Bestimmung der relativen und absoluten Konfiguration von Thuggacin A–C, hochaktiven Antibiotika aus dem Myxobakterium Sorangium cellulosum

Isolation, Structure Elucidation, and (Bio)Synthesis of Haprolid, a Cell-Type-Specific Myxobacterial Cytotoxin

Myxobacteria are well-established sources for novel natural products exhibiting intriguing bioactivities. We here report on haprolid (1) isolated from Byssovorax cruenta Har1. The compound exhibits an unprecedented macrolactone comprising four modified amino acids and a polyketide fragment. As configurational assignment proved difficult, a bioinformatic analysis of the biosynthetic gene cluster was chosen to predict the configuration of each stereocenter. In-depth analysis of the corresponding biosynthetic proteins established a hybrid polyketide synthase/nonribosomal peptide synthetase origin of haprolid and allowed for stereochemical assignments. A subsequent total synthesis yielded haprolid and corroborated all predictions made. Intriguingly, haprolid showed cytotoxicity against several cell lines in the nanomolar range whereas other cells were almost unaffected by treatment with the compound.

An Amidinohydrolase Provides the Missing Link in the Biosynthesis of Amino Marginolactone Antibiotics

Desertomycin A is an aminopolyol polyketide containing a macrolactone ring. We have proposed that desertomycin A and similar compounds (marginolactones) are formed by polyketide synthases primed not with γ-aminobutanoyl-CoA but with 4-guanidinylbutanoyl-CoA, to avoid facile cyclization of the starter unit. This hypothesis requires that there be a final-stage de-amidination of the corresponding guanidino-substituted natural product, but no enzyme for such a process has been described. We have now identified candidate amidinohydrolase genes within the desertomycin and primycin clusters. Deletion of the putative desertomycin amidinohydrolase gene dstH in Streptomyces macronensis led to the accumulation of desertomycin B, the guanidino form of the antibiotic. Also, purified DstH efficiently catalyzed the in vitro conversion of desertomycin B into the A form. Hence this amidinohydrolase furnishes the missing link in this proposed naturally evolved example of protective-group chemistry.

Total and semi-syntheses of antimicrobial thuggacin derivatives

The total and semi-synthesis of 13 new macrolactones derived from thuggacin, which is a secondary metabolite from the myxobacterium Sorangium cellulosum, are reported. The thuggacins have attracted much attention due to their strong antibacterial activity, particularly towards Mycobacterium tuberculosis. This study focuses on 1) thuggacin derivatives that cannot equilibrate by transacylation between the three natural thuggacins A-C, 2) the roles of the thiazole ring, and 3) the hexyl side chain at C2. Semi-synthetic O-methylation at C17 suppressed the transacylations without a substantial loss of antibacterial activity. Exchanging the C17-C25 side chain for simplified hydrophobic chains led to complete loss of antibacterial activity. Exchange of the thiazole by an oxazole ring or removal of the hexyl side chain at C2 had no substantial effect on the biological properties.

Structure and biosynthetic assembly of gulmirecins, macrolide antibiotics from the predatory bacterium Pyxidicoccus fallax

The gulmirecins constitute a new class of glycosylated macrolides that were isolated from the predatory bacterium Pyxidicoccus fallax HKI 727. Their structures were solved by a combination of NMR spectroscopic experiments and chemical derivatization. Analysis of the annotated gulmirecin gene cluster complemented the configurational assignment and provided insights into the stereochemical course of the biosynthetic assembly. The gulmirecins exhibit strong activity against staphylococci, including methicillin-resistant Staphylococcus aureus, but no cytotoxic effects on human cells.

Biosynthetic code for divergolide assembly in a bacterial mangrove endophyte

Divergolides are structurally diverse ansamycins produced by a bacterial endophyte (Streptomyces sp.) of the mangrove tree Bruguiera gymnorrhiza. By genomic analyses a gene locus coding for the divergolide pathway was detected. The div gene cluster encodes genes for the biosynthesis of 3-amino-5-hydroxybenzoate and the rare extender units ethylmalonyl-CoA and isobutylmalonyl-CoA, polyketide assembly by a modular type I polyketide synthase (PKS), and enzymes involved in tailoring reactions, such as a Baeyer-Villiger oxygenase. A detailed PKS domain analysis confirmed the stereochemical integrity of the divergolides and provided valuable new insights into the formation of the diverse aromatic chromophores. The bioinformatic analyses and the isolation and full structural elucidation of four new divergolide congeners led to a revised biosynthetic model that illustrates the formation of four different types of ansamycin chromophores from a single polyketide precursor.

Uncovering the origin of Z-configured double bonds in polyketides: intermediate E-double bond formation during borrelidin biosynthesis

The dehydratase domain BorDH3 is assayed with a synthetic surrogate of the predicted tetraketide substrate and shown to be E-selective. Detailed NMR spectroscopic analysis of pre-borrelidin assigns the timing of the E-5 Z-isomerization to the very final steps of borrelidin biosynthesis.