Astonishing diversity of natural surfactants: 2. Polyether glycosidic ionophores and macrocyclic glycosides

Polyether glycosidic ionophores and macrocyclic glycosides are of great interest, especially for the medicinal and pharmaceutical industries. These biologically active natural surfactants are good prospects for the future chemical preparation of compounds useful as antibiotics, anticancer agents, or in industry. More than 300 interesting and unusual natural surfactants are described in this review article, including their chemical structures and biological activities.

Polyene antibiotic biosynthesis gene clusters

Over the past 15 years the biosynthetic gene clusters for numerous bioactive polyketides have been intensively studied and recently this work has been extended to the antifungal polyene macrolides. These compounds consist of large macrolactone rings that have a characteristic series of conjugated double bonds, as well as an exocyclic carboxyl group and an unusual mycosamine sugar. The biosynthetic gene clusters for nystatin, pimaricin, amphotericin and candicidin have been investigated in detail. These clusters contain the largest modular polyketide synthase genes reported to date. This body of work also provides insights into the enzymes catalysing the unusual post-polyketide modifications, and the genes regulating antibiotic biosynthesis. The sequences also provide clues about the evolutionary origins of polyene biosynthetic genes. Successful genetic manipulation of the producing organisms leading to production of polyene analogues indicates good prospects for generating improved antifungal compounds via genetic engineering.

Engineered biosynthesis of novel polyenes: a pimaricin derivative produced by targeted gene disruption in Streptomyces natalensis

BACKGROUND

The post-polyketide synthase biosynthetic tailoring of polyene macrolides usually involves oxidations catalysed by cytochrome P450 monooxygenases (P450s). Although members from this class of enzymes are common in macrolide biosynthetic gene clusters, their specificities vary considerably toward the substrates utilised and the positions of the hydroxyl functions introduced. In addition, some of them may yield epoxide groups. Therefore, the identification of novel macrolide monooxygenases with activities toward alternative substrates, particularly epoxidases, is a fundamental aspect of the growing field of combinatorial biosynthesis. The specific alteration of these activities should constitute a further source of novel analogues. We investigated this possibility by directed inactivation of one of the P450s belonging to the biosynthetic gene cluster of an archetype polyene, pimaricin.

RESULTS

A recombinant mutant of the pimaricin-producing actinomycete Streptomyces natalensis produced a novel pimaricin derivative, 4,5-deepoxypimaricin, as a major product. This biologically active product resulted from the phage-mediated targeted disruption of the gene pimD, which encodes the cytochrome P450 epoxidase that converts deepoxypimaricin into pimaricin. The 4,5-deepoxypimaricin has been identified by mass spectrometry and nuclear magnetic resonance following high-performance liquid chromatography purification.

CONCLUSIONS

We have demonstrated that PimD is the epoxidase responsible for the conversion of 4,5-deepoxypimaricin to pimaricin in S. natalensis. The metabolite accumulated by the recombinant mutant, in which the epoxidase has been knocked out, constitutes the first designer polyene obtained by targeted manipulation of a polyene biosynthetic gene cluster. This novel epoxidase could prove to be valuable for the introduction of epoxy substituents into designer macrolides.

A complex multienzyme system encoded by five polyketide synthase genes is involved in the biosynthesis of the 26-membered polyene macrolide pimaricin in Streptomyces natalensis

BACKGROUND

Polyene macrolides are a class of large macrocyclic polyketides that interact with membrane sterols, having antibiotic activity against fungi but not bacteria. Their rings include a chromophore of 3-7 conjugated double bonds which constitute the distinct polyene structure. Pimaricin is an archetype polyene, important in the food industry as a preservative to prevent mould contamination of foods, produced by Streptomyces natalensis. We set out to clone, sequence and analyse the gene cluster responsible for the biosynthesis of this tetraene.

RESULTS

A large cluster of 16 open reading frames spanning 84985 bp of the S. natalensis genome has been sequenced and found to encode 13 homologous sets of enzyme activities (modules) of a polyketide synthase (PKS) distributed within five giant multienzyme proteins (PIMS0-PIMS4). The total of 60 constituent active sites, 25 of them on a single enzyme (PIMS2), make this an exceptional multienzyme system. Eleven additional genes appear to govern modification of the polyketide-derived framework and export. Disruption of the genes encoding the PKS abolished pimaricin production.

CONCLUSIONS

The overall architecture of the PKS gene cluster responsible for the biosynthesis of the 26-membered polyene macrolide pimaricin has been determined. Eleven additional tailoring genes have been cloned and analysed. The availability of the PKS cluster will facilitate the generation of designer pimaricins by combinatorial biosynthesis approaches. This work represents the extensive description of a second polyene macrolide biosynthetic gene cluster after the one for the antifungal nystatin.