Chemistry, bioactivity and biosynthesis of cyanobacterial alkylresorcinols

Abstract

Covering: up to 2019 Alkylresorcinols are amphiphilic metabolites, well-known for their diverse biological activities, produced by both prokaryotes and eukaryotes. A few classes of alkylresorcinol scaffolds have been reported from the photoautotrophic cyanobacteria, ranging from the relatively simple hierridins to the more intricate cylindrocyclophanes. Recently, it has emerged that cyanobacteria employ two different biosynthetic pathways to produce unique alkylresorcinol scaffolds. However, these convergent pathways intersect by sharing biosynthetic elements which lead to common structural motifs. To obtain a broader view of the biochemical diversity of these compounds in cyanobacteria, we comprehensively cover the isolation, structure, biological activity and biosynthesis of their mono- and dialkylresorcinols. Moreover, we provide an overview of the diversity and distribution of alkylresorcinol-generating biosynthetic gene clusters in this phylum and highlight opportunities for discovery of novel alkylresorcinol scaffolds. Because some of these molecules have inspired notable syntheses, different approaches used to build these molecules in the laboratory are showcased.

Fig. 1. Examples of MARs isolated from different heterotrophic bacteria, namely a glycolipid from the cysts of Azotobacter vinelandii (1), the main component of the beta-leprosol mixture from Mycobacterium leprae (2) and one of the components of panosialin from Streptomyces spp. (3). Compound 1 shows the numbering scheme used throughout the text to refer to resorcinol ring substituents.

Fig. 2. Cyanobacterial [7.7]paracyclophanes (16–47, 49–54) and related metabolites (48, 55–57).

Fig. 3. Biosynthesis of hierridins. Top – Hierridin biosynthetic gene cluster (hid). Bottom – Proposed biosynthesis of hierridins B (5) and C (6).

Fig. 4. Key findings that provided insight into the overall biosynthesis of cylindrocyclophanes. (A) Feeding experiments with ¹³C-labeled acetate produce cylindrocyclophanes with a distinctive labeling pattern. [1,2-¹³C]acetate produces adjacent ¹³C atoms in the positions indicated in green, with the 1- and 2-positions of acetate incorporated into specific positions. (B) Discovery of the cylindrofridins, which contain a chlorine atom at the site of carbon–carbon bond formation, matched a retrosynthetic hypothesis for the origin of cylindrocyclophanes. Cylindrofridin A (7) is the monomeric precursor to cylindrocyclophane D (23), while cylindrofridins B (55) and C (56) likely represent the product of partial macrocyclization.

Fig. 5. Biosynthesis of cyanobacterial cyclophanes. Overview of the BGCs for different cyclophane classes (top) and biosynthetic pathway for the cylindrocyclophanes (bottom), as illustrated by cylindrocylophane D (23). Enzymes that have been purified and characterized in vitro, are shown in bold.

Fig. 6. Biosynthetic hypotheses for the origin of nostocyclyne A. (A) The original hypothesis by Ploutno and Carmeli. Dashed lines indicate eventual ring-closing events. (B) An alternative hypothesis based on the elucidation of cylindrocyclophane biosynthesis.

Fig. 7. Biosynthesis of the bartolosides. Top – prototypical DAR-generating dar gene cluster from Pseudomonas aurantiaca and the two brt BGCs found in Synechocystis salina strains. Bottom – the brt pathway encodes the production of mono- and di-glycosylated bartolosides.

Fig. 8. The DAR microcarbonin (69) is hypothesized to be produced by a BGC found in Microcystis aeruginosa PCC 9808 (top). The biosynthetic events eventually leading to 69 are shown (bottom).

Fig. 9. Phylogenetic diversity of type III PKS enzymes in cyanobacteria. (A) Maximum-likelihood tree (JTT + G model) computed from an amino acid alignment containing sequences of type III PKS homologs from cyanobacteria (retrieved from the GenBank) as well as characterized bacterial type III PKSs (shown in bold). (B) – Selected BGCs from each clade (as represented in the phylogenetic tree) of cyanobacterial type III PKSs, the position of the type III PKS-encoding gene within each BGC is underlined.

Fig. 10. Phylogenetic diversity of BrtD enzymes in cyanobacteria. (A) Maximum-likelihood tree (JTT + G model) computed from an amino acid alignment containing sequences of the top BrtD (DarB) homologs from bacteria (retrieved from the GenBank), characterized DarB homologs (corresponding natural products shown next to taxa), and sequences of other ketosynthases from bacteria (selected based on Fuchs et al.). (B) Selected BGCs from each clade depicted in the phylogenetic tree, darA and darB homologs are underlined * the putative microcarbonin A BGC assignment is not supported by direct biochemical evidence.

Scheme 1. Synthesis of hierridins 4–6 and 8.,,

Scheme 2. Ring closing approaches implemented in the syntheses of [7.7]paracyclophanes. Reagents and conditions:–,, (a1) Na, Me₃SiCl, heat; (a2) Zn–Hg, HCl, AcOH, heat; (b) AlCl₃; (c) –25 °C to 20 °C, then HCl; (d) schrock cat.; (e) NaH, cat. 15-crown-5; (f1) NaOMe; (f2) H₂O₂, cat. (NH₄)₆Mo₇O₂₄; (f3) KOH/Al₂O₃, CF₂Br₂, 0 °C to 23 °C.

Scheme 3. Synthesis of cylindrocyclophane F (25) by Smith and co-workers.,,

Scheme 4. Total synthesis of cylindrocyclophane A (16) by Smith and co-workers (A and B) and Yamakoshi and co-workers (C and B).,

Scheme 5. Synthesis of cylindrocyclophane A (16) by Hoye and co-workers and Nicolaou's approach to synthesise cylindrocyclophanes A (16) and F (25).,

Teresa P. Martins

Caroline Rouger

Nathaniel R. Glasser

Sara Freitas

Nelly B. de Fraissinette

Emily P. Balskus

Deniz Tasdemir

Pedro N. Leão