Origin of the C3-unit in placidenes: further insights into taxa divergence of polypropionate biosynthesis in marine molluscs and fungi

Ocellatuspyrones A‒G, new antibacterial polypropionates from the Chinese mollusk Placobranchus ocellatus

Marine invertebrates serve as rich sources of secondary metabolites with intriguing chemical diversities and a wide spectrum of biological activities. Particularly, marine shell-less sacoglossan mollusks have attracted much attentions due to the fact that mollusks apply complex metabolites as chemical defense agents against to their predators. With the purpose of discovering bioactive secondary metabolites to develop marine-derived medicines from the South China Sea, we have conducted a chemical study on the photosynthetic mollusk Placobranchus ocellatus. As a result, seven new γ-pyrone polypropionates, namely ( ±)-ocellatuspyrone A (1), ( ±)-ocellatuspyrone B (2), and ocellatuspyrones C-G (5, 9-12), along with five known polypropionates, have been isolated and characterized from the South China Sea photosynthetic mollusk Placobranchus ocellatus. Extensive spectroscopic analysis, single crystal X-ray diffraction analysis, modified Mosher's method, ECD comparison, CD exciton chirality method, TDDFT-ECD calculation, and chemical conversion were used to determine the structures and absolute configurations of the new compounds and the stereochemistry of undefined known compounds 4, 6 and 7. All these isolated polypropionates were evaluated in bioassays for their biological activities, including antibacterial, neuroprotective effect, anti-inflammatory, PTP1B inhibitory, and antiviral activities. Compounds 7, 8 and 11 were found for the first time to show antibacterial activity against fish pathogenic bacteria Streptococcus parauberis (the main pathogen causing fish streptococcal infections and acute death) with MIC values of 35.8, 34.2, and 37.4 μg/mL, respectively, which might be potential novel antibacterial agents for the treatment of fish infectious diseases.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-023-00179-w.

Three new compounds from the marine-derived fungus Trichoderma atroviride G20-12

Three new compounds, 4'-(4,5-dimethyl-1,3-dioxolan-2-yl)methyl-phenol (1), (3'-hydroxybutan-2'-yl)5-oxopyrrolidine-2-carboxylate (2), and atroviridetide (3), have been isolated from the marine-derived fungus Trichoderma atroviride G 20-12. Their structures were determined on the basis of chemical and spectroscopic methods.

One metabolite, two pathways: convergence of polypropionate biosynthesis in fungi and marine molluscs

Structural similarity or even the identity of polyketide compounds does not necessarily imply unique biosynthesis. Feeding experiments with a (13)C labeled precursor establish that the C(3) units in 7-methyl-cyercene-1 (1) are derived from intact propionate in the marine mollusc Ercolania funerea. The same compound in the terrestrial fungus Leptosphaeria maculans/Phoma lingam is synthesized by an acetate/SAM pathway thus proving for the first time metabolic convergence of polyketide biosynthesis in eukaryotes. Traditional (1)H-(13)C NMR correlation spectroscopy has been successfully applied to estimate (13)C incorporation in biosynthetic experiments.

Expression pattern of polyketide synthase-2 during sea urchin development

Polyketide synthases (PKSs) are a large group of proteins responsible for the biosynthesis of polyketide compounds, which are mainly found in bacteria, fungi, and plants. Polyketides have a wide array of biological functions, including antibiotic, antifungal, predator defense, and light responses. In this study, we describe the developmental expression pattern of pks2, one of two pks found in the sea urchin genome. Throughout development, pks2 expression was restricted to skeletogenic cells and their precursors. Pks2 was first detected during the blastula stage. The transcript level peaked at hatched blastula, when all skeletogenic cell precursors expressed pks2. This was followed by a steady decline in expression in the skeletogenic cells on the aboral side of the embryo. By the prism stage, pks2 expression was limited to only 3-4 skeletogenic cells localized on the oral side.