A new [7.7]paracyclophane from Vietnamese marine snail Planaxis sulcatus (Born, 1780)

New Nostocyclophanes from Nostoc linckia

Six new nostocyclophanes and four known compounds have been isolated from Nostoc linckia (Nostocaceae) cyanobacterial strain UTEX B1932. The new compounds, nostocyclophanes E-J (1-6), were characterized by NMR and MS techniques. The known compounds were nostocyclophanes B-D, previously isolated from this strain, and dedichloronostocyclophane D. Structural modifications on the new [7.7]paracyclophane analogs 1-5, isolated from the 80% methanol fraction, range from simple changes such as the lack of methylation or halogenation to more unusual modifications such as those seen in nostocyclophane H (4), in which the exocyclic alkyl chains are of different length; this is the first time this modification has been observed in this family of natural products. In addition, nostocyclophane J (6) is a linear analog in which C-20 is chlorinated in preparation for the presumed enzymatic Friedel-Craft cyclization needed to form the final ring structure, analogous to the biosynthesis of the related cylindrocyclophanes. Nostocyclophane D, dedichloronostocyclophane D, and nostocyclophanes E-J demonstrated moderate to weak growth inhibition against MDA-MB-231 breast cancer cells.

Marine natural products

Covering: 2020This review covers the literature published in 2020 for marine natural products (MNPs), with 757 citations (747 for the period January to December 2020) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1407 in 420 papers for 2020), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. A meta analysis of bioactivity data relating to new MNPs reported over the last five years is also presented.

Aplydactylonins A-C, three new sesquiterpenes from the Vietnamese sea hare Aplysia dactylomela and their cytotoxicity

Aplydactylonins A-C (1-3), three new sesquiterpenes, were isolated from the Vietnamese sea hare Aplysia dactylomela. Their structures and absolute configurations were elucidated based on spectroscopic analysis, X-ray crystallography, and density functional theory (DFT) calculations of NMR and ECD data. Compound 2 exhibited cytotoxicity against HepG2, DU145 and A549 cells with respective IC₅₀ values of 4.08 ± 0.63, 38.64 ± 1.04 and 12.33 ± 0.95 µM. In addition, HepG2 cells treated with 5 μM compound 2 for 48 h showed a significant increase in early apoptotic cells (P < 0.05) and increased caspase 3 activity (P < 0.01). Moreover, compound 2 induced sub-G1 phase arrest in HepG2 cells.

Genome Mining and Evolutionary Analysis Reveal Diverse Type III Polyketide Synthase Pathways in Cyanobacteria

Cyanobacteria are prolific producers of natural products, including polyketides and hybrid compounds thereof. Type III polyketide synthases (PKSs) are of particular interest, due to their wide substrate specificity and simple reaction mechanism, compared with both type I and type II PKSs. Surprisingly, only two type III PKS products, hierridins, and (7.7)paracyclophanes, have been isolated from cyanobacteria. Here, we report the mining of 517 cyanobacterial genomes for type III PKS biosynthesis gene clusters. Approximately 17% of the genomes analyzed encoded one or more type III PKSs. Together with already characterized type III PKSs, the phylogeny of this group of enzymes was investigated. Our analysis showed that type III PKSs in cyanobacteria evolved into three major lineages, including enzymes associated with 1) (7.7)paracyclophane-like biosynthesis gene clusters, 2) hierridin-like biosynthesis gene clusters, and 3) cytochrome b5 genes. The evolutionary history of these enzymes is complex, with some sequences partitioning primarily according to speciation and others putatively according to their reaction type. Protein modeling showed that cyanobacterial type III PKSs generally have a smaller active site cavity (mean = 109.035 ų) compared with enzymes from other organisms. The size of the active site did not correlate well with substrate size, however, the “Gatekeeper” amino acid residues within the active site were strongly correlated to enzyme phylogeny. Our study provides unprecedented insight into the distribution, diversity, and molecular evolution of cyanobacterial type III PKSs, which could facilitate the discovery, characterization, and exploitation of novel enzymes, biochemical pathways, and specialized metabolites from this biosynthetically talented clade of microorganisms.