Fischerazoles A-C, cyanobacterial polychlorinated lipids featuring fatty acyl chain rearrangement

Abstract

Fatty acyl moieties are a very common feature of cyanobacterial secondary metabolites. Using a recently developed method to discover fatty acyl-containing natural products, we detected and isolated three new lipopeptides – fisch-erazoles A-C (1-3) – from the cyanobacterium Fischerella sp. PCC 9431. These metabolites present extensive halo-genation in their fatty acyl moiety as well as an unusual pendant allyl alcohol. We analyzed the genome sequence of the producing cyanobacterium and identified a putative biosynthetic gene cluster associated with the fischera-zoles, based on bioinformatic analysis. Using stable-isotope precursor feeding experiments, we identified the key substrates for the biosynthesis of 1-3. Surprisingly, we found that hexadecanoic acid was fully incorporated into fischerazoles, despite the fact that these compounds have a linear 15-carbon chain. Additional feeding experiments with stable-isotope labeled fatty acids established a one-carbon contraction of the palmitic acid-derived alkyl chain, and that the remaining carbon branches out as part of the pendant allyl alcohol group. This rearrangement, which leads to the functionalization of a mid-chain aliphatic carbon, might be of interest for biocatalysis. Our findings reinforce the utility of metabolomics-based methods to uncover natural product scaffolds with a high degree of novelty.

Incorporation and modification of fatty acids in cyanobacterial natural products biosynthesis

“…Hence, we developed a metabolomics-based method that allowed us to detect such compounds, some of which we have reported. 14,42 The method involves supplementation of cyanobacterial cultures with perdeuterated fatty acids, namely d 11 -hexanoic acid. The deuterons eventually get incorporated into lipids, including natural products, and the label is not scrambled due to the absence of fatty acid degradation (Fig.…”

“…aranazole C, 27) 41 or fischerazoles ( e.g. fischerazole A, 28) 42 illustrate the rich regioselective decoration of fatty acid-derived alkyl moieties with halogens. This is particularly remarkable in the biosynthesis of the aranazoles, where six halogenated positions are likely generated by the six nonheme iron/2OG-dependent halogenases encoded in the putative aranazole BGC ( arz ).…”

“…nocuolactylate A, 37) 14 and the fischerazoles ( e.g. 28) 42 are highly unusual fatty acid-incorporating structures that have been uncovered using this strategy.…”

“…36 BGCs that encode FAALs usually code for enzymes that modify and decorate acyl chains, including halogenases, cytochrome P450s, desaturases and other. 16,[40][41][42][43] Such modifications can influence the biological activity of the resulting natural product e.g. 44…”