin (1) inhibited migration of the metastatic prostate cancer (PC-3M) and breast cancer (MDA-MB-231) cells and showed antiangiogenic activity in HUVEC-2 cells at sublethal concentrations. Cytotoxicity-guided fractionation of an EtOAc extract of F. oxysporum strain CECIS occurring in Cylindropuntia echinocarpus afforded rhodolamprometrin (5), bikaverin (6), and the new natural product 6-deoxybikaverin (7). All compounds were evaluated for cytotoxicity in a panel of four sentinel cancer cell lines, NCI-H460 (non-small-cell lung), MIA Pa Ca-2 (pancreatic), MCF-7 (breast), and SF-268 (CNS glioma), and only beauvericin (1) and bikaverin (6) were active, with 1 and 6 showing selective toxicity toward NCI-H460 and MIA Pa Ca-2, respectively. Interestingly, 6-deoxybikaverin (7) was completely devoid of activity, suggesting the requirement of the C-6 hydroxy group of bikaverin for its cytotoxic activity.

SummaryPolyketides represent an important class of biologically active and structurally diverse compounds in nature. They are synthesized from acyl-coenzyme A substrates by polyketide synthases (PKSs). PKSs are classified into three groups: types I, II, and III. This article introduces recent studies on type III PKSs identified from plants, bacteria, and fungi, and describes the catalytic functions of these enzymes in detail. Plant type III PKSs have been widely studied, as exemplified by chalcone synthase, which plays an important role in the synthesis of plant metabolites. Bacterial type III PKSs fall into five groups, many of which were identified from Streptomyces, a genus that has been well known for its production of bioactive molecules and genetic alterability. Although it was believed that type III PKSs exist exclusively in plants and bacteria, recent fungal genome sequencing projects and biochemical studies revealed the presence of type III PKSs in filamentous fungi, which provides a new chance to study fungal secondary metabolism and synthesize ''unnatural'' natural products. Type III PKSs have been used for the biosynthesis of novel molecules through precursordirected and structure-based mutagenesis approaches.2012 IUBMB IUBMB Life, 64(4): [285][286][287][288][289][290][291][292][293][294][295] 2012

Resorcylic acid lactones and dihydroxyphenylacetic acid lactones represent important pharmacophores with heat shock response and immune system modulatory activities. The biosynthesis of these fungal polyketides involves a pair of collaborating iterative polyketide synthases (iPKSs): a highly reducing iPKS with product that is further elaborated by a nonreducing iPKS (nrPKS) to yield a 1,3-benzenediol moiety bridged by a macrolactone. Biosynthesis of unreduced polyketides requires the sequestration and programmed cyclization of highly reactive poly-β-ketoacyl intermediates to channel these uncommitted, pluripotent substrates to defined subsets of the polyketide structural space. Catalyzed by product template (PT) domains of the fungal nrPKSs and discrete aromatase/cyclase enzymes in bacteria, regiospecific first-ring aldol cyclizations result in characteristically different polyketide folding modes. However, a few fungal polyketides, including the dihydroxyphenylacetic acid lactone dehydrocurvularin, derive from a folding event that is analogous to the bacterial folding mode. The structural basis of such a drastic difference in the way a PT domain acts has not been investigated until now. We report here that the fungal vs. bacterial folding mode difference is portable on creating hybrid enzymes, and we structurally characterize the resulting unnatural products. Using structure-guided active site engineering, we unravel structural contributions to regiospecific aldol condensations and show that reshaping the cyclization chamber of a PT domain by only three selected point mutations is sufficient to reprogram the dehydrocurvularin nrPKS to produce polyketides with a fungal fold. Such rational control of first-ring cyclizations will facilitate efforts to the engineered biosynthesis of novel chemical diversity from natural unreduced polyketides.